Climate variability and change#External climate forcing

Climate variability is the term to describe variations in the mean state and other characteristics of climate (such as chances or possibility of extreme weather, etc.) "on all spatial and temporal scales beyond that of individual weather events." Some of the variability does not appear to be caused by known systems and occurs at seemingly random times. Such variability is called random variability or noise. On the other hand, periodic variability occurs relatively regularly and in distinct modes of variability or climate patterns.{{Sfn|Rohli|Vega|2018|p=274}}

The term climate change is often used to refer specifically to anthropogenic climate change. Anthropogenic climate change is caused by human activity, as opposed to changes in climate that may have resulted as part of Earth's natural processes.{{cite web |date=21 March 1994 |title=The United Nations Framework Convention on Climate Change |url=http://unfccc.int/resource/ccsites/zimbab/conven/text/art01.htm |quote=Climate change means a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods. |access-date=9 October 2018 |archive-date=20 September 2022 |archive-url=https://web.archive.org/web/20220920173907/https://unfccc.int/resource/ccsites/zimbab/conven/text/art01.htm |url-status=live }} Global warming became the dominant popular term in 1988, but within scientific journals global warming refers to surface temperature increases while climate change includes global warming and everything else that increasing greenhouse gas levels affect.{{cite web |title=What's in a Name? Global Warming vs. Climate Change |publisher=NASA |date=December 5, 2008 |url=http://www.nasa.gov/topics/earth/features/climate_by_any_other_name.html |access-date=23 July 2011 |archive-date=9 August 2010 |archive-url=https://web.archive.org/web/20100809221926/http://www.nasa.gov/topics/earth/features/climate_by_any_other_name.html |url-status=live }}

A related term, climatic change, was proposed by the World Meteorological Organization (WMO) in 1966 to encompass all forms of climatic variability on time-scales longer than 10 years, but regardless of cause. During the 1970s, the term climate change replaced climatic change to focus on anthropogenic causes, as it became clear that human activities had a potential to drastically alter the climate. Climate change was incorporated in the title of the Intergovernmental Panel on Climate Change (IPCC) and the UN Framework Convention on Climate Change (UNFCCC). Climate change is now used as both a technical description of the process, as well as a noun used to describe the problem.{{cite journal |last=Hulme |first=Mike |year=2016 |title=Concept of Climate Change, in: The International Encyclopedia of Geography |journal=The International Encyclopedia of Geography |page=1 |publisher=Wiley-Blackwell/Association of American Geographers (AAG) |url=https://www.academia.edu/10358797 |access-date=16 May 2016 |archive-date=29 September 2022 |archive-url=https://web.archive.org/web/20220929201908/https://www.academia.edu/10358797 |url-status=live }}

On the broadest scale, the rate at which energy is received from the Sun and the rate at which it is lost to space determine the equilibrium temperature and climate of Earth. This energy is distributed around the globe by winds, ocean currents,{{cite journal | title=Estimates of Global Oceanic Meridional Heat Transport | first1=Jane | last1=Hsiung | journal=Journal of Physical Oceanography | volume=15 | issue=11 | pages=1405–13 | date=November 1985 | doi=10.1175/1520-0485(1985)015<1405:EOGOMH>2.0.CO;2 | bibcode=1985JPO....15.1405H | doi-access=free }}{{cite journal | title=Meridional energy transport in the coupled atmosphere–ocean system: scaling and numerical experiments | first1=Geoffrey K. | last1=Vallis | first2=Riccardo | last2=Farneti | s2cid=122384001 | volume=135 | issue=644 | date=October 2009 | pages=1643–60 | journal=Quarterly Journal of the Royal Meteorological Society | doi=10.1002/qj.498 | bibcode=2009QJRMS.135.1643V }} and other mechanisms to affect the climates of different regions.{{cite journal | title=Earth's Global Energy Budget | last1=Trenberth | first1=Kevin E. | last2=Fasullo | first2=John T. | last3=Kiehl | first3=Jeffrey | display-authors=1 | journal=Bulletin of the American Meteorological Society | volume=90 | issue=3 | pages=311–23 | year=2009 | doi=10.1175/2008BAMS2634.1 | bibcode=2009BAMS...90..311T | doi-access=free }}

Factors that can shape climate are called climate forcings or "forcing mechanisms".{{cite book |last=Smith |first=Ralph C. |year=2013 |title=Uncertainty Quantification: Theory, Implementation, and Applications |series=Computational Science and Engineering |publisher=SIAM |isbn=978-1611973228 |volume=12 |page=23 |url=https://books.google.com/books?id=Tc1GAgAAQBAJ&pg=PA23}} These include processes such as variations in solar radiation, variations in the Earth's orbit, variations in the albedo or reflectivity of the continents, atmosphere, and oceans, mountain-building and continental drift and changes in greenhouse gas concentrations. External forcing can be either anthropogenic (e.g. increased emissions of greenhouse gases and dust) or natural (e.g., changes in solar output, the Earth's orbit, volcano eruptions).{{harvnb|Cronin|2010|pp=17–18}} There are a variety of climate change feedbacks that can either amplify or diminish the initial forcing. There are also key thresholds which when exceeded can produce rapid or irreversible change.

Some parts of the climate system, such as the oceans and ice caps, respond more slowly in reaction to climate forcings, while others respond more quickly. An example of fast change is the atmospheric cooling after a volcanic eruption, when volcanic ash reflects sunlight. Thermal expansion of ocean water after atmospheric warming is slow, and can take thousands of years. A combination is also possible, e.g., sudden loss of albedo in the Arctic Ocean as sea ice melts, followed by more gradual thermal expansion of the water.

Climate variability can also occur due to internal processes. Internal unforced processes often involve changes in the distribution of energy in the ocean and atmosphere, for instance, changes in the thermohaline circulation.

File:1951+ Percent of global area at temperature records - Seasonal comparison - NOAA.svg

Climatic changes due to internal variability sometimes occur in cycles or oscillations. For other types of natural climatic change, we cannot predict when it happens; the change is called random or stochastic.{{Sfn|Ruddiman|2008|pp=261–62}} From a climate perspective, the weather can be considered random.{{Cite journal|last=Hasselmann|first=K.|date=1976|title=Stochastic climate models Part I. Theory|journal=Tellus|volume=28|issue=6|pages=473–85|doi=10.1111/j.2153-3490.1976.tb00696.x|issn=2153-3490|bibcode=1976Tell...28..473H}} If there are little clouds in a particular year, there is an energy imbalance and extra heat can be absorbed by the oceans. Due to climate inertia, this signal can be 'stored' in the ocean and be expressed as variability on longer time scales than the original weather disturbances.{{Cite journal|last=Liu|first=Zhengyu|s2cid=53953041|date=14 October 2011|title=Dynamics of Interdecadal Climate Variability: A Historical Perspective|journal=Journal of Climate|volume=25|issue=6|pages=1963–95|doi=10.1175/2011JCLI3980.1|issn=0894-8755|doi-access=free}} If the weather disturbances are completely random, occurring as white noise, the inertia of glaciers or oceans can transform this into climate changes where longer-duration oscillations are also larger oscillations, a phenomenon called red noise.{{Sfn|Ruddiman|2008|p=262}} Many climate changes have a random aspect and a cyclical aspect. This behavior is dubbed stochastic resonance.{{Sfn|Ruddiman|2008|p=262}} Half of the 2021 Nobel prize on physics was awarded for this work to Klaus Hasselmann jointly with Syukuro Manabe for related work on climate modelling. While Giorgio Parisi who with collaborators introduced{{cite journal|vauthors=Benzi R, Parisi G, Sutera A, Vulpiani A|year=1982|title=Stochastic resonance in climatic change|journal=Tellus|volume=34|issue=1|pages=10–6|bibcode=1982Tell...34...10B|doi=10.1111/j.2153-3490.1982.tb01787.x|url=https://www.openaccessrepository.it/record/123925 |archive-url=https://web.archive.org/web/20241201230816/https://www.openaccessrepository.it/record/123925 |url-status=dead |archive-date=1 December 2024 |url-access=subscription}} the concept of stochastic resonance was awarded the other half but mainly for work on theoretical physics.

The ocean and atmosphere can work together to spontaneously generate internal climate variability that can persist for years to decades at a time.{{cite journal |last1=Brown |first1=Patrick T. |last2=Li |first2=Wenhong |last3=Cordero |first3=Eugene C. |last4=Mauget |first4=Steven A. |date=21 April 2015 |title=Comparing the model-simulated global warming signal to observations using empirical estimates of unforced noise |journal=Scientific Reports |issn=2045-2322 |doi=10.1038/srep09957 |pmc=4404682 |pmid=25898351 |volume=5|issue=1 |page=9957 |bibcode=2015NatSR...5.9957B }}{{cite journal |last=Hasselmann |first=K. |date=1 December 1976 |title=Stochastic climate models Part I. Theory |journal=Tellus |issn=2153-3490 |doi=10.1111/j.2153-3490.1976.tb00696.x |volume=28 |issue=6 |pages=473–85 |bibcode=1976Tell...28..473H }} These variations can affect global average surface temperature by redistributing heat between the deep ocean and the atmosphere{{cite journal |last1=Meehl |first1=Gerald A. |last2=Hu |first2=Aixue |last3=Arblaster |first3=Julie M. |last4=Fasullo |first4=John |last5=Trenberth |first5=Kevin E. |s2cid=16183172 |date=8 April 2013 |title=Externally Forced and Internally Generated Decadal Climate Variability Associated with the Interdecadal Pacific Oscillation |journal=Journal of Climate |issn=0894-8755 |doi=10.1175/JCLI-D-12-00548.1 |volume=26 |issue=18 |pages=7298–310 |bibcode=2013JCli...26.7298M |osti=1565088 |url=https://zenodo.org/record/1234599 |access-date=5 June 2020 |archive-date=11 March 2023 |archive-url=https://web.archive.org/web/20230311124210/https://zenodo.org/record/1234599 |url-status=live |doi-access=free }}{{cite journal |last1=England |first1=Matthew H. |last2=McGregor |first2=Shayne |last3=Spence |first3=Paul |last4=Meehl |first4=Gerald A. |last5=Timmermann |first5=Axel |author-link5= Axel Timmermann |last6=Cai |first6=Wenju |last7=Gupta |first7=Alex Sen |last8=McPhaden |first8=Michael J. |last9=Purich |first9=Ariaan |date=1 March 2014 |title=Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus |journal=Nature Climate Change |issn=1758-678X |doi=10.1038/nclimate2106 |volume=4 |issue=3 |pages=222–27|bibcode=2014NatCC...4..222E |hdl=1959.4/unsworks_13554 |hdl-access=free }} and/or by altering the cloud/water vapor/sea ice distribution which can affect the total energy budget of the Earth.{{cite journal |last1=Brown |first1=Patrick T. |last2=Li |first2=Wenhong |last3=Li |first3=Laifang |last4=Ming |first4=Yi |date=28 July 2014 |title=Top-of-atmosphere radiative contribution to unforced decadal global temperature variability in climate models |journal=Geophysical Research Letters |issn=1944-8007 |doi=10.1002/2014GL060625 |volume=41 |issue=14 |page=2014GL060625 |bibcode=2014GeoRL..41.5175B |hdl=10161/9167 |s2cid=16933795 |hdl-access=free }}{{cite journal |last1=Palmer |first1=M. D. |last2=McNeall |first2=D. J. |date=1 January 2014 |title=Internal variability of Earth's energy budget simulated by CMIP5 climate models |journal=Environmental Research Letters |issn=1748-9326 |doi=10.1088/1748-9326/9/3/034016 |volume=9 |issue=3 |page=034016 |bibcode=2014ERL.....9c4016P |doi-access=free }}

File:20210827 Global surface temperature bar chart - bars color-coded by El Niño and La Niña intensity.svg. The El Niño–Southern Oscillation has been linked to variability in longer-term global average temperature increase.]]

