Global dimming

{{Further|Irradiance|Insolation}}

File:Julsrud_2022_dimming.jpg

In the 1970s, numerous studies showed that atmospheric aerosols could affect the propagation of sunlight through the atmosphere, a measure also known as direct solar irradiance.{{cite journal |last1=Barnhardt |first1=E. A. |last2=Streete |first2=J. L. |year=1970 |title=A Method for Predicting Atmospheric Aerosol Scattering Coefficients in the Infrared |journal=Applied Optics |volume=9 |issue=6 |pages=1337–1344 |doi=10.1364/AO.9.001337 |pmid=20076382 |bibcode=1970ApOpt...9.1337B }}{{cite journal |last1=Herman |first1=Benjamin M. |last2=Browning |first2=Samuel R. |last3=Curran |first3=Robert J. |date=1 April 1971 |title=The Effect of Atmospheric Aerosols on Scattered Sunlight |journal=Journal of the Atmospheric Sciences |volume=28 |issue=3 |pages=419–428 |doi=10.1175/1520-0469(1971)028<0419:TEOAAO>2.0.CO;2 |bibcode=1971JAtS...28..419H |doi-access=free }} One study showed that less sunlight was filtering through at the height of {{cvt|1.7|km|mi}} above Los Angeles, even on those days when there was no visible smog.{{cite journal |last1=Hodge |first1=Paul W. |date=19 February 1971 |title=Large Decrease in the Clear Air Transmission of the Atmosphere 1.7 km above Los Angeles |journal=Nature |volume=229 |issue=5894 |pages=549 |doi=10.1038/229549a0 |pmid=16059347 |bibcode=1971Natur.229..549H }} Another suggested that sulfate pollution or a volcano eruption could provoke the onset of an ice age.{{cite journal |last1=Rasool |first1=Ichtiaque, S |last2=Schneider |first2=Stephen H. |s2cid=43228353 |date=July 1971 |title=Atmospheric Carbon Dioxide and Aerosols: Effects of Large Increases on Global Climate |journal=Science |volume=1 |issue=3992 |pages=138–141 |pmid=17739641 |doi=10.1126/science.173.3992.138 |bibcode=1971Sci...173..138R }}{{cite book |last=Lockwood |first=John G. |title=Causes of Climate |publisher=John Wiley & Sons |location=New York |series=Lecture Notes in mathematics 1358 |year=1979 |pages=162 |isbn=978-0-470-26657-1 |url=https://archive.org/details/causesofclimate0000lock/page/162 }} In the 1980s, Atsumu Ohmura, a geography researcher at the Swiss Federal Institute of Technology, found that solar radiation striking the Earth's surface had declined by more than 10% over the three previous decades, even as the global temperature had been generally rising since the 1970s.{{cite book|author1=Ohmura, A. |author2=Lang, H.|editor=Lenoble, J. |editor2=Geleyn, J.-F.|title=Secular variation of global radiation in Europe. In IRS '88: Current Problems in Atmospheric Radiation, A. Deepak Publ., Hampton, VA| date=June 1989 |publisher=Deepak Publ. |location=Hampton, VA|isbn=978-0-937194-16-4|pages=(635) pp. 298–301}} In the 1990s, this was followed by the papers describing multi-decade declines in Estonia,{{cite journal |last=Russak |first=V. |year=1990 |title=Trends of solar radiation, cloudiness and atmospheric transparency during recent decades in Estonia |journal=Tellus B |volume=42 |issue=2 |pages=206–210 |doi=10.1034/j.1600-0889.1990.t01-1-00006.x |id=1990TellB..42..206R |bibcode=1990TellB..42..206R |title-link=Estonia }} Germany,{{cite journal |last1=Liepert |first1=B. G. |first2=P. |last2=Fabian |last3=Grassi |first3=H. |year=1994 |title=Solar radiation in Germany – Observed trends and an assessment of their causes. Part 1. Regional approach |journal=Contributions to Atmospheric Physics |url=https://www.osti.gov/etdeweb/biblio/6912594 |volume=67 |pages=15–29 }} Israel{{cite journal |last1=Stanhill |first1=G. |last2=Moreshet |first2=S. |date=6 November 2004 |title=Global radiation climate changes in Israel |journal=Climatic Change |volume=22 |issue=2 |pages=121–138 |doi=10.1007/BF00142962 |bibcode=1992ClCh...22..121S |s2cid=154006620}} and across the former Soviet Union.{{cite journal |last=Abakumova |first=G.M. |year=1996 |title=Evaluation of long-term changes in radiation, cloudiness and surface temperature on the territory of the former Soviet Union |journal=Journal of Climate |volume=9 |issue=6 |pages=1319–1327 |doi=10.1175/1520-0442(1996)009<1319:EOLTCI>2.0.CO;2 |url=http://ams.allenpress.com/archive/1520-0442/9/6/pdf/i1520-0442-9-6-1319.pdf |bibcode = 1996JCli....9.1319A}}{{cite journal |last1=Wild |first1=Martin |date=27 June 2009 |title=Global dimming and brightening: A review |journal=Journal of Geophysical Research: Atmospheres |volume=114 |issue=D10 |doi=10.1029/2008JD011470 |bibcode=2009JGRD..114.0D16W }}

Subsequent research estimated an average reduction in sunlight striking the terrestrial surface of around 4–5% per decade over the late 1950s–1980s, and 2–3% per decade when 1990s were included.{{cite journal |last1=Gilgen |first1=H. |last2=Wild |first2=M. |last3=Ohmura |first3=A. |title=Means and trends of shortwave irradiance at the surface estimated from global energy balance archive data |journal=Journal of Climate |url=http://ams.allenpress.com/archive/1520-0442/11/8/pdf/i1520-0442-11-8-2042.pdf |volume=11 |issue=8 |year=1998 |pages=2042–2061 |doi=10.1175/1520-0442-11.8.2042 |bibcode = 1998JCli...11.2042G |doi-access=free}}{{cite journal |last1=Stanhill |first1=G. |last2=Cohen |first2=S. |title=Global dimming: a review of the evidence for a widespread and significant reduction in global radiation with discussion of its probable causes and possible agricultural consequences |journal=Agricultural and Forest Meteorology|volume=107 |year=2001 |issue=4 |doi=10.1016/S0168-1923(00)00241-0 |pages=255–278 |bibcode=2001AgFM..107..255S }}{{cite journal |last=Liepert |first=B. G. |title=Observed Reductions in Surface Solar Radiation in the United States and Worldwide from 1961 to 1990 |journal=Geophysical Research Letters |volume=29 |issue=12 |date=2 May 2002 |url=http://www.ldeo.columbia.edu/~liepert/pdf/2003GL019060.pdf |pages=61–1–61–4 |doi=10.1029/2002GL014910 |bibcode=2002GeoRL..29.1421L |doi-access=free}} Notably, solar radiation at the top of the atmosphere did not vary by more than 0.1-0.3% in all that time, strongly suggesting that the reasons for the dimming were on Earth.{{cite journal |last1=Eddy |first1=John A. |last2=Gilliland |first2=Ronald L. |last3=Hoyt |first3=Douglas V. |date=23 December 1982 |title=Changes in the solar constant and climatic effects |journal=Nature |volume=300 |issue=5894 |pages=689–693 |doi=10.1038/300689a0 |quote=Spacecraft measurements have established that the total radiative output of the Sun varies at the 0.1−0.3% level |bibcode=1982Natur.300..689E |s2cid=4320853 }} Additionally, only visible light and infrared radiation were dimmed, rather than the ultraviolet part of the spectrum.{{cite news |last=Adam |first=David |title=Goodbye sunshine |newspaper=The Guardian |date=18 December 2003 |url=https://www.theguardian.com/science/2003/dec/18/science.research1 |access-date=26 August 2009 }} Further, the dimming had occurred even when the skies were clear, and it was in fact stronger than during the cloudy days, proving that it was not caused by changes in cloud cover alone.{{cite journal |last1=Wild |first1=Martin |last2=Wacker |first2=Stephan |last3=Yang |first3=Su |last4=Sanchez-Lorenzo |first4=Arturo |date=1 February 2021 |title=Evidence for Clear-Sky Dimming and Brightening in Central Europe |journal=Geophysical Research Letters |volume=48 |issue=6 |doi=10.1029/2020GL092216 |bibcode=2021GeoRL..4892216W |hdl=20.500.11850/477374 |s2cid=233645438 |hdl-access=free }}{{cite web |date=18 February 2021 |title=Aerosol pollution has caused decades of global dimming |url=https://news.agu.org/press-release/aerosol-pollution-caused-decades-of-global-dimming/ |website=American Geophysical Union |access-date=18 December 2023 |archive-url=https://web.archive.org/web/20230327143716/https://news.agu.org/press-release/aerosol-pollution-caused-decades-of-global-dimming/ |archive-date=27 March 2023 }}

{{Further|Albedo|Atmospheric particulate matter}}

File:SulufrDioxide2017.png

Global dimming is primarily caused by the presence of sulfate particles which hang in the Earth's atmosphere as aerosols.{{Cite book |last1=Cohen |first1=Shabtai |chapter=Chapter 32 – Changes in the Sun's radiation: the role of widespread surface solar radiation trends in climate change: dimming and brightening |date=1 January 2021 |chapter-url=https://www.sciencedirect.com/science/article/pii/B9780128215753000323 |title=Climate Change |pages=687–709 |editor-last=Letcher |editor-first=Trevor M. |access-date=2023-04-26 |publisher=Elsevier |language=en |doi=10.1016/b978-0-12-821575-3.00032-3 |isbn=978-0-12-821575-3 |last2=Stanhill |first2=Gerald|edition=Third |s2cid=234180702 }} These aerosols have both a direct contribution to dimming, as they reflect sunlight like tiny mirrors.{{cite web|url=https://earthobservatory.nasa.gov/features/Aerosols/page3.php|title=Aerosols and Incoming Sunlight (Direct Effects)|publisher=NASA|date=2 November 2010}} They also have an indirect effect as nuclei, meaning that water droplets in clouds coalesce around the particles. Increased pollution causes more particulates and thereby creates clouds consisting of a greater number of smaller droplets (that is, the same amount of water is spread over more droplets). The smaller droplets make clouds more reflective, so that more incoming sunlight is reflected back into space and less reaches the Earth's surface.{{cite web |last1=Myhre |first1=Gunnar |last2=Lund Myhre |first2=Cathrine E. |last3=Samset |first3=Bjorn H. |last4=Storelvmo |first4=Trude |year=2013 |title=Aerosols and their Relation to Global Climate and Climate Sensitivity |url=https://www.nature.com/scitable/knowledge/library/aerosols-and-their-relation-to-global-climate-102215345/|publisher=The Nature Education Knowledge Project |access-date=6 January 2024 }} In models, these smaller droplets also decrease rainfall.{{cite web |url=http://www.physorg.com/news169474977.html|title=The Sky Is Not Falling: Pollution in eastern China cuts light, useful rainfall|author1=Yun Qian |author2=Daoyi Gong |publisher=Pacific Northwest National Laboratory|year=2009|access-date=2009-08-16}}

