Tipping points in the climate system

File:Fig 1 Forcing a system past a tipping point.png

File:Positive tipping point in society.png

{{See also|Abrupt climate change}}

The IPCC Sixth Assessment Report defines a tipping point as a "critical threshold beyond which a system reorganizes, often abruptly and/or irreversibly".{{Cite web |title=IPCC AR6 WG1 Ch4 |url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_04.pdf |url-status=live |page=95 |access-date=14 November 2021 |archive-date=5 September 2021 |archive-url=https://web.archive.org/web/20210905113907/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_04.pdf}} It can be brought about by a small disturbance causing a disproportionately large change in the system. It can also be associated with self-reinforcing feedbacks, which could lead to changes in the climate system irreversible on a human timescale.{{Cite web |date=10 February 2020 |title=Explainer: Nine "tipping points" that could be triggered by climate change |url=https://www.carbonbrief.org/explainer-nine-tipping-points-that-could-be-triggered-by-climate-change/ |access-date=16 July 2022 |website=Carbon Brief}} For any particular climate component, the shift from one state to a new stable state may take many decades or centuries.

The 2019 IPCC Special Report on the Ocean and Cryosphere in a Changing Climate defines a tipping point as: "A level of change in system properties beyond which a system reorganises, often in a non-linear manner, and does not return to the initial state even if the drivers of the change are abated. For the climate system, the term refers to a critical threshold at which global or regional climate changes from one stable state to another stable state.".{{Cite web |title=Glossary — Special Report on the Ocean and Cryosphere in a Changing Climate |url=https://www.ipcc.ch/srocc/chapter/glossary/ |access-date=10 July 2021 |archive-date=16 August 2021 |archive-url=https://web.archive.org/web/20210816015954/https://www.ipcc.ch/srocc/chapter/glossary/ |url-status=live}}

In ecosystems and in social systems, a tipping point can trigger a regime shift, a major systems reorganisation into a new stable state.{{Cite journal |last1=Heinze |first1=Christoph |last2=Blenckner |first2=Thorsten |last3=Martins |first3=Helena |last4=Rusiecka |first4=Dagmara |last5=Döscher |first5=Ralf |last6=Gehlen |first6=Marion |last7=Gruber |first7=Nicolas |last8=Holland |first8=Elisabeth |last9=Hov |first9=Øystein |last10=Joos |first10=Fortunat |last11=Matthews |first11=John Brian Robin |date=2021 |title=The quiet crossing of ocean tipping points |journal=Proceedings of the National Academy of Sciences |volume=118 |issue=9 |pages=e2008478118 |doi=10.1073/pnas.2008478118 |issn=0027-8424 |pmc=7936299 |pmid=33619085|bibcode=2021PNAS..11808478H |doi-access=free }} Such regime shifts need not be harmful. In the context of the climate crisis, the tipping point metaphor is sometimes used in a positive sense, such as to refer to shifts in public opinion in favor of action to mitigate climate change, or the potential for minor policy changes to rapidly accelerate the transition to a green economy.{{cite book|author=Michael E. Mann| title =The New Climate War: The Fight to Take Back Our Planet| year = 2021|pages = 231–238| isbn = 978-1-541-75822-3|publisher=PublicAffairs|ref = CITEREFMann2021}}{{Cite news |date=20 January 2023 |title='Super-tipping points' could trigger cascade of climate action |url=https://www.theguardian.com/environment/2023/jan/20/super-tipping-points-climate-electric-cars-meat-emissions |author = Damian Carrington|newspaper=the Guardian}}{{Cite journal |last1=Lenton |first1=Timothy M. |last2=Benson |first2=Scarlett |last3=Smith |first3=Talia |last4=Ewer |first4=Theodora |last5=Lanel |first5=Victor |last6=Petykowski |first6=Elizabeth |last7=Powell |first7=Thomas W. R. |last8=Abrams |first8=Jesse F. |last9=Blomsma |first9=Fenna |last10=Sharpe |first10=Simon |date=2022 |title=Operationalising positive tipping points towards global sustainability |url=https://www.cambridge.org/core/journals/global-sustainability/article/operationalising-positive-tipping-points-towards-global-sustainability/8E318C85A8E462AEC26913EC43FE60B1 |journal=Global Sustainability |language=en |volume=5 |doi=10.1017/sus.2021.30 |bibcode=2022GlSus...5E...1L |s2cid=235719545 |issn=2059-4798|hdl=10871/126085 |hdl-access=free }}

Scientists have identified many elements in the climate system which may have tipping points.Defined in [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_04.pdf IPCC_AR6_WGI_Chapter_04] {{Webarchive|url=https://web.archive.org/web/20210905113907/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_04.pdf |date=5 September 2021 }}, p.95, line 34. In the early 2000s the IPCC began considering the possibility of tipping points, originally referred to as large-scale discontinuities. At that time the IPCC concluded they would only be likely in the event of global warming of {{convert|4|C-change|F-change}} or more above preindustrial times, and another early assessment placed most tipping point thresholds at {{convert|3-5|C-change|F-change}} above 1980–1999 average warming.{{cite journal |last1=Lenton |first1=Timothy M. |last2=Held |first2=Hermann |last3=Kriegler |first3=Elmar |last4=Hall |first4=Jim W |last5=Lucht |first5=Wolfgang |last6=Rahmstorf |first6=Stefan |last7=Schellnhuber |first7=Hans Joachim |title=Tipping elements in the Earth's climate system |journal=PNAS |date=2008-02-12 |volume=105 |issue=6 |pages=1786–1793 |doi=10.1073/pnas.0705414105 |pmid=18258748 |pmc=2538841 |bibcode=2008PNAS..105.1786L |doi-access=free }} Since then estimates for global warming thresholds have generally fallen, with some thought to be possible in the Paris Agreement range ({{convert|1.5-2|C-change|F-change}}) by 2016.{{Cite journal |last1=Schellnhuber |first1=Hans Joachim |last2=Rahmstorf |first2=Stefan |last3=Winkelmann |first3=Ricarda |date=2016 |title=Why the right climate target was agreed in Paris |url=https://www.nature.com/articles/nclimate3013 |journal=Nature Climate Change |volume=6 |issue=7 |pages=649–653 |doi=10.1038/nclimate3013 |bibcode=2016NatCC...6..649S |issn=1758-6798}} As of 2021 tipping points are considered to have significant probability at today's warming level of just over {{convert|1|C-change|F-change}}, with high probability above {{convert|2|C-change|F-change}} of global warming. Some tipping points may be close to being crossed or have already been crossed, like those of the ice sheets in West Antarctic and Greenland, warm-water coral reefs, and the Amazon rainforest.{{Cite news |date=28 July 2021 |title=Critical measures of global heating reaching tipping point, study finds |url=http://www.theguardian.com/environment/2021/jul/27/global-heating-critical-measures-tipping-point-study |newspaper=the Guardian}}{{Cite journal |last1=Ripple |first1=William J |last2=Wolf |first2=Christopher |last3=Newsome |first3=Thomas M. |last4=Gregg |first4=Jillian W. |last5=Lenton |first5=Tim |author-link5=Tim Lenton |last6=Palomo |first6=Ignacio |last7=Eikelboom |first7=Jasper A. J. |last8=Law |first8=Beverly E. |last9=Huq |first9=Saleemul |last10=Duffy |first10=Philip B. |last11=Rockström |first11=Johan |date=28 July 2021 |title=World Scientists' Warning of a Climate Emergency 2021 |url=https://doi.org/10.1093/biosci/biab079 |journal=BioScience |volume=71 |issue=biab079 |pages=894–898 |doi=10.1093/biosci/biab079 |issn=0006-3568 |hdl-access=free |hdl=1808/30278}}

As of September 2022, nine global core tipping elements and seven regional impact tipping elements have been identified.{{Cite journal |last1=Armstrong McKay |first1=David|last2=Abrams |first2=Jesse |last3=Winkelmann |first3=Ricarda |last4=Sakschewski |first4=Boris |last5=Loriani |first5=Sina |last6=Fetzer |first6=Ingo|last7=Cornell|first7=Sarah |last8=Rockström |first8=Johan |last9=Staal |first9=Arie |last10=Lenton |first10=Timothy |date=9 September 2022 |title=Exceeding 1.5°C global warming could trigger multiple climate tipping points |url=https://www.science.org/doi/10.1126/science.abn7950 |journal=Science |language=en |volume=377 |issue=6611 |pages=eabn7950 |doi=10.1126/science.abn7950 |pmid=36074831 |hdl=10871/131584 |s2cid=252161375 |issn=0036-8075|hdl-access=free }} Out of those, one regional and three global climate elements are estimated to likely pass a tipping point if global warming reaches {{convert|1.5|C-change|F-change}}, namely Greenland ice sheet collapse, West Antarctic ice sheet collapse, tropical coral reef die off, and boreal permafrost abrupt thaw. Two further tipping points are forecast as likely if warming continues to approach {{convert|2|C-change|F-change}}: Barents sea ice abrupt loss, and the Labrador sea subpolar gyre collapse.{{Cite web |last=Baker |first=Harry |date=15 September 2022 |title=Climate "points of no return" may be much closer than we thought |url=https://www.livescience.com/climate-tipping-points-closer-than-realized |access-date=18 September 2022 |website=livescience.com |language=en}}{{Cite web |last=Armstrong McKay |first=David |date=9 September 2022 |title=Exceeding 1.5°C global warming could trigger multiple climate tipping points – paper explainer |url=https://climatetippingpoints.info/2022/09/09/climate-tipping-points-reassessment-explainer/ |access-date=2 October 2022 |website=climatetippingpoints.info |language=en}}

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File:Mean regional trends in ice thickness and front position.webp

The Greenland ice sheet is the second largest ice sheet in the world, and the water which it holds, if completely melted, would raise sea levels globally by 7.2 metres (24 ft).{{Cite web |title=Quick Facts on Ice Sheets |url=https://nsidc.org/cryosphere/quickfacts/icesheets.html |access-date=17 July 2022 |website=National Snow and Ice Data Center}}{{Cite web |title=New climate models suggest faster melting of the Greenland Ice Sheet |url=https://www.weforum.org/agenda/2020/12/new-climate-models-greenland-ice-sheet-sea-levels-global-warming/ |access-date=17 July 2022 |website=World Economic Forum|date=21 December 2020 }} Due to global warming, the ice sheet is melting at an accelerating rate, adding almost 1 mm to global sea levels every year.{{Cite journal |last1=Scambos |first1=Ted |last2=Straneo |first2=Fiamma |last3=Tedesco |first3=Marco |date=2021 |title=How fast is the Greenland ice sheet melting? |journal=Arctic, Antarctic, and Alpine Research |volume=53 |issue=1 |pages=221–222 |doi=10.1080/15230430.2021.1946241 |s2cid=242536272 |issn=1523-0430|doi-access=free |bibcode=2021AAAR...53..221S }} Around half of the ice loss occurs via surface melting, and the remainder occurs at the base of the ice sheet where it touches the sea, by calving (breaking off) icebergs from its margins.{{Cite journal |last1=Todd |first1=Joe |last2=Christoffersen |first2=Poul |last3=Zwinger |first3=Thomas |last4=Råback |first4=Peter |last5=Chauché |first5=Nolwenn |last6=Benn |first6=Doug |last7=Luckman |first7=Adrian |last8=Ryan |first8=Johnny |last9=Toberg |first9=Nick |last10=Slater |first10=Donald |last11=Hubbard |first11=Alun |date=2018 |title=A Full-Stokes 3-D Calving Model Applied to a Large Greenlandic Glacier |url=http://doi.wiley.com/10.1002/2017JF004349 |journal=Journal of Geophysical Research: Earth Surface |volume=123 |issue=3 |pages=410–432 |doi=10.1002/2017JF004349|bibcode=2018JGRF..123..410T |s2cid=54546830 }}

The Greenland ice sheet has a tipping point because of the melt-elevation feedback. Surface melting reduces the height of the ice sheet, and air at a lower altitude is warmer. The ice sheet is then exposed to warmer temperatures, accelerating its melt.{{Cite journal |last1=Boers |first1=Niklas |last2=Rypdal |first2=Martin |date=2021 |title=Critical slowing down suggests that the western Greenland Ice Sheet is close to a tipping point |journal=Proceedings of the National Academy of Sciences |volume=118 |issue=21 |pages=e2024192118 |doi=10.1073/pnas.2024192118 |issn=0027-8424 |pmc=8166178 |pmid=34001613|bibcode=2021PNAS..11824192B |doi-access=free }} A 2021 analysis of sub-glacial sediment at the bottom of a {{convert|1.4|km|mi}} Greenland ice core finds that the Greenland ice sheet melted away at least once during the last million years, and therefore strongly suggests that its tipping point is below the {{convert|2.5|C-change|F-change}} maximum temperature increase over the preindustrial conditions observed over that period.{{cite news |last1=Garric |first1=Audrey |title=La calotte glaciaire du Groenland a déjà fondu au moins une fois au cours du dernier million d'années|journal=Le Monde |date=15 March 2021 |url=https://www.lemonde.fr/planete/article/2021/03/15/la-calotte-glaciaire-du-groenland-a-deja-fondu-au-moins-une-fois-au-cours-du-dernier-million-d-annees_6073234_3244.html}}{{cite journal |last1=Christ |first1=Andrew J. |last2=Bierman |first2=Paul R. |last3=Schaefer |first3=Joerg M.|last4=Dahl-Jensen |first4=Dorthe |last5=Steffensen |first5=Jørgen P. |last6=Corbett |first6=Lee B. |last7=Peteet |first7=Dorothy M. |last8=Thomas |first8=Elizabeth K. |last9=Steig |first9=Eric J. |last10=Rittenour |first10=Tammy M. |last11=Tison |first11=Jean-Louis |last12=Blard |first12=Pierre-Henri |last13=Perdrial |first13=Nicolas |last14=Dethier |first14=David P. |last15=Lini |first15=Andrea |last16=Hidy |first16=Alan J. |last17=Caffee |first17=Marc W. |last18=Southon |first18=John |title=A multimillion-year-old record of Greenland vegetation and glacial history preserved in sediment beneath 1.4 km of ice at Camp Century |journal=Proceedings of the National Academy of Sciences of the United States|date=30 March 2021 |volume=118 |issue=13 |pages=e2021442118 |doi=10.1073/pnas.2021442118 |pmid=33723012 |pmc=8020747 |bibcode=2021PNAS..11821442C |doi-access=free }} There is some evidence that the Greenland ice sheet is losing stability, and getting close to a tipping point.