A climate oscillation or climate cycle is any recurring cyclical oscillation within global or regional climate. They are quasiperiodic (not perfectly periodic), so a Fourier analysis of the data does not have sharp peaks in the spectrum. Many oscillations on different time-scales have been found or hypothesized:{{Cite web|url=https://www.whoi.edu/main/topic/el-nino-other-oscillations|title=El Niño & Other Oscillations|website=Woods Hole Oceanographic Institution|access-date=6 April 2019|archive-date=6 April 2019|archive-url=https://web.archive.org/web/20190406082544/https://www.whoi.edu/main/topic/el-nino-other-oscillations|url-status=live}}

• the El Niño–Southern Oscillation (ENSO) – A large scale pattern of warmer (El Niño) and colder (La Niña) tropical sea surface temperatures in the Pacific Ocean with worldwide effects. It is a self-sustaining oscillation, whose mechanisms are well-studied.{{Cite journal|last=Wang|first=Chunzai|date=2018|title=A review of ENSO theories|journal=National Science Review|volume=5|issue=6|pages=813–825|doi=10.1093/nsr/nwy104|issn=2095-5138|doi-access=free}} ENSO is the most prominent known source of inter-annual variability in weather and climate around the world. The cycle occurs every two to seven years, with El Niño lasting nine months to two years within the longer term cycle.{{cite web|url=http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml#HOWOFTEN|title=ENSO FAQ: How often do El Niño and La Niña typically occur?|author=Climate Prediction Center|date=19 December 2005|publisher=National Centers for Environmental Prediction|url-status=dead|archive-url=https://web.archive.org/web/20090827143632/http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml#HOWOFTEN|archive-date=27 August 2009|access-date=26 July 2009|author-link=Climate Prediction Center}} The cold tongue of the equatorial Pacific Ocean is not warming as fast as the rest of the ocean, due to increased upwelling of cold waters off the west coast of South America.{{cite web|url=https://lamont.columbia.edu/news/part-pacific-ocean-not-warming-expected-why|title=Part of the Pacific Ocean Is Not Warming as Expected. Why|author=Kevin Krajick|publisher=Columbia University Lamont-Doherty Earth Observatory|access-date=2 November 2022|archive-date=5 March 2023|archive-url=https://web.archive.org/web/20230305101155/https://lamont.columbia.edu/news/part-pacific-ocean-not-warming-expected-why|url-status=live}}{{cite web|url=https://www.newsweek.com/mystery-stretch-pacific-ocean-warming-world-1445990?amp=1|title=Mystery Stretch of the Pacific Ocean Is Not Warming Like the Rest of the World's Waters|author=Aristos Georgiou|date=26 June 2019 |publisher=Newsweek|access-date=2 November 2022|archive-date=25 February 2023|archive-url=https://web.archive.org/web/20230225140142/https://www.newsweek.com/mystery-stretch-pacific-ocean-warming-world-1445990?amp=1|url-status=live}}
• the Madden–Julian oscillation (MJO) – An eastward moving pattern of increased rainfall over the tropics with a period of 30 to 60 days, observed mainly over the Indian and Pacific Oceans.{{Cite web|url=https://www.climate.gov/news-features/blogs/enso/what-mjo-and-why-do-we-care|title=What is the MJO, and why do we care?|website=NOAA Climate.gov|language=en|access-date=6 April 2019|archive-date=15 March 2023|archive-url=https://web.archive.org/web/20230315025156/https://www.climate.gov/news-features/blogs/enso/what-mjo-and-why-do-we-care|url-status=live}}
• the North Atlantic oscillation (NAO) – Indices of the NAO are based on the difference of normalized sea-level pressure (SLP) between Ponta Delgada, Azores and Stykkishólmur/Reykjavík, Iceland. Positive values of the index indicate stronger-than-average westerlies over the middle latitudes.National Center for Atmospheric Research. [http://www.cgd.ucar.edu/cas/jhurrell/indices.info.html Climate Analysis Section.] {{webarchive|url=https://web.archive.org/web/20060622232926/http://www.cgd.ucar.edu/cas/jhurrell/indices.info.html|date=22 June 2006}} Retrieved on 7 June 2007.
• the Quasi-biennial oscillation – a well-understood oscillation in wind patterns in the stratosphere around the equator. Over a period of 28 months the dominant wind changes from easterly to westerly and back.{{Cite journal|last1=Baldwin|first1=M. P.|last2=Gray|first2=L. J.|last3=Dunkerton|first3=T. J.|last4=Hamilton|first4=K.|last5=Haynes|first5=P. H.|last6=Randel|first6=W. J.|last7=Holton|first7=J. R.|last8=Alexander|first8=M. J.|last9=Hirota|first9=I.|s2cid=16727059|date=2001|title=The quasi-biennial oscillation|journal=Reviews of Geophysics|language=en|volume=39|issue=2|pages=179–229|doi=10.1029/1999RG000073|bibcode=2001RvGeo..39..179B|doi-access=free}}
• Pacific Centennial Oscillation - a climate oscillation predicted by some climate models
• the Pacific decadal oscillation – The dominant pattern of sea surface variability in the North Pacific on a decadal scale. During a "warm", or "positive", phase, the west Pacific becomes cool and part of the eastern ocean warms; during a "cool" or "negative" phase, the opposite pattern occurs. It is thought not as a single phenomenon, but instead a combination of different physical processes.{{Cite journal|last1=Newman|first1=Matthew|last2=Alexander|first2=Michael A.|last3=Ault|first3=Toby R.|last4=Cobb|first4=Kim M.|last5=Deser|first5=Clara|last6=Di Lorenzo|first6=Emanuele|last7=Mantua|first7=Nathan J.|last8=Miller|first8=Arthur J.|last9=Minobe|first9=Shoshiro|s2cid=4824093|date=2016|title=The Pacific Decadal Oscillation, Revisited|journal=Journal of Climate|volume=29|issue=12|pages=4399–4427|doi=10.1175/JCLI-D-15-0508.1|issn=0894-8755|bibcode=2016JCli...29.4399N}}
• the Interdecadal Pacific oscillation (IPO) – Basin wide variability in the Pacific Ocean with a period between 20 and 30 years.{{Cite web|url=https://www.niwa.co.nz/node/111124|title=Interdecadal Pacific Oscillation|date=19 January 2016|website=NIWA|language=en|access-date=6 April 2019|archive-date=17 March 2023|archive-url=https://web.archive.org/web/20230317140832/https://niwa.co.nz/node/111124|url-status=live}}
• the Atlantic multidecadal oscillation – A pattern of variability in the North Atlantic of about 55 to 70 years, with effects on rainfall, droughts and hurricane frequency and intensity.{{Cite journal|last1=Kuijpers|first1=Antoon|last2=Bo Holm Jacobsen|last3=Seidenkrantz|first3=Marit-Solveig|last4=Knudsen|first4=Mads Faurschou|date=2011|title=Tracking the Atlantic Multidecadal Oscillation through the last 8,000 years|journal=Nature Communications|language=en|volume=2|issue=1 |pages=178–|doi=10.1038/ncomms1186|pmid=21285956|issn=2041-1723|pmc=3105344|bibcode=2011NatCo...2..178K}}
• North African climate cycles – climate variation driven by the North African Monsoon, with a period of tens of thousands of years.{{cite journal|last1=Skonieczny|first1=C.|date=2 January 2019|title=Monsoon-driven Saharan dust variability over the past 240,000 years|journal=Science Advances|volume=5|issue=1|pages=eaav1887|doi=10.1126/sciadv.aav1887|pmc=6314818|pmid=30613782|bibcode=2019SciA....5.1887S}}
• the Arctic oscillation (AO) and Antarctic oscillation (AAO) – The annular modes are naturally occurring, hemispheric-wide patterns of climate variability. On timescales of weeks to months they explain 20–30% of the variability in their respective hemispheres. The Northern Annular Mode or Arctic oscillation (AO) in the Northern Hemisphere, and the Southern Annular Mode or Antarctic oscillation (AAO) in the southern hemisphere. The annular modes have a strong influence on the temperature and precipitation of mid-to-high latitude land masses, such as Europe and Australia, by altering the average paths of storms. The NAO can be considered a regional index of the AO/NAM.{{cite web |last1=Thompson |first1=David |title=Annular Modes – Introduction |url=http://www.atmos.colostate.edu/~davet/ao/introduction.html |access-date=11 February 2020 |archive-date=18 March 2023 |archive-url=https://web.archive.org/web/20230318094533/https://www.atmos.colostate.edu/~davet/ao/introduction.html |url-status=live }} They are defined as the first EOF of sea level pressure or geopotential height from 20°N to 90°N (NAM) or 20°S to 90°S (SAM).
• Dansgaard–Oeschger cycles – occurring on roughly 1,500-year cycles during the Last Glacial Maximum

{{See also|Thermohaline circulation}}

File:Ocean circulation conveyor belt.jpg. Tens of millions of years ago, continental-plate movement formed a land-free gap around Antarctica, allowing the formation of the ACC, which keeps warm waters away from Antarctica.]]

The oceanic aspects of climate variability can generate variability on centennial timescales due to the ocean having hundreds of times more mass than in the atmosphere, and thus very high thermal inertia. For example, alterations to ocean processes such as thermohaline circulation play a key role in redistributing heat in the world's oceans.

Ocean currents transport a lot of energy from the warm tropical regions to the colder polar regions. Changes occurring around the last ice age (in technical terms, the last glacial period) show that the circulation in the North Atlantic can change suddenly and substantially, leading to global climate changes, even though the total amount of energy coming into the climate system did not change much. These large changes may have come from so called Heinrich events where internal instability of ice sheets caused huge ice bergs to be released into the ocean. When the ice sheet melts, the resulting water is very low in salt and cold, driving changes in circulation.{{sfn|Burroughs|2001|pp=207–08}}

Life affects climate through its role in the carbon and water cycles and through such mechanisms as albedo, evapotranspiration, cloud formation, and weathering.{{cite journal |last1=Spracklen |first1=D. V. |last2=Bonn |first2=B. |last3=Carslaw |first3=K. S. |year=2008 |title=Boreal forests, aerosols and the impacts on clouds and climate |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |doi=10.1098/rsta.2008.0201 |pmid=18826917 |bibcode=2008RSPTA.366.4613S |volume=366 |issue=1885 |pages=4613–26 |s2cid=206156442 }}{{cite journal |last1=Christner |first1=B. C. |last2=Morris |first2=C. E. |last3=Foreman |first3=C. M. |last4=Cai |first4=R. |last5=Sands |first5=D. C. |year=2008 |title=Ubiquity of Biological Ice Nucleators in Snowfall |journal=Science |doi=10.1126/science.1149757 |pmid=18309078 |bibcode=2008Sci...319.1214C |volume=319 |issue=5867 |page=1214 |s2cid=39398426 |url=https://scholarworks.montana.edu/xmlui/bitstream/1/13209/1/08-006_Ubiquity_of_biological.pdf |archive-url=https://web.archive.org/web/20200305072355/https://scholarworks.montana.edu/xmlui/bitstream/1/13209/1/08-006_Ubiquity_of_biological.pdf |archive-date=2020-03-05 |url-status=live }}{{cite journal |last1=Schwartzman |first1=David W. |last2=Volk |first2=Tyler |year=1989 |title=Biotic enhancement of weathering and the habitability of Earth |journal=Nature |bibcode=1989Natur.340..457S |doi=10.1038/340457a0 |volume=340 |issue=6233 |pages=457–60 |s2cid=4314648 }} Examples of how life may have affected past climate include:

