Editing Cloud feedback (section)

== Role of aerosols ==
[[File:Bellouin_2019_aerosol_cloud_interactions.jpg|thumb|Air pollution, including from large-scale land clearing, has substantially increased the presence of aerosols in the atmosphere when compared to the preindustrial background levels. Different types of particles have different effects, and there is a variety of interactions in different atmospheric layers. Overall, they provide cooling, but complexity makes the exact strength of cooling very difficult to estimate.<ref name="Bellouin2019">{{cite journal |last1=Bellouin |first1=N. |last2=Quaas |first2=J. |last3=Gryspeerdt |first3=E. |last4=Kinne |first4=S. |last5=Stier |first5=P. |last6=Watson-Parris |first6=D. |last7=Boucher |first7=O. |last8=Carslaw |first8=K. S. |last9=Christensen |first9=M. |last10=Daniau |first10=A.-L. |last11=Dufresne |first11=J.-L. |last12=Feingold |first12=G. |last13=Fiedler |first13=S. |last14=Forster |first14=P. |last15=Gettelman |first15=A. |last16=Haywood |first16=J. M. |last17=Lohmann |first17=U. |last18=Malavelle |first18=F. |last19=Mauritsen |first19=T. |last20=McCoy |first20= D. T. |last21=Myhre |first21=G. |last22=Mülmenstädt |first22=J. |last23=Neubauer |first23=D. |last24=Possner |first24=A. |last25=Rugenstein |first25=M. |last26=Sato |first26=Y. |last27=Schulz |first27=M. |last28=Schwartz |first28=S. E. |last29=Sourdeval |first29=O. |last30=Storelvmo |first30= T. |last31=Toll |first31=V. |last32=Winker |first32=D. |last33=Stevens |first33=B. |date=1 November 2019 |title=Bounding Global Aerosol Radiative Forcing of Climate Change |journal=Reviews of Geophysics |volume=58 |issue=1 |page=e2019RG000660 |doi=10.1029/2019RG000660 |pmid=32734279 |pmc=7384191 }}</ref>]]

Atmospheric [[aerosol]]s—fine partices suspended in the air—affect cloud formation and properties, which also alters their impact on climate. While some aerosols, such as [[black carbon]] particles, make the clouds darker and thus contribute to warming,<ref>{{cite journal| title=Nature Geoscience: Global and regional climate changes due to black carbon|journal=Nature Geoscience| volume=1| issue=4| pages=221–227| doi=10.1038/ngeo156| year=2008| last1=Ramanathan| first1=V.| last2=Carmichael| first2=G.| s2cid=12455550| bibcode=2008NatGe...1..221R}}</ref> by far the strongest effect is from [[sulfate]]s, which increase the number of cloud droplets, making the clouds more reflective, and helping them cool the climate more. That is known as a ''direct'' aerosol effect; however, aerosols also have an ''indirect'' effect on [[liquid water path]], and determining it involves computationally heavy continuous calculations of evaporation and condensation within clouds. Climate models generally assume that aerosols increase liquid water path, which makes the clouds even more reflective.<ref name="McCoy2020" /> However, satellite observations taken in 2010s suggested that aerosols decreased liquid water path instead, and in 2018, this was reproduced in a model which integrated more complex cloud microphysics.<ref>{{cite journal |last1=Sato |first1=Yousuke |last2=Goto |first2=Daisuke |last3=Michibata |first3=Takuro |last4=Suzuki |first4=Kentaroh |last5=Takemura |first5=Toshihiko |last6=Tomita |first6=Hirofumi |last7=Nakajima |first7=Teruyuki |date=7 March 2018 |title=Aerosol effects on cloud water amounts were successfully simulated by a global cloud-system resolving model |journal=Nature Communications |volume=9 |issue=1 |page=985 |doi=10.1038/s41467-018-03379-6 |pmid=29515125 |pmc=5841301 |doi-access = free |bibcode=2018NatCo...9..985S }}</ref> Yet, 2019 research found that earlier satellite observations were biased by failing to account for the thickest, most water-heavy clouds naturally raining more and shedding more particulates: very strong aerosol cooling was seen when comparing clouds of the same thickness.<ref>{{cite journal | last1 = Rosenfeld | first1 = Daniel | last2 = Zhu | first2 = Yannian | last3 = Wang | first3 = Minghuai | last4 = Zheng | first4 = Youtong | last5 = Goren | first5 = Tom | last6 = Yu | first6 = Shaocai | year = 2019 | title = Aerosol-driven droplet concentrations dominate coverage and water of oceanic low level clouds | url = https://authors.library.caltech.edu/92390/2/aav0566_Rosenfeld_SM.pdf| journal = Science | volume =  363| issue = 6427| page =  eaav0566| doi = 10.1126/science.aav0566 | pmid = 30655446 | s2cid = 58612273 | doi-access = free }}</ref>