File:AntarcticBedrock.jpg map of Antarctica without its ice sheets, assuming constant sea levels and no post-glacial rebound]]

The West Antarctic Ice Sheet (WAIS) is a large ice sheet in Antarctica; in places more than {{convert|4|km|mi}} thick. It sits on bedrock mostly below sea level, having formed a deep subglacial basin due to the weight of the ice sheet over millions of years.{{Cite journal |last1=Fretwell |first1=P. |last2=Pritchard |first2=H. D. |last3=Vaughan |first3=D. G. |last4=Bamber |first4=J. L. |last5=Barrand |first5=N. E. |last6=Bell |first6=R. |last7=Bianchi |first7=C. |last8=Bingham |first8=R. G. |last9=Blankenship |first9=D. D. |last10=Casassa |first10=G. |last11=Catania |first11=G. |date=28 February 2013 |title=Bedmap2: improved ice bed, surface and thickness datasets for Antarctica |url=https://tc.copernicus.org/articles/7/375/2013/ |journal=The Cryosphere |volume=7 |issue=1 |pages=375–393 |doi=10.5194/tc-7-375-2013 |bibcode=2013TCry....7..375F |s2cid=13129041 |issn=1994-0416|doi-access=free }} As such, it is in contact with the heat from the ocean which makes it vulnerable to fast and irreversible ice loss. A tipping point could be reached once the WAIS's grounding lines (the point at which ice no longer sits on rock and becomes floating ice shelves) retreat behind the edge of the subglacial basin, resulting in self-sustaining retreat in to the deeper basin - a process known as the Marine Ice Sheet Instability (MISI).{{cite journal |last1=Hulbe |first1=Christina |title=Is ice sheet collapse in West Antarctica unstoppable? |journal=Science |date=2017 |volume=356 |issue=6341 |pages=910–911 |doi=10.1126/science.aam9728 |pmid=28572353 |bibcode=2017Sci...356..910H |s2cid=206658277 |url=http://www.sciencemag.org/lookup/doi/10.1126/science.aam9728}}{{cite journal |last1=Alley |first1=Richard B. |last2=Anandakrishnan |first2=Sridhar |last3=Christianson |first3=Knut |last4=Horgan |first4=Huw J. |last5=Muto |first5=Atsu |last6=Parizek |first6=Byron R. |last7=Pollard |first7=David |last8=Walker |first8=Ryan T. |title=Oceanic Forcing of Ice-Sheet Retreat: West Antarctica and More |journal=Annual Review of Earth and Planetary Sciences |date=2015 |volume=43 |issue=1 |pages=207–231 |doi=10.1146/annurev-earth-060614-105344|bibcode=2015AREPS..43..207A |s2cid=131486847 }} Thinning and collapse of the WAIS's ice shelves is helping to accelerate this grounding line retreat. If completely melted, the WAIS would contribute around {{convert|3.3|m|ft|frac=2}} of sea level rise over thousands of years.

Ice loss from the WAIS is accelerating, and some outlet glaciers are estimated to be close to or possibly already beyond the point of self-sustaining retreat.{{Cite journal |last1=Shepherd |first1=Andrew |last2=Ivins |first2=Erik |last3=Rignot |first3=Eric |last4=Smith |first4=Ben |last5=van den Broeke |first5=Michiel |last6=Velicogna |first6=Isabella |last7=Whitehouse |first7=Pippa |last8=Briggs |first8=Kate |last9=Joughin |first9=Ian |last10=Krinner |first10=Gerhard |last11=Nowicki |first11=Sophie |date=2018 |title=Mass balance of the Antarctic Ice Sheet from 1992 to 2017 |url=https://www.nature.com/articles/s41586-018-0179-y |journal=Nature |volume=558 |issue=7709 |pages=219–222 |doi=10.1038/s41586-018-0179-y |pmid=29899482 |bibcode=2018Natur.558..219I |hdl=2268/225208 |s2cid=186244208 |issn=1476-4687}}{{cite journal |last1=Feldmann |first1=Johannes |last2=Levermann |first2=Anders |title=Collapse of the West Antarctic Ice Sheet after local destabilization of the Amundsen Basin |journal=Proceedings of the National Academy of Sciences |date=17 November 2015 |volume=112 |issue=46 |pages=14191–14196 |doi=10.1073/pnas.1512482112 |pmid=26578762 |pmc=4655561 |bibcode=2015PNAS..11214191F |doi-access=free }}{{cite journal |last1=Joughin |first1=Ian |last2=Smith |first2=Benjamin E. |last3=Medley |first3=Brooke |last4=Seroussi |first4=H. |last5=Scheuchl |first5=B. |title=Marine Ice Sheet Collapse Potentially Under Way for the Thwaites Glacier Basin, West Antarctica |journal=Science |date=16 May 2014 |volume=344 |issue=6185 |pages=735–738 |doi=10.1126/science.1249055|pmid=24821948 |bibcode=2014Sci...344..735J |s2cid=206554077 |doi-access=free }} The paleo record suggests that during the past few hundred thousand years, the WAIS largely disappeared in response to similar levels of warming and {{CO2|link=Carbon dioxide}} emission scenarios projected for the next few centuries.{{Cite journal |last1=Joughin |first1=Ian |last2=Alley |first2=Richard B. |date=2011 |title=Stability of the West Antarctic ice sheet in a warming world |url=https://www.nature.com/articles/ngeo1194 |journal=Nature Geoscience |volume=4 |issue=8 |pages=506–513 |doi=10.1038/ngeo1194 |bibcode=2011NatGe...4..506J |issn=1752-0908}}

Like with the other ice sheets, there is a counteracting negative feedback - greater warming also intensifies the effects of climate change on the water cycle, which result in an increased precipitation over the ice sheet in the form of snow during the winter, which would freeze on the surface, and this increase in the surface mass balance (SMB) counteracts some fraction of the ice loss. In the IPCC Fifth Assessment Report, it was suggested that this effect could potentially overpower increased ice loss under the higher levels of warming and result in small net ice gain, but by the time of the IPCC Sixth Assessment Report, improved modelling had proven that the glacier breakup would consistently accelerate at a faster rate.Justin Gillis (March 22, 2016) [https://www.nytimes.com/2016/03/23/science/global-warming-sea-level-carbon-dioxide-emissions.html "Scientists Warn of Perilous Climate Shift Within Decades, Not Centuries"] New York Times{{Cite journal |last1=Fox-Kemper |first1=B. |last2=Hewitt |first2=H.T.|author2-link=Helene Hewitt |last3=Xiao |first3=C. |last4=Aðalgeirsdóttir |first4=G. |last5=Drijfhout |first5=S.S. |last6=Edwards |first6=T.L. |last7=Golledge |first7=N.R. |last8=Hemer |first8=M. |last9=Kopp |first9=R.E. |last10=Krinner |first10=G. |last11=Mix |first11=A. |date=2021 |editor-last=Masson-Delmotte |editor-first=V. |editor2-last=Zhai |editor2-first=P. |editor3-last=Pirani |editor3-first=A. |editor4-last=Connors |editor4-first=S.L. |editor5-last=Péan |editor5-first=C. |editor6-last=Berger |editor6-first=S. |editor7-last=Caud |editor7-first=N. |editor8-last=Chen |editor8-first=Y. |editor9-last=Goldfarb |editor9-first=L. |title=Chapter 9: Ocean, Cryosphere and Sea Level Change |journal=Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter09.pdf |publisher=Cambridge University Press, Cambridge, UK and New York, NY, USA |pages=1270–1272 }}

East Antarctic ice sheet is the largest and thickest ice sheet on Earth, with the maximum thickness of {{convert|4800|m|mi}}. A complete disintegration would raise the global sea levels by {{convert|53.3|m|ft}}, but this may not occur until global warming of {{convert|10|C-change|F-change}}, while the loss of two-thirds of its volume may require at least {{convert|6|C-change|F-change}} of warming to trigger.{{Cite journal |last1=Garbe |first1=Julius |last2=Albrecht |first2=Torsten |last3=Levermann |first3=Anders |last4=Donges |first4=Jonathan F. |last5=Winkelmann |first5=Ricarda |date=2020 |title=The hysteresis of the Antarctic Ice Sheet |url=https://www.nature.com/articles/s41586-020-2727-5 |journal=Nature |volume=585 |issue=7826 |pages=538–544 |bibcode=2020Natur.585..538G |doi=10.1038/s41586-020-2727-5 |pmid=32968257 |s2cid=221885420}} Its melt would also occur over a longer timescale than the loss of any other ice on the planet, taking no less than 10,000 years to finish. However, the subglacial basin portions of the East Antarctic ice sheet may be vulnerable to tipping at lower levels of warming. The Wilkes Basin is of particular concern, as it holds enough ice to raise sea levels by about {{convert|3-4|m|ft|frac=2}}.

{{Main|Arctic sea ice decline}}

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| image1 = NASA NH decadal ice extent 2022.png

| alt1 = Average decadal extent and area of the Arctic Ocean sea ice since 1979.

| caption1 = Average decadal extent and area of the Arctic Ocean sea ice since the start of satellite observations.

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| image2 = NASA NH yearly ice extent 2022.png

| alt2 = Annual trend in the Arctic sea ice extent and area for the 2011-2022 time period.

| caption2 = Annual trend in the Arctic sea ice extent and area for the 2011-2022 time period.

}}

Arctic sea ice was once identified as a potential tipping element. The loss of sunlight-reflecting sea ice during summer exposes the (dark) ocean, which would warm. Arctic sea ice cover is likely to melt entirely under even relatively low levels of warming, and it was hypothesised that this could eventually transfer enough heat to the ocean to prevent sea ice recovery even if the global warming is reversed. Modelling now shows that this heat transfer during the Arctic summer does not overcome the cooling and the formation of new ice during the Arctic winter. As such, the loss of Arctic ice during the summer is not a tipping point for as long as the Arctic winter remains cool enough to enable the formation of new Arctic sea ice.{{Cite web |date=17 December 2021 |title=Does Arctic sea ice have a tipping point? |url=https://nsidc.org/cryosphere/icelights/2011/03/arctic-sea-ice-and-tipping-point |access-date=19 July 2022 |website=National Snow and Ice Data Center}}{{Cite book |last1=Arias |first1=Paola A. |title=IPCC AR6 WG1 |last2=Bellouin |first2=Nicolas |last3=Coppola |first3=Erika |last4=Jones |first4=Richard G. |last5=Krinner |first5=Gerhard |year=2021 |pages=76 |chapter=Technical Summary |display-authors=4 |chapter-url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf}} However, if the higher levels of warming prevent the formation of new Arctic ice even during winter, then this change may become irreversible. Consequently, Arctic Winter Sea Ice is included as a potential tipping point in a 2022 assessment.

Additionally, the same assessment argued that while the rest of the ice in the Arctic Ocean may recover from a total summertime loss during the winter, ice cover in the Barents Sea may not reform during the winter even below {{convert|2|C-change|F-change}} of warming. This is because the Barents Sea is already the fastest-warming part of the Arctic: in 2021-2022 it was found that while the warming within the Arctic Circle has already been nearly four times faster than the global average since 1979,{{Cite journal |last1=Rantanen |first1=Mika |last2=Karpechko |first2=Alexey Yu |last3=Lipponen |first3=Antti |last4=Nordling |first4=Kalle |last5=Hyvärinen |first5=Otto |last6=Ruosteenoja |first6=Kimmo |last7=Vihma |first7=Timo |last8=Laaksonen |first8=Ari |date=11 August 2022 |title=The Arctic has warmed nearly four times faster than the globe since 1979 |journal=Communications Earth & Environment |language=en |volume=3 |issue=1 |page=168 |bibcode=2022ComEE...3..168R |doi=10.1038/s43247-022-00498-3 |issn=2662-4435 |s2cid=251498876 |doi-access=free}}{{cite web |date=2021-12-14 |title=The Arctic is warming four times faster than the rest of the world |url=https://www.science.org/content/article/arctic-warming-four-times-faster-rest-world |access-date=6 October 2022 |website=Science Magazine |language=en}} Barents Sea warmed up to seven times faster than the global average.{{Cite journal |last1=Isaksen |first1=Ketil |last2=Nordli |first2=Øyvind |display-authors=etal |date=15 June 2022 |title=Exceptional warming over the Barents area |journal=Scientific Reports |language=en |volume=12 |issue=1 |page=9371 |bibcode=2022NatSR..12.9371I |doi=10.1038/s41598-022-13568-5 |pmc=9200822 |pmid=35705593}}{{cite web |author=Damian Carrington |date=2022-06-15 |title=New data reveals extraordinary global heating in the Arctic |url=https://www.theguardian.com/environment/2022/jun/15/new-data-reveals-extraordinary-global-heating-in-the-arctic |access-date=7 October 2022 |website=The Guardian |language=en}} This tipping point matters because of the decade-long history of research into the connections between the state of Barents-Kara Sea ice and the weather patterns elsewhere in Eurasia.{{cite journal |last1=Petoukhov |first1=Vladimir |last2=Semenov |first2=Vladimir A. |year=2010 |title=A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents |url=http://oceanrep.geomar.de/8738/1/2009JD013568-pip.pdf |journal=Journal of Geophysical Research |volume=115 |issue=D21 |pages=D21111 |bibcode=2010JGRD..11521111P |doi=10.1029/2009JD013568 |doi-access=free}}{{Cite journal |last1=He |first1=Shengping |last2=Gao |first2=Yongqi |last3=Furevik |first3=Tore |last4=Wang |first4=Huijun |last5=Li |first5=Fei |date=16 December 2017 |title=Teleconnection between sea ice in the Barents Sea in June and the Silk Road, Pacific–Japan and East Asian rainfall patterns in August |url=https://link.springer.com/article/10.1007/s00376-017-7029-y |journal=Advances in Atmospheric Sciences |volume=35 |pages=52–64 |doi=10.1007/s00376-017-7029-y |s2cid=125312203}}{{Cite journal |last1=Zhang |first1=Ruonan |last2=Screen |first2=James A. |date=16 June 2021 |title=Diverse Eurasian Winter Temperature Responses to Barents-Kara Sea Ice Anomalies of Different Magnitudes and Seasonality |journal=Geophysical Research Letters |volume=48 |issue=13 |bibcode=2021GeoRL..4892726Z |doi=10.1029/2021GL092726 |s2cid=236235248 |doi-access=free}}{{Cite journal |last1=Song |first1=Mirong |last2=Wang |first2=Zhao-Yin |last3=Zhu |first3=Zhu |last4=Liu |first4=Ji-Ping |date=August 2021 |title=Nonlinear changes in cold spell and heat wave arising from Arctic sea-ice loss |url=https://www.sciencedirect.com/science/article/pii/S1674927821001118 |journal=Advances in Climate Change Research |volume=12 |issue=4 |pages=553–562 |bibcode=2021ACCR...12..553S |doi=10.1016/j.accre.2021.08.003 |s2cid=238716298}}{{Cite journal |last1=Sun |first1=Jianqi |last2=Liu |first2=Sichang |last3=Cohen |first3=Judah |last4=Yu |first4=Shui |date=2 August 2022 |title=Influence and prediction value of Arctic sea ice for spring Eurasian extreme heat events |journal=Communications Earth & Environment |volume=3 |issue=1 |page=172 |bibcode=2022ComEE...3..172S |doi=10.1038/s43247-022-00503-9 |s2cid=251230011 |doi-access=free}}