• glaciation 2.3 billion years ago triggered by the evolution of oxygenic photosynthesis, which depleted the atmosphere of the greenhouse gas carbon dioxide and introduced free oxygen{{cite journal |doi=10.1073/pnas.0504878102 |title=The Paleoproterozoic snowball Earth: A climate disaster triggered by the evolution of oxygenic photosynthesis |year=2005 |last1=Kopp |first1=R.E. |last2=Kirschvink |first2=J.L. |last3=Hilburn |first3=I.A. |last4=Nash |first4=C.Z. |journal=Proceedings of the National Academy of Sciences |volume=102 |issue=32 |pages=11131–36 |pmid=16061801 |pmc=1183582|bibcode = 2005PNAS..10211131K |doi-access=free }}{{cite journal |doi=10.1126/science.1071184 |title= Life and the Evolution of Earth's Atmosphere |year=2002 |last1= Kasting |first1=J.F. |journal= Science |volume=296 |issue=5570 |pages= 1066–68 |pmid=12004117 |last2=Siefert |first2=JL|s2cid=37190778 |bibcode = 2002Sci...296.1066K }}
• another glaciation 300 million years ago ushered in by long-term burial of decomposition-resistant detritus of vascular land-plants (creating a carbon sink and forming coal){{cite journal |doi=10.1126/science.271.5252.1105 |title= Middle to Late Paleozoic Atmospheric CO2 Levels from Soil Carbonate and Organic Matter |year=1996 |last1=Mora |first1=C.I. |last2=Driese |first2=S.G. |last3=Colarusso |first3=L. A. |journal=Science |volume=271 |issue=5252 |pages=1105–07 |bibcode= 1996Sci...271.1105M|s2cid=128479221 }}{{cite journal |doi=10.1073/pnas.96.20.10955 |title=Atmospheric oxygen over Phanerozoic time |year=1999 |last1=Berner |first1=R.A. |journal=Proceedings of the National Academy of Sciences |volume=96 |issue=20 |pages= 10955–57 |pmid=10500106 |pmc=34224|bibcode = 1999PNAS...9610955B |doi-access=free }}
• termination of the Paleocene–Eocene Thermal Maximum 55 million years ago by flourishing marine phytoplankton{{cite journal |doi=10.1038/35025035 |year=2000 |last1=Bains |first1=Santo |last2=Norris |first2=Richard D. |last3=Corfield |first3=Richard M. |last4=Faul |first4=Kristina L. |journal=Nature |volume=407 |issue=6801 |pages=171–74 |pmid=11001051 |title=Termination of global warmth at the Palaeocene/Eocene boundary through productivity feedback|bibcode = 2000Natur.407..171B |s2cid=4419536 }}{{cite journal |doi=10.1080/11035890001221188 |title=An assessment of the biogeochemical feedback response to the climatic and chemical perturbations of the LPTM |year= 2000 |last1=Zachos |first1= J.C. |last2= Dickens |first2=G.R. |journal= GFF |volume=122 |issue=1 |pages=188–89|bibcode=2000GFF...122..188Z |s2cid=129797785 }}
• reversal of global warming 49 million years ago by 800,000 years of arctic azolla blooms{{cite journal |doi=10.1111/j.1472-4669.2009.00195.x |title=The Eocene Arctic Azolla bloom: Environmental conditions, productivity and carbon drawdown |year=2009 |last1=Speelman |first1=E.N. |last2=Van Kempen |first2=M.M.L. |last3=Barke |first3=J. |last4=Brinkhuis |first4=H. |last5=Reichart |first5=G.J. |last6=Smolders |first6=A.J.P. |last7=Roelofs |first7=J.G.M. |last8=Sangiorgi |first8=F. |last9=De Leeuw |first9=J.W. |last10=Lotter |first10=A.F. |last11=Sinninghe Damsté |first11=J.S. |s2cid=13206343 |journal=Geobiology |volume=7 |issue=2 |pages=155–70 |pmid=19323694|bibcode=2009Gbio....7..155S }}{{cite journal |doi=10.1038/nature04692 |title=Episodic fresh surface waters in the Eocene Arctic Ocean |year=2006 |last1=Brinkhuis |first1=Henk |last2=Schouten |first2=Stefan |last3=Collinson |first3=Margaret E. |last4=Sluijs |first4=Appy |last5=Sinninghe Damsté |first5=Jaap S. Sinninghe |last6=Dickens |first6=Gerald R. |last7=Huber |first7=Matthew |last8=Cronin |first8=Thomas M. |last9=Onodera |first9=Jonaotaro |last10=Takahashi |first10=Kozo |last11=Bujak |first11=Jonathan P. |last12=Stein |first12=Ruediger |last13=Van Der Burgh |first13=Johan |last14=Eldrett |first14=James S. |last15=Harding |first15=Ian C. |last16=Lotter |first16=André F. |last17=Sangiorgi |first17=Francesca |last18=Van Konijnenburg-Van Cittert |first18=Han van Konijnenburg-van |last19=De Leeuw |first19=Jan W. |last20=Matthiessen |first20=Jens |last21=Backman |first21=Jan |last22=Moran |first22=Kathryn |last23=Expedition 302 |journal=Nature |volume=441 |issue=7093 |pages=606–09 |pmid=16752440 |first23=Scientists|bibcode = 2006Natur.441..606B |hdl=11250/174278 |s2cid=4412107 |hdl-access=free }}
• global cooling over the past 40 million years driven by the expansion of grass-grazer ecosystems{{cite journal |doi=10.1086/320791 |title=Cenozoic Expansion of Grasslands and Climatic Cooling |year=2001 |last1=Retallack |first1=Gregory J. |s2cid=15560105 |journal=The Journal of Geology |volume=109 |issue=4 |pages=407–26 |bibcode=2001JG....109..407R}}{{cite journal |doi=10.1130/0091-7613(1997)025<0039:MTPVCA>2.3.CO;2 |title= Miocene to present vegetation changes: A possible piece of the Cenozoic cooling puzzle |year=1997 |last1=Dutton |first1=Jan F. |last2=Barron |first2=Eric J. |journal=Geology |volume=25 |issue= 1 |page=39|bibcode = 1997Geo....25...39D }}

{{Main|Greenhouse gas}}

File:Carbon Dioxide 800kyr.svg

Whereas greenhouse gases released by the biosphere is often seen as a feedback or internal climate process, greenhouse gases emitted from volcanoes are typically classified as external by climatologists.{{harvnb|Cronin|2010|p=17}} Greenhouse gases, such as {{CO2}}, methane and nitrous oxide, heat the climate system by trapping infrared light. Volcanoes are also part of the extended carbon cycle. Over very long (geological) time periods, they release carbon dioxide from the Earth's crust and mantle, counteracting the uptake by sedimentary rocks and other geological carbon dioxide sinks.

Since the Industrial Revolution, humanity has been adding to greenhouse gases by emitting CO₂ from fossil fuel combustion, changing land use through deforestation, and has further altered the climate with aerosols (particulate matter in the atmosphere),{{cite web |url=https://www.science.org.au/learning/general-audience/science-booklets-0/science-climate-change/3-are-human-activities-causing |title=3. Are human activities causing climate change? |publisher=Australian Academy of Science |website=science.org.au |access-date=12 August 2017 |archive-date=8 May 2019 |archive-url=https://web.archive.org/web/20190508094624/https://www.science.org.au/learning/general-audience/science-booklets-0/science-climate-change/3-are-human-activities-causing |url-status=live }} release of trace gases (e.g. nitrogen oxides, carbon monoxide, or methane).{{cite book

|title = Climate Change, Human Systems and Policy Volume I

|chapter = Anthropogenic Climate Influences

|editor = Antoaneta Yotova

|date = 2009

|publisher = Eolss Publishers

|isbn = 978-1-905839-02-5

|url = https://www.eolss.net/ebooklib/bookinfo/climate-change-human-systems-policy.aspx

|access-date = 16 August 2020

|archive-date = 4 April 2023

|archive-url = https://web.archive.org/web/20230404081859/http://www.eolss.net/ebooklib/bookinfo/climate-change-human-systems-policy.aspx

|url-status = live

}} Other factors, including land use, ozone depletion, animal husbandry (ruminant animals such as cattle produce methane{{cite book |last=Steinfeld |first=H. |author2=P. Gerber |author3=T. Wassenaar |author4=V. Castel |author5=M. Rosales |author6=C. de Haan |title=Livestock's long shadow |year=2006 |url=http://www.fao.org/docrep/010/a0701e/a0701e00.HTM |access-date=21 July 2009 |archive-date=26 July 2008 |archive-url=https://web.archive.org/web/20080726214204/http://www.fao.org/docrep/010/a0701e/a0701e00.htm |url-status=live }}), and deforestation, also play a role.{{cite news |author=The Editorial Board |title=What the Paris Climate Meeting Must Do |url=https://www.nytimes.com/2015/11/29/opinion/sunday/what-the-paris-climate-meeting-must-do.html |date=28 November 2015 |work=The New York Times |access-date=28 November 2015 |archive-date=29 November 2015 |archive-url=https://web.archive.org/web/20151129034132/http://www.nytimes.com/2015/11/29/opinion/sunday/what-the-paris-climate-meeting-must-do.html |url-status=live }}

The US Geological Survey estimates are that volcanic emissions are at a much lower level than the effects of current human activities, which generate 100–300 times the amount of carbon dioxide emitted by volcanoes.{{cite web|url=http://volcanoes.usgs.gov/Hazards/What/VolGas/volgas.html|title=Volcanic Gases and Their Effects|date=10 January 2006|publisher=U.S. Department of the Interior|access-date=21 January 2008|archive-date=1 August 2013|archive-url=https://web.archive.org/web/20130801120440/http://volcanoes.usgs.govvolcanoes.usgs.gov/|url-status=live}} The annual amount put out by human activities may be greater than the amount released by supereruptions, the most recent of which was the Toba eruption in Indonesia 74,000 years ago.{{cite web|url=http://www.agu.org/news/press/pr_archives/2011/2011-22.shtml|title=Human Activities Emit Way More Carbon Dioxide Than Do Volcanoes|date=14 June 2011|publisher=American Geophysical Union|access-date=20 June 2011|archive-date=9 May 2013|archive-url=https://web.archive.org/web/20130509191429/http://www.agu.org/news/press/pr_archives/2011/2011-22.shtml|url-status=dead}}

File:MilankovitchCyclesOrbitandCores.png

Slight variations in Earth's motion lead to changes in the seasonal distribution of sunlight reaching the Earth's surface and how it is distributed across the globe. There is very little change to the area-averaged annually averaged sunshine; but there can be strong changes in the geographical and seasonal distribution. The three types of kinematic change are variations in Earth's eccentricity, changes in the tilt angle of Earth's axis of rotation, and precession of Earth's axis. Combined, these produce Milankovitch cycles which affect climate and are notable for their correlation to glacial and interglacial periods,{{cite web |url=http://www.homepage.montana.edu/~geol445/hyperglac/time1/milankov.htm|archive-url=https://web.archive.org/web/20110716144130/http://www.homepage.montana.edu/~geol445/hyperglac/time1/milankov.htm|archive-date=16 July 2011|title= Milankovitch Cycles and Glaciation|access-date=2 April 2009 |publisher= University of Montana}} their correlation with the advance and retreat of the Sahara, and for their appearance in the stratigraphic record.{{cite journal |doi=10.1111/j.1365-3121.1989.tb00403.x|title=A Milankovitch scale for Cenomanian time|year=1989|author=Gale, Andrew S. |journal=Terra Nova |volume=1|pages=420–25|issue=5|bibcode=1989TeNov...1..420G}}{{cite web|title=Same forces as today caused climate changes 1.4 billion years ago|url=http://www.sdu.dk/en/Om_SDU/Fakulteterne/Naturvidenskab/Nyheder/2015_03_10_climate_cycles|website=sdu.dk|publisher=University of Denmark.|url-status=dead|archive-url=https://web.archive.org/web/20150312163250/http://www.sdu.dk/en/Om_SDU/Fakulteterne/Naturvidenskab/Nyheder/2015_03_10_climate_cycles|archive-date=12 March 2015}}

During the glacial cycles, there was a high correlation between {{CO2}} concentrations and temperatures. Early studies indicated that {{CO2}} concentrations lagged temperatures, but it has become clear that this is not always the case.{{Cite journal|last1=van Nes|first1=Egbert H.|last2=Scheffer|first2=Marten|last3=Brovkin|first3=Victor|last4=Lenton|first4=Timothy M.|last5=Ye|first5=Hao|last6=Deyle|first6=Ethan|last7=Sugihara|first7=George|date=2015|title=Causal feedbacks in climate change|journal=Nature Climate Change|language=en|volume=5|issue=5|pages=445–48|doi=10.1038/nclimate2568|bibcode=2015NatCC...5..445V|issn=1758-6798}} When ocean temperatures increase, the solubility of {{CO2}} decreases so that it is released from the ocean. The exchange of {{CO2}} between the air and the ocean can also be impacted by further aspects of climatic change.[http://archive.ipcc.ch/publications_and_data/ar4/wg1/en/ch6s6-4.html Box 6.2: What Caused the Low Atmospheric Carbon Dioxide Concentrations During Glacial Times?] {{Webarchive|url=https://web.archive.org/web/20230108231413/https://archive.ipcc.ch/publications_and_data/ar4/wg1/en/ch6s6-4.html |date=8 January 2023 }} in {{Harvnb|IPCC AR4 WG1|2007}} . These and other self-reinforcing processes allow small changes in Earth's motion to have a large effect on climate.

