In the late 1980s and 1990s, scientists (and anyone traveling on airplanes) could see brown hazes hanging over many regions, particularly Eurasian megacities with millions of people. These layers of haze contain a wide range of chemical constituents, including carbon-rich organic aerosols generated by small stoves used for cooking and heating, by biomass burning, and by other activities.
Because dark carbon-rich aerosols absorb the Sun’s radiation, scientists initially suspected that these hazes heated the lower atmosphere and added to the net amplitude of global warming on a regional basis. Investigations in the late 1990s and early 2000s led, however, to a surprisingly different interpretation of these hazes, called brown clouds. The carbon-rich aerosols do absorb radiation, but they heat the layer of air 2-3 km above the surface. As a result, the clouds block a portion of the incoming solar radiation and prevent it from reaching Earth’s surface, which cools. One important consequence of this ongoing decrease in solar radiation has been a reduction of the intensity of the hydrological cycle. With less heating of the land, evaporation has decreased, contributing to greater sub-Saharan drought and a weakened Indian monsoon.
In regions of severe brown-cloud hazes, the reduction in solar radiation during peak seasons is almost an order of magnitude larger than the global average increase from the greenhouse-gas effect. The effects of the brown clouds have also been found to extend thousands of kilometers downwind from source regions.
During the late 1900s and early 2000s, satellite observations detected a related phenomenon called global dimming. The brightness of Earth’s surface viewed from space decreased by roughly 7% over four decades. Part of this trend is connected to the buildup of carbon aerosols in brown clouds. With the brown clouds intercepting more solar radiation, the land received less solar radiation and became dimmer. Other factors include blocking of solar radiation by contrails emitted from jets and other emissions from urban areas.
In several older industrial regions where aerosol emissions have been reduced, such as the closing of polluting industries in the former Soviet Union, the long-term dimming trend has reversed since the early 1990s and the surface is brightening. The dimming trend continues over Southeast Asia, Africa, and other regions where emissions from urbanization and other sources are still growing.
In summary: these findings indicate that part of the true greenhouse-gas warming effect has been masked in recent decades in regions where brown clouds occur.
