COVID-19 lockdowns have caused cleaner air – and warmed the planet

Research shows that reductions in aerosol emissions had a slight, temporary warming effect.

The lockdowns and reduction of societal activity linked COVID-19 The pandemic has affected pollutant emissions in a way that has warmed the planet slightly for several months last year, according to a new study from the National Center for Atmospheric Research (NCAR).

The counterintuitive finding highlights the influence of airborne particles, or aerosols, that block incoming sunlight. When aerosol emissions fell last spring, more of the Sun’s heat reached the planet, especially in heavily industrialized countries, such as the United States and Russia, which normally pump large amounts. aerosols in the atmosphere.

“There has been a sharp drop in emissions from the most polluting industries, and this has had both immediate and short-term effects on temperatures,” said Andrew Gettelman, NCAR scientist and lead author of the study. “Pollution cools the planet, so it makes sense that pollution reductions warm the planet.”

Temperatures over parts of the Earth’s land surface last spring were around 0.2 to 0.5 degrees Fahrenheit (0.1-0.3 degrees Celsius) hotter than expected in prevailing weather conditions, according to the study. The effect was most pronounced in regions that are normally associated with substantial aerosol emissions, with warming reaching about 0.7 ° F (0.37 C) over much of the United States and Russia.

The new study highlights the complex and often conflicting influences of different types of emissions from power plants, motor vehicles, industrial plants and other sources. While aerosols tend to clear clouds and send heat from the Sun back into space, carbon dioxide and other greenhouse gases have the opposite effect, trapping heat near the surface. of the planet and raising temperatures.

Despite the short-term warming effects, Gettelman pointed out that the long-term impact of the pandemic could be to slow climate change slightly due to reduced emissions of carbon dioxide, which persists in the atmosphere for decades. decades and has a more gradual influence on the climate. In contrast, aerosols – at the center of the new study – have a more immediate impact that fades in a few years.

The study was published in Geophysical research letters. It was funded in part by the National Science Foundation, sponsor of NCAR. In addition to scientists from NCAR, the study was co-authored by scientists from the University of Oxford, Imperial College and the University of Leeds.

Tease the impacts

Although scientists have long been able to quantify the effects of warming carbon dioxide, the climatic influence of various types of aerosols – including sulfates, nitrates, carbon black, and dust – has been more difficult to pin down. One of the main challenges in projecting the magnitude of future climate change is to estimate the extent to which society will continue to emit aerosols in the future and the influence of different types of aerosols on clouds and temperature.

To conduct this research, Gettelman and his co-authors used two of the world’s major climate models: the NCAR-based Community Earth System Model and a model known as ECHAM-HAMMOZ, which was developed by a consortium from European countries. They performed simulations on both models, adjusting aerosol emissions and incorporating actual weather conditions in 2020., like the winds.

This approach allowed them to identify the impact of reduced emissions on temperature changes that were too small to be detected in real observations, where they could be obscured by variability in atmospheric conditions.

The results showed that the warming effect was strongest in the middle and upper latitudes of the northern hemisphere. The effect has been mixed in the tropics and relatively minor over much of the southern hemisphere, where aerosol emissions are not as widespread.

Gettelman said the study will help scientists better understand the influence of various types of aerosols under different atmospheric conditions, thus helping to inform efforts to minimize climate change. While research shows how aerosols counter the influence of warming greenhouse gases, he stressed that releasing more of them into the lower atmosphere is not a viable strategy to slow climate change. .

“Aerosol emissions have major ramifications for health,” he said. “Saying that we should pollute is impractical.”

Reference: “Climate Impacts of COVID – 19 Induced Emission Changes” by A. Gettelman, R. Lamboll, CG Bardeen, PM Forster and D. Watson – Parris, December 29, 2020, Geophysical research letters.
DOI: 10.1029 / 2020GL091805