File:Solar Activity Proxies.pngs and beryllium isotopes. The period of extraordinarily few sunspots in the late 17th century was the Maunder minimum.|alt=]]The Sun is the predominant source of energy input to the Earth's climate system. Other sources include geothermal energy from the Earth's core, tidal energy from the Moon and heat from the decay of radioactive compounds. Both long term variations in solar intensity are known to affect global climate.{{Sfn|Rohli|Vega|2018|p=296}} Solar output varies on shorter time scales, including the 11-year solar cycle{{cite journal|last1=Willson|first1=Richard C.|last2=Hudson|first2=Hugh S.|year=1991|title=The Sun's luminosity over a complete solar cycle|journal=Nature|volume=351|issue=6321|pages=42–44|bibcode=1991Natur.351...42W|doi=10.1038/351042a0|s2cid=4273483}} and longer-term modulations.{{Cite journal|last1=Turner|first1=T. Edward|last2=Swindles|first2=Graeme T.|last3=Charman|first3=Dan J.|last4=Langdon|first4=Peter G.|last5=Morris|first5=Paul J.|last6=Booth|first6=Robert K.|last7=Parry|first7=Lauren E.|last8=Nichols|first8=Jonathan E.|date=5 April 2016|title=Solar cycles or random processes? Evaluating solar variability in Holocene climate records|journal=Scientific Reports|language=en|volume=6|issue=1|pages=23961|doi=10.1038/srep23961|pmid=27045989|issn=2045-2322|pmc=4820721}} Correlation between sunspots and climate and tenuous at best.{{Sfn|Rohli|Vega|2018|p=296}}

Three to four billion years ago, the Sun emitted only 75% as much power as it does today.{{Cite conference |last=Ribas |first=Ignasi |conference=IAU Symposium 264 'Solar and Stellar Variability – Impact on Earth and Planets' |title=The Sun and stars as the primary energy input in planetary atmospheres |journal=Proceedings of the International Astronomical Union |volume=264 |pages=3–18 |date=February 2010 |doi=10.1017/S1743921309992298 |bibcode=2010IAUS..264....3R |arxiv=0911.4872}} If the atmospheric composition had been the same as today, liquid water should not have existed on the Earth's surface. However, there is evidence for the presence of water on the early Earth, in the Hadean{{cite journal |doi=10.2138/rmg.2006.62.18 |title=Water in the Early Earth |year=2006 |author=Marty, B. |journal=Reviews in Mineralogy and Geochemistry |volume=62 |issue=1 |pages=421–450 |bibcode=2006RvMG...62..421M}}{{cite journal |doi=10.1126/science.1110873 |title=Zircon Thermometer Reveals Minimum Melting Conditions on Earliest Earth |year=2005 |last1=Watson |first1=E.B. |journal=Science |volume=308 |issue=5723 |pages=841–44 |pmid=15879213 |last2=Harrison |first2=TM|s2cid=11114317 |bibcode=2005Sci...308..841W}} and Archean{{cite journal |doi=10.1130/0091-7613(1994)022<1067:SWIISL>2.3.CO;2 |title=Surface-water influx in shallow-level Archean lode-gold deposits in Western, Australia |year=1994 |last1=Hagemann |first1=Steffen G. |last2=Gebre-Mariam |first2=Musie |last3=Groves |first3=David I. |journal=Geology |volume=22 |issue=12 |page=1067 |bibcode=1994Geo....22.1067H}} eons, leading to what is known as the faint young Sun paradox.{{cite journal | last = Sagan | first = C. | author2 = G. Mullen | title = Earth and Mars: Evolution of Atmospheres and Surface Temperatures | journal = Science | volume = 177 | issue = 4043 | pages = 52–6 | year = 1972 | url = http://www.sciencemag.org/cgi/content/abstract/177/4043/52?ck=nck | bibcode = 1972Sci...177...52S | doi = 10.1126/science.177.4043.52 | pmid = 17756316 | s2cid = 12566286 | access-date = 30 January 2009 | archive-date = 9 August 2010 | archive-url = https://web.archive.org/web/20100809113551/http://www.sciencemag.org/cgi/content/abstract/177/4043/52?ck=nck | url-status = live | url-access = subscription }} Hypothesized solutions to this paradox include a vastly different atmosphere, with much higher concentrations of greenhouse gases than currently exist.{{cite journal |doi=10.1126/science.276.5316.1217 |title=The Early Faint Sun Paradox: Organic Shielding of Ultraviolet-Labile Greenhouse Gases |year=1997 |last1=Sagan |first1=C. |journal=Science |volume=276 |issue=5316 |pages=1217–21 |pmid=11536805 |last2=Chyba |first2=C|bibcode = 1997Sci...276.1217S }} Over the following approximately 4 billion years, the energy output of the Sun increased. Over the next five billion years, the Sun's ultimate death as it becomes a red giant and then a white dwarf will have large effects on climate, with the red giant phase possibly ending any life on Earth that survives until that time.{{citation |last1=Schröder |first1=K.-P. |last2=Connon Smith |first2=Robert |date=2008 |title=Distant future of the Sun and Earth revisited |journal=Monthly Notices of the Royal Astronomical Society |volume=386 |issue=1 |pages=155–63 |doi=10.1111/j.1365-2966.2008.13022.x |doi-access=free |bibcode=2008MNRAS.386..155S |arxiv=0801.4031 |s2cid=10073988}}

File:Msu 1978-2010.jpg NASA satellites, effects appear from aerosols released by major volcanic eruptions (El Chichón and Pinatubo). El Niño is a separate event, from ocean variability.]]

The volcanic eruptions considered to be large enough to affect the Earth's climate on a scale of more than 1 year are the ones that inject over 100,000 tons of SO₂ into the stratosphere.{{cite journal

| last1 = Miles | first1 = M.G.

| last2 = Grainger | first2 = R.G.

| last3 = Highwood | first3 = E.J.

| title = The significance of volcanic eruption strength and frequency for climate

| journal = Quarterly Journal of the Royal Meteorological Society

| date = 2004

| volume = 130 | pages = 2361–76

| issue = 602

| doi = 10.1256/qj.03.60

| bibcode = 2004QJRMS.130.2361M

| s2cid = 53005926

}} This is due to the optical properties of SO₂ and sulfate aerosols, which strongly absorb or scatter solar radiation, creating a global layer of sulfuric acid haze.{{cite web

| title = Volcanic Gases and Climate Change Overview

| url = http://volcanoes.usgs.gov/hazards/gas/climate.php

| website = usgs.gov

| publisher = USGS

| access-date = 31 July 2014

| archive-date = 29 July 2014

| archive-url = https://web.archive.org/web/20140729142333/http://volcanoes.usgs.gov/hazards/gas/climate.php

| url-status = live

}} On average, such eruptions occur several times per century, and cause cooling (by partially blocking the transmission of solar radiation to the Earth's surface) for a period of several years. Although volcanoes are technically part of the lithosphere, which itself is part of the climate system, the IPCC explicitly defines volcanism as an external forcing agent.[http://archive.ipcc.ch/publications_and_data/ar4/syr/en/annexes.html Annexes] {{Webarchive|url=https://web.archive.org/web/20190706041420/https://archive.ipcc.ch/publications_and_data/ar4/syr/en/annexes.html |date=6 July 2019 }}, in {{Harvnb|IPCC AR4 SYR|2008|p=58}}.

Notable eruptions in the historical records are the 1991 eruption of Mount Pinatubo which lowered global temperatures by about 0.5 °C (0.9 °F) for up to three years,{{cite web

|url=http://pubs.usgs.gov/fs/1997/fs113-97/

|title=The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines

|last=Diggles

|first=Michael

|date=28 February 2005

|work=U.S. Geological Survey Fact Sheet 113-97

|publisher=United States Geological Survey

|access-date=8 October 2009

|archive-date=25 August 2013

|archive-url=https://web.archive.org/web/20130825233934/http://pubs.usgs.gov/fs/1997/fs113-97/

|url-status=live

}}{{cite web

| last1 = Diggles

| first1 = Michael

| title = The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines

| url = http://pubs.usgs.gov/fs/1997/fs113-97/

| website = usgs.gov

| access-date = 31 July 2014

| archive-date = 25 August 2013

| archive-url = https://web.archive.org/web/20130825233934/http://pubs.usgs.gov/fs/1997/fs113-97/

| url-status = live

}} and the 1815 eruption of Mount Tambora causing the Year Without a Summer.{{cite journal

|doi=10.1191/0309133303pp379ra

|title=Climatic, environmental and human consequences of the largest known historic eruption: Tambora volcano (Indonesia) 1815

|year=2003

|last1=Oppenheimer|first1=Clive

|journal=Progress in Physical Geography

|volume=27

|pages=230–59

|issue=2

|bibcode=2003PrPG...27..230O

|s2cid=131663534

}}

At a larger scale—a few times every 50 million to 100 million years—the eruption of large igneous provinces brings large quantities of igneous rock from the mantle and lithosphere to the Earth's surface. Carbon dioxide in the rock is then released into the atmosphere.{{Cite journal|title=Deep Carbon and the Life Cycle of Large Igneous Provinces|last1=Black|first1=Benjamin A.|last2=Gibson|first2=Sally A.|date=2019|journal=Elements|doi=10.2138/gselements.15.5.319|volume=15|issue=5|pages=319–324|doi-access=free|bibcode=2019Eleme..15..319B }}

{{cite journal

|doi=10.1016/S0012-8252(00)00037-4

|title=Large igneous provinces and mass extinctions

|year=2001

|last1=Wignall|first1=P

|journal=Earth-Science Reviews

|volume=53

|issue=1

|pages=1–33

|bibcode=2001ESRv...53....1W

}} Small eruptions, with injections of less than 0.1 Mt of sulfur dioxide into the stratosphere, affect the atmosphere only subtly, as temperature changes are comparable with natural variability. However, because smaller eruptions occur at a much higher frequency, they too significantly affect Earth's atmosphere.{{cite journal

| last1 = Graf | first1 = H.-F.

| last2 = Feichter | first2 = J.

| last3 = Langmann | first3 = B.

| title = Volcanic sulphur emissions: Estimates of source strength and its contribution to the global sulphate distribution

| journal = Journal of Geophysical Research: Atmospheres

| date = 1997

| volume = 102 | issue = D9

| pages = 10727–38

| doi = 10.1029/96JD03265

| bibcode=1997JGR...10210727G

| hdl = 21.11116/0000-0003-2CBB-A

| hdl-access = free

}}

{{Main|Plate tectonics}}

Over the course of millions of years, the motion of tectonic plates reconfigures global land and ocean areas and generates topography. This can affect both global and local patterns of climate and atmosphere-ocean circulation.{{Cite journal| year =1999| title = Paleoaltimetry incorporating atmospheric physics and botanical estimates of paleoclimate| journal = Geological Society of America Bulletin| volume = 111| pages = 497–511| issue = 4 | doi = 10.1130/0016-7606(1999)111<0497:PIAPAB>2.3.CO;2| first4 = K.A.| last2 = Wolfe | first1 = C.E.| last3 = Molnar | first2 = J.A.| first3 = P.| last4 = Emanuel| last1 = Forest|bibcode = 1999GSAB..111..497F | hdl = 1721.1/10809| hdl-access = free}}

