Global dimming
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Global dimming is a decline in the amount of sunlight reaching the Earth's surface.<ref name="AGU2021" /><ref name="Sington2004">Template:Cite news</ref> It is caused by atmospheric particulate matter, predominantly sulfate aerosols, which are components of air pollution.<ref name="Myhre2013" /> Global dimming was observed soon after the first systematic measurements of solar irradiance began in the 1950s. This weakening of visible sunlight proceeded at the rate of 4–5% per decade until the 1980s.<ref name="IPCC_WGI_Ch11">Template:Cite journal</ref> During these years, air pollution increased due to post-war industrialization. Solar activity did not vary more than the usual during this period.<ref name="AGU2021" /><ref name="Eddy1982"/>
Aerosols have a cooling effect on the earth's atmosphere, and global dimming has masked the extent of global warming experienced to date, with the most polluted regions even experiencing cooling in the 1970s.<ref name="IPCC_WGI_Ch11" /><ref name="Wild2007" /> Global dimming has interfered with the water cycle by lowering evaporation, and thus has probably reduced rainfall in certain areas.<ref name="IPCC_WGI_Ch11"/> It may have weakened the Monsoon of South Asia and caused the entire tropical rain belt to shift southwards between 1950 and 1985, with a limited recovery afterwards.<ref name=Lau2006 /><ref name=Fadnavis2021 /><ref name="Peace2022" /> Record levels of particulate pollution in the Northern Hemisphere caused or at least exacerbated the monsoon failure behind the 1984 Ethiopian famine.<ref name="Rotstayn and Lohmann 2002" /><ref name="Hirasawa2022" /><ref name="BBC2020" /><ref name="Herman2020" />
Since the 1980s, a decrease in air pollution has led to a partial reversal of the dimming trend, sometimes referred to as global brightening.<ref name="IPCC_WGI_Ch11" /> This global brightening had contributed to the acceleration of global warming, which began in the 1990s.<ref name="IPCC_WGI_Ch11"/><ref name="Wild2007" /> According to climate models, the dimming effect of aerosols most likely offsets around Template:Convert of warming as of 2021.<ref name="IPCC_WGI_SPM" /> As nations act to reduce the toll of air pollution on the health of their citizens, the masking effect on global warming is expected to decline further.<ref name="XuRamanathanVictor2018" /> The scenarios for climate action required to meet Template:Convert and Template:Convert targets incorporate the predicted decrease in aerosol levels.<ref name="IPCC_WGI_SPM">IPCC, 2021: Summary for Policymakers. In: 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, {{#invoke:doi|main}}.</ref> However, model simulations of the effects of aerosols on weather systems remain uncertain.<ref name="Julsrud2022" /><ref name="Persad2022" />
The processes behind global dimming are similar to stratospheric aerosol injection. This is a proposed solar geoengineering intervention which aims to counteract global warming through intentional releases of reflective aerosols.<ref name="Visioni2020" /> Stratospheric aerosol injection could be very effective at stopping or reversing warming but it would also have substantial effects on the global water cycle, regional weather, and ecosystems. Furthermore, it would have to be carried out over centuries to prevent a rapid and violent return of the warming.<ref name="IPCC_WGI_SRM" />
HistoryEdit
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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> One study showed that less sunlight was filtering through at the height of Template:Cvt above Los Angeles, even on those days when there was no visible smog.<ref>Template:Cite journal</ref> Another suggested that sulfate pollution or a volcano eruption could provoke the onset of an ice age.<ref>Template:Cite journal</ref><ref>Template:Cite book</ref> 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.<ref name="Ohmura and Lang 1989">Template:Cite book</ref><ref name="Wild2009" /> In the 1990s, this was followed by the papers describing multi-decade declines in Estonia,<ref name="Russak1990">Template:Cite journal</ref> Germany,<ref name="Liepert1994">Template:Cite journal</ref> Israel<ref name="Stanhill1992">Template:Cite journal</ref> and across the former Soviet Union.<ref name="Abakumova1996">Template:Cite journal</ref><ref name="Wild2009">Template:Cite journal</ref>
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.<ref name="Stanhill1992"/><ref name="Gilgen1998">Template:Cite journal</ref><ref name="Stanhill2001">Template:Cite journal</ref><ref name="Liepert2002">Template:Cite journal</ref> 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.<ref name="Eddy1982">Template:Cite journal</ref><ref name="AGU2021" /> Additionally, only visible light and infrared radiation were dimmed, rather than the ultraviolet part of the spectrum.<ref>Template:Cite news</ref> 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.<ref>Template:Cite journal</ref><ref name="AGU2021">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Julsrud2022" />
CausesEdit
Anthropogenic sulfatesEdit
