When pandemic-related lockdowns grounded planes and brought car traffic to a near standstill in early 2020, transport emissions plummeted, leading to a drop in levels of a short-lived gas that scrubs methane from the atmosphere. The slump in traffic helped fuel a spike in methane, a new study finds.

Nitrogen oxides, common pollutants from cars and planes, combine with water to form the hydroxyl radical, a chemical that breaks down methane, a highly potent heat-trapping gas. In 2020, transport emissions fell sharply, leading to a drop in levels of the hydroxyl radical.

That same year, methane levels rose by 15 parts per billion, up from 9.7 parts per billion the year before. The drop in transport emissions accounts for most of the growth, an estimated 2.9 parts per billion, modeling showed. The findings were published in the journal Atmospheric Chemistry and Physics.

“Understanding why methane is growing so fast is crucial for devising policies to limit climate change,” David Stevenson, an atmospheric scientist at the University of Edinburgh, said in a statement. “The recent jump in methane’s growth rate is worrying, but a big chunk of the jump in 2020 was probably related to the reduced emissions of other gases, especially nitrogen oxides from transport, during Covid-19 lockdowns.”

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