To quantify the change in emissions during shelter-in-place, the researchers looked at Google’s and Apple’s anonymized mobility data from cell phones. As a proxy for human activity, these figures showed how the emissions of greenhouse gases like CO2 and NOx (aka nitrogen oxides, which trail from cars) from 123 countries changed between February and June, 2020. The team found that traffic patterns would indicate that these and other gases fell soemwhere between 10 and 30 percent globally as civilization locked down.
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But why look at mobility data, and not just train a satellite on the planet to measure CO2? The problem is that carbon dioxide is a very long-lived gas in the atmosphere, persisting for hundreds of years. (Compare that to methane, which is a more potent greenhouse gas, but disappears after about a decade.) These few months of lockdown have been but a blip in the atmospheric timescale of CO2. “It takes a very long time for these changes in carbon dioxide to change the concentrations,” says Piers Forster, director of the Priestley International Centre for Climate at the University of Leeds and a lead author on the new paper. (He actually codesigned the study with his daughter, Harriet Forster, a day school student. They began working on the research when her A levels were canceled because of Covid-19.)In a frustrating bit of irony, the reduction in emissions during the pandemic has in a way warmed the planet. While CO2 emissions have fallen, so too have sulfur dioxide (SO2) emissions from the burning of coal, as heavy industries reduced their energy use. This air pollution from power plants normally forms aerosols in the atmosphere, which bounces some of the sun's energy back into space. But once industry dropped off, so did the aerosols, “and that causes a reduction in the amount of sunlight being reflected, and that causes an increase in the temperature” at the surface of Earth, says Forster. “Really, the first effect of reducing emissions is in fact an increase, we think, in the surface temperature.”
That warming, combined with the fact that CO2 lasts so darn long in the atmosphere, means that the lockdown will lead to a minuscule net amount of cooling by 2030. “The temperature change from this lockdown will only be one-hundredth of a degree,” says Harriet Forster. “But the point is what we can do recovering from this. If we invest in green energy, we could really improve our chances of reaching the 1.5 degree target.”