{{Main|Retreat of glaciers since 1850}}

File:2015-2100 Impacts of global warming on glaciers and sea level rise.svg

Mountain glaciers are the largest repository of land-bound ice after the Greenland and the Antarctica ice sheets, and they are also undergoing melting as the result of climate change. A glacier tipping point is when it enters a disequilibrium state with the climate and will melt away unless the temperatures go down.{{cite book |last1=Hubbard |first1=Bryn |url=https://books.google.com/books?id=Y9MWQT0gdBMC&pg=PA179 |title=Field Techniques in Glaciology and Glacial Geomorphology |author2=Neil F. Glasser |date=May 20, 2005 |publisher=Wiley |isbn=978-0470844274 |pages=179–198 |access-date=November 23, 2020}}{{cite journal |author=Pelto, M.S. |year=2010 |title=Forecasting temperate alpine glacier survival from accumulation zone observations |url=https://tc.copernicus.org/articles/4/67/2010/ |journal=The Cryosphere |volume=4 |issue=1 |pages=67–75 |bibcode=2010TCry....4...67P |doi=10.5194/tc-4-67-2010 |access-date=November 23, 2020 |doi-access=free}} Examples include glaciers of the North Cascade Range, where even in 2005 67% of the glaciers observed were in disequilibrium and will not survive the continuation of the present climate,{{cite web |author=Mauri S. Pelto |title=North Cascade Glacier Terminus Behavior |url=http://www.nichols.edu/departments/glacier/north%20cascade%20glacier%20retreat.htm |access-date=August 7, 2016 |publisher=Nichols College}} or the French Alps, where The Argentière and Mer de Glace glaciers are expected to disappear completely by end of the 21st century if current climate trends persist.{{cite web |last1=Vaughn |first1=Adam |date=September 18, 2019 |title=Special report: How climate change is melting France's largest glacier |url=https://www.newscientist.com/article/mg24332483-600-special-report-how-climate-change-is-melting-frances-largest-glacier/ |access-date=February 3, 2021 |publisher=New Scientist}} Altogether, it was estimated in 2023 that 49% of the world's glaciers would be lost by 2100 at {{convert|1.5|C-change|F-change}} of global warming, and 83% of glaciers would be lost at {{convert|4|C-change|F-change}}. This would amount to one quarter and nearly half of mountain glacier *mass* loss, respectively, as only the largest, most resilient glaciers would survive the century. This ice loss would also contribute ~{{cvt|9|cm|in|frac=2}} and ~{{cvt|15|cm|in|frac=2}} to sea level rise, while the current likely trajectory of {{convert|2.7|C-change|F-change}} would result in the SLR contribution of ~{{cvt|11|cm|in|frac=2}} by 2100.

The absolute largest amount of glacier ice is located in the Hindu Kush Himalaya region, which is colloquially known as the Earth's Third Pole as the result. It is believed that one third of that ice will be lost by 2100 even if the warming is limited to {{convert|1.5|C-change|F-change}}, while the intermediate and severe climate change scenarios (Representative Concentration Pathways (RCP) 4.5 and 8.5) are likely to lead to the losses of 50% and >67% of the region's glaciers over the same timeframe. Glacier melt is projected to accelerate regional river flows until the amount of meltwater peaks around 2060, going into an irreversible decline afterwards. Since regional precipitation will continue to increase even as the glacier meltwater contribution declines, annual river flows are only expected to diminish in the western basins where contribution from the monsoon is low: however, irrigation and hydropower generation would still have to adjust to greater interannual variability and lower pre-monsoon flows in all of the region's rivers.{{cite web |author=Damian Carrington |date=4 February 2019 |title=A third of Himalayan ice cap doomed, finds report |url=https://www.theguardian.com/environment/2019/feb/04/a-third-of-himalayan-ice-cap-doomed-finds-shocking-report |access-date=20 October 2022 |website=TheGuardian.com}}{{Cite book |last1=Bolch |first1=Tobias |title=The Hindu Kush Himalaya Assessment: Mountains, Climate Change, Sustainability and People |last2=Shea |first2=Joseph M. |last3=Liu |first3=Shiyin |last4=Azam |first4=Farooq M. |last5=Gao |first5=Yang |last6=Gruber |first6=Stephan |last7=Immerzeel |first7=Walter W. |last8=Kulkarni |first8=Anil |last9=Li |first9=Huilin |date=5 January 2019 |publisher=Springer |isbn=9783319922881 |pages=209–255 |chapter=Status and Change of the Cryosphere in the Extended Hindu Kush Himalaya Region |doi=10.1007/978-3-319-92288-1_3 |chapter-url=https://link.springer.com/chapter/10.1007/978-3-319-92288-1_3 |last10=Tahir |first10=Adnan A. |last11=Zhang |first11=Guoqing |last12=Zhang |first12=Yinsheng |s2cid=134572569}}{{Cite book |last1=Scott |first1=Christopher A. |title=The Hindu Kush Himalaya Assessment: Mountains, Climate Change, Sustainability and People |last2=Zhang |first2=Fan |last3=Mukherji |first3=Aditi |last4=Immerzeel |first4=Walter |last5=Mustafa |first5=Daanish |last6=Bharati |first6=Luna |date=5 January 2019 |isbn=978-3-319-92287-4 |pages=257–299 |chapter=Water in the Hindu Kush Himalaya |doi=10.1007/978-3-319-92288-1_8 |chapter-url=https://link.springer.com/chapter/10.1007/978-3-319-92288-1_8 |s2cid=133800578}}

File:Permafrost in Herschel Island 001.jpg, Canada, 2013]]

{{See also|Permafrost carbon cycle}}

File:Fig 1.2.15 Schematic showing feedback processes related to land and subsea permafrost..png

Perennially frozen ground, or permafrost, covers large fractions of land – mainly in Siberia, Alaska, northern Canada and the Tibetan plateau – and can be up to a kilometre thick.{{Cite journal |last1=Zhang |first1=T. |last2=Barry |first2=R. G. |last3=Knowles |first3=K. |last4=Heginbottom |first4=J. A. |last5=Brown |first5=J. |date=2008 |title=Statistics and characteristics of permafrost and ground-ice distribution in the Northern Hemisphere |url=https://doi.org/10.1080/10889370802175895 |journal=Polar Geography |volume=31 |issue=1–2 |pages=47–68 |doi=10.1080/10889370802175895 |bibcode=2008PolGe..31...47Z |issn=1088-937X |s2cid=129146972}} Subsea permafrost up to 100 metres thick also occurs on the sea floor under part of the Arctic Ocean.{{Cite web |title=Where is Frozen Ground? |url=https://nsidc.org/cryosphere/frozenground/whereis_fg.html |access-date=17 July 2022 |website=National Snow and Ice Data Center}} This frozen ground holds vast amounts of carbon from plants and animals that have died and decomposed over thousands of years. Scientists believe there is nearly twice as much carbon in permafrost than is present in Earth's atmosphere.

As the climate warms and the permafrost begins to thaw, carbon dioxide and methane are released into the atmosphere. With higher temperatures, microbes become active and decompose the biological material in the permafrost, some of which is irreversibly lost.{{Cite journal |last1=Schuur |first1=E. a. G. |last2=McGuire |first2=A. D. |last3=Schädel |first3=C. |last4=Grosse |first4=G. |last5=Harden |first5=J. W. |last6=Hayes |first6=D. J. |last7=Hugelius |first7=G. |last8=Koven |first8=C. D. |last9=Kuhry |first9=P. |last10=Lawrence |first10=D. M. |last11=Natali |first11=S. M. |last12=Olefeldt |first12=D. |last13=Romanovsky |first13=V. E. |last14=Schaefer |first14=K. |last15=Turetsky |first15=M. R. |date=April 2015 |title=Climate change and the permafrost carbon feedback |url=https://www.nature.com/articles/nature14338 |journal=Nature |language=en |volume=520 |issue=7546 |pages=171–179 |doi=10.1038/nature14338 |pmid=25855454 |bibcode=2015Natur.520..171S |issn=1476-4687}} While most thaw is gradual and will take centuries, abrupt thaw can occur in some places where permafrost is rich in large ice masses, which once melted cause the ground to slump or form 'thermokarst' lakes over years to decades.{{Cite web |last1=Winkelmann |first1=Ricarda |last2=Steinert |first2=Norman J |last3=Armstrong McKay |first3=David I |last4=Brovkin |first4=Victor |last5=Kääb |first5=Andreas |last6=Notz |first6=Dirk |last7=Aksenov |first7=Yevgeny |last8=Arndt |first8=Sandra |last9=Bathiany |first9=Sebastian |last10=Burke |first10=Eleanor |last11=Garbe |first11=Julius |last12=Gasson |first12=Ed |last13=Goelzer |first13=Heiko |last14=Hugelius |first14=Gustaf |display-authors=etal |date=2023-06-13 |title=Global Tipping Points Report 2023: Chapter 1.2 - Cryosphere tipping points |url=https://report-2023.global-tipping-points.org/section1/1-earth-system-tipping-points/1-2-tipping-points-in-the-cryosphere/1-2-2-current-state-of-knowledge-on-cryosphere-tipping-points/1-2-2-4-permafrost/ |access-date=2024-10-14 |website=Global Tipping Points |language=en}}{{Cite journal |last1=Turetsky |first1=Merritt R. |last2=Abbott |first2=Benjamin W. |last3=Jones |first3=Miriam C. |last4=Walter Anthony |first4=Katey |last5=Olefeldt |first5=David |last6=Schuur |first6=Edward A. G. |last7=Koven |first7=Charles |last8=McGuire |first8=A. David |last9=Grosse |first9=Guido |last10=Kuhry |first10=Peter |last11=Hugelius |first11=Gustaf |last12=Lawrence |first12=David M. |last13=Gibson |first13=Carolyn |last14=Sannel |first14=A. Britta K. |date=May 2019 |title=Permafrost collapse is accelerating carbon release |url=https://www.nature.com/articles/d41586-019-01313-4 |journal=Nature |language=en |volume=569 |issue=7754 |pages=32–34 |doi=10.1038/d41586-019-01313-4|pmid=31040419 |bibcode=2019Natur.569...32T }} These processes can become self-sustaining, leading to localised tipping dynamics, and could increase greenhouse gas emissions by around 40%.{{Cite journal |last1=Turetsky |first1=Merritt R. |last2=Abbott |first2=Benjamin W. |last3=Jones |first3=Miriam C. |last4=Anthony |first4=Katey Walter |last5=Olefeldt |first5=David |last6=Schuur |first6=Edward A. G. |last7=Grosse |first7=Guido |last8=Kuhry |first8=Peter |last9=Hugelius |first9=Gustaf |last10=Koven |first10=Charles |last11=Lawrence |first11=David M. |last12=Gibson |first12=Carolyn |last13=Sannel |first13=A. Britta K. |last14=McGuire |first14=A. David |date=February 2020 |title=Carbon release through abrupt permafrost thaw |url=https://www.nature.com/articles/s41561-019-0526-0 |journal=Nature Geoscience |language=en |volume=13 |issue=2 |pages=138–143 |doi=10.1038/s41561-019-0526-0 |bibcode=2020NatGe..13..138T |issn=1752-0908}} Because {{CO2}} and methane are both greenhouse gases, they act as a self-reinforcing feedback on permafrost thaw, but are unlikely to lead to a global tipping point or runaway warming process.{{Cite web |last=Viglione |first=Giuliana |date=14 March 2022 |title='Imminent' tipping point threatening Europe's permafrost peatlands |url=https://www.carbonbrief.org/imminent-tipping-point-threatening-europes-permafrost-peatlands/ |access-date=16 July 2022 |website=Carbon Brief}}{{Cite journal |last1=Fewster |first1=Richard E. |last2=Morris |first2=Paul J. |last3=Ivanovic |first3=Ruza F. |last4=Swindles |first4=Graeme T. |last5=Peregon |first5=Anna M. |last6=Smith |first6=Christopher J. |date=2022 |title=Imminent loss of climate space for permafrost peatlands in Europe and Western Siberia |url=https://www.nature.com/articles/s41558-022-01296-7 |url-status=bot: unknown |journal=Nature Climate Change |volume=12 |issue=4 |pages=373–379 |bibcode=2022NatCC..12..373F |doi=10.1038/s41558-022-01296-7 |issn=1758-6798 |s2cid=247440316 |archive-url=https://web.archive.org/web/20220909122005/https://www.nature.com/articles/s41558-022-01296-7 |archive-date=9 September 2022 |access-date=16 July 2022}}{{Cite journal |last1=Nitzbon |first1=Jan |last2=Schneider von Deimling |first2=Thomas |last3=Aliyeva |first3=Mehriban |last4=Chadburn |first4=Sarah E. |last5=Grosse |first5=Guido |last6=Laboor |first6=Sebastian |last7=Lee |first7=Hanna |last8=Lohmann |first8=Gerrit |last9=Steinert |first9=Norman J. |last10=Stuenzi |first10=Simone M. |last11=Werner |first11=Martin |last12=Westermann |first12=Sebastian |last13=Langer |first13=Moritz |date=June 2024 |title=No respite from permafrost-thaw impacts in the absence of a global tipping point |url=https://www.nature.com/articles/s41558-024-02011-4 |journal=Nature Climate Change |language=en |volume=14 |issue=6 |pages=573–585 |doi=10.1038/s41558-024-02011-4 |bibcode=2024NatCC..14..573N |issn=1758-6798}}

