COVID-19 cleared the skies but also supercharged methane emissions

During the spring of 2020, the COVID-19 pandemic caused a near-total shutdown of industries and travel worldwide, leading to a significant drop in nitrogen dioxide levels. This reduction, primarily from vehicles and heavy industries, resulted in some of the cleanest air seen in decades. However, alongside this temporary improvement in air quality, an unexpected phenomenon emerged: a sharp increase in methane emissions. According to a new study published in the journal Science, methane—the second most significant greenhouse gas after carbon dioxide—experienced a growth rate of 16.2 parts per billion in 2020, marking the highest annual increase since the early 1980s. Researchers investigated the intricate chemical processes in the troposphere, the lowest layer of the atmosphere, to understand the connection between the decline in nitrogen dioxide and the rise in methane. Since the late 1960s, scientists have known that atmospheric methane does not simply dissipate. Instead, it is broken down by hydroxyl radicals, highly reactive molecules that convert methane into harmless water vapor and carbon dioxide. Shushi Peng, a co-author of the study from Peking University, explained that hydroxyl radicals have a very short lifespan—less than one second. These radicals are regenerated through chemical reactions initiated by sunlight, which require nitrogen oxides as a critical component. With the 2020 lockdowns significantly reducing nitrogen oxide emissions by approximately 15 to 20 percent globally, the formation of hydroxyl radicals slowed dramatically. As a result, methane molecules that would typically be eliminated from the atmosphere lingered longer, contributing to the observed spike in emissions. Peng and his team estimate that the decrease in the effectiveness of this methane-clearing process was responsible for about 80 percent of the increase in methane levels during that year. Thus, as air quality improved, methane remained in the atmosphere for extended periods, exacerbating its warming effects on the planet.