Moreover, large-scale observations can be confounded by changes in other atmospheric factors, like humidity: i.e. it was found that while post-1980 improvements in air quality would have reduced the number of clouds over the [[East Coast of the United States]] by around 20%, this was offset by the increase in relative humidity caused by atmospheric response to [[AMOC]] slowdown.<ref name="Cao2021">{{cite journal |last1=Cao |first1=Yang |last2=Wang |first2=Minghuai |last3=Rosenfeld |first3=Daniel |last4=Zhu |first4=Yannian |last5=Liang |first5=Yuan |last6=Liu |first6=Zhoukun |last7=Bai |first7=Heming |date=10 March 2021 |title=Strong Aerosol Effects on Cloud Amount Based on Long-Term Satellite Observations Over the East Coast of the United States |journal=Geophysical Research Letters | volume=48 |issue=6 | page=e2020GL091275 |doi=10.1029/2020GL091275 |doi-access = free |bibcode=2021GeoRL..4891275C }}</ref> Similarly, while the initial research looking at sulfates from the [[2014–2015 eruption of Bárðarbunga]] found that they caused no change in liquid water path,<ref>{{Cite journal |last1=Malavelle |first1=Florent F. |last2=Haywood |first2=Jim M. |last3=Jones |first3=Andy |last4=Gettelman |first4=Andrew |last5=Clarisse |first5=Lieven |last6=Bauduin |first6=Sophie |last7=Allan |first7=Richard P. |last8=Karset |first8=Inger Helene H. |last9=Kristjánsson |first9=Jón Egill |last10=Oreopoulos |first10=Lazaros |last11=Cho |first11=Nayeong |last12=Lee |first12=Dongmin |last13=Bellouin |first13=Nicolas |last14=Boucher |first14=Olivier |last15=Grosvenor |first15=Daniel P. |last16=Carslaw |first16=Ken S. |last17=Dhomse |first17=Sandip |last18=Mann |first18=Graham W. |last19=Schmidt |first19=Anja |last20=Coe |first20=Hugh |last21=Hartley |first21=Margaret E. |last22=Dalvi |first22=Mohit |last23=Hill |first23=Adrian A. |last24=Johnson |first24=Ben T. |last25=Johnson |first25=Colin E. |last26=Knight |first26=Jeff R. |last27=O'Connor |first27=Fiona M. |last28=Partridge |first28=Daniel G. |last29=Stier |first29=Philip |last30=Myhre |first30=Gunnar |last31=Platnick |first31=Steven |last32=Stephens |first32=Graeme L. |last33=Takahashi |first33=Hanii |last34=Thordarson |first34=Thorvaldur |date=22 June 2017 |title=Strong constraints on aerosol–cloud interactions from volcanic eruptions |journal=Nature |volume=546 |issue=7659 |pages=485–491 |language=en |doi=10.1038/nature22974 |pmid=28640263 |bibcode=2017Natur.546..485M |s2cid=205257279 |hdl=10871/28042 |hdl-access=free }}</ref> it was later suggested that this finding was confounded by counteracting changes in humidity.<ref name="Cao2021"/> 
[[File:ShipTracks.jpg|thumb|left|Visible ship tracks in the Northern Pacific, on 4 March 2009]]
To avoid confounders, many observations of aerosol effects focus on [[ship tracks]], but post-2020 research found that visible ship tracks are a poor proxy for other clouds, and estimates derived from them overestimate aerosol cooling by as much as 200%.<ref>{{cite journal | last1=Glassmeier |first1=Franziska |last2=Hoffmann |first2=Fabian |last3=Johnson |first3=Jill S. |last4=Yamaguchi |first4=Takanobu |last5=Carslaw |first5=Ken S. |last6=Feingold |first6=Graham | date=29 January 2021 |title=Aerosol-cloud-climate cooling overestimated by ship-track data | journal=Science |volume =371 |issue=6528 |pages=485–489 |doi=10.1126/science.abd3980 |pmid=33510021 |doi-access = free |bibcode=2021Sci...371..485G }}</ref> At the same time, other research found that the majority of ship tracks are "invisible" to satellites, meaning that the earlier research had underestimated aerosol cooling by overlooking them.