Do humans also exert a cooling influence on Earth's climate?

January 23, 2014

Yes, human activities exert a cooling influence on Earth in several ways. Overall, this cooling influence is smaller than the warming influence of the heat-trapping gases humans put into the air.

Particulate pollution (aerosols) is humanity's greatest cooling influence. Plumes of aerosols are produced by: power plants and large-scale industrial processes; smoke and gases from biomass burning; windblown dust from deforested areas, dried wetlands, and crop fields; and exhaust from ships, cars, trucks, buses, and trains. Most aerosol particles scatter the sun's rays back to space, thereby directly exerting a cooling influence by reducing the amount of sunlight reaching the surface. Aerosols also have an indirect cooling influence by producing brighter white (more reflective) and longer-lived clouds that reduce the amount of sunlight reaching the surface. Aerosols' indirect cooling influence via clouds contributes twice as much cooling as their direct cooling influence outside of clouds. But to keep aerosols' cooling influence in proper perspective, their cooling influence is only about one-third as large as the current warming effect of human-produced heat-trapping gases.

Aerosols along the Himalaya

Aerosols can result from natural processes such as dust storms and lightning-sparked wildfires. Aerosols can also result from human activities, everything from cooking fires to industrial smokestacks. This image, captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on the NASA Terra satellite on December 6, 2018, shows a natural-color image of smoke hugging the southern face of the Himalaya. Superimposed on the natural-color image is a MODIS measurement of aerosol optical depth (AOD). AOD values over 3 indicate aerosols dense enough to obscure sunlight.

Whereas aerosols linger in the atmosphere from days to a few weeks, heat-trapping gases that humans release to the atmosphere linger from decades to centuries. Plus, when it was recognized that human particle pollution causes other undesired harmful side effects—such as acid rain and human respiratory diseases and deaths—the United States and other nations adopted regulations to reduce emissions of some aerosols.

Another measurable way in which humans exert a cooling influence is by changing land cover over large areas in ways that increase the land's reflectance, thereby reducing the amount of sunlight absorbed. By comparison, the cooling influence of humanity's land cover changes only offsets about 5% of the warming caused by human-emitted heat-trapping gases.


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