Before the Industrial Revolution, the main source of sulfate aerosols was dimethyl sulfide produced by some types of oceanic plankton. Emissions from volcano activity were the second largest source, although large volcanic eruptions, such as the 1991 eruption of Mount Pinatubo, dominate in the years when they occur. In 1990, the IPCC First Assessment Report estimated dimethyl sulfide emissions at 40 million tons per year, while volcano emissions were estimated at 10 million tons. These annual levels have been largely stable for a long time. On the other hand, global human-caused emissions of sulfur into the atmosphere increased from less than 3 million tons per year in 1860 to 15 million tonnes in 1900, 40 million tonnes in 1940 and about 80 million tonnes in 1980. This meant that by 1980, the human-caused emissions from the burning of sulfur-containing fuels (mostly coal and bunker fuel) became at least as large as all natural emissions of sulfur-containing compounds. The report also concluded that "in the industrialized regions of Europe and North America, anthropogenic emissions dominate over natural emissions by about a factor of ten or even more".IPCC, 1990: [https://www.ipcc.ch/site/assets/uploads/2018/03/ipcc_far_wg_I_chapter_01.pdf Chapter 1: Greenhouse Gases and Aerosols] [R.T. Watson, H. Rodhe, H. Oeschger and U. Siegenthaler]. In: [https://www.ipcc.ch/site/assets/uploads/2018/03/ipcc_far_wg_I_full_report.pdf Climate Change: The IPCC Scientific Assessment] [J.T.Houghton, G.J.Jenkins and J.J.Ephraums (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 31–34,

{{Further|Anthropogenic cloud}}

File:Bellouin 2019 smoke albedo effects.jpg

Another important type of aerosol is black carbon, colloquially known as soot. It is formed due to incomplete combustion of fossil fuels, as well as of wood and other plant matter.{{Cite journal |last1=Zeng |first1=Linghan |last2=Tan |first2=Tianyi |last3=Zhao |first3=Gang |last4=Du |first4=Zhuofei |last5=Hu |first5=Shuya |last6=Shang |first6=Dongjie |last7=Hu |first7=Ming |date=2 January 2024 |title=Overestimation of black carbon light absorption due to mixing state heterogeneity |journal=npj Climate and Atmospheric Science |volume=7 |issue=1 |page=2 |doi=10.1038/s41612-023-00535-8 |bibcode=2024npCAS...7....2Z |doi-access=free }} Globally, the single largest source of black carbon is from grassland and forest fires, including both wildfires and intentional burning. However, coal use is responsible for the majority (60 to 80%) of black carbon emissions in Asia and Africa, while diesel combustion produces 70% of black carbon in Europe and The Americas.{{Cite journal |last1=Bond |first1=T. C. |last2=Doherty |first2=S. J. |last3=Fahey |first3=D. W. |last4=Forster |first4=P. M. |last5=Berntsen |first5=T. |last6=DeAngelo |first6=B. J. |last7=Flanner |first7=M. G. |last8=Ghan |first8=S. |last9=Kärcher |first9=B. |last10=Koch |first10=D. |last11=Kinne |first11=S. |last12=Kondo |first12=Y. |last13=Quinn |first13=P. K. |last14=Sarofim |first14=M. C. |last15=Schultz |first15=M. G. |last16=Schulz |first16=M. |last17=Venkataraman |first17=C. |last18=Zhang |first18=H. |last19=Zhang |first19=S. |last20=Bellouin |first20=N. |last21=Guttikunda |first21=S. K. |last22=Hopke |first22=P. K. |last23=Jacobson |first23=M. Z. |last24=Kaiser |first24=J. W. |last25=Klimont |first25=Z. |last26=Lohmann |first26=U. |last27=Schwarz |first27=J. P. |last28=Shindell |first28=D. |last29=Storelvmo |first29=T. |last30=Warren |first30=S. G. |last31=Zender |first31=C. S.|title=Bounding the role of black carbon in the climate system: A scientific assessment |date=15 January 2013 |journal= Journal of Geophysical Research: Atmospheres|doi=10.1002/jgrd.50171 |bibcode=2013JGRD..118.5380B |volume=118 |issue=11pages=5380–5552 |pages=5380–5552 |doi-access=free |hdl=2027.42/99106 |hdl-access=free }}

Black carbon in the lower atmosphere is a major contributor to 7 million premature deaths caused by air pollution every year.{{Cite journal |last1=Gustafsson |first1=Örjan |last2=Ramanathan |first2=Veerabhadran |date=1 April 2016 |title=Convergence on climate warming by black carbon aerosols |journal=PNAS |volume=113 |issue=16 |pages=4243–4245 |doi=10.1073/pnas.1603570113 |doi-access=free |pmid=27071127 |pmc=4843464 |bibcode=2016PNAS..113.4243G }} Its presence is particularly visible, as the so-called "brown clouds" appear in heavily polluted areas. In fact, it was 1970s research into the Denver brown cloud which had first found that black carbon particles absorb solar energy and so can affect the amount of visible sunlight. Later research found that black carbon is 190 times more effective at absorbing sunlight within clouds than the regular dust from soil particles.{{Cite journal |last1=Jacobson |first1=Mark Z. |date=21 March 2012 |title=Investigating cloud absorption effects: Global absorption properties of black carbon, tar balls, and soil dust in clouds and aerosols |journal= Journal of Geophysical Research: Atmospheres|volume=117 |issue=D6 |doi=10.1029/2011JD017218 |bibcode=2012JGRD..117.6205J }} At worst, all clouds within an atmospheric layer {{cvt|3-5|km|mi}} thick are visibly darkened, and the plume can reach transcontinental scale{{Cite journal |last1=Ramanathan |first1=V. |last2=Carmichael |first2=G. |date=23 March 2008 |title=Global and regional climate changes due to black carbon |journal=Nature Geoscience |volume=1 |issue=16 |pages=221–227 |doi=10.1038/ngeo156 |bibcode=2008NatGe...1..221R }} (i.e. the Asian brown cloud.) Even so, the overall dimming from black carbon is much lower than that from the sulfate particles.

{{further|Clean Air Act (United States)}}

Image:Aerosol dimming.jpg around the world steadily declined (red line) since the 1991 eruption of Mount Pinatubo, according to satellite estimates.]]

After 1990, the global dimming trend had clearly switched to global brightening.{{cite web|title=Earth lightens up|work=Pacific Northwest National Laboratory|url=http://www.pnnl.gov/news/release.aspx?id=20|access-date=8 May 2005|archive-date=16 September 2012|archive-url=https://web.archive.org/web/20120916004426/http://www.pnnl.gov/news/release.aspx?id=20|url-status=dead}}{{cite journal |author=Wild, M |year=2005 |title=From Dimming to Brightening: Decadal Changes in Solar Radiation at Earth's Surface |url=https://www.science.org/doi/10.1126/science.1103215 |journal=Science |volume=308 |issue=2005–05–06 |pages=847–850 |bibcode=2005Sci...308..847W |doi=10.1126/science.1103215 |pmid=15879214 |s2cid=13124021}}{{cite journal |title=Do Satellites Detect Trends in Surface Solar Radiation? | author=Pinker | bibcode=2005Sci...308..850P | last2=Zhang | first2=B | last3=Dutton | first3=EG | journal=Science | year=2005 | volume=308 | issue=6 May 2005 | pages=850–854 | doi=10.1126/science.1103159 | pmid=15879215 | s2cid=10644227}}{{cite web | url=http://www.realclimate.org/index.php?p=154 | title=Global Dimming may have a brighter future |date=15 May 2005 | access-date=2006-06-12 | website=RealClimate}}{{cite news |date=15 March 2007 |title=Global 'Sunscreen' Has Likely Thinned, Report NASA Scientists |publisher=NASA |url=https://www.giss.nasa.gov/research/news/archive/2007031 }}{{Dead link|date=June 2024 |bot=InternetArchiveBot |fix-attempted=yes }} This followed measures taken to combat air pollution by the developed nations, typically through flue-gas desulfurization installations at thermal power plants, such as wet scrubbers or fluidized bed combustion.{{cite journal |last1=Lin |first1=Cheng-Kuan |last2=Lin |first2=Ro-Ting |last3=Chen |first3=Pi-Cheng |last4=Wang |first4=Pu |last5=De Marcellis-Warin |first5=Nathalie |last6=Zigler |first6=Corwin |last7=Christiani |first7=David C. |date=8 February 2018 |title=A Global Perspective on Sulfur Oxide Controls in Coal-Fired Power Plants and Cardiovascular Disease |journal=Scientific Reports |language=en |volume=8 |issue=1 |page=2611 |doi=10.1038/s41598-018-20404-2 |pmid=29422539 |issn=2045-2322 |pmc=5805744|bibcode=2018NatSR...8.2611L}}{{cite journal |last1=Henneman |first1=Lucas R.F. |last2=Liu |first2=Cong |last3=Mulholland |first3=James A. |last4=Russell |first4=Armistead G. |date=7 October 2016 |title=Evaluating the effectiveness of air quality regulations: A review of accountability studies and frameworks |journal=Journal of the Air & Waste Management Association |language=en |volume=67 |issue=2 |pages=144–172 |doi=10.1080/10962247.2016.1242518 |pmid=27715473 }}{{cite book |last1=Gulyurtlu |first1=I. |last2=Pinto |first2=F. |last3=Abelha |first3=P. |last4=Lopes |first4=H. |last5=Crujeira |first5=A.T. |title=Fluidized Bed Technologies for Near-Zero Emission Combustion and Gasification |chapter=Pollutant emissions and their control in fluidised bed combustion and gasification |publisher=Woodhead Publishing |year=2013 |pages=435–480 |isbn=978-0-85709-541-1 |doi=10.1533/9780857098801.2.435 }} In the United States, sulfate aerosols have declined significantly since 1970 with the passage of the Clean Air Act, which was strengthened in 1977 and 1990. According to the EPA, from 1970 to 2005, total emissions of the six principal air pollutants, including sulfates, dropped by 53% in the US.{{cite web | access-date=2007-03-17 | archive-date=2007-03-17 | archive-url=https://web.archive.org/web/20070317212933/http://www.epa.gov/airtrends/econ-emissions.html | url=http://www.epa.gov/airtrends/econ-emissions.html | title=Air Emissions Trends – Continued Progress Through 2005 | publisher=U.S. Environmental Protection Agency | date=8 July 2014}} By 2010, this reduction in sulfate pollution led to estimated healthcare cost savings valued at $50 billion annually.{{cite web |date=2 June 2006 |title=Effects of Acid Rain – Human Health |url=http://www.epa.gov/acidrain/effects/health.html |publisher=EPA |access-date=2 September 2013 |archive-url=https://web.archive.org/web/20080118120242/http://www.epa.gov/acidrain/effects/health.html |archive-date=18 January 2008 }} Similar measures were taken in Europe, such as the 1985 Helsinki Protocol on the Reduction of Sulfur Emissions under the Convention on Long-Range Transboundary Air Pollution, and with similar improvements.{{cite web|last1=Moses|first1=Elizabeth|last2=Cardenas|first2=Beatriz |last3=Seddon|first3=Jessica|date=25 February 2020|title=The Most Successful Air Pollution Treaty You've Never Heard Of |url=https://www.wri.org/insights/most-successful-air-pollution-treaty-youve-never-heard|language=en}}

