Research shows that the reduction of aerosol emissions has affected slight, temporary warming.
The closures and the reduced social activity associated with the COVID-19 pandemic affected emissions in a way that warmed the planet for several months last year, according to a new study by the National Center for Atmospheric Research (NCAR).
The counter-intuitive finding emphasizes the influence of air particles, or aerosols, which block incoming sunlight. When aerosol emissions dropped last spring, more of the sun’s heat reached the planet, especially in heavily industrialized countries, such as the United States and Russia, which usually pump high amounts of aerosols into the atmosphere.
“There was a huge decrease in emissions from the most polluting industries, and that had immediate short-term effects on temperature,” said Andrew Gettelman, an NCAR scientist. “Pollution cools the planet, so it makes sense that reducing pollution will warm the planet.”
The temperature during the past spring on parts of the earth’s surface was about 0.2-0.5 degrees Fahrenheit (0.1-0.3 degrees Celsius) warmer than would be expected with the prevailing weather conditions, the study found. The effect was particularly noticeable in regions normally associated with significant aerosol emissions, with temperatures of about 0.37 ° C over much of the United States and Russia.
The new study highlights the complex and often conflicting impacts of different types of emissions from power stations, motor vehicles, industrial facilities and other sources. While aerosols tend to brighten clouds and reflect the sun’s heat back into space, carbon dioxide and other greenhouse gases have the opposite effect, trapping heat near the earth’s surface and raising the temperature.
Despite the short-term warming effects, Gettelman stressed that the long-term effect of the pandemic could be to slow down climate change slightly due to reduced carbon dioxide emissions, which hang in the atmosphere for decades and have a more gradual impact on the climate. In contrast, aerosols – the focus of the new study – have a more immediate impact that disappears within a few years.
The study was published in Geophysical research letters. It was partially funded by the National Science Foundation, the sponsor of NCAR. In addition to NCAR scientists, the study was compiled by scientists at Oxford University, Imperial College and the University of Leeds.
Tease the consequences
Although scientists have long been able to quantify the warming effects of carbon dioxide, the climatic influence of different types of aerosols – sulfates, nitrates, black carbon and dust – has been more difficult to determine. One of the biggest challenges in projecting the extent of future climate change is to estimate the extent to which society will release aerosols in the future and the influence of different types of aerosols on clouds and temperature.
To conduct the research, Gettelman and his co-authors used two of the world’s leading climate models: the NCAR-based Community Earth System Model and a model known as ECHAM-HAMMOZ, developed by a consortium of European countries. They did simulations on both models, to adjust the emission of aerosols and record the actual weather conditions in 2020, like winds.
This approach enabled them to identify the impact of reduced emissions on temperature changes that were too small to bother in actual observations, where it could be obscured by the volatility in atmospheric conditions.
The results showed that the warming effect was strongest in the middle and upper latitudes of the Northern Hemisphere. The effect was mixed in the tropics and relatively small in much of the Southern Hemisphere, where aerosol emissions are not as pervasive.
Gettelman said the study would help scientists better understand the impact of different types of aerosols in different atmospheric conditions, thus helping to minimize efforts to address climate change. Although the research illustrates how aerosols counteract the warming effects of greenhouse gases, he emphasizes that releasing more of them into the lower atmosphere is not a viable strategy to slow down climate change.
“Aerosol emissions have major health consequences,” he said. “To say we have to pollute is not practical.”
Reference: “Climate Impact of COVID – 19-Induced Emission Changes” by A. Gettelman, R. Lamboll, CG Bardeen, Prime Minister Forster and D. Watson – Parris, 29 December 2020, Geophysical research letters.
DOI: 10.1029 / 2020GL091805