The past half-century has seen remarkable improvements in air quality in many parts of the world, thanks largely to legislation like the U.S. Clean Air Act. Efforts like these took aim at pollutants like the group of chemicals known as aerosols, which include sulfur dioxide, nitrogen dioxide, and other compounds that are harmful to human health.

Like greenhouse gases, aerosols are produced by cars, factories, and power plants — but unlike greenhouse gases, they make the earth cooler rather than warmer. This is because aerosols reflect the sun’s rays, rather than trapping its heat like carbon. Some studies estimate that, without aerosol pollution, the world might have already warmed by another half a degree Celsius. 

This creates a tricky paradox, which renowned climate scientist James Hansen has called a “Faustian bargain.” If you remove aerosols from the air, you …

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