{{Main|Atlantic meridional overturning circulation#Slowdown or possible shutdown of the thermohaline circulation}}

File:OCP07 Fig-6.jpg

The Atlantic Meridional Overturning Circulation (AMOC), also known as the Gulf Stream System, is a large system of ocean currents.{{Cite web |last=Potsdam Institute for Climate Impact Research |title=Gulf Stream System at its weakest in over a millennium |url=https://www.sciencedaily.com/releases/2021/02/210225113357.htm |access-date=17 July 2022 |website=ScienceDaily}}{{Cite web |title=What is the Atlantic Meridional Overturning Circulation? |url=https://www.metoffice.gov.uk/weather/learn-about/weather/oceans/amoc |access-date=26 November 2021 |website=Met Office}} It is driven by differences in the density of water; colder and more salty water is heavier than warmer fresh water. The AMOC acts as a conveyor belt, sending warm surface water from the tropics north, and carrying cold fresh water back south. As warm water flows northwards, some evaporates which increases salinity. It also cools when it is exposed to cooler air. Cold, salty water is more dense and slowly begins to sink. Several kilometres below the surface, cold, dense water begins to move south. Increased rainfall and the melting of ice due to global warming dilutes the salty surface water, and warming further decreases its density. The lighter water is less able to sink, slowing down the circulation.

Theory, simplified models, and reconstructions of abrupt changes in the past suggest the AMOC has a tipping point. If freshwater input from melting glaciers reaches a certain threshold, it could collapse into a state of reduced flow. Even after melting stops, the AMOC may not return to its current state. It is unlikely that the AMOC will tip in the 21st century,{{Cite web |last= |date=December 2019 |title=Risk management of climate thresholds and feedbacks: Atlantic Meridional Overturning Circulation (AMOC) |url=https://www.metoffice.gov.uk/binaries/content/assets/metofficegovuk/pdf/weather/learn-about/climate/deliverables/amoc.pdf |access-date=25 November 2020 |website=Met Office}} but it may do so before 2300 if greenhouse gas emissions are very high. A weakening of 24% to 39% is expected depending on greenhouse emissions, even without tipping behaviour.{{Cite book |last1=Fox-Kemper |first1=Baylor |title=IPCC AR6 WG1 |last2=Hewitt |first2=Helene T. |last3=Xiao |first3=Cunde |last4=Aðalgeirsdóttir |first4=Guðfinna |last5=Drijfhout |first5=Sybren S. |last6=Edwards |first6=Tamsin L. |last7=Golledge |first7=Nicholas R. |year=2021 |at=Section 9.2.3.1 |chapter=Chapter 9: Ocean, cryosphere, and sea level change |ref={{harvid|IPCC AR6 WG1 Ch9|2021}} |display-authors=4 |chapter-url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_09.pdf}} If the AMOC does shut down, a new stable state could emerge that lasts for thousands of years, possibly triggering other tipping points.

In 2021, a study which used a primitive finite-difference ocean model estimated that AMOC collapse could be invoked by a sufficiently fast increase in ice melt even if it never reached the common thresholds for tipping obtained from slower change. Thus, it implied that the AMOC collapse is more likely than what is usually estimated by the complex and large-scale climate models.{{Cite journal |last1=Lohmann |first1=Johannes |last2=Ditlevsen |first2=Peter D. |date=2021-03-02 |title=Risk of tipping the overturning circulation due to increasing rates of ice melt |journal=Proceedings of the National Academy of Sciences |language=en |volume=118 |issue=9 |pages=e2017989118 |bibcode=2021PNAS..11817989L |doi=10.1073/pnas.2017989118 |issn=0027-8424 |pmc=7936283 |pmid=33619095 |doi-access=free}} Another 2021 study found early-warning signals in a set of AMOC indices, suggesting that the AMOC may be close to tipping.{{Cite journal |last=Boers |first=Niklas |date=2021 |title=Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation |url=https://www.nature.com/articles/s41558-021-01097-4 |journal=Nature Climate Change |volume=11 |issue=8 |pages=680–688 |bibcode=2021NatCC..11..680B |doi=10.1038/s41558-021-01097-4 |issn=1758-6798 |s2cid=236930519}} However, it was contradicted by another study published in the same journal the following year, which found a largely stable AMOC which had so far not been affected by climate change beyond its own natural variability.{{Cite journal |last1=Latif |first1=Mojib |last2=Sun |first2=Jing |last3=Visbeck |first3=Martin |last4=Bordbar |first4=M. Hadi |date=25 April 2022 |title=Natural variability has dominated Atlantic Meridional Overturning Circulation since 1900 |journal=Nature Climate Change |volume=12 |issue=5 |pages=455–460 |bibcode=2022NatCC..12..455L |doi=10.1038/s41558-022-01342-4 |s2cid=248385988 |doi-access=free}} Two more studies published in 2022 have also suggested that the modelling approaches commonly used to evaluate AMOC appear to overestimate the risk of its collapse.{{Cite journal |last1=He |first1=Feng |last2=Clark |first2=Peter U. |date=7 April 2022 |title=Freshwater forcing of the Atlantic Meridional Overturning Circulation revisited |url=https://www.nature.com/articles/s41558-022-01328-2 |journal=Nature Climate Change |volume=12 |issue=5 |pages=449–454 |bibcode=2022NatCC..12..449H |doi=10.1038/s41558-022-01328-2 |s2cid=248004571}}{{Cite journal |last1=Kim |first1=Soong-Ki |last2=Kim |first2=Hyo-Jeong |last3=Dijkstra |first3=Henk A. |last4=An |first4=Soon-Il |date=11 February 2022 |title=Slow and soft passage through tipping point of the Atlantic Meridional Overturning Circulation in a changing climate |journal=npj Climate and Atmospheric Science |volume=5 |issue=13 |doi=10.1038/s41612-022-00236-8 |s2cid=246705201 |doi-access=free|bibcode=2022npCAS...5...13K }}

{{excerpt|North Atlantic Current#Climate change|paragraphs=2-3}}

File:NOAA SMOC changes.png }}]]

{{excerpt|Southern Ocean overturning circulation|paragraphs=2,4|files=no}}

File:20220910 Amazon deforestation and degradation, by country - Amazon Watch.svg to warn that the Amazonia is in the midst of a tipping point crisis.{{cite web |title=Amazon Against the Clock: A Regional Assessment on Where and How to Protect 80% by 2025 |url=https://amazonwatch.org/assets/files/2022-amazonia-against-the-clock-executive-summary.pdf |website=Amazon Watch |archive-url=https://web.archive.org/web/20220910025229/https://amazonwatch.org/assets/files/2022-amazonia-against-the-clock-executive-summary.pdf |archive-date=10 September 2022 |page=8 |date=September 2022 |quote=Graphic 2: Current State of the Amazon by country, by percentage / Source: RAISG (Red Amazónica de Información Socioambiental Georreferenciada) Elaborated by authors. |url-status=live }}]]

{{See also|Deforestation of the Amazon rainforest|Climate change in Brazil}}

The Amazon rainforest is the largest tropical rainforest in the world. It is twice as big as India and spans nine countries in South America. It produces around half of its own rainfall by recycling moisture through evaporation and transpiration as air moves across the forest. This moisture recycling expands the area in which there is enough rainfall for rainforest to be maintained, and without it one model indicates around 40% of the current forest area would be too dry to sustain rainforest.{{Cite journal |last1=Staal |first1=Arie |last2=Fetzer |first2=Ingo |last3=Wang-Erlandsson |first3=Lan |last4=Bosmans |first4=Joyce H. C. |last5=Dekker |first5=Stefan C. |last6=van Nes |first6=Egbert H. |last7=Rockström |first7=Johan |last8=Tuinenburg |first8=Obbe A. |date=2020-10-05 |title=Hysteresis of tropical forests in the 21st century |journal=Nature Communications |language=en |volume=11 |issue=1 |pages=4978 |doi=10.1038/s41467-020-18728-7 |issn=2041-1723 |pmc=7536390 |pmid=33020475|bibcode=2020NatCo..11.4978S }} However, when forest is lost via climate change (from droughts and wildfires) or deforestation, there will be less rain in downwind regions, increasing tree stress and mortality there. Eventually, if enough forest is lost a threshold can be reached beyond which large parts of the remaining rainforest may die off and transform into drier degraded forest or savanna landscapes, particularly in the drier south and east.{{Cite journal |last=Amigo |first=Ignacio |date=2020 |title=When will the Amazon hit a tipping point? |journal=Nature |volume=578 |issue=7796 |pages=505–507 |doi=10.1038/d41586-020-00508-4|pmid=32099130 |bibcode=2020Natur.578..505A |s2cid=211265824 |doi-access=free }}{{Cite journal |last1=Flores |first1=Bernardo M. |last2=Montoya |first2=Encarni |last3=Sakschewski |first3=Boris |last4=Nascimento |first4=Nathália |last5=Staal |first5=Arie |last6=Betts |first6=Richard A. |last7=Levis |first7=Carolina |last8=Lapola |first8=David M. |last9=Esquível-Muelbert |first9=Adriane |last10=Jakovac |first10=Catarina |last11=Nobre |first11=Carlos A. |last12=Oliveira |first12=Rafael S. |last13=Borma |first13=Laura S. |last14=Nian |first14=Da |last15=Boers |first15=Niklas |date=February 2024 |title=Critical transitions in the Amazon forest system |journal=Nature |language=en |volume=626 |issue=7999 |pages=555–564 |doi=10.1038/s41586-023-06970-0 |pmid=38356065 |pmc=10866695 |bibcode=2024Natur.626..555F |issn=1476-4687}} In 2022, a study reported that the rainforest has been losing resilience since the early 2000s.{{cite journal |last1=Boulton |first1=Chris A. |last2=Lenton |first2=Tim |author-link2=Tim Lenton |last3=Boers |first3=Niklas |date=March 2022 |title=Pronounced loss of Amazon rainforest resilience since the early 2000s |journal=Nature Climate Change |volume=12 |issue=3 |pages=271–278 |bibcode=2022NatCC..12..271B |doi=10.1038/s41558-022-01287-8 |issn=1758-6798 |s2cid=247255222 |doi-access=free}} Resilience is measured by recovery-time from short-term perturbations, with delayed return to equilibrium of the rainforest termed as critical slowing down. The observed loss of resilience reinforces the theory that the rainforest could be approaching a critical transition, although it cannot determine exactly when or if a tipping point will be reached.{{cite news |title=Climate crisis: Amazon rainforest tipping point is looming, data shows |url=https://www.theguardian.com/environment/2022/mar/07/climate-crisis-amazon-rainforest-tipping-point |access-date=18 April 2022 |work=The Guardian |date=7 March 2022}}{{Cite journal |last1=Dakos |first1=Vasilis |last2=Boulton |first2=Chris A. |last3=Buxton |first3=Joshua E. |last4=Abrams |first4=Jesse F. |last5=Arellano-Nava |first5=Beatriz |last6=Armstrong McKay |first6=David I. |last7=Bathiany |first7=Sebastian |last8=Blaschke |first8=Lana |last9=Boers |first9=Niklas |last10=Dylewsky |first10=Daniel |last11=López-Martínez |first11=Carlos |last12=Parry |first12=Isobel |last13=Ritchie |first13=Paul |last14=van der Bolt |first14=Bregje |last15=van der Laan |first15=Larissa |date=2024-08-19 |title=Tipping point detection and early warnings in climate, ecological, and human systems |url=https://esd.copernicus.org/articles/15/1117/2024/ |journal=Earth System Dynamics |language=English |volume=15 |issue=4 |pages=1117–1135 |doi=10.5194/esd-15-1117-2024 |doi-access=free |bibcode=2024ESD....15.1117D |issn=2190-4979}}