File:Pollution over east China.jpgs in Eastern China. Such smoke is full of black carbon, which contributes to dimming trends but has an overall warming effect.]]

On the other hand, a 2009 review found that dimming continued to increase in China after stabilizing in the 1990s and intensified in India, consistent with their continued industrialization, while the US, Europe, and South Korea continued to brighten. Evidence from Zimbabwe, Chile and Venezuela also pointed to increased dimming during that period, albeit at a lower confidence level due to the lower number of observations.{{cite journal |last1=Wild |first1=Martin |last2=Trüssel| first2 = Barbara |last3=Ohmura |first3=Atsumu |last4=Long |first4=Charles N. |last5=König-Langlo |first5=Gert |last6=Dutton |first6=Ellsworth G. |last7=Tsvetkov |first7=Anatoly |title=Global dimming and brightening: An update beyond 2000 |journal=Journal of Geophysical Research: Atmospheres |date=16 May 2009 |doi=10.1029/2008JD011382 |volume=114 |issue=D10 |pages=D00D13 |bibcode=2009JGRD..114.0D13W |doi-access=free }}{{cite journal |last1=Carnell |first1=R. E. |last2=Senior |first2=C. A. |date=April 1998 |title=Changes in mid-latitude variability due to increasing greenhouse gases and sulphate aerosols |journal=Climate Dynamics |volume=14 |issue=5 |pages=369–383 |doi=10.1007/s003820050229 |bibcode=1998ClDy...14..369C |s2cid=129699440 }} Later research found that over China, the dimming trend continued at a slower rate after 1990,{{cite journal |last1=He |first1=Yanyi |last2=Wang |first2= Kaicun |last3=Zhou |first3=Chunlüe |last4=Wild |first4=Martin |date=19 April 2018 |title=A Revisit of Global Dimming and Brightening Based on the Sunshine Duration |journal=Geophysical Research Letters |volume=6 |issue=9 |page=6346 |doi=10.1029/2018GL077424 |bibcode=2018GeoRL..45.4281H |s2cid=134001797 |hdl=20.500.11850/268470 |hdl-access=free }} and did not begin to reverse until around 2005.{{cite journal |last1=He |first1=Yanyi |last2=Wang |first2=Kaicun |last3=Zhou |first3=Chunlüe |last4=Wild |first4=Martin |date=15 April 2022 |title=Evaluation of surface solar radiation trends over China since the 1960s in the CMIP6 models and potential impact of aerosol emissions |journal=Atmospheric Research |volume=268 |page=105991 |doi=10.1016/j.atmosres.2021.105991 |s2cid=245483347 |doi-access=free |bibcode=2022AtmRe.26805991W }} Due to these contrasting trends, no statistically significant change had occurred on a global scale from 2001 to 2012. Post-2010 observations indicate that the global decline in aerosol concentrations and global dimming continued, with pollution controls on the global shipping industry playing a substantial role in the recent years.{{cite journal |last1=Quaas |first1=Johannes |last2=Jia |first2=Hailing |last3=Smith |first3=Chris |last4=Albright |first4=Anna Lea |last5=Aas |first5=Wenche |last6=Bellouin |first6=Nicolas |last7=Boucher |first7=Olivier |last8=Doutriaux-Boucher |first8=Marie |last9=Forster |first9=Piers M. |last10=Grosvenor |first10=Daniel |last11=Jenkins |first11=Stuart |last12=Klimont |first12=Zbigniew |last13=Loeb |first13=Norman G. |last14=Ma |first14=Xiaoyan |last15=Naik |first15=Vaishali |last16=Paulot |first16=Fabien |last17=Stier |first17=Philip |last18=Wild |first18=Martin |last19=Myhre |first19=Gunnar |last20=Schulz |first20=Michael |date=21 September 2022 |title=Robust evidence for reversal of the trend in aerosol effective climate forcing |url=https://acp.copernicus.org/articles/22/12221/2022/ |journal=Atmospheric Chemistry and Physics |volume=22 |issue=18 |pages=12221–12239 |language=en |doi=10.5194/acp-22-12221-2022 |s2cid=252446168 |hdl=20.500.11850/572791 |hdl-access=free |doi-access=free |bibcode=2022ACP....2212221Q }} Since nearly 90% of the human population lives in the Northern Hemisphere, clouds there are far more affected by aerosols than in the Southern Hemisphere, but these differences have halved in the two decades since 2000, providing further evidence for the ongoing global brightening.{{cite journal |last1=Cao |first1=Yang |last2=Zhu |first2=Yannian |last3=Wang |first3=Minghuai |last4=Rosenfeld |first4=Daniel |last5=Liang |first5=Yuan |last6=Liu |first6=Jihu |last7=Liu |first7=Zhoukun |last8=Bai |first8=Heming |date=7 January 2023 | title=Emission Reductions Significantly Reduce the Hemispheric Contrast in Cloud Droplet Number Concentration in Recent Two Decades |journal= Journal of Geophysical Research: Atmospheres| volume=128 |issue=2 |page=e2022JD037417 |doi=10.1029/2022JD037417 |doi-access=free |bibcode=2023JGRD..12837417C }}

File:Bellouin_2019_aerosol_cloud_interactions.jpg

Aerosols have a cooling effect, which has masked the total extent of global warming experienced to date.

It has been understood for a long time that any effect on solar irradiance from aerosols would necessarily impact Earth's radiation balance. Reductions in atmospheric temperatures have already been observed after large volcanic eruptions such as the 1963 eruption of Mount Agung in Bali, 1982 El Chichón eruption in Mexico, 1985 Nevado del Ruiz eruption in Colombia and 1991 eruption of Mount Pinatubo in the Philippines. However, even the major eruptions only result in temporary jumps of sulfur particles, unlike the more sustained increases caused by anthropogenic pollution.

In 1990, the IPCC First Assessment Report acknowledged that "Human-made aerosols, from sulphur emitted largely in fossil fuel combustion can modify clouds and this may act to lower temperatures", while "a decrease in emissions of sulphur might be expected to increase global temperatures". However, lack of observational data and difficulties in calculating indirect effects on clouds left the report unable to estimate whether the total impact of all anthropogenic aerosols on the global temperature amounted to cooling or warming. By 1995, the IPCC Second Assessment Report had confidently assessed the overall impact of aerosols as negative (cooling);{{cite web |author=Zeke Hausfather |date=5 October 2017 |title=Analysis: How well have climate models projected global warming? |url=https://www.carbonbrief.org/analysis-how-well-have-climate-models-projected-global-warming/ |access-date=2023-03-21 |publisher=Carbon Brief}} however, aerosols were recognized as the largest source of uncertainty in future projections in that report and the subsequent ones.

Unlike sulfate pollution, black carbon contributes to both global dimming and global warming, since its particles absorb sunlight and heat up instead of reflecting it away. These particles also develop thick coatings over time, which can increase the initial absorption by up to 40%. Because the rate at which these coatings are formed varies depending on the season, the warming from black carbon varies seasonally as well.{{cite journal |last1=Mbengue |first1=Saliou |last2=Zikova |first2=Nadezda |last3=Schwarz |first3=Jaroslav |last4=Vodička |first4=Petr |last5=Šmejkalová |first5=Adéla Holubová |last6=Holoubek |first6=Ivan |date=28 June 2021 |title=Mass absorption cross-section and absorption enhancement from long term black and elemental carbon measurements: A rural background station in Central Europe |journal=Science of the Total Environment |volume=794 |issue=1 |page=148365 |doi=10.1016/j.scitotenv.2021.148365|pmid=34198082 |pmc=8434419 |bibcode=2021ScTEn.79448365M }}

Though this warming is weaker than the {{CO2}}-induced warming or the cooling from sulfates, it can be regionally significant when black carbon is deposited over ice masses like mountain glaciers and the Greenland ice sheet. There, it reduces their albedo and increases their absorption of solar radiation, which accelerates their melting. Black carbon also has an outsized contribution to local warming inside polluted cities.{{Cite journal |last1=Peng |first1=Jianfei |last2=Hu |first2=Min |last3=Guo |first3=Song |last4=Zhang |first4=Renyi |date=28 March 2016 |title=Markedly enhanced absorption and direct radiative forcing of black carbon under polluted urban environments |journal=PNAS |volume=113 |issue=16 |pages=4266–4271 |doi=10.1073/pnas.1602310113|doi-access=free |pmid=27035993 |pmc=4843448 |bibcode=2016PNAS..113.4266P }} Even the indirect effect of soot particles acting as cloud nuclei is not strong enough to provide cooling: the "brown clouds" formed around soot particles were known to have a net warming effect since the 2000s.{{cite web |author=National Science Foundation |author-link=National Science Foundation |date=1 August 2007 |title="Brown Cloud" Particulate Pollution Amplifies Global Warming |url=https://www.nsf.gov/news/news_summ.jsp?cntn_id=109712 |access-date=2008-04-03}} Black carbon pollution is particularly strong over India: thus, it is considered to be one of the few regions where cleaning up air pollution would reduce, rather than increase, warming.{{cite journal |last1=Miinalainen |first1=Tuuli |last2=Kokkola |first2=Harri |last3=Lipponen |first3=Antti |last4=Hyvärinen |first4=Antti-Pekka |last5=Kumar Soni |first5=Vijay |last6=Lehtinen |first6=Kari E. J. |last7=Kühn |first7=Thomas |date=20 March 2023 |title=Assessing the climate and air quality effects of future aerosol mitigation in India using a global climate model combined with statistical downscaling |url=https://acp.copernicus.org/articles/23/3471/2023/ |journal=Atmospheric Chemistry and Physics |language=en |volume=23 |issue=6 |pages=3471–3491 |bibcode=2023ACP....23.3471M |doi=10.5194/acp-23-3471-2023 |s2cid=253222600 |doi-access=free}}

{{Main|Contrail#Impacts on climate}}

File:Sfc.contrail.1.26.01.JPGs (white lines) and natural clouds.]]