<ref>{{cite journal |last1=Manshausen |first1=Peter |last2=Watson-Parris |first2=Duncan |last3=Christensen |first3=Matthew W. |last4=Jalkanen |first4=Jukka-Pekka |last5=Stier  |first5=Philip Stier |date=7 March 2018 |title=Invisible ship tracks show large cloud sensitivity to aerosol |journal=Nature |volume=610 |issue=7930 |pages=101–106 |doi=10.1038/s41586-022-05122-0 |pmid=36198778 |pmc=9534750 |doi-access=free }}</ref> Finally, 2023 research indicates that all climate models have underestimated sulfur emissions from volcanoes which occur in the background, outside of major eruptions, and so had consequently overestimated the cooling provided by anthropogenic aerosols, especially in the Arctic climate.<ref>{{cite journal |last1=Jongebloed |first1=U. A. |last2=Schauer |first2=A. J. |last3=Cole-Dai |first3=J. |last4=Larrick |first4=C. G. |last5=Wood |first5=R. |last6=Fischer |first6=T. P. |last7=Carn |first7=S. A. |last8=Salimi |first8=S. |last9=Edouard |first9=S. R. |last10=Zhai |first10=S. |last11=Geng |first11=L. |last12=Alexander |first12=B. |title=Underestimated Passive Volcanic Sulfur Degassing Implies Overestimated Anthropogenic Aerosol Forcing | date=2 January 2023 |journal=Geophysical Research Letters | volume=50 |issue=1 |pages=e2022GL102061 |doi=10.1029/2022GL102061 |s2cid=255571342 |doi-access=free |bibcode=2023GeoRL..5002061J }}</ref>
[[File:Estimates of past and future SO2 global anthropogenic emissions.png|thumb|upright=1.25|Early 2010s estimates of past and future anthropogenic global sulfur dioxide emissions, including the [[Representative Concentration Pathway]]s. 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.<ref name="XuRamanathanVictor2018">{{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}}</ref>]]
Estimates of how much aerosols affect cloud cooling are very important, because the amount of sulfate aerosols in the air had undergone dramatic changes in the recent decades. First, it had increased greatly from 1950s to 1980s, largely due to the widespread burning of [[sulfur]]-heavy coal, which caused an observable reduction in visible sunlight that had been described as [[global dimming]].<ref name="AGU2021" /><ref name="Julsrud2022" /> Then, it started to decline substantially from the 1990s onwards and is expected to continue to decline in the future, due to the measures to combat [[acid rain]] and other impacts of [[air pollution]].<ref name="EPA">{{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=[[United States Environmental Protection Agency|U.S. Environmental Protection Agency]] | date=8 July 2014}}</ref> Consequently, the aerosols provided a considerable cooling effect which counteracted or "masked" some of the [[greenhouse effect]] from human emissions, and this effect had been declining as well, which contributed to acceleration of [[climate change]].<ref name="IPCC_WGI_SPM">IPCC, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM.pdf Summary for Policymakers]. In: [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 3–32, {{doi|10.1017/9781009157896.001}}.</ref>

Climate models do account for the presence of aerosols and their recent and future decline in their projections, and typically estimate that the cooling they provide in 2020s is similar to the warming from human-added [[atmospheric methane]], meaning that simultaneous reductions in both would effectively cancel each other out.<ref name="CB2021">{{cite web|url=https://www.carbonbrief.org/explainer-will-global-warming-stop-as-soon-as-net-zero-emissions-are-reached |title=Explainer: Will global warming 'stop' as soon as net-zero emissions are reached? |author=Zeke Hausfather|publisher=[[Carbon Brief]]|date= 29 April 2021 |access-date=2023-03-23}}</ref> However, the existing uncertainty about aerosol-cloud interactions likewise introduces uncertainty into models, particularly when concerning predictions of changes in weather events over the regions with a poorer historical record of atmospheric observations.<ref name="Wang2021">{{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 |doi=10.1029/2021JD035476 |doi-access=free |hdl=10852/97300 |hdl-access=free }}</ref><ref name="Julsrud2022">{{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 }}</ref><ref name=Persad2022>{{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 }}</ref><ref name="Ramachandran2022">{{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 }}</ref>