During the last quarter of the twentieth century, the zone of latitude occupied by taiga experienced some of the greatest temperature increases on Earth. Winter temperatures have increased more than summer temperatures. In summer, the daily low temperature has increased more than the daily high temperature.{{Cite journal|url=http://www.arcus.org/witness-the-arctic/2009/3/article/507 |title=Coincidence and Contradiction in the Warming Boreal Forest |journal=Geophysical Research Letters |volume=32 |issue=15 |doi=10.1029/2005GL023331 |access-date=2012-01-14|bibcode=2005GeoRL..3215715W |date=2009-10-09 |last1=Wilmking |first1=M. |pages=L15715 |doi-access=free }} It has been hypothesised that the boreal environments have only a few states which are stable in the long term - a treeless tundra/steppe, a forest with >75% tree cover and an open woodland with ≈20% and ≈45% tree cover. Thus, continued climate change would be able to force at least some of the presently existing taiga forests into one of the two woodland states or even into a treeless steppe - but it could also shift tundra areas into woodland or forest states as they warm and become more suitable for tree growth.{{Cite journal |last1=Scheffer |first1=Marten |last2=Hirota |first2=Marina |last3=Holmgren |first3=Milena |author-link3=Milena Holmgren |last4=Van Nes |first4=Egbert H. |last5=Chapin |first5=F. Stuart |date=26 December 2012 |title=Thresholds for boreal biome transitions |journal=Proceedings of the National Academy of Sciences |volume=109 |issue=52 |pages=21384–21389 |bibcode=2012PNAS..10921384S |doi=10.1073/pnas.1219844110 |issn=0027-8424 |pmc=3535627 |pmid=23236159 |doi-access=free}}

File:2018 boreal tree species trends.jpg

These trends were first detected in the Canadian boreal forests in the early 2010s,{{Cite journal |last1=Peng |first1=Changhui |last2=Ma |first2=Zhihai |last3=Lei |first3=Xiangdong |last4=Zhu |first4=Qiuan |last5=Chen |first5=Huai |last6=Wang |first6=Weifeng |last7=Liu |first7=Shirong |last8=Li |first8=Weizhong |last9=Fang |first9=Xiuqin |last10=Zhou |first10=Xiaolu |date=20 November 2011 |title=A drought-induced pervasive increase in tree mortality across Canada's boreal forests |url=https://www.nature.com/articles/nclimate1293 |journal=Nature Climate Change |language=en |volume=1 |issue=9 |pages=467–471 |doi=10.1038/nclimate1293 |bibcode=2011NatCC...1..467P }}{{Cite journal |last1=Ma |first1=Zhihai |last2=Peng |first2=Changhui |last3=Zhu |first3=Qiuan |last4=Chen |first4=Huai |last5=Yu |first5=Guirui |last6=Li |first6=Weizhong |last7=Zhou |first7=Xiaolu |last8=Wang |first8=Weifeng |last9=Zhang |first9=Wenhua |date=30 January 2012 |title=Regional drought-induced reduction in the biomass carbon sink of Canada's boreal forests |journal=Proceedings of the National Academy of Sciences |language=en |volume=109 |issue=7 |pages=2423–2427 |doi=10.1073/pnas.1111576109 |pmid=22308340 |pmc=3289349 |bibcode=2012PNAS..109.2423M |doi-access=free }}{{Cite journal |last1= Chen |first1=Han Y. H. |last2=Luo |first2=Yong |date=2 July 2015 |title=Net aboveground biomass declines of four major forest types with forest ageing and climate change in western Canada's boreal forests |url=https://onlinelibrary.wiley.com/doi/10.1111/gcb.12994 |journal=Global Change Biology |language=en |volume=21 |issue=10 |pages=3675–3684 |doi=10.1111/gcb.12994 |pmid=26136379 |bibcode=2015GCBio..21.3675C |s2cid=25403205 }}{{Cite journal |last1=Sulla-Menashe |first1=Damien |last2=Woodcock |first2=Curtis E |last3=Friedl |first3=Mark A |date=4 January 2018 |title=Canadian boreal forest greening and browning trends: an analysis of biogeographic patterns and the relative roles of disturbance versus climate drivers |journal=Environmental Research Letters |language=en |volume=13 |issue=1 |pages=014007 |doi=10.1088/1748-9326/aa9b88 |bibcode=2018ERL....13a4007S |s2cid=158470300 |doi-access=free }} and summer warming had also been shown to increase water stress and reduce tree growth in dry areas of the southern boreal forest in central Alaska and portions of far eastern Russia.{{cite web |url=http://www.libraryindex.com/pages/3196/Boreal-Forests-Climate-Change.html |title=Boreal Forests and Climate Change - Changes in Climate Parameters and Some Responses, Effects of Warming on Tree Growth on Productive Sites |access-date=2011-03-25 |url-status=dead |archive-url=https://web.archive.org/web/20110727080310/http://www.libraryindex.com/pages/3196/Boreal-Forests-Climate-Change.html |archive-date=2011-07-27 }} In Siberia, the taiga is converting from predominantly needle-shedding larch trees to evergreen conifers in response to a warming climate.

Subsequent research in Canada found that even in the forests where biomass trends did not change, there was a substantial shift towards the deciduous broad-leaved trees with higher drought tolerance over the past 65 years.{{Cite journal |last1=Hisano |first1=Masumi |last2=Ryo |first2=Masahiro |last3=Chen |first3=Xinli |last4=Chen |first4=Han Y. H. |date=16 May 2021 |title=Rapid functional shifts across high latitude forests over the last 65 years |url=https://onlinelibrary.wiley.com/doi/10.1111/gcb.15710 |journal=Global Change Biology |language=en |volume=27 |issue=16 |pages=3846–3858 |doi=10.1111/gcb.15710 |pmid=33993581 |s2cid=234744857 }} A Landsat analysis of 100,000 undisturbed sites found that the areas with low tree cover became greener in response to warming, but tree mortality (browning) became the dominant response as the proportion of existing tree cover increased.{{Cite journal |last1=Berner |first1=Logan T. |last2=Goetz |first2=Scott J. |date=24 February 2022 |title=Satellite observations document trends consistent with a boreal forest biome shift |journal=Global Change Biology |language=en |volume=28 |issue=10 |pages=3846–3858 |doi=10.1111/gcb.16121 |pmid=35199413 |pmc=9303657 }} A 2018 study of the seven tree species dominant in the Eastern Canadian forests found that while {{convert|2|C-change|F-change}} warming alone increases their growth by around 13% on average, water availability is much more important than temperature. Also, further warming of up to {{convert|4|C-change|F-change}} would result in substantial declines unless matched by increases in precipitation.{{Cite journal |last1=D'Orangeville |first1=Loïc |last2=Houle |first2=Daniel |last3=Duchesne |first3=Louis |last4=Phillips |first4=Richard P. |last5=Bergeron |first5=Yves |last6=Kneeshaw |first6=Daniel |date=10 August 2018 |title=Beneficial effects of climate warming on boreal tree growth may be transitory |journal=Nature Communications |language=en |volume=9 |issue=1 |page=3213 |doi=10.1038/s41467-018-05705-4 |pmid=30097584 |pmc=6086880 |bibcode=2018NatCo...9.3213D }}

A 2021 paper had confirmed that the boreal forests are much more strongly affected by climate change than the other forest types in Canada and projected that most of the eastern Canadian boreal forests would reach a tipping point around 2080 under the RCP 8.5 scenario, which represents the largest potential increase in anthropogenic emissions.{{Cite journal |last1=Boulanger |first1=Yan |last2=Puigdevall |first2=Jesus Pascual |date=3 April 2021 |title=Boreal forests will be more severely affected by projected anthropogenic climate forcing than mixedwood and northern hardwood forests in eastern Canada |url=https://link.springer.com/article/10.1007/s10980-021-01241-7 |journal=Landscape Ecology |language=en |volume=36 |issue=6 |pages=1725–1740 |doi=10.1007/s10980-021-01241-7 |bibcode=2021LaEco..36.1725B |s2cid=226959320 }} Another 2021 study projected that under the moderate SSP2-4.5 scenario, boreal forests would experience a 15% worldwide increase in biomass by the end of the century, but this would be more than offset by the 41% biomass decline in the tropics.{{Cite journal |last1=Larjavaara |first1=Markku |last2=Lu |first2=Xiancheng |last3=Chen |first3=Xia |last4=Vastaranta |first4=Mikko |date=12 October 2021 |title=Impact of rising temperatures on the biomass of humid old-growth forests of the world |journal=Carbon Balance and Management |language=en |volume=16 |issue=1 |page=31 |doi=10.1186/s13021-021-00194-3 |pmid=34642849 |pmc=8513374 |doi-access=free |bibcode=2021CarBM..16...31L }} In 2022, the results of a 5-year warming experiment in North America had shown that the juveniles of tree species which currently dominate the southern margins of the boreal forests fare the worst in response to even {{convert|1.5|C-change|F-change}} or {{convert|3.1|C-change|F-change}} of warming and the associated reductions in precipitation. While the temperate species which would benefit from such conditions are also present in the southern boreal forests, they are both rare and have slower growth rates.{{Cite journal |last1=Reich |first1=Peter B. |last2=Bermudez |first2=Raimundo |last3=Montgomery |first3=Rebecca A. |last4=Rich |first4=Roy L. |last5=Rice |first5=Karen E. |last6=Hobbie |first6=Sarah E. |last7=Stefanski |first7=Artur |date=10 August 2022 |title=Even modest climate change may lead to major transitions in boreal forests |url=https://www.nature.com/articles/s41586-022-05076-3 |journal=Nature |language=en |volume=608 |issue=7923 |pages=540–545 |doi=10.1038/s41586-022-05076-3 |pmid=35948640 |bibcode=2022Natur.608..540R |s2cid=251494296 }}

{{See also|African humid period#Implications for future global warming}}

File:Greening Sahel 1982-1999.jpg

The Special Report on Global Warming of 1.5 °C and the IPCC Fifth Assessment Report indicate that global warming will likely result in increased precipitation across most of East Africa, parts of Central Africa and the principal wet season of West Africa.{{rp|16–17}} However, there is significant uncertainty related to these projections especially for West Africa.ODI and CDKN (2014) [https://cdkn.org/sites/default/files/files/AR5_IPCC_Whats_in_it_for_Africa.pdf The IPCC's Fifth Assessment Report - What's in it for Africa?] Overseas Development Institute and Climate and Development Knowledge Network{{rp|16–17}}Currently, the Sahel is becoming greener but precipitation has not fully recovered to levels reached in the mid-20th century.{{Cite journal |last1=Brooks |first1=Nick |last2=Chiapello |first2=Isabelle |last3=Lernia |first3=Savino Di |last4=Drake |first4=Nick |last5=Legrand |first5=Michel |last6=Moulin |first6=Cyril |last7=Prospero |first7=Joseph |date=2005 |title=The climate-environment-society nexus in the Sahara from prehistoric times to the present day |url=https://www.tandfonline.com/doi/full/10.1080/13629380500336680 |journal=The Journal of North African Studies |language=en |volume=10 |issue=3–4 |pages=253–292 |doi=10.1080/13629380500336680 |issn=1362-9387}}{{rp|267}}

A study from 2022 concluded: "Clearly the existence of a future tipping threshold for the WAM (West African Monsoon) and Sahel remains uncertain as does its sign but given multiple past abrupt shifts, known weaknesses in current models, and huge regional impacts but modest global climate feedback, we retain the Sahel/WAM as a potential regional impact tipping element (low confidence)."

Some simulations of global warming and increased carbon dioxide concentrations have shown a substantial increase in precipitation in the Sahel/Sahara.{{Cite journal |last1=Renssen |first1=H. |last2=Brovkin |first2=V. |last3=Fichefet |first3=T. |last4=Goosse |first4=H. |date=2003 |title=Holocene climate instability during the termination of the African Humid Period |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2002GL016636 |journal=Geophysical Research Letters |language=en |volume=30 |issue=4 |page=1184 |doi=10.1029/2002GL016636 |bibcode=2003GeoRL..30.1184R |issn=0094-8276}}{{rp|4}} This and the increased plant growth directly induced by carbon dioxide{{Cite journal |last1=Pausata |first1=Francesco S.R. |last2=Gaetani |first2=Marco |last3=Messori |first3=Gabriele |last4=Berg |first4=Alexis |last5=Maia de Souza |first5=Danielle |last6=Sage |first6=Rowan F. |last7=deMenocal |first7=Peter B. |date=2020 |title=The Greening of the Sahara: Past Changes and Future Implications |url=https://linkinghub.elsevier.com/retrieve/pii/S2590332220301007 |journal=One Earth |language=en |volume=2 |issue=3 |pages=235–250 |doi=10.1016/j.oneear.2020.03.002|bibcode=2020OEart...2..235P }}{{rp|236}} could lead to an expansion of vegetation into present-day desert, although it might be accompanied by a northward shift of the desert, i.e. a drying of northernmost Africa.{{rp|267}}

