Global dimming
Global dimming is a decline in the amount of sunlight reaching the Earth's surface.[2][3] It is caused by atmospheric particulate matter, predominantly sulfate aerosols, which are components of air pollution.[4] 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.[1] During these years, air pollution increased due to post-war industrialization. Solar activity did not vary more than the usual during this period.[5][2]
As aerosols have a cooling effect, global dimming has masked the extent of global warming experienced to date, with the most-polluted regions even experiencing cooling in the 1970s.[1][6] Global dimming has interfered with the water cycle by lowering evaporation, and thus has probably reduced rainfall in certain areas.[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.[7][8][9] Record levels of particulate pollution in the Northern Hemisphere caused or at least exacerbated the monsoon failure behind the 1984 Ethiopian famine.[10][11][12][13]
Since the 1980s, a decrease in air pollution has led to a partial reversal of the dimming trend, sometimes referred to as global brightening.[1] This global brightening had contributed to the acceleration of global warming which began in the 1990s.[1][6] According to climate models, the dimming effect of aerosols most likely offsets around 0.5 °C (0.9 °F) of warming as of 2021.[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.[15] The scenarios for climate action required to meet 1.5 °C (2.7 °F) and 2 °C (3.6 °F) targets incorporate the predicted decrease in aerosol levels.[14] However, model simulations of the effects of aerosols on weather systems remain uncertain.[16][17]
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.[18] 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.[19]