Aircraft leave behind visible contrails (also known as vapor trails) as they travel. These contrails both reflect incoming solar radiation and trap outgoing longwave radiation that is emitted by the Earth. Because contrails reflect sunlight only during the day, but trap heat day and night, they are normally considered to cause net warming, albeit very small. A 1992 estimate was between 3.5 mW/m² and 17 mW/m² – hundreds of times smaller than the radiative forcing from major greenhouse gases.{{cite journal |last=Ponater |first=M. |year=2005 |title=On contrail climate sensitivity |journal=Geophysical Research Letters |volume=32 |issue=10 |pages=L10706 |bibcode=2005GeoRL..3210706P |doi=10.1029/2005GL022580 |doi-access=free}}

However, some scientists argued that the daytime cooling effect from contrails was much stronger than usually estimated, and this argument attracted attention following the September 11 attacks. Because no commercial aircraft flew across the US in the immediate aftermath of the attacks, this period was considered a real-world demonstration of contrail-free weather.{{cite news |last=Perkins |first=Sid |date=11 May 2002 |title=September's Science: Shutdown of airlines aided contrail studies |url=https://www.sciencenews.org/article/septembers-science-shutdown-airlines-aided-contrail-studies |access-date=13 October 2021 |journal=Science News |publisher=Science News Online}} Across 4,000 weather stations in the continental United States, the diurnal temperature variation (the difference in the day's highs and lows at a fixed station) was widened by {{cvt|1.1|C-change|F-change}} – the largest recorded increase in 30 years.{{cite journal |last1=Travis |first1=David J. |last2=Carleton |first2=Andrew M. |last3=Lauritsen |first3=Ryan G. |year=2002 |title=Contrails reduce daily temperature range |url=http://facstaff.uww.edu/travisd/pdf/jetcontrailsrecentresearch.pdf |url-status=dead |journal=Nature |volume=418 |issue=6898 |page=601 |bibcode=2002Natur.418..601T |doi=10.1038/418601a |pmid=12167846 |s2cid=4425866 |archive-url=https://web.archive.org/web/20060503192714/http://facstaff.uww.edu/travisd/pdf/jetcontrailsrecentresearch.pdf |archive-date=3 May 2006}} In the southern US, the difference was diminished by about {{cvt|6|F-change|C-change|order=flip}}, and by {{cvt|5|F-change|C-change|order=flip}} in the US midwest.{{citation |title=Jet contrails affect surface temperatures |date=18 June 2015 |work=Science Daily |url=https://www.sciencedaily.com/releases/2015/06/150618122236.htm |access-date=13 October 2021}} This was interpreted by some scientists as a proof of a strong cooling influence of aircraft contrails.{{cite journal |last=Travis |first=D.J. |author2=A.M. Carleton |author3=R.G. Lauritsen |date=March 2004 |title=Regional Variations in U.S. Diurnal Temperature Range for the 11–14 September 2001 Aircraft Groundings: Evidence of Jet Contrail Influence on Climate |url=https://journals.ametsoc.org/view/journals/clim/17/5/1520-0442_2004_017_1123_rviudt_2.0.co_2.xml |journal=J. Clim. |volume=17 |issue=5 |page=1123 |bibcode=2004JCli...17.1123T |doi=10.1175/1520-0442(2004)017<1123:RVIUDT>2.0.CO;2}}

Ultimately, follow-up studies found that a natural change in cloud cover which occurred at the time was sufficient to explain these findings.{{cite journal |last1=Kalkstein |last2=Balling Jr. |year=2004 |title=Impact of unusually clear weather on United States daily temperature range following 9/11/2001 |url=http://www.int-res.com/abstracts/cr/v26/n1/p1-4/ |journal=Climate Research |volume=26 |page=1 |bibcode=2004ClRes..26....1K |doi=10.3354/cr026001 |doi-access=free}}{{cite journal |last1=Hong |first1=Gang |last2=Yang |first2=Ping |last3=Minnis |first3=Patrick |last4=Hu |first4=Yong X. |last5=North |first5=Gerald |year=2008 |title=Do contrails significantly reduce daily temperature range? |journal=Geophysical Research Letters |volume=35 |issue=23 |pages=L23815 |bibcode=2008GeoRL..3523815H |doi=10.1029/2008GL036108 |doi-access=free }} When the global response to the 2020 coronavirus pandemic led to a reduction in global air traffic of nearly 70% relative to 2019, multiple studies found "no significant response of diurnal surface air temperature range" as the result of contrail changes, and either "no net significant global ERF" (effective radiative forcing) or a very small warming effect.{{cite journal |last1=Digby |first1=Ruth A. R. |last2=Gillett |first2=Nathan P. |last3=Monahan |first3=Adam H. |last4=Cole |first4=Jason N. S. |date=29 September 2021 |title=An Observational Constraint on Aviation-Induced Cirrus From the COVID-19-Induced Flight Disruption |journal=Geophysical Research Letters |volume=48 |issue=20 |pages=e2021GL095882 |bibcode=2021GeoRL..4895882D |doi=10.1029/2021GL095882 |pmc=8667656 |pmid=34924638 |doi-access=free}}{{cite journal |last1=Gettelman |first1=Andrew |last2=Chen |first2=Chieh-Chieh |last3=Bardeen |first3=Charles G. |date=18 June 2021 |title=The climate impact of COVID-19-induced contrail changes |url=https://acp.copernicus.org/articles/21/9405/2021/ |journal=Atmospheric Chemistry and Physics |volume=21 |issue=12 |pages=9405–9416 |bibcode=2021ACP....21.9405G |doi=10.5194/acp-21-9405-2021 |doi-access=free}}{{cite journal |last1=Zhu |first1=Jialei |last2=Penner |first2=Joyce E. |last3=Garnier |first3=Anne |last4=Boucher |first4=Olivier |last5=Gao |first5=Meng |last6=Song |first6=Lei |last7=Deng |first7=Junjun |last8=Liu |first8=Cong-qiang |last9=Fu |first9=Pingqing |date=18 March 2022 |title=Decreased Aviation Leads to Increased Ice Crystal Number and a Positive Radiative Effect in Cirrus Clouds |journal=AGU Advances |volume=3 |issue=2 |page=ee2020GL089788 |bibcode=2022AGUA....300546Z |doi=10.1029/2021AV000546 |doi-access=free|hdl=2027.42/172020 |hdl-access=free }}

File:Physical Drivers of climate change.svg. For example, sulfur dioxide causes cooling because it reacts to form a variety of sunlight-reflecting sulfates. Its large error bar shows that there is a lot of uncertainty regarding the strength of cooling caused by sulfur dioxide in the atmosphere.]]

At the peak of global dimming, sulfur dioxide was able to counteract the warming trend completely. By 1975, the continually increasing concentrations of greenhouse gases had overcome the masking effect, and have dominated ever since. Even then, regions with high concentrations of sulfate aerosols due to air pollution had initially experienced cooling, in contradiction to the overall warming trend.{{cite web|url = http://www.ucsusa.org/global_warming/science/crichton-thriller-state-of-fear.html|title = Crichton's Thriller State of Fear: Separating Fact from Fiction|access-date = 12 June 2006|archive-url = https://web.archive.org/web/20060614125559/http://www.ucsusa.org/global_warming/science/crichton-thriller-state-of-fear.html|archive-date = 14 June 2006}} The eastern United States was a prominent example: the temperatures there declined by {{convert|0.7|C-change|F-change}} between 1970 and 1980, and by up to {{convert|1|C-change|F-change}} in the Arkansas and Missouri.{{cite web |date=18 May 2012 |title="Warming Hole" Over the Eastern U.S. Due to Air Pollution |url=https://earthobservatory.nasa.gov/images/77966/warming-hole-over-the-eastern-us-due-to-air-pollution |publisher=NASA}}

Since the 1980s, the reduction in global dimming has contributed to higher global temperatures. Hot extremes have accelerated as global dimming has abated. It has been estimated that since the mid-1990s, peak daily temperatures in northeast Asia and hottest days of the year in Western Europe would have been substantially cooler if aerosol concentrations had stayed the same as before. Some of the acceleration of sea level rise, as well as Arctic amplification and the associated Arctic sea ice decline, was also attributed to the reduction in aerosol masking.{{cite journal |last1=Wild |first1=M. |last2=Ohmura |first2=A. |last3=Makowski |first3=K. |title=Impact of global dimming and brightening on global warming |journal=Geophysical Research Letters |year=2007 |volume=34 |issue=4 |pages=L04702 |doi=10.1029/2006GL028031 |bibcode=2007GeoRL..34.4702W |doi-access=free }}{{cite journal |last=Kerr |first=Richard A. | date=16 March 2007 |title=Climate change: Is a Thinning Haze Unveiling the Real Global Warming? |journal=Science |volume=315 |issue=5818 |page=1480 |doi=10.1126/science.315.5818.1480 |pmid=17363636 |s2cid=40829354 |doi-access=free }}{{cite journal |last1=Krishnan |first1=Srinath |last2=Ekman |first2=Annica M. L. |last3=Hansson |first3=Hans-Christen |last4=Riipinen |first4=Ilona |last5=Lewinschal |first5=Anna |last6=Wilcox |first6=Laura J. |last7=Dallafior |first7=Tanja |date=28 March 2020 |title=The Roles of the Atmosphere and Ocean in Driving Arctic Warming Due to European Aerosol Reductions |journal=Geophysical Research Letters |language=en |volume=47 |issue=11 |pages=e2019GL086681 |doi= 10.1029/2019GL086681|bibcode=2020GeoRL..4786681K |s2cid=216171731 }}{{cite web |date=14 December 2021 |title=The Arctic is warming four times faster than the rest of the world |url=https://www.science.org/content/article/arctic-warming-four-times-faster-rest-world |language=en |access-date=6 October 2022}}