Global dimming is primarily caused by the presence of sulfate particles which hang in the Earth's atmosphere as aerosols.<ref>Template:Cite book</ref> These aerosols have both a direct contribution to dimming, as they reflect sunlight like tiny mirrors.<ref name=NASA2010>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref name="Myhre2013">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In models, these smaller droplets also decrease rainfall.<ref name="YunQian">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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.<ref name="IPCC_FAR" /> 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.<ref name="IPCC_FAR" /> 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".<ref name="IPCC_FAR">IPCC, 1990: Chapter 1: Greenhouse Gases and Aerosols [R.T. Watson, H. Rodhe, H. Oeschger and U. Siegenthaler]. In: 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,</ref>
Black carbonEdit
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.<ref>Template:Cite journal</ref> 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.<ref name="Bond2013">Template:Cite journal</ref>
Black carbon in the lower atmosphere is a major contributor to 7 million premature deaths caused by air pollution every year.<ref>Template:Cite journal</ref> 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.<ref name="Bond2013" /> Later research found that black carbon is 190 times more effective at absorbing sunlight within clouds than the regular dust from soil particles.<ref>Template:Cite journal</ref> At worst, all clouds within an atmospheric layer Template:Cvt thick are visibly darkened, and the plume can reach transcontinental scale<ref name="Ramanathan2008">Template:Cite journal</ref> (i.e. the Asian brown cloud.) Even so, the overall dimming from black carbon is much lower than that from the sulfate particles.<ref name="IPCC_WGI_SPM" />
ReversalEdit
After 1990, the global dimming trend had clearly switched to global brightening.<ref name=LightensUp>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Wild2005">Template:Cite journal</ref><ref name="Pinker2005">Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="NASA2007">Template:Cite newsTemplate:Dead link</ref> 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite book</ref> 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.<ref name="EPA">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> By 2010, this reduction in sulfate pollution led to estimated healthcare cost savings valued at $50 billion annually.<ref name="EPAHealth">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Similar measures were taken in Europe,<ref name="EPA"/> 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Later research found that over China, the dimming trend continued at a slower rate after 1990,<ref>Template:Cite journal</ref> and did not begin to reverse until around 2005.<ref>Template:Cite journal</ref> Due to these contrasting trends, no statistically significant change had occurred on a global scale from 2001 to 2012.<ref name="IPCC_WGI_Ch11"/> 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.<ref name="Quaas2022">Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
Relationship to climate changeEdit
Cooling from sulfate aerosolsEdit
Aerosols have a cooling effect, which has masked the total extent of global warming experienced to date.<ref name="Bellouin2019" />
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.<ref name="NASA2007" />
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.<ref name="IPCC_FAR" /> By 1995, the IPCC Second Assessment Report had confidently assessed the overall impact of aerosols as negative (cooling);<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> however, aerosols were recognized as the largest source of uncertainty in future projections in that report and the subsequent ones.<ref name="IPCC_WGI_Ch11" />
Warming from black carbonEdit
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.<ref name="Bond2013" /> 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.<ref>Template:Cite journal</ref>
Though this warming is weaker than the Template:CO2-induced warming or the cooling from sulfates,<ref name="IPCC_WGI_SPM" /> 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.<ref name="Ramanathan2008" /> Black carbon also has an outsized contribution to local warming inside polluted cities.<ref>Template:Cite journal</ref> 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref>Template:Cite journal</ref>
Minor role of aircraft contrailsEdit
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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.<ref>Template:Cite journal</ref>
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.<ref name="Sington2004" /> 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.<ref>Template:Cite news</ref> 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 Template:Cvt – the largest recorded increase in 30 years.<ref>Template:Cite journal</ref> In the southern US, the difference was diminished by about Template:Cvt, and by Template:Cvt in the US midwest.<ref>Template:Citation</ref> This was interpreted by some scientists as a proof of a strong cooling influence of aircraft contrails.<ref>Template:Cite journal</ref>