The position of the continents determines the geometry of the oceans and therefore influences patterns of ocean circulation. The locations of the seas are important in controlling the transfer of heat and moisture across the globe, and therefore, in determining global climate. A recent example of tectonic control on ocean circulation is the formation of the Isthmus of Panama about 5 million years ago, which shut off direct mixing between the Atlantic and Pacific Oceans. This strongly affected the ocean dynamics of what is now the Gulf Stream and may have led to Northern Hemisphere ice cover.{{cite web|url=http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16401 |title=Panama: Isthmus that Changed the World |access-date=1 July 2008 |publisher=NASA Earth Observatory |url-status=dead |archive-url=https://web.archive.org/web/20070802015424/http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16401 |archive-date=2 August 2007 }}{{cite journal |url=http://www.whoi.edu/oceanus/viewArticle.do?id=2508 |title=How the Isthmus of Panama Put Ice in the Arctic |first1=Gerald H. |last1=Haug |first2=Lloyd D. |last2=Keigwin |date=22 March 2004 |journal=Oceanus |volume=42 |issue=2 |publisher=Woods Hole Oceanographic Institution |access-date=1 October 2013 |archive-date=5 October 2018 |archive-url=https://web.archive.org/web/20181005081528/http://www.whoi.edu/oceanus/viewArticle.do?id=2508 |url-status=live }} During the Carboniferous period, about 300 to 360 million years ago, plate tectonics may have triggered large-scale storage of carbon and increased glaciation.{{cite journal|title=Isotope stratigraphy of the European Carboniferous: proxy signals for ocean chemistry, climate and tectonics|date=30 September 1999|volume=161|issue=1–3|doi=10.1016/S0009-2541(99)00084-4|pages=127–63|first1=Peter |last1=Bruckschen|first2=Susanne |last2=Oesmanna|first3=Ján |last3=Veizer |journal=Chemical Geology|bibcode=1999ChGeo.161..127B}} Geologic evidence points to a "megamonsoonal" circulation pattern during the time of the supercontinent Pangaea, and climate modeling suggests that the existence of the supercontinent was conducive to the establishment of monsoons.{{cite journal|first=Judith T. |last=Parrish|title=Climate of the Supercontinent Pangea|journal=The Journal of Geology|year=1993|volume=101|pages=215–33 |doi=10.1086/648217|issue=2|publisher=The University of Chicago Press|jstor=30081148|bibcode = 1993JG....101..215P |s2cid=128757269}}

The size of continents is also important. Because of the stabilizing effect of the oceans on temperature, yearly temperature variations are generally lower in coastal areas than they are inland. A larger supercontinent will therefore have more area in which climate is strongly seasonal than will several smaller continents or islands.

It has been postulated that ionized particles known as cosmic rays could impact cloud cover and thereby the climate. As the sun shields the Earth from these particles, changes in solar activity were hypothesized to influence climate indirectly as well. To test the hypothesis, CERN designed the CLOUD experiment, which showed the effect of cosmic rays is too weak to influence climate noticeably.{{Cite web|url=https://www.carbonbrief.org/why-the-sun-is-not-responsible-for-recent-climate-change|title=Explainer: Why the sun is not responsible for recent climate change|last=Hausfather|first=Zeke|date=18 August 2017|website=Carbon Brief|access-date=5 September 2019|archive-date=17 March 2023|archive-url=https://web.archive.org/web/20230317140828/https://www.carbonbrief.org/why-the-sun-is-not-responsible-for-recent-climate-change/|url-status=live}}{{Cite journal|last=Pierce|first=J. R.|date=2017|title=Cosmic rays, aerosols, clouds, and climate: Recent findings from the CLOUD experiment|journal=Journal of Geophysical Research: Atmospheres|volume=122|issue=15|pages=8051–55|doi=10.1002/2017JD027475|bibcode=2017JGRD..122.8051P|s2cid=125580175 |issn=2169-8996}}

Evidence exists that the Chicxulub asteroid impact some 66 million years ago had severely affected the Earth's climate. Large quantities of sulfate aerosols were kicked up into the atmosphere, decreasing global temperatures by up to 26 °C and producing sub-freezing temperatures for a period of 3–16 years. The recovery time for this event took more than 30 years.{{citation

| contribution=Severe environmental effects of Chicxulub impact imply key role in end-Cretaceous mass extinction

| last1=Brugger | first1=Julia

| last2=Feulner | first2=Georg | last3=Petri | first3=Stefan

| title=19th EGU General Assembly, EGU2017, proceedings from the conference, 23–28 April 2017|location=Vienna, Austria

| volume=19 | pages=17167 | date=April 2017 | bibcode=2017EGUGA..1917167B | postscript=. }} The large-scale use of nuclear weapons has also been investigated for its impact on the climate. The hypothesis is that soot released by large-scale fires blocks a significant fraction of sunlight for as much as a year, leading to a sharp drop in temperatures for a few years. This possible event is described as nuclear winter.{{sfn|Burroughs|2001|p=232}}

Humans' use of land impact how much sunlight the surface reflects and the concentration of dust. Cloud formation is not only influenced by how much water is in the air and the temperature, but also by the amount of aerosols in the air such as dust.{{Cite web|url=https://www.chemistryworld.com/news/mineral-dust-plays-key-role-in-cloud-formation-and-chemistry/6157.article|title=Mineral dust plays key role in cloud formation and chemistry|last=Hadlington|first=Simon 9|date=May 2013|website=Chemistry World|access-date=5 September 2019|archive-date=24 October 2022|archive-url=https://web.archive.org/web/20221024053651/https://www.chemistryworld.com/news/mineral-dust-plays-key-role-in-cloud-formation-and-chemistry/6157.article|url-status=live}} Globally, more dust is available if there are many regions with dry soils, little vegetation and strong winds.{{Cite journal|last1=Mahowald|first1=Natalie|author-link=Natalie Mahowald|last2=Albani|first2=Samuel|last3=Kok|first3=Jasper F.|last4=Engelstaeder|first4=Sebastian|last5=Scanza|first5=Rachel|last6=Ward|first6=Daniel S.|last7=Flanner|first7=Mark G.|date=1 December 2014|title=The size distribution of desert dust aerosols and its impact on the Earth system|journal=Aeolian Research|volume=15|pages=53–71|bibcode=2014AeoRe..15...53M|doi=10.1016/j.aeolia.2013.09.002|issn=1875-9637|doi-access=free}}

Paleoclimatology is the study of changes in climate through the entire history of Earth. It uses a variety of proxy methods from the Earth and life sciences to obtain data preserved within things such as rocks, sediments, ice sheets, tree rings, corals, shells, and microfossils. It then uses the records to determine the past states of the Earth's various climate regions and its atmospheric system. Direct measurements give a more complete overview of climate variability.

Climate changes that occurred after the widespread deployment of measuring devices can be observed directly. Reasonably complete global records of surface temperature are available beginning from the mid-late 19th century. Further observations are derived indirectly from historical documents. Satellite cloud and precipitation data has been available since the 1970s.{{cite journal|last1=New |first1=M. |last2=Todd |first2=M. |last3=Hulme |first3=M |last4=Jones |first4=P. |s2cid=56212756|date=December 2001|title=Review: Precipitation measurements and trends in the twentieth century|journal=International Journal of Climatology|volume=21|issue=15|pages=1889–922|bibcode=2001IJCli..21.1889N|doi=10.1002/joc.680}}

Historical climatology is the study of historical changes in climate and their effect on human history and development. The primary sources include written records such as sagas, chronicles, maps and local history literature as well as pictorial representations such as paintings, drawings and even rock art. Climate variability in the recent past may be derived from changes in settlement and agricultural patterns.{{Cite journal|last1=Demenocal|first1=P.B.|year=2001|title=Cultural Responses to Climate Change During the Late Holocene|url=http://www.ldeo.columbia.edu/~peter/Resources/Publications/deMenocal.2001.pdf|journal=Science|volume=292|issue=5517|pages=667–73|bibcode=2001Sci...292..667D|doi=10.1126/science.1059827|pmid=11303088|s2cid=18642937 |access-date=28 August 2015|archive-date=17 December 2008|archive-url=https://web.archive.org/web/20081217162859/http://www.ldeo.columbia.edu/~peter/Resources/Publications/deMenocal.2001.pdf|url-status=dead}} Archaeological evidence, oral history and historical documents can offer insights into past changes in the climate. Changes in climate have been linked to the rise{{Cite journal|last1=Sindbaek|first1=S.M.|year=2007|title=Networks and nodal points: the emergence of towns in early Viking Age Scandinavia|journal=Antiquity|volume=81|issue=311|pages=119–32|doi=10.1017/s0003598x00094886|doi-access=free}} and the collapse of various civilizations.

File:Vostok Petit data.svg, temperature and dust from the Vostok ice core over the last 450,000 years.]]

Various archives of past climate are present in rocks, trees and fossils. From these archives, indirect measures of climate, so-called proxies, can be derived. Quantification of climatological variation of precipitation in prior centuries and epochs is less complete but approximated using proxies such as marine sediments, ice cores, cave stalagmites, and tree rings.{{cite journal|last1=Dominic |first1=F. |last2=Burns |first2=S.J. |last3=Neff |first3=U. |last4=Mudulsee |first4=M. |last5=Mangina |first5=A |last6=Matter |first6=A. |date=April 2004 |title=Palaeoclimatic interpretation of high-resolution oxygen isotope profiles derived from annually laminated speleothems from Southern Oman|journal=Quaternary Science Reviews |volume=23 |issue=7–8 |pages=935–45 |bibcode=2004QSRv...23..935F |doi=10.1016/j.quascirev.2003.06.019}} Stress, too little precipitation or unsuitable temperatures, can alter the growth rate of trees, which allows scientists to infer climate trends by analyzing the growth rate of tree rings. This branch of science studying this called dendroclimatology.{{Cite book|url=https://books.google.com/books?id=e9Ez2dRGmioC|title=Dendroclimatology: progress and prospect|publisher=Springer Science & Business Media|year=2010|isbn=978-1-4020-4010-8|editor1-last=Hughes|editor1-first=Malcolm K.|series=Developments in Paleoenvironmental Research|volume=11|location=New York|editor2-last=Swetnam|editor2-first=Thomas W.|editor3-last=Diaz|editor3-first=Henry F.}} Glaciers leave behind moraines that contain a wealth of material—including organic matter, quartz, and potassium that may be dated—recording the periods in which a glacier advanced and retreated.

Analysis of ice in cores drilled from an ice sheet such as the Antarctic ice sheet, can be used to show a link between temperature and global sea level variations. The air trapped in bubbles in the ice can also reveal the CO₂ variations of the atmosphere from the distant past, well before modern environmental influences. The study of these ice cores has been a significant indicator of the changes in CO₂ over many millennia, and continues to provide valuable information about the differences between ancient and modern atmospheric conditions. The ¹⁸O/¹⁶O ratio in calcite and ice core samples used to deduce ocean temperature in the distant past is an example of a temperature proxy method.