{{excerpt|Cuvette Centrale#Climate change}}

{{Main|Coral bleaching}}

File:Keppelbleaching.jpg

Around 500 million people around the world depend on coral reefs for food, income, tourism and coastal protection.{{Cite web |last=Gibbens |first=Sarah |date=4 June 2020 |title=The world's coral reefs are dying—here's how scientists plan to save them |url=https://www.nationalgeographic.com/science/article/scientists-work-to-save-coral-reefs-climate-change-marine-parks |url-status=dead |archive-url=https://web.archive.org/web/20210219015044/https://www.nationalgeographic.com/science/article/scientists-work-to-save-coral-reefs-climate-change-marine-parks |archive-date=19 February 2021 |access-date=17 July 2022 |website=National Geographic}} Since the 1980s, this is being threatened by the increase in sea surface temperatures which is triggering mass bleaching of coral, especially in sub-tropical regions.{{Cite journal |last1=Hughes |first1=Terry P. |last2=Kerry |first2=James T. |last3=Álvarez-Noriega |first3=Mariana |last4=Álvarez-Romero |first4=Jorge G. |last5=Anderson |first5=Kristen D. |last6=Baird |first6=Andrew H. |last7=Babcock |first7=Russell C. |last8=Beger |first8=Maria |last9=Bellwood |first9=David R. |last10=Berkelmans |first10=Ray |last11=Bridge |first11=Tom C. |date=2017 |title=Global warming and recurrent mass bleaching of corals |url=https://www.nature.com/articles/nature21707 |journal=Nature |volume=543 |issue=7645 |pages=373–377 |bibcode=2017Natur.543..373H |doi=10.1038/nature21707 |issn=1476-4687 |pmid=28300113 |s2cid=205254779 |hdl-access=free |hdl=20.500.11937/52828}} A sustained ocean temperature spike of {{convert|1|C-change|F-change}} above average is enough to cause bleaching.{{Cite magazine |last=Worland |first=Justin |title=Explore This Coral Reef Before it Disappears |url=https://time.com/coral/ |access-date=17 July 2022 |magazine=Time}} Under heat stress, corals expel the small colourful algae which live in their tissues, which causes them to turn white. The algae, known as zooxanthellae, have a symbiotic relationship with coral such that without them, the corals slowly die.{{cite web |last1=Gilmour |first1=James Paton |last2=Green |first2=Rebecca |date=21 May 2019 |title='Bright white skeletons': some Western Australian reefs have the lowest coral cover on record |url=http://theconversation.com/bright-white-skeletons-some-western-australian-reefs-have-the-lowest-coral-cover-on-record-116423 |access-date=17 July 2022 |website=The Conversation}} After these zooxanthellae have disappeared, the corals are vulnerable to a transition towards a seaweed-dominated ecosystem, making it very difficult to shift back to a coral-dominated ecosystem.{{Cite journal |last1=Holbrook |first1=Sally J. |last2=Schmitt |first2=Russell J. |last3=Adam |first3=Thomas C. |last4=Brooks |first4=Andrew J. |date=2016 |title=Coral Reef Resilience, Tipping Points and the Strength of Herbivory |journal=Scientific Reports |volume=6 |issue=1 |pages=35817 |bibcode=2016NatSR...635817H |doi=10.1038/srep35817 |issn=2045-2322 |pmc=5090207 |pmid=27804977}} The IPCC estimates that by the time temperatures have risen to {{convert|1.5|C-change|F-change}} above pre-industrial times, "Coral reefs... are projected to decline by a further 70–90%"; and that if the world warms by {{convert|2|C-change|F-change}}, they will become extremely rare.{{cite book |author=IPCC |title=Global warming of 1.5°C: An IPCC Special Report on the impacts of global warming of 1.5°C |year=2018 |pages=8 |chapter=Summary for Policymakers |author-link=IPCC |chapter-url=https://www.ipcc.ch/site/assets/uploads/sites/2/2019/05/SR15_SPM_version_report_HR.pdf}}

{{excerpt|Cloud feedback#Possible break-up of equatorial stratocumulus clouds}}

File:Wunderling 2021 tipping cascade.png

Crossing a threshold in one part of the climate system may trigger another tipping element to tip into a new state. Such sequences of thresholds are called cascading tipping points, an example of a domino effect.{{Cite journal |last1=Rocha |first1=Juan C. |last2=Peterson |first2=Garry |last3=Bodin |first3=Örjan |last4=Levin |first4=Simon |date=2018 |title=Cascading regime shifts within and across scales |journal=Science |volume=362 |issue=6421 |pages=1379–1383 |bibcode=2018Sci...362.1379R |doi=10.1126/science.aat7850 |issn=0036-8075 |pmid=30573623 |s2cid=56582186 |doi-access=free}} Ice loss in West Antarctica and Greenland will significantly alter ocean circulation. Sustained warming of the northern high latitudes as a result of this process could activate tipping elements in that region, such as permafrost degradation, and boreal forest dieback. Thawing permafrost is a threat multiplier because it holds roughly twice as much carbon as the amount currently circulating in the atmosphere.{{Cite web |date=18 February 2020 |title=The irreversible emissions of a permafrost "tipping point" |url=https://www.weforum.org/agenda/2020/02/irreversible-emissions-permafrost-tipping-point/ |access-date=17 July 2022 |website=World Economic Forum}} Loss of ice in Greenland likely destabilises the West Antarctic ice sheet via sea level rise, and vice-versa, especially if Greenland were to melt first as West Antarctica is particularly vulnerable to contact with warm sea water.

A 2021 study with three million computer simulations of a climate model showed that nearly one-third of those simulations resulted in domino effects, even when temperature increases were limited to {{convert|2|C-change|F-change}} – the upper limit set by the Paris Agreement in 2015.{{Cite web |last=Turner |first=Ben |date=12 June 2021 |title=Dramatic climate domino effects could be unleashed after less than 2 degrees of warming, a new study reveals |url=https://www.livescience.com/climate-domino-effects-close.html |access-date=23 July 2022 |website=livescience.com |language=en}} The authors of the study said that the science of tipping points is so complex that there is great uncertainty as to how they might unfold, but nevertheless, argued that the possibility of cascading tipping points represents "an existential threat to civilisation".{{Cite news |last=Carrington |first=Damian |date=27 November 2019 |title=Climate emergency: world "may have crossed tipping points" |url=http://www.theguardian.com/environment/2019/nov/27/climate-emergency-world-may-have-crossed-tipping-points |newspaper=the Guardian}} A network model analysis suggested that temporary overshoots of climate change – increasing global temperature beyond Paris Agreement goals temporarily as often projected – can substantially increase risks of climate tipping cascades ("by up to 72% compared with non-overshoot scenarios").{{cite news |title=Overshooting climate targets could significantly increase risk for tipping cascades |url=https://phys.org/news/2022-12-overshooting-climate-significantly-cascades.html |access-date=17 January 2023 |work=Potsdam Institute for Climate Impact Research via phys.org |language=en}}{{cite journal |last1=Wunderling |first1=Nico |last2=Winkelmann |first2=Ricarda |last3=Rockström |first3=Johan |last4=Loriani |first4=Sina |last5=Armstrong McKay |first5=David I. |last6=Ritchie |first6=Paul D. L. |last7=Sakschewski |first7=Boris |last8=Donges |first8=Jonathan F. |date=January 2023 |title=Global warming overshoots increase risks of climate tipping cascades in a network model |url=https://www.researchgate.net/publication/366520998 |journal=Nature Climate Change |language=en |volume=13 |issue=1 |pages=75–82 |bibcode=2023NatCC..13...75W |doi=10.1038/s41558-022-01545-9 |issn=1758-6798 |s2cid=255045153 |url-access=subscription}}

File:Climate-tipping-points-en.svg, Indian summer monsoon, Arctic ozone hole and all of Arctic sea ice were all listed as tipping points. Labrador-Irminger circulation, mountain glaciers and East Antarctic ice however were not included. This 2008 list also includes Antarctic bottom water (part of the Southern Ocean overturning circulation), which was left out of the 2022 list, but included in some subsequent ones.]]

The possibility that the El Niño–Southern Oscillation (ENSO) is a tipping element had attracted attention in the past. Normally strong winds blow west across the South Pacific Ocean from South America to Australia. Every two to seven years, the winds weaken due to pressure changes and the air and water in the middle of the Pacific warms up, causing changes in wind movement patterns around the globe. This is known as El Niño and typically leads to droughts in India, Indonesia and Brazil, and increased flooding in Peru. In 2015/2016, this caused food shortages affecting over 60 million people.{{Cite web |title=Tipping Points: Why we might not be able to reverse climate change |url=https://climatescience.org/ |access-date=17 July 2022 |website=ClimateScience }} El Niño-induced droughts may increase the likelihood of forest fires in the Amazon.{{Cite journal |last1=Duque-Villegas |first1=Mateo |last2=Salazar |first2=Juan Fernando |last3=Rendón |first3=Angela Maria |date=2019 |title=Tipping the ENSO into a permanent El Niño can trigger state transitions in global terrestrial ecosystems |url=https://esd.copernicus.org/articles/10/631/2019/ |journal=Earth System Dynamics |volume=10 |issue=4 |pages=631–650 |doi=10.5194/esd-10-631-2019 |bibcode=2019ESD....10..631D |s2cid=210348791 |issn=2190-4979|doi-access=free }} The threshold for tipping was estimated to be between {{convert|3.5|C-change|F-change}} and {{convert|7|C-change|F-change}} of global warming in 2016. After tipping, the system would be in a more permanent El Niño state, rather than oscillating between different states. This has happened in Earth's past, in the Pliocene, but the layout of the ocean was significantly different from now. So far, there is no definitive evidence indicating changes in ENSO behaviour, and the IPCC Sixth Assessment Report concluded that it is "virtually certain that the ENSO will remain the dominant mode of interannual variability in a warmer world."{{Cite book |chapter= Technical Summary |last1=Arias |first1=Paola A. |last2 = Bellouin |first2= Nicolas |last3 = Coppola |first3 = Erika |last4 = Jones |first4 = Richard G. |last5 = Krinner |first5 = Gerhard |display-authors=4 |chapter-url= https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf |year= 2021 |title= IPCC AR6 WG1 |pages=88 }} Consequently, the 2022 assessment no longer includes it in the list of likely tipping elements.

The Indian summer monsoon is another part of the climate system which was considered suspectible to irreversible collapse in the earlier research.{{Cite journal |last1=Stolbova |first1=Veronika |last2=Surovyatkina |first2=Elena |last3=Bookhagen |first3=Bodo |last4=Kurths |first4=Jürgen |date=2016 |title=Tipping elements of the Indian monsoon: Prediction of onset and withdrawal |url=http://doi.wiley.com/10.1002/2016GL068392 |journal=Geophysical Research Letters |volume=43 |issue=8 |pages=3982–3990 |doi=10.1002/2016GL068392|bibcode=2016GeoRL..43.3982S |s2cid=51811076 |hdl=2164/9132 |hdl-access=free }} However, more recent research has demonstrated that warming tends to strengthen the Indian monsoon,{{Cite journal |last1=Katzenberger |first1=Anja |last2=Schewe |first2=Jacob |last3=Pongratz |first3=Julia |last4=Levermann |first4=Anders |date=2021 |title=Robust increase of Indian monsoon rainfall and its variability under future warming in CMIP-6 models |url=https://esd.copernicus.org/articles/12/367/2021/ |journal=Earth System Dynamics |volume=12 |issue=2 |pages=367–386 |doi=10.5194/esd-12-367-2021|bibcode=2021ESD....12..367K |s2cid=235080216 |doi-access=free }} and it is projected to strengthen in the future.{{Cite book |chapter= Technical Summary |last1=Arias |first1=Paola A. |last2 = Bellouin |first2= Nicolas |last3 = Coppola |first3 = Erika |last4 = Jones |first4 = Richard G. |last5 = Krinner |first5 = Gerhard |display-authors=4 |chapter-url= https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf |year= 2021 |title= IPCC AR6 WG1 |pages=100}}

Methane hydrate deposits in the Arctic were once thought to be vulnerable to a rapid dissociation which would have a large impact on global temperatures, in a dramatic scenario known as a clathrate gun hypothesis. Later research found that it takes millennia for methane hydrates to respond to warming, while methane emissions from the seafloor rarely transfer from the water column into the atmosphere.{{cite journal |last1=Sparrow |first1=Katy J. |last2=Kessler |first2=John D. |last3=Southon |first3=John R. |last4=Garcia-Tigreros |first4=Fenix |last5=Schreiner |first5=Kathryn M. |last6=Ruppel |first6=Carolyn D. |last7=Miller |first7=John B. |last8=Lehman |first8=Scott J. |last9=Xu |first9=Xiaomei |date=17 January 2018 |title=Limited contribution of ancient methane to surface waters of the U.S. Beaufort Sea shelf |url=https://www.science.org/doi/10.1126/sciadv.aay7934 |journal=Science Advances |volume=4 |issue=1 |pages=eaao4842 |doi=10.1126/sciadv.aao4842 |pmid=29349299 |pmc=5771695 |bibcode=2018SciA....4.4842S }}{{cite journal |last1=Mau |first1=S. |last2=Römer |first2=M. |last3=Torres |first3=M. E. |last4=Bussmann |first4=I. |last5=Pape |first5=T. |last6=Damm |first6=E. |last7=Geprägs |first7=P. |last8=Wintersteller |first8=P. |last9=Hsu |first9=C.-W. |last10=Loher |first10=M. |last11=Bohrmann |first11=G. |date=23 February 2017 |title=Widespread methane seepage along the continental margin off Svalbard - from Bjørnøya to Kongsfjorden |journal=Scientific Reports |volume=7 |page=42997 |doi=10.1038/srep42997 |pmid=28230189 |pmc=5322355 |bibcode=2017NatSR...742997M }}{{cite journal |last1=Silyakova |first1=Anna |last2=Jansson |first2=Pär |last3=Serov |first3=Pavel |last4=Ferré |first4=Benedicte |last5=Pavlov |first5=Alexey K. |last6=Hattermann |first6=Tore |last7=Graves |first7=Carolyn A. |last8=Platt |first8=Stephen M. |last9=Lund Myhre |first9=Cathrine |last10=Gründger |first10=Friederike |last11=Niemann |first11=Helge |date=1 February 2020 |title=Physical controls of dynamics of methane venting from a shallow seep area west of Svalbard |url=https://www.sciencedirect.com/science/article/pii/S0278434319304133 |journal=Continental Shelf Research |volume=194 |page=104030 |doi=10.1016/j.csr.2019.104030 |bibcode=2020CSR...19404030S |hdl=10037/16975 |s2cid=214097236 |hdl-access=free }} IPCC Sixth Assessment Report states "It is very unlikely that gas clathrates (mostly methane) in deeper terrestrial permafrost and subsea clathrates will lead to a detectable departure from the emissions trajectory during this century".{{Cite journal |last1=Fox-Kemper |first1=B. |last2=Hewitt |first2=H.T.|author2-link=Helene Hewitt |last3=Xiao |first3=C. |last4=Aðalgeirsdóttir |first4=G. |last5=Drijfhout |first5=S.S. |last6=Edwards |first6=T.L. |last7=Golledge |first7=N.R. |last8=Hemer |first8=M. |last9=Kopp |first9=R.E. |last10=Krinner |first10=G. |last11=Mix |first11=A. |date=2021 |editor-last=Masson-Delmotte |editor-first=V. |editor2-last=Zhai |editor2-first=P. |editor3-last=Pirani |editor3-first=A. |editor4-last=Connors |editor4-first=S.L. |editor5-last=Péan |editor5-first=C. |editor6-last=Berger |editor6-first=S. |editor7-last=Caud |editor7-first=N. |editor8-last=Chen |editor8-first=Y. |editor9-last=Goldfarb |editor9-first=L. |title=Chapter 5: Global Carbon and other Biogeochemical Cycles and Feedbacks |journal=Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Full_Report.pdf |publisher=Cambridge University Press, Cambridge, UK and New York, NY, USA |page=5 |doi=10.1017/9781009157896.011}}