Ultimately, follow-up studies found that a natural change in cloud cover which occurred at the time was sufficient to explain these findings.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Historical coolingEdit
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.<ref name="EPA"/> Even then, regions with high concentrations of sulfate aerosols due to air pollution had initially experienced cooling, in contradiction to the overall warming trend.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The eastern United States was a prominent example: the temperatures there declined by Template:Convert between 1970 and 1980, and by up to Template:Convert in the Arkansas and Missouri.<ref name=":0">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Brightening and accelerated warmingEdit
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.<ref name="IPCC_WGI_Ch11" /> 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.<ref name="Wild2007">Template:Cite journal</ref><ref name="Kerr 2007">Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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.<ref>Template:Cite journal</ref> Because aerosol cooling depends on reflecting sunlight, air quality improvements had a negligible impact on wintertime temperatures,<ref>Template:Cite journal</ref> but had increased temperatures from April to September by around Template:Convert in Central and Eastern Europe.<ref name="Glantz2022">Template:Cite journal</ref> The central and eastern United States experienced warming of Template:Convert between 1980 and 2010 as sulfate pollution was reduced,<ref name=":0" /> even as sulfate particles still accounted for around 25% of all particulates.<ref name="EPAHealth" /> 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.<ref>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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 Template:Convert initially and up to Template:Convert by 2023, before disappearing. Regionally, the lockdowns were estimated to increase temperatures by Template:Convert in eastern China over January–March, and then by Template:Convert over Europe, eastern United States, and South Asia in March–May, with the peak impact of Template:Convert in some regions of the United States and Russia.<ref>Template:Cite journal</ref><ref name="Yang2020">Template:Cite journal</ref> In the city of Wuhan, the urban heat island effect was found to have decreased by Template:Convert at night and by Template:Convert overall during the strictest lockdowns.<ref>Template:Cite journal</ref>
FutureEdit
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.<ref name="XuRamanathanVictor2018" /><ref name="Bellouin2019" />
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.<ref name="NASA2010" /> By 2021, CMIP6 models estimated total aerosol cooling in the range from Template:Convert to Template:Convert;<ref>Template:Cite journal</ref> The IPCC Sixth Assessment Report selected the best estimate of a Template:Convert cooling provided by sulfate aerosols, while black carbon amounts to about Template:Convert of warming.<ref name="IPCC_WGI_SPM" /> While these values are based on combining model estimates with observational constraints, including those on ocean heat content,<ref name="Quaas2022" /> the matter is not yet fully settled. The difference between model estimates mainly stems from disagreements over the indirect effects of aerosols on clouds.<ref name="Andrew">Template:Cite news</ref><ref>Template:Cite journal</ref>
Regardless of the current strength of aerosol cooling, all future climate change scenarios project decreases in particulates and this includes the scenarios where Template:Convert and Template:Convert targets are met: their specific emission reduction targets assume the need to make up for lower dimming.<ref name="IPCC_WGI_SPM"/> 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.<ref name="CB2021">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> <ref name="Hassan2022">Template:Cite journal</ref> Yet, in the recent years, methane concentrations had been increasing at rates exceeding their previous period of peak growth in the 1980s,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite magazine</ref> with wetland methane emissions driving much of the recent growth,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> while air pollution is getting cleaned up aggressively.<ref name="Quaas2022"/> These trends are some of the main reasons why Template:Convert warming is now expected around 2030, as opposed to the mid-2010s estimates where it would not occur until 2040.<ref name="XuRamanathanVictor2018">Template:Cite journal</ref>
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.<ref name=Persad2022>Template:Cite journal</ref> For instance, a climate change scenario with high greenhouse gas emissions but strong reductions in air pollution would see Template:Convert 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.<ref>Template:Cite journal</ref> 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.<ref name="Luo2020">Template:Cite journal</ref>