The remnants of plants, and specifically pollen, are also used to study climatic change. Plant distributions vary under different climate conditions. Different groups of plants have pollen with distinctive shapes and surface textures, and since the outer surface of pollen is composed of a very resilient material, they resist decay. Changes in the type of pollen found in different layers of sediment indicate changes in plant communities. These changes are often a sign of a changing climate.{{cite journal|last1=Langdon|first1=P.G.|last2=Barber|first2=K.E.|last3=Lomas-Clarke|first3=S.H.|last4=Lomas-Clarke|first4=S.H.|date=August 2004|title=Reconstructing climate and environmental change in northern England through chironomid and pollen analyses: evidence from Talkin Tarn, Cumbria|journal=Journal of Paleolimnology|volume=32|issue=2|pages=197–213|bibcode=2004JPall..32..197L|doi=10.1023/B:JOPL.0000029433.85764.a5|s2cid=128561705}}{{cite journal|last=Birks|first=H.H.|date=March 2003|title=The importance of plant macrofossils in the reconstruction of Lateglacial vegetation and climate: examples from Scotland, western Norway, and Minnesota, US|url=https://bora.uib.no/bitstream/1956/387/4/1956-387.pdf|journal=Quaternary Science Reviews|volume=22|issue=5–7|pages=453–73|bibcode=2003QSRv...22..453B|doi=10.1016/S0277-3791(02)00248-2|hdl=1956/387|hdl-access=free|access-date=20 April 2018|archive-date=11 June 2007|archive-url=https://web.archive.org/web/20070611133600/https://bora.uib.no/bitstream/1956/387/4/1956-387.pdf|url-status=dead}} As an example, pollen studies have been used to track changing vegetation patterns throughout the Quaternary glaciations{{cite journal|last1=Miyoshi|first1=N|last2=Fujiki|first2=Toshiyuki|last3=Morita|first3=Yoshimune|year=1999|title=Palynology of a 250-m core from Lake Biwa: a 430,000-year record of glacial–interglacial vegetation change in Japan|journal=Review of Palaeobotany and Palynology|volume=104|issue=3–4|pages=267–83|doi=10.1016/S0034-6667(98)00058-X|bibcode=1999RPaPa.104..267M}} and especially since the last glacial maximum.{{cite journal|last=Prentice|first=I. Colin|author2=Bartlein, Patrick J|author3=Webb, Thompson|year=1991|title=Vegetation and Climate Change in Eastern North America Since the Last Glacial Maximum|journal=Ecology|volume=72|issue=6|pages=2038–56|doi=10.2307/1941558|jstor=1941558|bibcode=1991Ecol...72.2038P }} Remains of beetles are common in freshwater and land sediments. Different species of beetles tend to be found under different climatic conditions. Given the extensive lineage of beetles whose genetic makeup has not altered significantly over the millennia, knowledge of the present climatic range of the different species, and the age of the sediments in which remains are found, past climatic conditions may be inferred.{{cite journal |last1=Coope |first1=G.R. |last2=Lemdahl |first2=G. |last3=Lowe |first3=J.J. |last4=Walkling |first4=A. |date=4 May 1999 |title=Temperature gradients in northern Europe during the last glacial – Holocene transition (14–9 14 C kyr BP) interpreted from coleopteran assemblages |journal=Journal of Quaternary Science |volume=13 |issue=5 |pages=419–33 |bibcode=1998JQS....13..419C |doi=10.1002/(SICI)1099-1417(1998090)13:5<419::AID-JQS410>3.0.CO;2-D}}

One difficulty in detecting climate cycles is that the Earth's climate has been changing in non-cyclic ways over most paleoclimatological timescales. Currently we are in a period of anthropogenic global warming. In a larger timeframe, the Earth is emerging from the latest ice age, cooling from the Holocene climatic optimum and warming from the "Little Ice Age", which means that climate has been constantly changing over the last 15,000 years or so. During warm periods, temperature fluctuations are often of a lesser amplitude. The Pleistocene period, dominated by repeated glaciations, developed out of more stable conditions in the Miocene and Pliocene climate. Holocene climate has been relatively stable. All of these changes complicate the task of looking for cyclical behavior in the climate.

Positive feedback, negative feedback, and ecological inertia from the land-ocean-atmosphere system often attenuate or reverse smaller effects, whether from orbital forcings, solar variations or changes in concentrations of greenhouse gases. Certain feedbacks involving processes such as clouds are also uncertain; for contrails, natural cirrus clouds, oceanic dimethyl sulfide and a land-based equivalent, competing theories exist concerning effects on climatic temperatures, for example contrasting the Iris hypothesis and CLAW hypothesis.

File:Aridity ice age vs early holocene vs modern.jpg ice age climate{{Clear}}Middle: Atlantic Period, warm and wet{{Clear}}Bottom: Potential vegetation in climate now if not for human effects like agriculture.{{cite web | editor1-last=Adams | editor1-first=J.M. | editor2-last=Faure | editor2-first=H. | year=1997 | url=http://www.esd.ornl.gov/projects/qen/nerc.html | title=Global land environments since the last interglacial | publisher=Oak Ridge National Laboratory | location=Tennessee | url-status=dead | archive-url=https://web.archive.org/web/20080116122058/http://www.esd.ornl.gov/projects/qen/nerc.html | archive-date=16 January 2008 | df=dmy-all }} QEN members.]]

A change in the type, distribution and coverage of vegetation may occur given a change in the climate. Some changes in climate may result in increased precipitation and warmth, resulting in improved plant growth and the subsequent sequestration of airborne CO₂. Though an increase in CO₂ may benefit plants, some factors can diminish this increase. If there is an environmental change such as drought, increased CO₂ concentrations will not benefit the plant.{{Cite journal |last=Swann |first=Abigail L. S. |date=2018-06-01 |title=Plants and Drought in a Changing Climate |url=https://doi.org/10.1007/s40641-018-0097-y |journal=Current Climate Change Reports |language=en |volume=4 |issue=2 |pages=192–201 |doi=10.1007/s40641-018-0097-y |bibcode=2018CCCR....4..192S |issn=2198-6061}} So even though climate change does increase CO₂ emissions, plants will often not use this increase as other environmental stresses put pressure on them.{{Cite journal |last1=Ainsworth |first1=E. A. |last2=Lemonnier |first2=P. |last3=Wedow |first3=J. M. |date=January 2020 |editor-last=Tausz-Posch |editor-first=S. |title=The influence of rising tropospheric carbon dioxide and ozone on plant productivity |journal=Plant Biology |language=en |volume=22 |issue=S1 |pages=5–11 |doi=10.1111/plb.12973 |issn=1435-8603 |pmc=6916594 |pmid=30734441|bibcode=2020PlBio..22S...5A }} However, sequestration of CO₂ is expected to affect the rate of many natural cycles like plant litter decomposition rates.{{cite journal |last1=Ochoa-Hueso |first1=R |last2=Delgado-Baquerizo |first2=N |last3=King |first3=PTA |last4=Benham |first4=M |last5=Arca |first5=V |last6=Power |first6=SA |title=Ecosystem type and resource quality are more important than global change drivers in regulating early stages of litter decomposition |journal=Soil Biology and Biochemistry |date=2019 |volume=129 |pages=144–52 |doi=10.1016/j.soilbio.2018.11.009 |bibcode=2019SBiBi.129..144O |hdl=10261/336676 |s2cid=92606851 |hdl-access=free }} A gradual increase in warmth in a region will lead to earlier flowering and fruiting times, driving a change in the timing of life cycles of dependent organisms. Conversely, cold will cause plant bio-cycles to lag.{{cite web |last=Kinver |first=Mark |date=15 November 2011 |title=UK trees' fruit ripening '18 days earlier' |publisher=Bbc.co.uk |url=https://www.bbc.co.uk/news/science-environment-15721263 |access-date=1 November 2012 |archive-date=17 March 2023 |archive-url=https://web.archive.org/web/20230317140816/https://www.bbc.co.uk/news/science-environment-15721263 |url-status=live }}

Larger, faster or more radical changes, however, may result in vegetation stress, rapid plant loss and desertification in certain circumstances.{{cite journal |last1=Sahney |first1=S. |last2=Benton |first2=M.J. |last3=Falcon-Lang |first3=H.J. |year=2010 |title=Rainforest collapse triggered Pennsylvanian tetrapod diversification in Euramerica |journal=Geology |doi=10.1130/G31182.1 |bibcode=2010Geo....38.1079S |volume=38 |issue=12 |pages=1079–82 |url=https://www.academia.edu/368820 |format=PDF |access-date=27 November 2013 |archive-date=17 March 2023 |archive-url=https://web.archive.org/web/20230317140814/https://www.academia.edu/368820 |url-status=live }}{{cite journal |last1=Bachelet |first1=D. |author-link1=Dominique Bachelet|last2=Neilson |first2=R. |last3=Lenihan |first3=J. M. |last4=Drapek |first4=R.J. |year=2001 |title=Climate Change Effects on Vegetation Distribution and Carbon Budget in the United States |journal=Ecosystems |doi=10.1007/s10021-001-0002-7 |volume=4 |issue=3 |pages=164–85 |bibcode=2001Ecosy...4..164B |s2cid=15526358 }}{{Cite journal |last1=Ridolfi |first1=Luca |last2=D'Odorico |first2=P. |last3=Porporato |first3=A. |last4=Rodriguez-Iturbe |first4=I. |date=2000-07-27 |title=Impact of climate variability on the vegetation water stress |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2000JD900206 |journal=Journal of Geophysical Research: Atmospheres |language=en |volume=105 |issue=D14 |pages=18013–18025 |doi=10.1029/2000JD900206 |bibcode=2000JGR...10518013R |issn=0148-0227}} An example of this occurred during the Carboniferous Rainforest Collapse (CRC), an extinction event 300 million years ago. At this time vast rainforests covered the equatorial region of Europe and America. Climate change devastated these tropical rainforests, abruptly fragmenting the habitat into isolated 'islands' and causing the extinction of many plant and animal species.

One of the most important ways animals can deal with climatic change is migration to warmer or colder regions.{{Sfn|Burroughs|2007|p=273}} On a longer timescale, evolution makes ecosystems including animals better adapted to a new climate.{{Cite journal|last1=Millington|first1=Rebecca|last2=Cox|first2=Peter M.|last3=Moore|first3=Jonathan R.|last4=Yvon-Durocher|first4=Gabriel|date=10 May 2019|title=Modelling ecosystem adaptation and dangerous rates of global warming|journal=Emerging Topics in Life Sciences|language=en|volume=3|issue=2|pages=221–31|doi=10.1042/ETLS20180113|pmid=33523155|issn=2397-8554|hdl=10871/36988|s2cid=150221323|hdl-access=free}} Rapid or large climate change can cause mass extinctions when creatures are stretched too far to be able to adapt.{{Sfn|Burroughs|2007|p=267}}

Collapses of past civilizations such as the Maya may be related to cycles of precipitation, especially drought, that in this example also correlates to the Western Hemisphere Warm Pool. Around 70 000 years ago the Toba supervolcano eruption created an especially cold period during the ice age, leading to a possible genetic bottleneck in human populations.

Glaciers are considered among the most sensitive indicators of a changing climate.{{cite report|last=Seiz |first=G. |author2=N. Foppa |title=The activities of the World Glacier Monitoring Service (WGMS) |year=2007 |url=http://www.meteoswiss.admin.ch/web/en/climate/climate_international/gcos/inventory/wgms.Par.0008.DownloadFile.tmp/gcosreportwgmse.pdf |access-date=21 June 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090325100331/http://www.meteoswiss.admin.ch/web/en/climate/climate_international/gcos/inventory/wgms.Par.0008.DownloadFile.tmp/gcosreportwgmse.pdf |archive-date=25 March 2009 }} Their size is determined by a mass balance between snow input and melt output. As temperatures increase, glaciers retreat unless snow precipitation increases to make up for the additional melt. Glaciers grow and shrink due both to natural variability and external forcings. Variability in temperature, precipitation and hydrology can strongly determine the evolution of a glacier in a particular season.

The most significant climate processes since the middle to late Pliocene (approximately 3 million years ago) are the glacial and interglacial cycles. The present interglacial period (the Holocene) has lasted about 11,700 years.{{cite web|url=http://www.stratigraphy.org/column.php?id=Chart/Time%20Scale|title=International Stratigraphic Chart|year=2008|publisher=International Commission on Stratigraphy|access-date=3 October 2011|url-status=dead|archive-url=https://web.archive.org/web/20111015042711/http://www.stratigraphy.org/column.php?id=Chart%2FTime%20Scale|archive-date=15 October 2011}} Shaped by orbital variations, responses such as the rise and fall of continental ice sheets and significant sea-level changes helped create the climate. Other changes, including Heinrich events, Dansgaard–Oeschger events and the Younger Dryas, however, illustrate how glacial variations may also influence climate without the orbital forcing.

