March 2, 2021 3:59 PM EST

It’s awfully hard to find any upside in a global pandemic that’s sickened nearly 115 million people and killed more than 2.5 million. But throughout 2020, there was some good news buried in the bad concerning that other great infirmity: the sickly state of the earthly climate. When economies are booming and people are moving, greenhouse gasses soar. It follows then that economic slowdowns and global lockdowns would lead energy use and greenhouses gas emissions to plummet—and they did. At least, at first.

According to a report released March 2 by the International Energy Agency, 2020 on the whole saw a total drop-off in global CO2 emissions of 6%—the largest annual decline since World War II—keeping almost 2 billion tons of planet-warming gasses out of the sky. That’s about the same as eliminating the entirety of the European Union’s annual CO2 output.

The bad news is that as the global economy begins to stir—thanks in part to the uneven if undeniable success of lockdowns, social distancing and the availability of vaccines—emissions are on the rise again. In December of 2020, not only had CO2 output rebounded, it actually rose to a level 2% higher than in the same month in 2019. “The rebound in global carbon emissions toward the end of last year is a stark warning that not enough is being done to accelerate clean energy transitions worldwide,” said Fatih Birol, executive director of the IEA, in a statement that accompanied the release of the study. “If governments don’t move quickly with the right energy policies, this could put at risk the world’s historic opportunity to make 2019 the definitive peak in global emissions.”

The overall arc of emissions in 2020 closely mirrored the severity of the pandemic. The global nadir came in April, when worldwide CO2 emissions were roughly 14% lower than they were in April of 2019—just at the point at which most countries were retreating into their first and strictest wave of lockdowns.

Different sources of CO2 were affected in different ways throughout the year. Aviation was down by a staggering 70% during the April low. Overall, emissions from the sector fell by 45% in 2020, the equivalent of taking 100 million cars off the road. Car, bus and motorcycle transportation accounted for 50% of the year’s total drop in CO2 emissions. Demand for oil fell by 8.6% in 2020. Demand for coal was down by 4%.

Geography mattered, too. China, the world’s largest emitter of greenhouse gasses and the first country hit by the pandemic, went into lockdown in February, resulting in its CO2 output falling by 12% compared to the same month a year prior. The country’s swift action and strict quarantine rules allowed it to begin returning to something closer to normal by April, when CO2 emissions in the country rose above 2019 levels. China finished 2020 with overall CO2 output 5% higher than it was in 2019.

India, with its 1.4 billion people, saw its emissions fall by a dizzying 40% in April, the largest decline of any major economy. The slowdown in industrial and freight output kept an estimated 50 million tons of CO2 out of the air, with major Indian cities experiencing clearer skies than they have seen in years. But the good environmental times didn’t last and in September, CO2 output returned to 2019 levels. Brazil similarly saw its output fall by more than 20% in April and was similarly back above 2019 levels by the end of the year.

In general, emerging economies were less likely to reduce their CO2 output, with emissions falling by only 4% there in 2020 compared to 10% in developed economies. The main cause for that difference was the rebound in road transportation in the emerging world in the second half of 2020.

In the U.S., the scattershot, state-by-state lockdown rules meant less dramatic fluctuations in output, with year-over-year emissions drops never reaching 20%, and a 10% reduction overall for 2020. And there, too, emissions approached 2019 levels by December.

One thing the pandemic didn’t seem to affect was the ongoing increase in the share of energy coming from renewable sources. In 2020, renewables accounted for 29% of all energy generated, compared to 27% in 2019—the largest year-over-year increase on record. But the upward drift of fossil fuel use at the end of the year nonetheless discourages experts.

“In March 2020, the IEA urged governments to put clean energy at the heart of their economic stimulus plans to ensure a sustainable recovery,” said Birol. “But our numbers show we are returning to carbon-intensive business-as-usual.”

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Write to Jeffrey Kluger at jeffrey.kluger@time.com.

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