In Europe, the declines in aerosol concentrations since the 1980s had also reduced the associated fog, mist and haze: altogether, it was responsible for about 10–20% of daytime warming across Europe, and about 50% of the warming over the more polluted Eastern Europe.{{cite journal |last1=Vautard |first1=Robert |last2=Yiou |first2=Pascal |last3=Oldenborgh |first3=Geert Jan van |date=3 December 2021 |title=Decline of fog, mist and haze in Europe over the past 30 years |journal=Nature Geoscience |url=https://www.nature.com/articles/ngeo414 | volume=2 |issue=2 |pages=115–119 |doi=10.1038/ngeo414 }} Because aerosol cooling depends on reflecting sunlight, air quality improvements had a negligible impact on wintertime temperatures,{{cite journal| last1=Markowicz |first1=Krzysztof M. |last2=Zawadzka-Manko | first2=Olga |last3=Posyniak |first3=Michal |date=3 December 2021 |title=A large reduction of direct aerosol cooling over Poland in the last decades |journal=International Journal of Climatology | volume=42 |issue=7 |pages=4129–4146 |doi=10.1002/joc.7488 |s2cid=244881291 |doi-access=free }} but had increased temperatures from April to September by around {{convert|1|C-change|F-change}} in Central and Eastern Europe.{{cite journal |last1=Glantz |first1=P. |last2=Fawole |first2=O. G. |last3=Ström |first3=J. |last4=Wild |first4=M. |last5=Noone |first5=K. J. |date=27 November 2022 | title=Unmasking the Effects of Aerosols on Greenhouse Warming Over Europe |url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2021JD035889 |journal = Journal of Geophysical Research: Atmospheres|volume =127 |issue=22 |pages=e2021JD035889 | doi=10.1029/2021JD035889 |bibcode=2022JGRD..12735889G |s2cid=253357109 |hdl=20.500.11850/584879 |hdl-access=free }} The central and eastern United States experienced warming of {{convert|0.3|C-change|F-change}} between 1980 and 2010 as sulfate pollution was reduced, even as sulfate particles still accounted for around 25% of all particulates. By 2021, the northeastern coast of the United States was one of the fastest-warming regions of North America, as the slowdown of the Atlantic Meridional Overturning Circulation increased temperatures in that part of the North Atlantic Ocean.{{cite journal |last1=Karmalkar |first1=Ambarish V. |last2=Horton |first2=Radley M. |date=23 September 2021 |title=Drivers of exceptional coastal warming in the northeastern United States |journal=Nature Climate Change |volume=11 |issue=10 |pages=854–860 |bibcode=2021NatCC..11..854K |doi=10.1038/s41558-021-01159-7 |s2cid=237611075}}{{cite web |last1=Krajick |first1=Kevin |date=23 September 2021 |title=Why the U.S. Northeast Coast Is a Global Warming Hot Spot |url=https://news.climate.columbia.edu/2021/09/23/the-u-s-northeast-coast-is-a-global-warming-hot-spot-says-study/ |access-date=2023-03-23 |publisher=Columbia Climate School}}

File:Yang_2020_China_lockdowns_temperature.jpg

In 2020, COVID-19 lockdowns provided a notable "natural experiment", as there had been a marked decline in sulfate and black carbon emissions caused by the curtailed road traffic and industrial output. That decline did have a detectable warming impact: it was estimated to have increased global temperatures by {{convert|0.01-0.02|C-change|F-change}} initially and up to {{convert|0.03|C-change|F-change}} by 2023, before disappearing. Regionally, the lockdowns were estimated to increase temperatures by {{convert|0.05-0.15|C-change|F-change}} in eastern China over January–March, and then by {{convert|0.04-0.07|C-change|F-change}} over Europe, eastern United States, and South Asia in March–May, with the peak impact of {{convert|0.3|C-change|F-change}} in some regions of the United States and Russia.{{cite journal |last1=Gettelman |first1=A. |last2=Lamboll |first2=R. |last3=Bardeen |first3=C. G. |last4=Forster |first4=P. M. |last5=Watson-Parris |first5=D. |date=29 December 2020 |title=Climate Impacts of COVID-19 Induced Emission Changes |journal=Geophysical Research Letters |volume=48 |issue=3 |page=e2020GL091805 |doi=10.1029/2020GL091805 |doi-access=free}}{{cite journal |last1=Yang |first1=Yang |last2=Ren |first2=Lili |last3=Li |first3=Huimin |last4=Wang |first4=Hailong |last5=Wang |first5=Pinya |last6=Chen |first6=Lei |last7=Yue |first7=Xu |last8=Liao |first8=Hong |date=17 September 2020 |title=Fast Climate Responses to Aerosol Emission Reductions During the COVID-19 Pandemic |journal=Geophysical Research Letters |volume=47 |issue=19 |page=ee2020GL089788 |bibcode=2020GeoRL..4789788Y |doi=10.1029/2020GL089788 |doi-access=free}} In the city of Wuhan, the urban heat island effect was found to have decreased by {{convert|0.24|C-change|F-change}} at night and by {{convert|0.12|C-change|F-change}} overall during the strictest lockdowns.{{cite journal |last1=Sun |first1=Shanlei |last2=Zhou |first2=Decheng |last3=Chen |first3=Haishan |last4=Li |first4=Jinjian |last5=Ren |first5=Yongjian |last6=Liao |first6=Hong |last7=Liu |first7=Yibo |date=25 June 2022 |title=Decreases in the urban heat island effect during the Coronavirus Disease 2019 (COVID-19) lockdown in Wuhan, China: Observational evidence |journal=International Journal of Climatology |volume=42 |issue=16 |pages=8792–8803 |bibcode=2022IJCli..42.8792S |doi=10.1002/joc.7771 |doi-access=free}}

Since changes in aerosol concentrations already have an impact on the global climate, they would necessarily influence future projections as well. In fact, it is impossible to fully estimate the warming impact of all greenhouse gases without accounting for the counteracting cooling from aerosols.

File:Estimates of past and future SO2 global anthropogenic emissions.pngs. While no climate change scenario may reach Maximum Feasible Reductions (MFRs), all assume steep declines from today's levels. By 2019, sulfate emission reductions were confirmed to proceed at a very fast rate.]]

Climate models started to account for the effects of sulfate aerosols around the IPCC Second Assessment Report; when the IPCC Fourth Assessment Report was published in 2007, every climate model had integrated sulfates, but only 5 were able to account for less impactful particulates like black carbon. By 2021, CMIP6 models estimated total aerosol cooling in the range from {{convert|0.1|C-change|F-change}} to {{convert|0.7|C-change|F-change}};{{cite journal |last1=Gillett |first1=Nathan P. |last2=Kirchmeier-Young |first2=Megan |last3=Ribes |first3=Aurélien |last4=Shiogama |first4=Hideo |last5=Hegerl |first5=Gabriele C. |last6=Knutti |first6=Reto |last7=Gastineau |first7=Guillaume |last8=John |first8=Jasmin G. |last9=Li |first9=Lijuan |last10=Nazarenko |first10=Larissa |last11=Rosenbloom |first11=Nan |last12=Seland |first12=Øyvind |last13=Wu |first13=Tongwen |last14=Yukimoto |first14=Seiji |last15=Ziehn |first15=Tilo |url=https://repository.library.noaa.gov/view/noaa/32874/noaa_32874_DS1.pdf |title=Constraining human contributions to observed warming since the pre-industrial period | date=18 January 2021 |journal=Nature Climate Change | volume=11|issue=3 | pages=207–212| doi=10.1038/s41558-020-00965-9 |bibcode=2021NatCC..11..207G |s2cid=231670652 }} The IPCC Sixth Assessment Report selected the best estimate of a {{convert|0.5|C-change|F-change}} cooling provided by sulfate aerosols, while black carbon amounts to about {{convert|0.1|C-change|F-change}} of warming. While these values are based on combining model estimates with observational constraints, including those on ocean heat content, the matter is not yet fully settled. The difference between model estimates mainly stems from disagreements over the indirect effects of aerosols on clouds.{{cite news |last1=Andrew |first1=Tawana |title=Behind the Forecast: How clouds affect temperatures |url=https://www.wave3.com/2019/09/27/behind-forecast-how-clouds-affect-temperatures/ |access-date=4 January 2023 |work=Science Behind the Forecast |publisher=LOUISVILLE, Ky. (WAVE) |date=27 September 2019 |language=en}}{{cite journal |last1=Zhang |first1=Jie |last2=Furtado |first2=Kalli |last3=Turnock |first3=Steven T. |last4=Mulcahy |first4=Jane P. |last5=Wilcox |first5=Laura J. |last6=Booth |first6=Ben B. |last7=Sexton |first7=David |last8=Wu |first8=Tongwen |last9=Zhang |first9=Fang |last10=Liu |first10=Qianxia |date=22 December 2021 |title=The role of anthropogenic aerosols in the anomalous cooling from 1960 to 1990 in the CMIP6 Earth system models |url=https://acp.copernicus.org/articles/21/18609/2021/ |journal=Atmospheric Chemistry and Physics |volume=21 |issue=4 |pages=18609–18627 |language=en |doi=10.5194/acp-21-18609-2021 |doi-access=free |bibcode=2021ACP....2118609Z }}