During the Last Glacial Maximum, some 25,000 years ago, sea levels were roughly 130 m lower than today. The deglaciation afterwards was characterized by rapid sea level change.{{Sfn|Burroughs|2007|p=279}} In the early Pliocene, global temperatures were 1–2˚C warmer than the present temperature, yet sea level was 15–25 meters higher than today.{{cite web|url=http://www.giss.nasa.gov/research/briefs/hansen_15/|archive-url=https://web.archive.org/web/20110724050602/http://www.giss.nasa.gov/research/briefs/hansen_15/|url-status=dead|archive-date=24 July 2011|title=Science Briefs: Earth's Climate History|last=Hansen|first=James|publisher=NASA GISS|access-date=25 April 2013}}

Sea ice plays an important role in Earth's climate as it affects the total amount of sunlight that is reflected away from the Earth.{{Cite journal|last1=Belt|first1=Simon T.|last2=Cabedo-Sanz|first2=Patricia|last3=Smik|first3=Lukas|last4=Navarro-Rodriguez|first4=Alba|last5=Berben|first5=Sarah M. P.|last6=Knies|first6=Jochen|last7=Husum|first7=Katrine|display-authors=3|date=2015|title=Identification of paleo Arctic winter sea ice limits and the marginal ice zone: Optimised biomarker-based reconstructions of late Quaternary Arctic sea ice|journal=Earth and Planetary Science Letters|volume=431|pages=127–39|doi=10.1016/j.epsl.2015.09.020|bibcode=2015E&PSL.431..127B|issn=0012-821X|hdl=10026.1/4335|hdl-access=free}} In the past, the Earth's oceans have been almost entirely covered by sea ice on a number of occasions, when the Earth was in a so-called Snowball Earth state,{{Cite journal|last1=Warren|first1=Stephen G.|last2=Voigt|first2=Aiko|last3=Tziperman|first3=Eli|last4=Sadler|first4=Peter M.|last5=Rose|first5=Catherine V.|last6=Rose|first6=Brian E. J.|last7=Ramstein|first7=Gilles|last8=Partin|first8=Camille A.|last9=Maloof|first9=Adam C.|display-authors=3|date=1 November 2017|title=Snowball Earth climate dynamics and Cryogenian geology-geobiology|journal=Science Advances|volume=3|issue=11|pages=e1600983|doi=10.1126/sciadv.1600983|pmid=29134193|issn=2375-2548|pmc=5677351|bibcode=2017SciA....3E0983H}} and completely ice-free in periods of warm climate.{{Cite journal|last1=Caballero|first1=R.|last2=Huber|first2=M.|date=2013|title=State-dependent climate sensitivity in past warm climates and its implications for future climate projections|journal=Proceedings of the National Academy of Sciences|volume=110|issue=35|pages=14162–67|doi=10.1073/pnas.1303365110|pmid=23918397|pmc=3761583|bibcode=2013PNAS..11014162C|issn=0027-8424|doi-access=free}} When there is a lot of sea ice present globally, especially in the tropics and subtropics, the climate is more sensitive to forcings as the ice–albedo feedback is very strong.{{Cite journal|last1=Hansen James|last2=Sato Makiko|last3=Russell Gary|last4=Kharecha 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|pages=20120294|doi=10.1098/rsta.2012.0294|pmc=3785813|pmid=24043864|arxiv=1211.4846|bibcode=2013RSPTA.37120294H}}

{{See also|List of periods and events in climate history|Paleoclimatology}}

Various climate forcings are typically in flux throughout geologic time, and some processes of the Earth's temperature may be self-regulating. For example, during the Snowball Earth period, large glacial ice sheets spanned to Earth's equator, covering nearly its entire surface, and very high albedo created extremely low temperatures, while the accumulation of snow and ice likely removed carbon dioxide through atmospheric deposition. However, the absence of plant cover to absorb atmospheric CO₂ emitted by volcanoes meant that the greenhouse gas could accumulate in the atmosphere. There was also an absence of exposed silicate rocks, which use CO₂ when they undergo weathering. This created a warming that later melted the ice and brought Earth's temperature back up.

File:65 Myr Climate Change.png ratios from foraminifera.]]

The Paleocene–Eocene Thermal Maximum (PETM) was a time period with more than 5–8 °C global average temperature rise across the event.{{cite journal|author=McInherney, F.A..|author2=Wing, S.|year=2011|title=A perturbation of carbon cycle, climate, and biosphere with implications for the future|url=http://www.whoi.edu/fileserver.do?id=136084&pt=2&p=148709|journal=Annual Review of Earth and Planetary Sciences|volume=39|issue=1 |pages=489–516|bibcode=2011AREPS..39..489M|doi=10.1146/annurev-earth-040610-133431|access-date=26 October 2019|archive-date=14 September 2016|archive-url=https://web.archive.org/web/20160914003526/http://www.whoi.edu/fileserver.do?id=136084&pt=2&p=148709|url-status=live|url-access=subscription}} This climate event occurred at the time boundary of the Paleocene and Eocene geological epochs.{{cite journal|author=Westerhold, T..|author2=Röhl, U.|author3=Raffi, I.|author4=Fornaciari, E.|author5=Monechi, S.|author6=Reale, V.|author7=Bowles, J.|author8=Evans, H. F.|year=2008|title=Astronomical calibration of the Paleocene time|url=https://www.geo.arizona.edu/~reiners/fortransfer6/WesterholdEtAl_PPP2008.pdf |archive-url=https://web.archive.org/web/20170809094938/http://www.geo.arizona.edu/~reiners/fortransfer6/WesterholdEtAl_PPP2008.pdf |archive-date=2017-08-09 |url-status=live|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|volume=257|issue=4|pages=377–403|bibcode=2008PPP...257..377W|doi=10.1016/j.palaeo.2007.09.016}} During the event large amounts of methane was released, a potent greenhouse gas.{{Sfn|Burroughs|2007|p=|pp=190–91}} The PETM represents a "case study" for modern climate change as in the greenhouse gases were released in a geologically relatively short amount of time. During the PETM, a mass extinction of organisms in the deep ocean took place.{{Cite journal|last1=Ivany|first1=Linda C.|last2=Pietsch|first2=Carlie|last3=Handley|first3=John C.|last4=Lockwood|first4=Rowan|last5=Allmon|first5=Warren D.|last6=Sessa|first6=Jocelyn A.|date=1 September 2018|title=Little lasting impact of the Paleocene-Eocene Thermal Maximum on shallow marine molluscan faunas|journal=Science Advances|language=en|volume=4|issue=9|pages=eaat5528|doi=10.1126/sciadv.aat5528|issn=2375-2548|pmid=30191179|pmc=6124918|bibcode=2018SciA....4.5528I}}

Throughout the Cenozoic, multiple climate forcings led to warming and cooling of the atmosphere, which led to the early formation of the Antarctic ice sheet, subsequent melting, and its later reglaciation. The temperature changes occurred somewhat suddenly, at carbon dioxide concentrations of about 600–760 ppm and temperatures approximately 4 °C warmer than today. During the Pleistocene, cycles of glaciations and interglacials occurred on cycles of roughly 100,000 years, but may stay longer within an interglacial when orbital eccentricity approaches zero, as during the current interglacial. Previous interglacials such as the Eemian phase created temperatures higher than today, higher sea levels, and some partial melting of the West Antarctic ice sheet.

Climatological temperatures substantially affect cloud cover and precipitation. At lower temperatures, air can hold less water vapour, which can lead to decreased precipitation.{{Cite journal|last1=Haerter|first1=Jan O.|last2=Moseley|first2=Christopher|last3=Berg|first3=Peter|date=2013|title=Strong increase in convective precipitation in response to higher temperatures|journal=Nature Geoscience|volume=6|issue=3|pages=181–85|doi=10.1038/ngeo1731|bibcode=2013NatGe...6..181B|issn=1752-0908}} During the Last Glacial Maximum of 18,000 years ago, thermal-driven evaporation from the oceans onto continental landmasses was low, causing large areas of extreme desert, including polar deserts (cold but with low rates of cloud cover and precipitation). In contrast, the world's climate was cloudier and wetter than today near the start of the warm Atlantic Period of 8000 years ago.

File:Holocene Temperature Variations.png

The Holocene is characterized by a long-term cooling starting after the Holocene Optimum, when temperatures were probably only just below current temperatures (second decade of the 21st century),{{Cite journal|last1=Kaufman|first1=Darrell|last2=McKay|first2=Nicholas|last3=Routson|first3=Cody|last4=Erb|first4=Michael|last5=Dätwyler|first5=Christoph|last6=Sommer|first6=Philipp S.|last7=Heiri|first7=Oliver|last8=Davis|first8=Basil|date=30 June 2020|title=Holocene global mean surface temperature, a multi-method reconstruction approach|journal=Scientific Data|language=en|volume=7|issue=1|page=201|doi=10.1038/s41597-020-0530-7|pmid=32606396|pmc=7327079|bibcode=2020NatSD...7..201K|issn=2052-4463|doi-access=free}} and a strong African Monsoon created grassland conditions in the Sahara during the Neolithic Subpluvial. Since that time, several cooling events have occurred, including:

• the Piora Oscillation
• the Middle Bronze Age Cold Epoch
• the Iron Age Cold Epoch
• the Little Ice Age
• the phase of cooling c. 1940–1970, which led to global cooling hypothesis

In contrast, several warm periods have also taken place, and they include but are not limited to:

• a warm period during the apex of the Minoan civilization
• the Roman Warm Period
• the Medieval Warm Period
• Modern warming during the 20th century

Certain effects have occurred during these cycles. For example, during the Medieval Warm Period, the American Midwest was in drought, including the Sand Hills of Nebraska which were active sand dunes. The black death plague of Yersinia pestis also occurred during Medieval temperature fluctuations, and may be related to changing climates.

Solar activity may have contributed to part of the modern warming that peaked in the 1930s. However, solar cycles fail to account for warming observed since the 1980s to the present day.{{Citation needed|date=September 2016}} Events such as the opening of the Northwest Passage and recent record low ice minima of the modern Arctic shrinkage have not taken place for at least several centuries, as early explorers were all unable to make an Arctic crossing, even in summer. Shifts in biomes and habitat ranges are also unprecedented, occurring at rates that do not coincide with known climate oscillations {{Citation needed|date=September 2016}}.

{{main|Climate change}}

As a consequence of humans emitting greenhouse gases, global surface temperatures have started rising. Global warming is an aspect of modern climate change, a term that also includes the observed changes in precipitation, storm tracks and cloudiness. As a consequence, glaciers worldwide have been found to be shrinking significantly.{{cite report|url=http://www.grid.unep.ch/glaciers/pdfs/summary.pdf|title=United Nations Environment Programme – Global Glacier Changes: facts and figures|last=Zemp|first=M.|author2=I.Roer|author3=A.Kääb|author4=M.Hoelzle|author5=F.Paul|author6=W. Haeberli|access-date=21 June 2009|archive-url=https://web.archive.org/web/20090325100332/http://www.grid.unep.ch/glaciers/pdfs/summary.pdf|year=2008|archive-date=25 March 2009|url-status=dead}}{{cite web|url=https://www.epa.gov/climate-indicators/climate-change-indicators-glaciers|title=Climate Change Indicators: Glaciers|last=EPA, OA|first=US|website=US EPA|date=July 2016|access-date=26 January 2018|archive-date=29 September 2019|archive-url=https://web.archive.org/web/20190929003522/https://www.epa.gov/climate-indicators/climate-change-indicators-glaciers|url-status=live}} Land ice sheets in both Antarctica and Greenland have been losing mass since 2002 and have seen an acceleration of ice mass loss since 2009.{{cite web|url=https://climate.nasa.gov/vital-signs/land-ice/|title=Land ice – NASA Global Climate Change|access-date=10 December 2017|archive-date=23 February 2017|archive-url=https://web.archive.org/web/20170223211832/https://climate.nasa.gov/vital-signs/land-ice/|url-status=live}} Global sea levels have been rising as a consequence of thermal expansion and ice melt. The decline in Arctic sea ice, both in extent and thickness, over the last several decades is further evidence for rapid climate change.{{cite web|url=https://climate.nasa.gov/evidence/|title=Climate Change: How do we know?|editor1-last=Shaftel|editor1-first=Holly|website=NASA Global Climate Change|publisher=Earth Science Communications Team at NASA's Jet Propulsion Laboratory|access-date=16 December 2017|archive-date=18 December 2019|archive-url=https://web.archive.org/web/20191218104252/https://climate.nasa.gov/evidence/|url-status=live}}

