“The vegetation basically responded immediately to the change in conditions,” said John P. Wilson, a professor of spatial sciences at the University of Southern California and an author of the study. The findings, Wilson and his co-authors wrote, show that “short-term changes in human activity can have a relatively rapid ecological impact.”
During the lockdown, the study found, there was a reduction in nitrogen dioxide, which is derived from burning fossil fuels and often used as a proxy for greenhouse gas emissions more broadly. Aerosol optical depth, a measure of the degree to which smog and dust impede radiant heat, was also lower. That meant clearer skies and more sunlight reaching Earth’s surface.
To see the ecological response, researchers used satellite data to compare the advent of spring in 2020 with the previous five years (2015 to 2019). The annual seasonal shift, they reported, came 8.4 days earlier than average and included a 17.45 percent increase in leaf area coverage.
“Suddenly, across a big swath of China, emissions were cut off,” Wilson said. “We thought of this as sort of a natural experiment.”
“It’s very intriguing,” said David Wilcove, a conservation biologist at Princeton University who was not involved in the study. “It potentially shows the complex set of cascading events that can follow from human activity or the cessation of human activity.”
Jianguo Wu, an ecologist at Arizona State University, said the study has its shortcomings. Comparing beyond the previous five years, for example, could better account for potential natural fluctuations in spring.
Wilson agreed that a comparison against more years would be ideal but pointed out that data availability was a limiting factor. China has also grown so rapidly in just the past decade, he said, that older data may not necessarily be representative.
Wu also noted that teasing out precise change across such a large country can be complicated. “The specific numbers, to me, are not that important,” he said. But he said the broad conclusions are sound and ecologically plausible. “If radiation increases, then the photosynthetic activity will be greater.”
China is not alone in seeing a steep drop in its emissions amid coronavirus shutdowns. In hard-hit northern Italy, a Washington Post analysis of satellite data found, nitrogen dioxide levels also plummeted. Globally, one study found that carbon dioxide emissions dropped by about 2.5 billion tons through December of last year, the equivalent of taking about 500 million cars off the road. Another study reported a peak drop in daily global emissions of 17 percent in early April 2020.
As for this latest study, Wilcove sees it as an impetus for more research in this area. “What I’d be really keen to see is additional work along these lines in other cities, and see if it emerges as a general pattern,” he said.
Wu agreed, and noted that when short-term changes occur — such as when officials in Beijing took steps to reduce air pollution for the 2008 Summer Olympics — the moves are often only temporary.
“These short-term actions need to be sustained,” he said. “You cannot just do this once. Things get worse again.”
The International Energy Agency says that emissions have “rebounded strongly” across the globe. A recent report from the London-based think tank Ember found that by the end of the year, China was the only major economy to experience an increase in its greenhouse gas emissions in 2020, with a bump of 1.7 percent.
But Wilson said that this study should still be taken as an indicator of what is possible, even in the near term.
“I take comfort from a natural experiment like this that we can change the world,” he said. “What we’re trying to do is motivate people to take those first few steps.”