When countries began going into lockdown last winter and spring, clearer skies from reduced traffic and industry were hailed as a rare bright spot during a difficult time.
But a study published in Geophysical Research Letters in December 2020 shows that those blue skies had an unexpected side effect: They made the Earth slightly warmer.
“There was a big decline in emissions from the most polluting industries, and that had immediate, short-term effects on temperatures,” said Andrew Gettelman, lead author and National Center for Atmospheric Research (NCAR) scientist. “Pollution cools the planet, so it makes sense that pollution reductions would warm the planet.”
Soot and sulfate air pollution had the biggest impact, the study authors explained. Known as aerosols, these types of pollutants release particles into the atmosphere that either scatter sunlight on clear days or brighten clouds, reflecting sunlight. Both of these impacts mean less sunlight reaches Earth and temperatures cool.
In 2020, a reduction of these pollutants warmed global temperatures by about 0.1 to 0.3 degrees Celsius, the press release explained. The effect increased in places with higher aerosol emissions. Temperatures over China, Russia and the U.S. were as much as 0.37 degrees Celsius warmer. All told, aerosol reduction may have contributed to 2020 experiencing one of the warmest years on record, NASA Climate Scientist Gavin Schmidt, who was not involved in the research, told The Associated Press.
To reach their conclusions, the researchers compared the actual weather with climate models reproducing the same conditions without the lockdowns and subsequent emission reductions. This allowed them to calculate the impact of reduced aerosols on temperature changes that were too small to identify based solely on observations, the press release explained.
The study found that aerosol reduction had a bigger impact on 2020 temperatures than the reduction of greenhouse gas emissions such as carbon dioxide. However, that may change in the future. Because carbon dioxide stays in the atmosphere longer, the lockdown dip in greenhouse gases may still slow down the climate crisis in the long term.
Gettelman emphasized that the study’s message is not that we should pollute more.
“Clean air warms the planet a tiny bit, but it kills a lot fewer people with air pollution,” Gettelman told The Associated Press.
Instead, the value of the study involves understanding aerosols’ impact on the climate, according to the press release. This can then help scientists more effectively combat climate change.