Regardless of the current strength of aerosol cooling, all future climate change scenarios project decreases in particulates and this includes the scenarios where {{convert|1.5|C-change|F-change}} and {{convert|2|C-change|F-change}} targets are met: their specific emission reduction targets assume the need to make up for lower dimming. Since models estimate that the cooling caused by sulfates is largely equivalent to the warming caused by atmospheric methane (and since methane is a relatively short-lived greenhouse gas), it is believed that simultaneous reductions in both would effectively cancel each other out.{{cite web |last=Hausfather |first=Zeke |date=29 April 2021 |title=Explainer: Will global warming 'stop' as soon as net-zero emissions are reached? |url=https://www.carbonbrief.org/explainer-will-global-warming-stop-as-soon-as-net-zero-emissions-are-reached |access-date=3 March 2023 |publisher=Carbon Brief}}

{{Cite journal |last1=Hassan |first1=Taufiq |last2=Allen |first2=Robert J. |display-authors=etal |date=27 June 2022 |title=Air quality improvements are projected to weaken the Atlantic meridional overturning circulation through radiative forcing effects |journal=Communications Earth & Environment|volume=3 |issue=3 |page=149 |doi=10.1038/s43247-022-00476-9|bibcode=2022ComEE...3..149H |s2cid=250077615 |doi-access=free }} Yet, in the recent years, methane concentrations had been increasing at rates exceeding their previous period of peak growth in the 1980s,{{cite web |title=Trends in Atmospheric Methane |url=https://gml.noaa.gov/ccgg/trends_ch4/ |access-date=14 October 2022 |website=NOAA}}{{cite magazine |vauthors=Tollefson J |date=8 February 2022 |title=Scientists raise alarm over 'dangerously fast' growth in atmospheric methane |url=https://www.nature.com/articles/d41586-022-00312-2 |access-date=14 October 2022 |magazine=Nature}} with wetland methane emissions driving much of the recent growth,{{cite journal |vauthors=Lan X, Basu S, Schwietzke S, Bruhwiler LM, Dlugokencky EJ, Michel SE, Sherwood OA, Tans PP, Thoning K, Etiope G, Zhuang Q, Liu L, Oh Y, Miller JB, Pétron G, Vaughn BH, Crippa M |date=8 May 2021 |title=Improved Constraints on Global Methane Emissions and Sinks Using δ¹³C-CH₄ |journal=Global Biogeochemical Cycles |volume=35 |issue=6 |pages=e2021GB007000 |doi=10.1029/2021GB007000 |pmid=34219915 |pmc=8244052 |bibcode=2021GBioC..3507000L |doi-access=free}}{{cite journal |last1=Feng |first1=Liang |last2=Palmer |first2=Paul I. |last3=Zhu |first3=Sihong |last4=Parker |first4=Robert J. |last5=Liu |first5=Yi |date=16 March 2022 |title=Tropical methane emissions explain large fraction of recent changes in global atmospheric methane growth rate |journal=Nature Communications |language=en |volume=13 |issue=1 |page=1378 |doi=10.1038/s41467-022-28989-z |pmid=35297408 |pmc=8927109 |bibcode=2022NatCo..13.1378F }} while air pollution is getting cleaned up aggressively. These trends are some of the main reasons why {{convert|1.5|C-change|F-change}} warming is now expected around 2030, as opposed to the mid-2010s estimates where it would not occur until 2040.{{cite journal|last1=Xu|first1=Yangyang|last2=Ramanathan|first2=Veerabhadran|last3=Victor|first3=David G.|date=5 December 2018|title=Global warming will happen faster than we think|journal=Nature|language=en|volume=564|issue=7734|pages=30–32 |url=https://www.researchgate.net/publication/329411074 |doi=10.1038/d41586-018-07586-5|pmid=30518902|bibcode=2018Natur.564...30X|doi-access=free}}

File:Luo 2020 aerosol temps Europe.jpg

It has also been suggested that aerosols are not given sufficient attention in regional risk assessments, in spite of being more influential on a regional scale than globally.{{cite journal|last1=Persad|first1=Geeta G.|last2=Samset|first2=Bjørn H.|last3=Wilcox|first3=Laura J.|date=21 November 2022 |title=Aerosols must be included in climate risk assessments|journal=Nature|language=en|volume=611 |issue=7937 |pages=662–664 |doi=10.1038/d41586-022-03763-9 |pmid=36411334 |doi-access=free|bibcode=2022Natur.611..662P }} For instance, a climate change scenario with high greenhouse gas emissions but strong reductions in air pollution would see {{convert|0.2|C-change|F-change}} more global warming by 2050 than the same scenario with little improvement in air quality, but regionally, the difference would add 5 more tropical nights per year in northern China and substantially increase precipitation in northern China and northern India.{{cite journal |last1=Li |first1=Yingfang |last2=Wang |first2=Zhili |last3=Lei |first3=Yadong |last4=Che |first4=Huizheng |last5=Zhang |first5=Xiaoye |date=23 February 2023 |title=Impacts of reductions in non-methane short-lived climate forcers on future climate extremes and the resulting population exposure risks in eastern and southern Asia |url=https://acp.copernicus.org/articles/23/2499/2023/ |journal=Atmospheric Chemistry and Physics |volume=23 |issue=4 |pages=2499–2523 |language=en |doi=10.5194/acp-23-2499-2023 |s2cid=257180147 |doi-access=free |bibcode=2023ACP....23.2499L }} Likewise, a paper comparing current level of clean air policies with a hypothetical maximum technically feasible action under otherwise the same climate change scenario found that the latter would increase the risk of temperature extremes by 30–50% in China and in Europe.{{cite journal |last1=Luo |first1=Feifei |last2=Wilcox |first2=Laura |last3=Dong |first3=Buwen |last4=Su |first4=Qin |last5=Chen |first5=Wei |last6=Dunstone |first6=Nick |last7=Li |first7=Shuanglin |last8=Gao |first8=Yongqi |date=19 February 2020 |title=Projected near-term changes of temperature extremes in Europe and China under different aerosol emissions |journal=Environmental Research Letters | volume=15 |issue=3 |page=4013 |doi=10.1088/1748-9326/ab6b34 |doi-access=free |bibcode=2020ERL....15c4013L }}

Unfortunately, because historical records of aerosols are sparser in some regions than in others, accurate regional projections of aerosol impacts are difficult. Even the latest CMIP6 climate models can only accurately represent aerosol trends over Europe, but struggle with representing North America and Asia. This means that their near-future projections of regional impacts are likely to contain errors as well.{{cite journal |last1=Wang |first1=Zhili |last2=Lin |first2=Lei |last3=Xu |first3=Yangyang |last4=Che |first4=Huizheng |last5=Zhang |first5=Xiaoye |last6=Zhang |first6=Hua |last7=Dong |first7=Wenjie |last8=Wang |first8=Chense |last9=Gui |first9=Ke |last10=Xie |first10=Bing |date=12 January 2021 | title=Incorrect Asian aerosols affecting the attribution and projection of regional climate change in CMIP6 models |journal=npj Climate and Atmospheric Science | volume=4 |issue=21 |doi=10.1029/2021JD035476 |doi-access=free |bibcode=2022JGRD..12735476J |hdl=10852/97300 |hdl-access=free }}{{cite journal |last1=Julsrud |first1=I. R. |last2=Storelvmo |first2=T. |last3=Schulz |first3=M. |last4=Moseid |first4=K. O. |last5=Wild |first5=M. |date=20 October 2022 | title=Disentangling Aerosol and Cloud Effects on Dimming and Brightening in Observations and CMIP6 |journal= Journal of Geophysical Research: Atmospheres| volume=127 |issue=21 |page=e2021JD035476 |doi=10.1029/2021JD035476 |doi-access=free |bibcode=2022JGRD..12735476J |hdl=10852/97300 |hdl-access=free }}{{cite journal |last1=Ramachandran |first1=S. |last2=Rupakheti |first2=Maheswar |last3=Cherian |first3=R. |date=10 February 2022 |title=Insights into recent aerosol trends over Asia from observations and CMIP6 simulations |journal=Science of the Total Environment |volume=807 |issue=1 |page=150756 |doi=10.1016/j.scitotenv.2021.150756 |pmid=34619211 |s2cid=238474883 |doi-access=free |bibcode=2022ScTEn.80750756R }}

{{Further|Water cycle}}

File:Xie_et_al_2022_Asian_aerosols.png

On regional and global scale, air pollution can affect the water cycle, in a manner similar to some natural processes. One example is the impact of Sahara dust on hurricane formation: air laden with sand and mineral particles moves over the Atlantic Ocean, where they block some of the sunlight from reaching the water surface, slightly cooling it and dampening the development of hurricanes.{{cite journal|url = https://www.sciencedaily.com/releases/2018/07/180720154909.htm |title = Sahara dust may make you cough, but it's a storm killer|year = 2018|doi=10.1175/JCLI-D-16-0776.1|journal=Journal of Climate |volume=31 |issue=18|pages=7621–7644|last1=Pan|first1=Bowen|last2=Wang|first2=Yuan|last3=Hu|first3=Jiaxi|last4=Lin|first4=Yun|last5=Hsieh|first5=Jen-Shan|last6=Logan|first6=Timothy|last7=Feng|first7=Xidan|last8=Jiang|first8=Jonathan H.|last9=Yung|first9=Yuk L.|last10=Zhang|first10=Renyi |doi-access=free}} Likewise, it has been suggested since the early 2000s that since aerosols decrease solar radiation over the ocean and hence reduce evaporation from it, they would be "spinning down the hydrological cycle of the planet."{{cite web|url=http://ens-newswire.com/ens/dec2001/2001-12-07-07.asp |title=Aerosol Pollution Could Drain Earth's Water Cycle |author=Cat Lazaroff|publisher=Environment News Service|date=7 December 2001|access-date=2007-03-24 |archive-date=2016-06-03 |archive-url=https://web.archive.org/web/20160603035532/http://ens-newswire.com/ens/dec2001/2001-12-07-07.asp }}{{cite news| url=http://www.ldeo.columbia.edu/news/2006/04_14_06.htm| title=Could Reducing Global Dimming Mean a Hotter, Dryer World?| publisher=Lamont–Doherty Earth Observatory News| author1=Kostel, Ken| author2=Oh, Clare| date=14 April 2006| access-date=2006-06-12| archive-url=https://web.archive.org/web/20160303233341/http://www.ldeo.columbia.edu/news/2006/04_14_06.htm| archive-date=2016-03-03 }}

