[1] 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.
[14] 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.
[23][24] 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.
[29][31][32][33] 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.
[34] 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.
[41] Globally, the single largest source of black carbon is from grassland and forest fires, including both wildfires and intentional burning.
[42] Black carbon in the lower atmosphere is a major contributor to 7 million premature deaths caused by air pollution every year.
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.
[42] Later research found that black carbon is 190 times more effective at absorbing sunlight within clouds than the regular dust from soil particles.
[46][47][48][49][50] 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.
However, even the major eruptions only result in temporary jumps of sulfur particles, unlike the more sustained increases caused by anthropogenic pollution.
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.
[68] 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.
[72] Ultimately, follow-up studies found that a natural change in cloud cover which occurred at the time was sufficient to explain these findings.
[54] Even then, regions with high concentrations of sulfate aerosols due to air pollution had initially experienced cooling, in contradiction to the overall warming trend.
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 less hot if aerosol concentrations had stayed the same as before.
[86][87] 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.
[14] While these values are based on combining model estimates with observational constraints, including those on ocean heat content,[61] the matter is not yet fully settled.
[101] 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.
[105] 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.
Reductions from the stronger air quality policies could exacerbate this expected decline by around 10%, unless methane emissions are reduced by an equivalent amount.
[95] 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.
[10][12][11] However, model simulations of Sahel climate are very inconsistent,[110] so it's difficult to prove that the drought would not have occurred without aerosol pollution, although it would have clearly been less severe.
[111][13] 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.
[8] 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.
[111] 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.
[124][125] 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.
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 1 °C (1.8 °F) 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.
[19] Lower sunlight would affect crop yields and carbon sinks due to reduced photosynthesis,[120] but this would likely be offset by lack of thermal stress from warming and the greater CO2 fertilization effect relative to now.
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.