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,<ref name="Julsrud2022"/> 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.<ref name="Wang2021">Template:Cite journal</ref><ref name="Julsrud2022">Template:Cite journal</ref><ref name="Ramachandran2022">Template:Cite journal</ref>
Relationship with water cycleEdit
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.<ref>Template:Cite journal</ref> 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."<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite news</ref>
In 2011, it was found that anthropogenic aerosols had been the predominant factor behind 20th century changes in rainfall over the Atlantic Ocean sector,<ref>Template:Cite journal</ref> when the entire tropical rain belt shifted southwards between 1950 and 1985, with a limited northwards shift afterwards.<ref name="Peace2022">Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref> 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.<ref name="Hassan2022" />
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.<ref name="Rotstayn and Lohmann 2002">Template:Cite journal</ref><ref name="BBC2020">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Hirasawa2022">Template:Cite journal</ref> However, model simulations of Sahel climate are very inconsistent,<ref>Template:Cite journal</ref> so it's difficult to prove that the drought would not have occurred without aerosol pollution, although it would have clearly been less severe.<ref name=gbd1>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Herman2020">Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
Another dramatic finding had connected the impact of aerosols with the weakening of the Monsoon of South Asia. It was first advanced in 2006,<ref name=Lau2006>Template:Cite journal</ref> yet it also remained difficult to prove.<ref>Template:Cite journal</ref> In particular, some research suggested that warming itself increases the risk of monsoon failure, potentially pushing it past a tipping point.<ref>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> By 2021, however, it was concluded that global warming consistently strengthened the monsoon,<ref>Template:Cite journal</ref> and some strengthening was already observed in the aftermath of lockdown-caused aerosol reductions.<ref name=Fadnavis2021>Template:Cite journal</ref>
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.<ref name="YunQian" /> The phenomenon of aerosols suppressing rainfall through reducing cloud droplet size has been confirmed by subsequent studies.<ref>Template:Cite journal</ref> 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.<ref name="Xie2022">Template:Cite journal</ref> 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.<ref name="Risser2024">Template:Cite journal</ref> The IPCC Sixth Assessment Report had also linked changes in aerosol concentrations to altered precipitation in the Mediterranean region.<ref name="IPCC_WGI_Ch11"/>
Relevance for solar geoengineeringEdit
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Global dimming is also relevant for certain proposals about slowing, halting, or reversing global warming.<ref name="Gramling2018">Template:Cite news</ref> 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.<ref name="RoyalSociety2009">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref name="Lawrence2016">Template:Cite journal</ref>
Because the historical levels of global dimming were associated with high mortality from air pollution and issues such as acid rain,<ref name="Ramanathan2006">Template:Cite journal</ref> the concept of relying on cooling directly from pollution has been described as a "Faustian bargain" and is not seriously considered by modern research.<ref name=gbd1/> 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.<ref>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref name="Lawrence2016" /> 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.<ref name="Visioni2020" />
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.<ref name="Lawrence2016" /> It has been estimated that the amount of sulfur needed to offset a warming of around Template:Convert relative to now (and Template:Convert 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.<ref name="Visioni2020">Template:Cite journal</ref> 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 Template:Convert 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.<ref name="Smith2020" >Template:Cite journal</ref><ref>Template:Cite journal</ref> Even so, these approaches would still be "orders of magnitude" cheaper than greenhouse gas mitigation,<ref>Template:Cite journal</ref> let alone the costs of unmitigated effects of climate change.<ref name="RoyalSociety2009" />
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.<ref name="IPCC_WGI_SRM"/> Lower sunlight would affect crop yields and carbon sinks due to reduced photosynthesis,<ref name="Gramling2018" /> but this would likely be offset by lack of thermal stress from warming and the greater CO2 fertilization effect relative to now.<ref name="IPCC_WGI_SRM">Template:Cite journal</ref> Most importantly, the warming from Template: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.<ref name="IPCC_WGI_SRM" />
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