The lockdown in response to the coronavirus pandemic created a revealing natural experiment. Analyzing satellite data from 2015 to 2019 with the spring of 2020, Professor John P. Wilson of University of Southern California found that reductions in burning fossil fuels caused nitrogen dioxide levels to lower. Aerosol optical depth, a measurement for how much smog and dust impede radiant heat, lowered as well. In short the skies are clearer and more sunlight is touching the Earth. “It’s very intriguing,” said David Wilcove, a conservation biologist at Princeton University. “It potentially shows the complex set of cascading events that can follow from human activity or the cessation of human activity.” Another study, too, found that global carbon dioxide emissions plummeted by about 2.5 billion tons in 2020. Although such responses need to be sustained for such impacts to have real effects, Wilson says, “I take comfort from a natural experiment like this that we can change the world."

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