File:IPCC schematic wikipedia.pdf

Tipping point behaviour in the climate can be described in mathematical terms. Three types of tipping points have been identified—bifurcation, noise-induced and rate-dependent.{{Cite journal |last1=Ashwin |first1=Peter |last2=Wieczorek |first2=Sebastian |last3=Vitolo |first3=Renato |last4=Cox |first4=Peter |date=13 March 2012 |title=Tipping points in open systems: bifurcation, noise-induced and rate-dependent examples in the climate system |url=https://royalsocietypublishing.org/doi/10.1098/rsta.2011.0306 |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=370 |issue=1962 |pages=1166–1184 |doi=10.1098/rsta.2011.0306 |pmid=22291228 |issn=1364-503X |bibcode=2012RSPTA.370.1166A |arxiv=1103.0169 |s2cid=2324694}}{{Cite journal |last1=Rietkerk |first1=Max |last2=Bastiaansen |first2=Robbin |last3=Banerjee |first3=Swarnendu |last4=van de Koppel |first4=Johan |last5=Baudena |first5=Mara |last6=Doelman |first6=Arjen |date=8 October 2021 |title=Evasion of tipping in complex systems through spatial pattern formation |url=https://www.science.org/doi/10.1126/science.abj0359 |journal=Science |language=en |volume=374 |issue=6564 |pages=eabj0359 |doi=10.1126/science.abj0359 |pmid=34618584 |hdl=1874/413153 |s2cid=238476226 |issn=0036-8075}}

Bifurcation-induced tipping happens when a particular parameter in the climate (for instance a change in environmental conditions or forcing), passes a critical level – at which point a bifurcation takes place – and what was a stable state loses its stability or simply disappears.{{Cite journal |last1=O'Keeffe |first1=Paul E. |last2=Wieczorek |first2=Sebastian |date=1 January 2020 |title=Tipping Phenomena and Points of No Return in Ecosystems: Beyond Classical Bifurcations |url=https://epubs.siam.org/doi/10.1137/19M1242884 |journal=SIAM Journal on Applied Dynamical Systems |volume=19 |issue=4 |pages=2371–2402 |arxiv=1902.01796v7 |doi=10.1137/19M1242884|hdl=10468/10788 |s2cid=119316104 }} The Atlantic Meridional Overturning Circulation (AMOC) is an example of a tipping element that can show bifurcation-induced tipping. Slow changes to the bifurcation parameters in this system – the salinity and temperature of the water – may push the circulation towards collapse.{{Cite journal |last1=Boulton |first1=Chris A. |last2=Allison |first2=Lesley C. |last3=Lenton |first3=Tim |author-link3=Tim Lenton |date=December 2014 |title=Early warning signals of Atlantic Meridional Overturning Circulation collapse in a fully coupled climate model |journal=Nature Communications |volume=5 |issue=1 |pages=5752 |bibcode=2014NatCo...5.5752B |doi=10.1038/ncomms6752 |issn=2041-1723 |pmc=4268699 |pmid=25482065}}{{Cite journal |last1=Bathiany |first1=Sebastian |last2=Dijkstra |first2=Henk |last3=Crucifix |first3=Michel |last4=Dakos |first4=Vasilis |last5=Brovkin |first5=Victor |last6=Williamson |first6=Mark S. |last7=Lenton |first7=Tim |author-link7=Tim Lenton |last8=Scheffer |first8=Marten |date=2016 |title=Beyond bifurcation: using complex models to understand and predict abrupt climate change |journal=Dynamics and Statistics of the Climate System |volume=1 |issue=1 |pages=dzw004 |doi=10.1093/climsys/dzw004 |issn=2059-6987|doi-access=free }}

Many types of bifurcations show hysteresis,{{Cite journal |last1=Smith |first1=Adam B. |last2=Revilla |first2=Eloy |last3=Mindell |first3=David P. |last4=Matzke |first4=Nicholas |last5=Marshall |first5=Charles |last6=Kitzes |first6=Justin |last7=Gillespie |first7=Rosemary |last8=Williams |first8=John W. |last9=Vermeij |first9=Geerat |date=2012 |title=Approaching a state shift in Earth's biosphere |journal=Nature |volume=486 |issue=7401 |pages=52–58 |bibcode=2012Natur.486...52B |doi=10.1038/nature11018 |issn=1476-4687 |pmid=22678279 |hdl-access=free |hdl=10261/55208 |s2cid=4788164}} which is the dependence of the state of a system on its history. For instance, depending on how warm it was in the past, there can be differing amounts of ice on the poles at the same concentration of greenhouse gases or temperature.{{Cite journal |last1=Pollard |first1=David |last2=DeConto |first2=Robert M. |date=2005 |title=Hysteresis in Cenozoic Antarctic ice-sheet variations |journal=Global and Planetary Change |volume=45 |issue=1–3 |pages=9–12 |bibcode=2005GPC....45....9P |doi=10.1016/j.gloplacha.2004.09.011}}

For tipping points that occur because of a bifurcation, it may be possible to detect whether a system is getting closer to a tipping point, as it becomes less resilient to perturbations on approach of the tipping threshold. These systems display critical slowing down, with an increased memory (rising autocorrelation) and variance. Depending on the nature of the tipping system, there may be other types of early warning signals.{{Cite journal |last=Thomas |first=Zoë A. |date=15 November 2016 |title=Using natural archives to detect climate and environmental tipping points in the Earth System |url=http://www.sciencedirect.com/science/article/pii/S0277379116303766 |url-status=live |journal=Quaternary Science Reviews |volume=152 |pages=60–71 |bibcode=2016QSRv..152...60T |doi=10.1016/j.quascirev.2016.09.026 |issn=0277-3791 |archive-url=https://web.archive.org/web/20211121182818/https://www.sciencedirect.com/science/article/abs/pii/S0277379116303766 |archive-date=21 November 2021 |access-date=20 April 2020}}{{Cite journal |last1=Lenton |first1=Tim |author1-link=Tim Lenton |last2=Livina |first2=V.N. |last3=Dakos |first3=V. |last4=Van Nes |first4=E.H. |last5=Scheffer |first5=M. |date=2012 |title=Early warning of climate tipping points from critical slowing down: comparing methods to improve robustness |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=370 |issue=1962 |pages=1185–1204 |bibcode=2012RSPTA.370.1185L |doi=10.1098/rsta.2011.0304 |issn=1364-503X |pmc=3261433 |pmid=22291229}} Abrupt change is not an early warning signal (EWS) for tipping points, as abrupt change can also occur if the changes are reversible to the control parameter.{{cite news |last1=Rosier |first1=Sebastian |date=6 April 2021 |title=Guest post: Identifying three "tipping points" in Antarctica's Pine Island glacier |work=Carbon Brief |url=https://www.carbonbrief.org/guest-post-identifying-three-tipping-points-in-antarcticas-pine-island-glacier |url-status=live |access-date=1 August 2021 |archive-url=https://web.archive.org/web/20210731194805/https://www.carbonbrief.org/guest-post-identifying-three-tipping-points-in-antarcticas-pine-island-glacier |archive-date=31 July 2021}}{{cite journal |last1=Rosier |first1=Sebastian H. R. |last2=Reese |first2=Ronja |last3=Donges |first3=Jonathan F. |last4=De Rydt |first4=Jan |last5=Gudmundsson |first5=G. Hilmar |last6=Winkelmann |first6=Ricarda |date=25 March 2021 |title=The tipping points and early warning indicators for Pine Island Glacier, West Antarctica |url=https://tc.copernicus.org/articles/15/1501/2021/tc-15-1501-2021.html |url-status=live |journal=The Cryosphere |volume=15 |issue=3 |pages=1501–1516 |bibcode=2021TCry...15.1501R |doi=10.5194/tc-15-1501-2021 |issn=1994-0416 |archive-url=https://web.archive.org/web/20210801184802/https://tc.copernicus.org/articles/15/1501/2021/tc-15-1501-2021.html |archive-date=1 August 2021 |access-date=1 August 2021 |s2cid=233738686|doi-access=free }}

These EWSs are often developed and tested using time series from the paleo record, like sediments, ice caps, and tree rings, where past examples of tipping can be observed.{{cite journal |last1=Brovkin |first1=Victor |last2=Brook |first2=Edward |last3=Williams |first3=John W. |last4=Bathiany |first4=Sebastian |last5=Lenton |first5=Tim |author-link5=Tim Lenton |last6=Barton |display-authors=4 |date=29 July 2021 |title=Past abrupt changes, tipping points and cascading impacts in the Earth system |url=https://www.nature.com/articles/s41561-021-00790-5 |url-status=live |journal=Nature Geoscience |volume=14 |issue=8 |pages=550–558 |bibcode=2021NatGe..14..550B |doi=10.1038/s41561-021-00790-5 |archive-url=https://web.archive.org/web/20210730154644/https://www.nature.com/articles/s41561-021-00790-5 |archive-date=30 July 2021 |access-date=1 August 2021 |s2cid=236504982}} It is not always possible to say whether increased variance and autocorrelation is a precursor to tipping, or caused by internal variability, for instance in the case of the collapse of the AMOC. Quality limitations of paleodata further complicate the development of EWSs. They have been developed for detecting tipping due to drought in forests in California,{{cite journal |last1=Liu |first1=Yanlan |last2=Kumar |first2=Mukesh |last3=Katul |first3=Gabriel G. |last4=Porporato |first4=Amilcare |date=November 2019 |title=Reduced resilience as an early warning signal of forest mortality |url=https://www.nature.com/articles/s41558-019-0583-9 |url-status=live |journal=Nature Climate Change |volume=9 |issue=11 |pages=880–885 |bibcode=2019NatCC...9..880L |doi=10.1038/s41558-019-0583-9 |issn=1758-6798 |archive-url=https://web.archive.org/web/20210801191402/https://www.nature.com/articles/s41558-019-0583-9 |archive-date=1 August 2021 |access-date=1 August 2021 |s2cid=203848411}} and melting of the Pine Island Glacier in West Antarctica, among other systems. Using early warning signals (increased autocorrelation and variance of the melt rate time series), it has been suggested that the Greenland ice sheet is currently losing resilience, consistent with modelled early warning signals of the ice sheet.{{cite journal |last1=Boers |first1=Niklas |last2=Rypdal |first2=Martin |date=25 May 2021 |title=Critical slowing down suggests that the western Greenland Ice Sheet is close to a tipping point |journal=Proceedings of the National Academy of Sciences |volume=118 |issue=21 |pages=e2024192118 |bibcode=2021PNAS..11824192B |doi=10.1073/pnas.2024192118 |issn=0027-8424 |pmc=8166178 |pmid=34001613|doi-access=free }}

Human-induced changes in the climate system may be too fast for early warning signals to become evident, especially in systems with inertia.{{Cite book |last1=Chen |first1=D. |title=Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |last2=Rojas |first2=M. |last3=Samset |first3=B.H. |last4=Cobb |first4=K. |year=2021 |editor1-last=Masson-Delmotte |editor1-first=V. |at=Section 1.4.4.3 |chapter=Chapter 1: Framing, context, and methods |display-authors=etal |chapter-url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_01.pdf}}

Noise-induced tipping is the transition from one state to another due to random fluctuations or internal variability of the system. Noise-induced transitions do not show any of the early warning signals which occur with bifurcations. This means they are unpredictable because the underlying potential does not change. Because they are unpredictable, such occurrences are often described as a "one-in-x-year" event.{{Cite journal |last1=Lenton |first1=Tim |author1-link=Tim Lenton |date=2011 |title=Early warning of climate tipping points |url=https://www.nature.com/articles/nclimate1143 |journal=Nature Climate Change |volume=1 |issue=4 |pages=201–209 |citeseerx=10.1.1.666.244 |doi=10.1038/nclimate1143 |bibcode=2011NatCC...1..201L |issn=1758-6798}} An example is the Dansgaard–Oeschger events during the last ice age, with 25 occurrences of sudden climate fluctuations over a 500-year period.{{Cite journal |last1=Ditlevsen |first1=Peter D. |last2=Johnsen |first2=Sigfus J. |date=2010 |title=Tipping points: Early warning and wishful thinking |journal=Geophysical Research Letters |volume=37 |issue=19 |pages=n/a |doi=10.1029/2010GL044486 |issn=1944-8007 |bibcode=2010GeoRL..3719703D |doi-access=free}}

