Editing Cloud feedback (section)

== Possible break-up of equatorial stratocumulus clouds ==
{{See also|Tipping points in the climate system}}
In 2019, a study employed a [[large eddy simulation]] model to estimate that equatorial [[stratocumulus cloud]]s could break up and scatter when [[carbon dioxide|{{CO2}}]] levels rise above 1,200 [[Parts per million|ppm]] (almost three times higher than the current levels, and over 4 times greater than the preindustrial levels). The study estimated that this would cause a surface warming of about {{convert|8|C-change|F-change}} globally and {{convert|10|C-change|F-change}} in the subtropics, which would be in addition to at least {{convert|4|C-change|F-change}} already caused by such {{CO2}} concentrations. In addition, stratocumulus clouds would not reform until the {{CO2}} concentrations drop to a much lower level.<ref>{{Cite journal |last1=Schneider |first1=Tapio |last2=Kaul |first2=Colleen M. |last3=Pressel |first3=Kyle G. |date=2019 |title=Possible climate transitions from breakup of stratocumulus decks under greenhouse warming |journal=Nature Geoscience |volume=12 |issue=3 |pages=163–167 |doi=10.1038/s41561-019-0310-1|bibcode=2019NatGe..12..163S |s2cid=134307699 }}</ref> It was suggested that this finding could help explain past episodes of unusually rapid warming such as [[Paleocene-Eocene Thermal Maximum]].<ref>{{cite web |date=25 February 2019 |url=https://www.quantamagazine.org/cloud-loss-could-add-8-degrees-to-global-warming-20190225/|title=A World Without Clouds|first=Natalie|last=Wolchover|website=[[Quanta Magazine]] |access-date=2 October 2022}}</ref> In 2020, further work from the same authors revealed that in their large eddy simulation, this [[Tipping_points_in_the_climate_system|tipping point]] cannot be stopped with [[solar radiation modification]]: in a hypothetical scenario where very high {{CO2}} emissions continue for a long time but are offset with extensive solar radiation modification, the break-up of stratocumulus clouds is simply delayed until {{CO2}} concentrations hit 1,700 ppm, at which point it would still cause around {{convert|5|C-change|F-change}} of unavoidable warming.<ref>{{Cite journal |last1=Schneider |first1=Tapio |last2=Kaul |first2=Colleen M. |last3=Pressel |first3=Kyle G. |date=2020 |title=Solar geoengineering may not prevent strong warming from direct effects of {{CO2}} on stratocumulus cloud cover |journal=PNAS |volume=117 |issue=48 |pages=30179–30185 |doi=10.1073/pnas.2003730117|pmid=33199624 |pmc=7720182 |bibcode=2020PNAS..11730179S |doi-access=free }}</ref>

However, because large eddy simulation models are simpler and smaller-scale than the [[general circulation model]]s used for climate projections, with limited representation of atmospheric processes like [[Subsidence (atmosphere)|subsidence]], this finding is currently considered speculative.<ref name="CB" /> Other scientists say that the model used in that study unrealistically extrapolates the behavior of small cloud areas onto all cloud decks, and that it is incapable of simulating anything other than a rapid transition, with some comparing it to "a knob with two settings".<ref>{{cite news |url=https://www.science.org/content/article/world-without-clouds-hardly-clear-climate-scientists-say |title=A world without clouds? Hardly clear, climate scientists say |date=February 26, 2019|website=Science Magazine |first=Paul |last=Voosen}}</ref> Additionally, {{CO2}} concentrations would only reach 1,200 ppm if the world follows [[Representative Concentration Pathway]] 8.5, which represents the highest possible greenhouse gas emission scenario and involves a massive expansion of [[coal]] infrastructure. In that case, 1,200 ppm would be passed shortly after 2100.<ref name="CB">{{Cite web |date=25 February 2019 |title=Extreme {{CO2}} levels could trigger clouds 'tipping point' and 8C of global warming |url=https://www.carbonbrief.org/extreme-co2-levels-could-trigger-clouds-tipping-point-and-8c-of-global-warming/ |access-date=2 October 2022 |website=[[Carbon Brief]]}}</ref>