{{Gallery

|align=right | height=150 |mode=packed

|title= Examples of regional climate variability

| File:Land vs Ocean Temperature.svg

| Land-ocean. Surface air temperatures over land masses have been increasing faster than those over the ocean,{{cite web |title=GISS Surface Temperature Analysis (v4) / Annual Mean Temperature Change over Land and over Ocean |url=https://data.giss.nasa.gov/gistemp/graphs_v4/ |website=NASA GISS |archive-url=https://web.archive.org/web/20200416074510/https://data.giss.nasa.gov/gistemp/graphs_v4/ |archive-date=16 April 2020 |url-status=live}} the ocean absorbing about 90% of excess heat.{{cite magazine |last1=Harvey |first1=Chelsea |title=The Oceans Are Heating Up Faster Than Expected |url=https://www.scientificamerican.com/article/the-oceans-are-heating-up-faster-than-expected/ |magazine=Scientific American |date=1 November 2018 |archive-url=https://web.archive.org/web/20200303222236/https://www.scientificamerican.com/article/the-oceans-are-heating-up-faster-than-expected/ |archive-date=3 March 2020 |url-status=live }} Data from [https://web.archive.org/web/20200416074510/https://data.giss.nasa.gov/gistemp/graphs_v4/ NASA GISS].

|File:20200505 Global warming variability - Northern vs Southern hemispheres.svg

| Hemispheres. The Hemispheres' average temperature changes{{cite web |title=GISS Surface Temperature Analysis (v4) / Annual Mean Temperature Change for Hemispheres |url=https://data.giss.nasa.gov/gistemp/graphs_v4/ |website=NASA GISS |archive-url=https://web.archive.org/web/20200416074510/https://data.giss.nasa.gov/gistemp/graphs_v4/ |archive-date=16 April 2020 |url-status=live}} have diverged because of the North's greater percentage of landmass, and due to global ocean currents.{{cite web |last1=Freedman |first1=Andrew |title=In Warming, Northern Hemisphere is Outpacing the South |url=https://www.climatecentral.org/news/in-global-warming-northern-hemisphere-is-outpacing-the-south-15850 |website=Climate Central |archive-url=https://web.archive.org/web/20191031123759/https://www.climatecentral.org/news/in-global-warming-northern-hemisphere-is-outpacing-the-south-15850 |archive-date=31 October 2019 |date=9 April 2013 |url-status=live }}

| File:20200314 Temperature changes for three latitude bands (5MA, 1880- ) GISS.svg

| Latitude bands. Three latitude bands that respectively cover 30, 40 and 30 percent of the global surface area show mutually distinct temperature growth patterns in recent decades.{{cite web |title=GISS Surface Temperature Analysis (v4) / Temperature Change for Three Latitude Bands |url=https://data.giss.nasa.gov/gistemp/graphs_v4/ |website=NASA GISS |archive-url=https://web.archive.org/web/20200416074510/https://data.giss.nasa.gov/gistemp/graphs_v4/ |archive-date=16 April 2020 |url-status=live}}

| File:1960- Warming stripes global temperature graphic - atmospheric heights and ocean depths.png

| Altitude. A warming stripes graphic ({{blue|blues}} denote cool, {{red|reds}} denote warm) shows how the greenhouse effect traps heat in the lower atmosphere and oceans, so that the upper atmosphere, receiving less reflected energy, cools.{{cite journal |last1=Hawkins |first1=Ed |last2=Williams |first2=Richard G. |last3=Young |first3=Paul J. |last4=Berardelli |first4=Jeff |last5=Burgess |first5=Samantha N. |last6=Highwood |first6=Ellie |last7=Randel |first7=William |last8=Roussenov |first8=Vassil |last9=Smith |first9=Doug |last10=Placky |first10=Bernadette Woods |title=Warming Stripes Spark Climate Conversations: From the Ocean to the Stratosphere |journal=Bulletin of the American Meteorological Society |volume=6 |issue=5 |date=1 May 2025 |pages=E964-E970 |doi=10.1175/BAMS-D-24-0212.1}}{{cite web |last1=Hawkins |first1=Ed |title=Atmospheric temperature trends |url=http://www.climate-lab-book.ac.uk/2019/atmospheric-temperature-trends/ |website=Climate Lab Book |archive-url=https://web.archive.org/web/20190912192530/http://www.climate-lab-book.ac.uk/2019/atmospheric-temperature-trends/ |archive-date=12 September 2019 |date=12 September 2019 |url-status=live }} (Higher-altitude cooling differences attributed to ozone depletion and greenhouse gas increases; spikes occurred with volcanic eruptions of 1982–83 (El Chichón) and 1991–92 (Pinatubo).)

| File:20200505 Global warming variability - global vs Caribbean.svg

| Global versus regional. For geographical and statistical reasons, larger year-to-year variations are expected{{cite news |last1=Meduna |first1=Veronika |title=The climate visualisations that leave no room for doubt or denial |url=https://thespinoff.co.nz/science/17-09-2018/the-climate-visualisations-that-leave-no-room-for-doubt-or-denial/ |work=The Spinoff |date=17 September 2018 |archive-url=https://web.archive.org/web/20190517104250/https://thespinoff.co.nz/science/17-09-2018/the-climate-visualisations-that-leave-no-room-for-doubt-or-denial/ |archive-date=17 May 2019 |location=New Zealand |url-status=live }} for localized geographic regions (e.g., the Caribbean) than for global averages.{{cite web |title=Climate at a Glance / Global Time Series |url=https://www.ncdc.noaa.gov/cag/global/time-series/globe/land_ocean/12/12/1880-2019 |website=NCDC / NOAA |archive-url=https://web.archive.org/web/20200223062050/https://www.ncdc.noaa.gov/cag/global/time-series/globe/land_ocean/12/12/1880-2019 |archive-date=23 February 2020 |url-status=live}}

| File:20200509 Emergence of temperatures from range of normal historical variability - tropical vs northern Americas (Hawkins).gif

| Relative deviation. Though northern America has warmed more than its tropics, the tropics have more clearly departed from normal historical variability (colored bands: 1σ, 2σ standard deviations).{{cite web |last1=Hawkins |first1=Ed |title=From the familiar to the unknown |url=https://www.climate-lab-book.ac.uk/2020/from-the-familiar-to-the-unknown/ |website=Climate Lab Book (professional blog) |archive-url=https://web.archive.org/web/20200423232229/https://www.climate-lab-book.ac.uk/2020/from-the-familiar-to-the-unknown/ |archive-date=23 April 2020 |date=10 March 2020 |url-status=live }} ([https://web.archive.org/web/20200502073245/http://www.climate-lab-book.ac.uk/files/2020/03/emerge_example.png Direct link to image]; Hawkins credits Berkeley Earth for data.) "The emergence of observed temperature changes over both land and ocean is clearest in tropical regions, in contrast to the regions of largest change which are in the northern extra-tropics. As an illustration, northern America has warmed more than tropical America, but the changes in the tropics are more apparent and have more clearly emerged from the range of historical variability. The year-to-year variations in the higher latitudes have made it harder to distinguish the long-term changes."

}}

File:1880- Global warming by latitude zone - NASA - GISS data.webm

In addition to global climate variability and global climate change over time, numerous climatic variations occur contemporaneously across different physical regions.

The oceans' absorption of about 90% of excess heat has helped to cause land surface temperatures to grow more rapidly than sea surface temperatures. The Northern Hemisphere, having a larger landmass-to-ocean ratio than the Southern Hemisphere, shows greater average temperature increases. Variations across different latitude bands also reflect this divergence in average temperature increase, with the temperature increase of northern extratropics exceeding that of the tropics, which in turn exceeds that of the southern extratropics.

Upper regions of the atmosphere have been cooling contemporaneously with a warming in the lower atmosphere, confirming the action of the greenhouse effect and ozone depletion.

Observed regional climatic variations confirm predictions concerning ongoing changes, for example, by contrasting (smoother) year-to-year global variations with (more volatile) year-to-year variations in localized regions. Conversely, comparing different regions' warming patterns to their respective historical variabilities, allows the raw magnitudes of temperature changes to be placed in the perspective of what is normal variability for each region.

Regional variability observations permit study of regionalized climate tipping points such as rainforest loss, ice sheet and sea ice melt, and permafrost thawing.{{Cite journal|last1=Lenton|first1=Timothy M.|last2=Rockström|first2=Johan |last3=Gaffney|first3=Owen|last4=Rahmstorf|first4=Stefan|last5=Richardson|first5=Katherine|last6=Steffen |first6=Will|last7=Schellnhuber|first7=Hans Joachim|date=27 November 2019|title=Climate tipping points – too risky to bet against|journal=Nature|language=en|volume=575|issue=7784|pages=592–595|pmid=31776487 |bibcode=2019Natur.575..592L|doi=10.1038/d41586-019-03595-0|doi-access=free|hdl=10871/40141|hdl-access=free}} Correction dated 9 April 2020 Such distinctions underlie research into a possible global cascade of tipping points.

{{Portal|Environment|Global warming|Energy}}

• Climatological normal
• Anthropocene
• Ocean heat content

{{clear}}

{{Reflist}}

• {{cite book |last=Cronin |first=Thomas N. |title=Paleoclimates: understanding climate change past and present |location=New York |publisher=Columbia University Press |year=2010 |isbn=978-0-231-14494-0 }}
• {{Cite book |ref= {{harvid|IPCC AR4 WG1|2007}} |title = Climate Change 2007: The Physical Science Basis |series = Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change |author = IPCC |author-link = IPCC |year = 2007 |display-editors= 4 |editor-first1= S. |editor-last1= Solomon |editor-first2= D. |editor-last2= Qin |editor-first3= M. |editor-last3= Manning |editor-first4= Z. |editor-last4= Chen |editor-first5= M. |editor-last5= Marquis |editor-first6= K.B. |editor-last6= Averyt |editor-first7= M. |editor-last7= Tignor |editor-first8= H.L. |editor-last8= Miller |publisher = Cambridge University Press |url = https://archive.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4_wg1_full_report.pdf |isbn = 978-0-521-88009-1}} (pb: {{ISBN|978-0-521-70596-7}}).
• {{Cite book |ref= {{harvid|IPCC AR4 SYR|2008}} |author= IPCC |author-link= IPCC |year= 2008 |title= Climate Change 2008: Synthesis Report |series= Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change |editor1= The Core Writing Team |editor-first2= R.K. |editor-last2= Pachauri |editor-first3= A.R. |editor-last3= Reisinger |publisher= IPCC |place= Geneva, Switzerland |isbn= 978-92-9169-122-7 |url= https://www.ipcc.ch/site/assets/uploads/2018/02/ar4_syr_full_report.pdf}}
• {{Cite book|title=Climate Change : A multidisciplinary approach|last=Burroughs|first=William James|publisher=Cambridge university press|year=2001|isbn=0521567718|location=Cambridge}}
• {{Cite book|title=Climate Change : A multidisciplinary approach|last=Burroughs|first=William James|publisher=Cambridge University Press|year=2007|isbn=978-0-511-37027-4|location=Cambridge}}
• {{Cite book|title=Earth's climate : Past and Future|last=Ruddiman|first=William F.|publisher=W. H. Freeman and Company|year=2008|isbn=978-0716784906|location=New York}}
• {{cite book|title=Climatology|last1=Rohli|first1=Robert. V.|last2=Vega|first2=Anthony J.|publisher=Jones & Bartlett Learning|year=2018|isbn=978-1284126563|edition=4th}}

{{Library resources box}}

• {{Commonscatinline|Climate variability and change}}
• [https://climate.nasa.gov/ Global Climate Change] from NASA (US)
• [http://ipcc.ch/ Intergovernmental Panel on Climate Change (IPCC)]
• [https://science.nasa.gov/earth-science/oceanography/ocean-earth-system/climate-variability Climate Variability] {{Webarchive|url=https://web.archive.org/web/20230530121638/https://science.nasa.gov/earth-science/oceanography/ocean-earth-system/climate-variability |date=30 May 2023 }} – NASA Science
• [https://www.ncdc.noaa.gov/climate-information/climate-change-and-variability Climate Change and Variability, National Centers for Environmental Information] {{Webarchive|url=https://web.archive.org/web/20210921083150/https://www.ncdc.noaa.gov/climate-information/climate-change-and-variability |date=21 September 2021 }}

{{climate oscillations}}

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