Rate-induced tipping occurs when a change in the environment is faster than the force that restores the system to its stable state. In peatlands, for instance, after years of relative stability, rate-induced tipping can lead to an "explosive release of soil carbon from peatlands into the atmosphere" – sometimes known as "compost bomb instability".{{Cite journal |last1=Wieczorek |first1=S. |last2=Ashwin |first2=P. |last3=Luke |first3=C. M. |last4=Cox |first4=P. M. |date=8 May 2011 |title=Excitability in ramped systems: the compost-bomb instability |journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=467 |issue=2129 |pages=1243–1269 |doi=10.1098/rspa.2010.0485 |issn=1364-5021 |bibcode=2011RSPSA.467.1243W |doi-access=free}}{{Cite journal |last1=Luke |first1=C. M. |last2=Cox |first2=P. M. |date=2011 |title=Soil carbon and climate change: from the Jenkinson effect to the compost-bomb instability |journal=European Journal of Soil Science |volume=62 |issue=1 |pages=5–12 |doi=10.1111/j.1365-2389.2010.01312.x |bibcode=2011EuJSS..62....5L |s2cid=55462001 |issn=1365-2389 }} The AMOC may also show rate-induced tipping: if the rate of ice melt increases too fast, it may collapse, even before the ice melt reaches the critical value where the system would undergo a bifurcation.{{Cite journal |last1=Lohmann |first1=Johannes |last2=Ditlevsen |first2=Peter D. |date=2021 |title=Risk of tipping the overturning circulation due to increasing rates of ice melt |journal=Proceedings of the National Academy of Sciences |volume=118 |issue=9 |pages=e2017989118 |doi=10.1073/pnas.2017989118 |issn=0027-8424 |pmc=7936283 |pmid=33619095|bibcode=2021PNAS..11817989L |doi-access=free }}

File:Franzke 2022 Environ Res Lett Fig2 - Schematic of some possible interactions and cascading effects between the Earth system and the Human system.jpg

{{See also|Effects of climate change}}

Tipping points can have very severe impacts. They can exacerbate current dangerous impacts of climate change, or give rise to new impacts. Some potential tipping points would take place abruptly, such as disruptions to the Indian monsoon, with severe impacts on food security for hundreds of millions. Other impacts would likely take place over longer timescales, such as the melting of the ice caps. The circa {{convert|10|m|ft|frac=2}} of sea level rise from the combined melt of Greenland and West Antarctica would require moving many cities inland over the course of centuries, but would also accelerate sea level rise this century, with Antarctic ice sheet instability projected to expose 120 million more people to annual floods in a mid-emissions scenario.{{Cite journal |last1=Kulp |first1=Scott A. |last2=Strauss |first2=Benjamin H. |date=2019-10-29 |title=New elevation data triple estimates of global vulnerability to sea-level rise and coastal flooding |journal=Nature Communications |language=en |volume=10 |issue=1 |pages=4844 |doi=10.1038/s41467-019-12808-z |pmid=31664024 |pmc=6820795 |bibcode=2019NatCo..10.4844K |issn=2041-1723}} A collapse of the Atlantic Overturning Circulation would cause over 10 degrees Celsius of cooling in parts of Europe, cause drying in Europe, Central America, West Africa, and southern Asia, and lead to about {{convert|1|m|ft|frac=2}} of sea level rise in the North Atlantic.{{Cite journal |last1=van Westen |first1=René M. |last2=Kliphuis |first2=Michael |last3=Dijkstra |first3=Henk A. |date=2024-02-09 |title=Physics-based early warning signal shows that AMOC is on tipping course |journal=Science Advances |language=en |volume=10 |issue=6 |pages=eadk1189 |doi=10.1126/sciadv.adk1189 |issn=2375-2548 |pmc=10857529 |pmid=38335283|arxiv=2308.01688 |bibcode=2024SciA...10K1189V }}{{Cite journal |display-authors=etal |last1=Loriani |first1=Sina |last2=Aksenov |first2=Yevgeny |last3=Dijkstra |first3=Henk |last4=England |first4=Matt |last5=Fedoroc |first5=Alexey |last6=Messori |first6=Gabriele |last7=Pausata |first7=Francesco |last8=Sallée |first8=JB |last9=Sinha |first9=Bablu |last10=Sherwood |first10=Steven |last11=Tharammal |first11=Thejna |last12=Armstrong McKay |first12=David |last13=Bala |first13=Govindasamy |last14=Born |first14=Andreas |date=2023-12-06 |title=Global Tipping Points Report 2023 - Chapter 1.4: Tipping points in ocean and atmosphere circulations |url=https://report-2023.global-tipping-points.org/section1/1-earth-system-tipping-points/1-4-tipping-points-in-ocean-and-atmosphere-circulations/ |journal=Global Tipping Points Report}} The impacts of AMOC collapse would have serious implications for food security, with one projection showing reduced yields of key crops across most world regions, with for example arable agriculture becoming economically infeasible in Britain.{{Cite book |last=OECD |url=https://www.oecd-ilibrary.org/environment/climate-tipping-points_abc5a69e-en |title=Climate Tipping Points: Insights for Effective Policy Action |date=2022 |publisher=Organisation for Economic Co-operation and Development |location=Paris |language=en |doi=10.1787/abc5a69e-en|isbn=978-92-64-85876-3 }}{{Cite journal |last1=Ritchie |first1=Paul D. L. |last2=Smith |first2=Greg S. |last3=Davis |first3=Katrina J. |last4=Fezzi |first4=Carlo |last5=Halleck-Vega |first5=Solmaria |last6=Harper |first6=Anna B. |last7=Boulton |first7=Chris A. |last8=Binner |first8=Amy R. |last9=Day |first9=Brett H. |last10=Gallego-Sala |first10=Angela V. |last11=Mecking |first11=Jennifer V. |last12=Sitch |first12=Stephen A. |last13=Lenton |first13=Timothy M. |last14=Bateman |first14=Ian J. |date=2020-01-13 |title=Shifts in national land use and food production in Great Britain after a climate tipping point |url=https://www.nature.com/articles/s43016-019-0011-3 |journal=Nature Food |language=en |volume=1 |issue=1 |pages=76–83 |doi=10.1038/s43016-019-0011-3 |issn=2662-1355}} These impacts could happen simultaneously in the case of cascading tipping points.{{Cite journal |last1=Schellnhuber |first1=Hans Joachim |last2=Winkelmann |first2=Ricarda |last3=Scheffer |first3=Marten |last4=Lade |first4=Steven J. |last5=Fetzer |first5=Ingo |last6=Donges |first6=Jonathan F. |last7=Crucifix |first7=Michel |last8=Cornell |first8=Sarah E. |last9=Barnosky |first9=Anthony D. |author-link9=Anthony David Barnosky |date=2018 |title=Trajectories of the Earth System in the Anthropocene |journal=Proceedings of the National Academy of Sciences |volume=115 |issue=33 |pages=8252–8259 |bibcode=2018PNAS..115.8252S |doi=10.1073/pnas.1810141115 |issn=0027-8424 |pmc=6099852 |pmid=30082409 |doi-access=free}} A review of abrupt changes over the last 30,000 years showed that tipping points can lead to a large set of cascading impacts in climate, ecological and social systems. For instance, the abrupt termination of the African humid period cascaded, and desertification and regime shifts led to the retreat of pastoral societies in North Africa and a change of dynasty in Egypt.

Some scholars have proposed a threshold which, if crossed, could trigger multiple tipping points and self-reinforcing feedback loops that would prevent stabilisation of the climate, causing much greater warming and sea-level rises and leading to severe disruption to ecosystems, society, and economies. This scenario is sometimes called the Hothouse Earth scenario. The researchers proposed that this scenario could unfold beyond a threshold of around 2 °C above pre-industrial levels. However, while this scenario is possible, the existence and value of this threshold remains speculative, and doubts have been raised if tipping points would lock in much extra warming in the shorter term.{{Cite web |last=Betts |first=Richard |date=2018-08-09 |title=Hothouse Earth: here's what the science actually does – and doesn't – say |url=https://theconversation.com/hothouse-earth-heres-what-the-science-actually-does-and-doesnt-say-101341 |access-date=2024-10-16 |website=The Conversation |language=en-US}}{{Cite journal |last1=Wang |first1=Seaver |last2=Foster |first2=Adrianna |last3=Lenz |first3=Elizabeth A. |last4=Kessler |first4=John D. |last5=Stroeve |first5=Julienne C. |last6=Anderson |first6=Liana O. |last7=Turetsky |first7=Merritt |last8=Betts |first8=Richard |last9=Zou |first9=Sijia |last10=Liu |first10=Wei |last11=Boos |first11=William R. |last12=Hausfather |first12=Zeke |date=2023-02-15 |title=Mechanisms and Impacts of Earth System Tipping Elements |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021RG000757 |journal=Reviews of Geophysics |language=en |volume=61 |issue=1 |doi=10.1029/2021RG000757 |bibcode=2023RvGeo..6100757W |issn=8755-1209}} Decisions taken over the next decade could influence the climate of the planet for tens to hundreds of thousands of years and potentially even lead to conditions which are inhospitable to current human societies. The report also states that there is a possibility of a cascade of tipping points being triggered even if the goal outlined in the Paris Agreement to limit warming to {{convert|1.5|-|2.0|C|F}} is achieved.{{Cite journal |last1=Steffen |first1=Will |last2=Rockström |first2=Johan |last3=Richardson |first3=Katherine |last4=Lenton |first4=Timothy M. |last5=Folke |first5=Carl |last6=Liverman |first6=Diana |last7=Summerhayes |first7=Colin P. |last8=Barnosky |first8=Anthony D. |last9=Cornell |first9=Sarah E. |last10=Crucifix |first10=Michel |last11=Donges |first11=Jonathan F. |date=2018-08-14 |title=Trajectories of the Earth System in the Anthropocene |journal=Proceedings of the National Academy of Sciences |language=en |volume=115 |issue=33 |pages=8252–8259 |bibcode=2018PNAS..115.8252S |doi=10.1073/pnas.1810141115 |pmc=6099852 |pmid=30082409 |doi-access=free}}

File:Post-Glacial Sea Level.png was a period of abrupt sea level rise around 14,000 years ago. It may be an example of a tipping point.{{Cite journal |last1=Brovkin |first1=Victor |last2=Brook |first2=Edward |last3=Williams |first3=John W. |last4=Bathiany |first4=Sebastian |last5=Lenton |first5=Tim |author-link5=Tim Lenton |last6=Barton |first6=Michael |last7=DeConto |first7=Robert M. |last8=Donges |first8=Jonathan F. |last9=Ganopolski |first9=Andrey |last10=McManus |first10=Jerry |last11=Praetorius |first11=Summer |date=2021 |title=Past abrupt changes, tipping points and cascading impacts in the Earth system |url=https://www.nature.com/articles/s41561-021-00790-5 |journal=Nature Geoscience |volume=14 |issue=8 |pages=550–558 |bibcode=2021NatGe..14..550B |doi=10.1038/s41561-021-00790-5 |issn=1752-0908 |s2cid=236504982}}]]

The geological record shows many abrupt changes that suggest tipping points may have been crossed in pre-historic times. For instance, the Dansgaard–Oeschger events during the last ice age were periods of abrupt warming (within decades) in Greenland and Europe, that may have involved the abrupt changes in major ocean currents. During the deglaciation in the early Holocene, sea level rise was not smooth, but rose abruptly during meltwater pulses. The monsoon in North Africa saw abrupt changes on decadal timescales during the African humid period. This period, spanning from 15,000 to 5,000 years ago, also ended suddenly in a drier state.

A runaway greenhouse effect is a tipping point so extreme that oceans evaporate{{Cite web |title=What can Venus tell us about climate change on Earth? |url=https://www.skyatnightmagazine.com/space-science/venus-climate-change-earth/ |access-date=18 July 2022 |website=BBC Sky at Night Magazine}} and the water vapour escapes to space, an irreversible climate state that happened on Venus.{{Cite web |last=Dunbar |first=Brian |date=6 May 2015 |title=Venus |url=http://www.nasa.gov/venus |access-date=18 July 2022 |website=NASA}} A runaway greenhouse effect has virtually no chance of being caused by people.{{cite report |url=https://www.ipcc.ch/site/assets/uploads/2018/03/inf3-6.pdf |title=Scoping of the IPCC 5th Assessment Report Cross Cutting Issues |access-date=24 March 2019 |archive-url=https://web.archive.org/web/20091109215503/http://www.ipcc.ch/meetings/session31/inf3.pdf |archive-date=9 November 2009 |url-status=live |work=Thirty-first Session of the IPCC Bali, 26–29 October 2009}}{{explain|date=October 2023}} Venus-like conditions on the Earth require a large long-term forcing that is unlikely to occur until the sun brightens by a ten of percents, which will take 600 - 700 million years.{{cite journal |last1=Hansen |first1=James |last2=Sato |first2=Makiko |last3=Russell |first3=Gary |last4=Kharecha |first4=Pushker |date=2013 |title=Climate sensitivity, sea level and atmospheric carbon dioxide |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=371 |issue=2001 |at=20120294 |arxiv=1211.4846 |bibcode=2013RSPTA.37120294H |doi=10.1098/rsta.2012.0294 |pmc=3785813 |pmid=24043864}}

• {{annotated link|Climate change scenario}}
• {{annotated link|Climate sensitivity}}
• {{annotated link|Greenhouse and icehouse Earth}}
• {{annotated link|Planetary boundaries}}
• {{annotated link|World Scientists' Warning to Humanity}}

{{Reflist|2}}

{{wikiquote}}

• [https://global-tipping-points.org/ Global tipping points] and [https://www.exeter.ac.uk/research/tippingpoints/ research on tipping points] at the University of Exeter
• [https://www.bbc.co.uk/programmes/w3ct42r6 The climate tipping points] - 2022 BBC radio documentary
• {{TED talk | johan_rockstrom_the_tipping_points_of_climate_change_and_where_we_stand | The tipping points of climate change — and where we stand | Johan Rockström }}

{{Global warming}}

{{Doomsday}}

{{DEFAULTSORT:Tipping Point (Climatology)}}

Category:Climate change feedbacks

Category:Climatology

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