In 2011, it was found that anthropogenic aerosols had been the predominant factor behind 20th century changes in rainfall over the Atlantic Ocean sector,{{cite journal |last1=Chang |first1=C.-Y. |last2=Chiang |first2=J. C. H. |last3=Wehner |first3=M. F. |last4=Friedman |first4=A. R. |last5=Ruedy |first5=R. |title=Sulfate Aerosol Control of Tropical Atlantic Climate over the Twentieth Century |date=15 May 2011 |journal=Journal of Climate |volume=24 |issue=10 |pages=2540–2555 |doi=10.1175/2010JCLI4065.1 |doi-access=free|bibcode=2011JCli...24.2540C }} when the entire tropical rain belt shifted southwards between 1950 and 1985, with a limited northwards shift afterwards.{{cite journal |last1=Peace |first1=Amy H. |last2=Booth |first2=Ben B. B. |last3=Regayre |first3=Leighton A. |last4=Carslaw |first4=Ken S. |last5=Sexton |first5=David M. H. |last6=Bonfils |first6=Céline J. W. |last7=Rostron |first7=John W. |date=26 August 2022 |title=Evaluating uncertainty in aerosol forcing of tropical precipitation shifts |url=https://esd.copernicus.org/articles/13/1215/2022/ |journal=Earth System Dynamics |volume=13 |issue=3 |pages=1215–1232 |doi=10.5194/esd-13-1215-2022 |doi-access=free |bibcode=2022ESD....13.1215P }} Future reductions in aerosol emissions are expected to result in a more rapid northwards shift, with limited impact in the Atlantic but a substantially greater impact in the Pacific.{{cite journal |last1=Allen |first1=Robert J. |date=20 August 2015 | title=A 21st century northward tropical precipitation shift caused by future anthropogenic aerosol reductions |journal= Journal of Geophysical Research: Atmospheres|volume=120 |issue=18 |pages=9087–9102 |doi=10.1002/2015JD023623 |doi-access=free |bibcode=2015JGRD..120.9087A }} Some research also suggests that these reductions would affect the AMOC (already expected to weaken due to climate change). Reductions from the stronger air quality policies could exacerbate this expected decline by around 10%, unless methane emissions are reduced by an equivalent amount.

Most notably, multiple studies connect aerosols from the Northern Hemisphere to the failed monsoon in sub-Saharan Africa during the 1970s and 1980s, which then led to the Sahel drought and the associated famine.{{cite journal | author = Rotstayn and Lohmann| year = 2002 | title = Tropical Rainfall Trends and the Indirect Aerosol Effect| journal = Journal of Climate| volume = 15| issue =15| pages = 2103–2116| doi = 10.1175/1520-0442(2002)015<2103:TRTATI>2.0.CO;2 | last2 = Lohmann | first2 = Ulrike|author-link2=Ulrike Lohmann| s2cid = 55802370|bibcode = 2002JCli...15.2103R | doi-access = free}}{{cite web|url=http://www.bbc.co.uk/sn/tvradio/programmes/horizon/dimming_trans.shtml|title=Global Dimming|work=bbc.co.uk|publisher=BBC|access-date=2020-01-05}}{{cite journal |last1=Hirasawa |first1=Haruki |last2=Kushner |first2=Paul J. |last3=Sigmond |first3=Michael |last4=Fyfe |first4=John |last5=Deser |first5=Clara |title=Evolving Sahel Rainfall Response to Anthropogenic Aerosols Driven by Shifting Regional Oceanic and Emission Influences |date=2 May 2022 |journal=Journal of Climate |volume=35 |issue=11 |pages=3181–3193 |doi=10.1175/JCLI-D-21-0795.1 |doi-access=free|bibcode=2022JCli...35.3181H }} However, model simulations of Sahel climate are very inconsistent,{{cite journal |last1=Monerie |first1=Paul-Arthur |last2=Dittus |first2=Andrea J. |last3=Wilcox |first3=Laura J. |last4=Turner |first4=Andrew G. |date=22 January 2023 | title=Uncertainty in Simulating Twentieth Century West African Precipitation Trends: The Role of Anthropogenic Aerosol Emissions |journal=Earth's Future |volume=11 |issue=2 |page=e2022EF002995 |doi=10.1029/2022EF002995 |doi-access=free |bibcode=2023EaFut..1102995M }} so it's difficult to prove that the drought would not have occurred without aerosol pollution, although it would have clearly been less severe.{{cite web |url=http://www.realclimate.org/index.php?p=105 |website=RealClimate |title=Global Dimming? |date=18 January 2005 |access-date=2007-04-05 |first=Gavin |last=Schmidt |author-link=Gavin Schmidt}}{{cite journal |last1=Herman |first1=Rebecca Jean |last2=Giannini |first2=Alessandra |last3=Biasutti |first3=Michela |last4=Kushnir |first4=Yochanan |date=22 July 2020 |title=The effects of anthropogenic and volcanic aerosols and greenhouse gases on twentieth century Sahel precipitation |journal=Scientific Reports | volume=10 |issue=1 |page=12203 |doi=10.1038/s41598-020-68356-w |pmid=32699339 |pmc=7376254 |bibcode=2020NatSR..1012203H }} Some research indicates that those models which demonstrate warming alone driving strong precipitation increases in the Sahel are the most accurate, making it more likely that sulfate pollution was to blame for overpowering this response and sending the region into drought.{{cite journal |last1=Schewe |first1=Jacob |last2=Levermann |first2=Anders |date=15 September 2022 |title=Sahel Rainfall Projections Constrained by Past Sensitivity to Global Warming |journal=Earth's Future |volume=11 |issue=2 |page=e2022GL098286 |doi=10.1029/2022GL098286 |doi-access=free |bibcode=2022GeoRL..4998286S }}

File:Risser_2024_USA_aerosol_GHG_rainfall.png

Another dramatic finding had connected the impact of aerosols with the weakening of the Monsoon of South Asia. It was first advanced in 2006,{{cite journal |last1=Lau |first1=K. M. |last2=Kim |first2=K. M. |date=8 November 2006 |title=Observational relationships between aerosol and Asian monsoon rainfall, and circulation |journal=Geophysical Research Letters |volume=33 |issue=21 |doi=10.1029/2006GL027546 |s2cid=129282371 |doi-access=free |bibcode=2006GeoRL..3321810L }} yet it also remained difficult to prove.{{cite journal |last1=Tao |first1=Wei-Kuo |last2=Chen |first2=Jen-Ping |last3=Li |first3=Zhanqing |last4=Wang |first4=Chien |last5=Zhang |first5=Chidong |date=17 April 2012 |title=Impact of aerosols on convective clouds and precipitation |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2011RG000369 |journal=Reviews of Geophysics |volume=50 |issue=2 |doi=10.1029/2011RG000369 |bibcode=2012RvGeo..50.2001T |s2cid=15554383 |hdl=2060/20120011727 |hdl-access=free }} In particular, some research suggested that warming itself increases the risk of monsoon failure, potentially pushing it past a tipping point.{{cite journal |last1=Schewe |first1=Jacob |last2=Levermann |first2=Anders |date=5 November 2012 |title=A statistically predictive model for future monsoon failure in India |url=https://iopscience.iop.org/article/10.1088/1748-9326/7/4/044023 |journal=Environmental Research Letters |volume=7 |issue=4 |page=4023 |doi=10.1088/1748-9326/7/4/044023 |bibcode=2012ERL.....7d4023S |s2cid=5754559 }}{{cite web|url=https://www.pik-potsdam.de/en/news/latest-news/archive-news/2012/monsoon-might-fail-more-often-due-to-climate-change |title=Monsoon might fail more often due to climate change |publisher=Potsdam Institute for Climate Impact Research|date=6 November 2012|access-date=2023-03-25}} By 2021, however, it was concluded that global warming consistently strengthened the monsoon,{{cite journal |last1=Katzenberger |first1=Anja |last2=Schewe |first2=Jacob |last3=Pongratz |first3=Julia |last4=Levermann |first4=Anders |date=2021 |title=Robust increase of Indian monsoon rainfall and its variability under future warming in CMIP-6 models |url=https://esd.copernicus.org/articles/12/367/2021/ |journal=Earth System Dynamics |volume=12 |issue=2 |pages=367–386 |doi=10.5194/esd-12-367-2021|bibcode=2021ESD....12..367K |s2cid=235080216 |doi-access=free }} and some strengthening was already observed in the aftermath of lockdown-caused aerosol reductions.{{cite journal |last1=Fadnavis |first1=Suvarna |last2=Sabin |first2=T. P. |last3=Rap |first3=Alexandru |last4=Müller |first4=Rolf |last5=Kubin |first5=Anne |last6=Heinold |first6=Bernd |date=16 July 2021 |title=The impact of COVID-19 lockdown measures on the Indian summer monsoon |url=https://iopscience.iop.org/article/10.1088/1748-9326/ac109c |journal=Environmental Research Letters |volume=16 |issue=7 |page=4054 |doi=10.1088/1748-9326/ac109c |bibcode=2021ERL....16g4054F |s2cid=235967722 }}

In 2009, an analysis of 50 years of data found that light rains had decreased over eastern China, even though there was no significant change in the amount of water held by the atmosphere. This was attributed to aerosols reducing droplet size within clouds, which led to those clouds retaining water for a longer time without raining. The phenomenon of aerosols suppressing rainfall through reducing cloud droplet size has been confirmed by subsequent studies.{{cite journal |last1=Fan |first1=Chongxing |last2=Wang |first2=Minghuai |last3=Rosenfeld |first3=Daniel |last4=Zhu |first4=Yannian |last5=Liu |first5=Jihu |last6=Chen |first6=Baojun |title=Strong Precipitation Suppression by Aerosols in Marine Low Clouds |journal=Geophysical Research Letters | date=18 March 2020 |doi=10.1029/2019GL086207 |volume=47 |issue=7 |pages=e2019GL086207 |doi-access = free |bibcode=2020GeoRL..4786207F |hdl=2027.42/154630 |hdl-access=free }} Later research found that aerosol pollution over South and East Asia didn't just suppress rainfall there, but also resulted in more moisture transferred to Central Asia, where summer rainfall had increased as the result.{{cite journal |last1=Xie |first1= Xiaoning |last2=Myhre |first2=Gunnar |last3=Shindell |first3=Drew |last4=Faluvegi |first4=Gregory |last5=Takemura |first5=Toshihiko |last6=Voulgarakis |first6=Apostolos |last7=Shi |first7=Zhengguo |last8=Li |first8=Xinzhou |last9=Xie |first9=Xiaoxun |last10=Liu |first10=Heng |last11=Liu |first11=Xiaodong |last12=Liu |first12=Yangang |date=27 December 2022 |title=Anthropogenic sulfate aerosol pollution in South and East Asia induces increased summer precipitation over arid Central Asia |journal=Communications Earth & Environment |volume=3 |issue= 1 |page= 328 |language=en |doi=10.1038/s43247-022-00660-x |pmid= 36588543 |pmc= 9792934 |bibcode= 2022ComEE...3..328X }} In the United States, effects of climate change on the water cycle would typically increase both mean and extreme precipitation across the country, but these effects have so far been "masked" by the drying due to historically strong aerosol concentrations.{{cite journal |last1=Risser |first1=Mark D. |last2=Collins |first2=William D. |last3=Wehner |first3=Michael F. |last4=O'Brien |first4=Travis A. |last5=Huang |first5=Huanping |last6=Ullrich |first6=Paul A. |date=22 February 2024 |title=Anthropogenic aerosols mask increases in US rainfall by greenhouse gases |journal=Nature Communications |language=en |volume=15 |issue=1 |page=1318 |doi=10.1038/s41467-024-45504-8 |pmid=38388495 |pmc=10884021 |bibcode=2024NatCo..15.1318R }} The IPCC Sixth Assessment Report had also linked changes in aerosol concentrations to altered precipitation in the Mediterranean region.

{{main|Stratospheric aerosol injection}}

File:Smith_2020_SAI_RCP_scenarios.jpg

Global dimming is also relevant for certain proposals about slowing, halting, or reversing global warming.{{cite news |last=Gramling |first=Carolyn |date=8 August 2018 |title=Global dimming may mitigate warming, but could hurt crop yields |url=https://www.sciencenews.org/article/geoengineering-tactic-cool-planet-could-hurt-crops |publisher=Science News Online |access-date=6 January 2024 }} An increase in planetary albedo of 1% would eliminate most of radiative forcing from anthropogenic greenhouse gas emissions and thereby global warming, while a 2% albedo increase would negate the warming effect of doubling the atmospheric carbon dioxide concentration.{{cite web|title=The Royal Society|url=https://royalsociety.org/~/media/Royal_Society_Content/policy/publications/2009/8693.pdf|website=royalsociety.org|access-date=20 October 2015|archive-date=21 July 2015|page=23|archive-url=https://web.archive.org/web/20150721043235/https://royalsociety.org/~/media/Royal_Society_Content/policy/publications/2009/8693.pdf|url-status=live}} This is the theory behind solar radiation modification (or solar geoengineering), and the high reflective potential of sulfate aerosols means that they were considered in this capacity starting from the 1970s.{{cite journal |last1=Lawrence |first1=Mark G. |last2=Crutzen |first2=Paul J. |date=17 November 2016 | title=Was breaking the taboo on research on climate engineering via albedo modification a moral hazard, or a moral imperative? |journal=Earth's Future |volume=5 |issue=2 |pages=136–143 |doi=10.1002/2016EF000463 |doi-access=free }}

Because the historical levels of global dimming were associated with high mortality from air pollution and issues such as acid rain,{{cite journal |last=Ramanathan |first=V. |year=2006 |title=Atmospheric Brown Clouds: Health, Climate and Agriculture Impacts |journal=Pontifical Academy of Sciences Scripta Varia (Pontifica Academia Scientiarvm) |volume=106 |issue=Interactions Between Global Change and Human Health |pages=47–60 |url=http://www-ramanathan.ucsd.edu/PASScriptaVaria106.pdf |archive-url=https://web.archive.org/web/20070730012326/http://www-ramanathan.ucsd.edu/PASScriptaVaria106.pdf |archive-date=30 July 2007 }} the concept of relying on cooling directly from pollution has been described as a "Faustian bargain" and is not seriously considered by modern research. Instead, the seminal 2006 paper by Paul Crutzen suggested that the way to avoid increased warming as the sulfate pollution decreased was to revisit the 1974 proposal by the Soviet researcher Mikhail Budyko.{{cite journal | author = Crutzen, P. | date = August 2006 | title = Albedo enhancement by stratospheric sulfur injections: a contribution to resolve a policy dilemma? | journal = Climatic Change | volume = 77 | issue = 3–4 | pages = 211–220 | doi = 10.1007/s10584-006-9101-y | url = https://link.springer.com/content/pdf/10.1007%2Fs10584-006-9101-y.pdf | bibcode = 2006ClCh...77..211C | s2cid = 154081541 | doi-access = free }}{{cite web |url=https://www.nytimes.com/2006/06/27/science/earth/27cool.html |title=How to Cool a Planet (Maybe) |date=27 June 2006 |access-date=6 April 2009 |work=The New York Times |author=William J. Broad }} The proposal involved releasing sulfates from the airplanes flying in the upper layers of the atmosphere, in what is now described as stratospheric aerosol injection, or SAI. In comparison, most air pollution is in the lower atmospheric layer (the troposphere), and only resides there for weeks. Because aerosols deposited in the stratosphere would last for years, far less sulfur would have to be emitted to produce the same amount of cooling.

While Crutzen's initial proposal focused on avoiding the warming caused by the reductions in air pollution, it was immediately understood that scaling up this proposal could slow, stop, or reverse warming. It has been estimated that the amount of sulfur needed to offset a warming of around {{convert|4|C-change|F-change}} relative to now (and {{convert|5|C-change|F-change}} relative to the preindustrial), under the highest-emission scenario RCP 8.5 would be less than what is already emitted through air pollution today, and that reductions in sulfur pollution from future air quality improvements already expected under that scenario would offset the sulfur used for geoengineering.{{cite journal |last1=Visioni |first1=Daniele |last2=Slessarev |first2=Eric |last3=MacMartin |first3=Douglas G |last4=Mahowald |first4=Natalie M |last5=Goodale |first5=Christine L |author-link5=Christine Goodale |last6=Xia |first6=Lili |date=1 September 2020 |title=What goes up must come down: impacts of deposition in a sulfate geoengineering scenario |journal=Environmental Research Letters |volume=15 |issue=9 |page=094063 |bibcode=2020ERL....15i4063V |doi=10.1088/1748-9326/ab94eb |issn=1748-9326 |doi-access=free}} The trade-off is increased cost. Although there's a popular narrative that stratospheric aerosol injection can be carried out by individuals, small states, or other non-state rogue actors, scientific estimates suggest that cooling the atmosphere by {{convert|1|C-change|F-change}} through stratospheric aerosol injection would cost at least $18 billion annually (at 2020 USD value), meaning that only the largest economies or economic blocs could afford this intervention.{{cite journal |last=Smith |first=Wake |date=October 2020 |title=The cost of stratospheric aerosol injection through 2100 |journal=Environmental Research Letters |language=en |volume=15 |issue=11 |page=114004 |doi=10.1088/1748-9326/aba7e7 |bibcode=2020ERL....15k4004S |s2cid=225534263 |issn=1748-9326|doi-access=free }}{{cite journal|last1=Robock|first1=Alan|last2=Marquardt|first2=Allison|last3=Kravitz|first3=Ben|last4=Stenchikov|first4=Georgiy|title=Benefits, risks, and costs of stratospheric geoengineering|journal=Geophysical Research Letters|volume=36|issue=19|year=2009|pages=L19703 |doi=10.1029/2009GL039209|url=http://climate.envsci.rutgers.edu/pdf/2009GL039209.pdf|hdl=10754/552099|bibcode=2009GeoRL..3619703R|doi-access=free}} Even so, these approaches would still be "orders of magnitude" cheaper than greenhouse gas mitigation,{{cite journal |last1=Grieger |first1=Khara D. |last2=Felgenhauer |first2=Tyler |last3=Renn | first3=Ortwin |last4=Wiener |first4=Jonathan | last5=Borsuk | first5=Mark |date=30 April 2019 |title=Emerging risk governance for stratospheric aerosol injection as a climate management technology |journal=Environment Systems and Decisions |volume=39 |issue=4 |pages=371–382 |doi=10.1007/s10669-019-09730-6 |doi-access=free |bibcode=2019EnvSD..39..371G }} let alone the costs of unmitigated effects of climate change.

Even if SAI were to stop or outright reverse global warming, weather patterns in many areas would still change substantially. The habitat of mosquitoes and other disease vectors would shift. However, it's unclear how it would compare to the shifts that are otherwise likely to occur from climate change. Lower sunlight would affect crop yields and carbon sinks due to reduced photosynthesis, but this would likely be offset by lack of thermal stress from warming and the greater CO₂ fertilization effect relative to now.{{cite journal |last1=Trisos |first1=Christopher H. |last2=Geden |first2=Oliver |last3=Seneviratne |first3=Sonia I. |last4=Sugiyama |first4=Masahiro |last5=van Aalst |first5=Maarten |last6=Bala |first6=Govindasamy |last7=Mach |first7=Katharine J. |last8=Ginzburg |first8=Veronika |last9=de Coninck |first9=Heleen |last10=Patt |first10=Anthony |title=Cross-Working Group Box SRM: Solar Radiation Modification |url=https://www.ipcc.ch/report/ar6/wg2/downloads/report/IPCC_AR6_WGII_Chapter16.pdf |journal=Climate Change 2022: Impacts, Adaptation and Vulnerability |year=2022 |volume=2021 |pages=2473–2478 |doi=10.1017/9781009157896.007|bibcode=2021AGUFM.U13B..05K }} Most importantly, the warming from {{CO2}} emissions lasts for hundreds to thousands of years, while the cooling from SAI stops 1–3 years after the last aerosol injection. This means that neither stratospheric aerosol injection nor other forms of solar geoengineering can be used as a substitute for reducing greenhouse gas emissions because if solar geoengineering were to cease. In contrast, if greenhouse gas levels remained high, it would lead to "large and extremely rapid" warming and similarly abrupt changes to the water cycle. Many thousands of species would likely go extinct as a result. Instead, any solar geoengineering would act as a temporary measure to limit warming. At the same time, emissions of greenhouse gases are reduced and carbon dioxide is removed, which may well take hundreds of years.

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