Science Briefs

COVID-19 Lockdown Effects on Climate Appear Limited and Short-Lived

As COVID-19 took hold in China and then spread across the rest of the world in early 2020, the day-to-day lives of people took a dramatic turn. Many governments imposed policies restricting the operations of businesses, domestic and international travel, and even when people were permitted to leave their homes, while many businesses required their employees to work from home. Some industries suffered as a result, while others prospered; almost universally, people were driving and flying less, at least for a time.

These changes had an intriguing effect: it seemed the air was cleaner. In Nairobi, Kenya, people were able to see as far as Mt. Kenya and Mt. Kilimanjaro. And in New Delhi, India, an Air Quality Index that was typically an extremely unhealthy 200 or higher fell below 20. Satellites and surface air quality monitors saw distinct reductions in the amounts of particulate matter (PM) — also known as aerosols — and lower-atmospheric ozone pollution in many parts of the world. In some cases, the improvements were strongly influenced by meteorological patterns and longer-term trends, but the COVID lockdowns were also having an effect, often reducing concentrations of pollutants by 10-60%, and emissions of carbon dioxide by about 8%.

Maps of aerosols in the atmosphere over India during March 31-April 5 in 2019 and 2020

Fig. 1: The maps above show aerosol optical depth measured in the atmosphere over India from March 31 to April 5 in pre-lockdown 2019 (left) and during lockdown 2020 (center), as well as the anomaly (right) in 2020 compared to the 2016-2019 average. Although there were decreases in 2020 across most of the region, they are especially notable in the decline along the Ganges River valley. (Credit: NASA/GSFC/EO/Joshua Stevens using Terra MODIS data analysis by NASA/USRA/Pawan Gupta)

Decreasing pollution is good news for human health, and it can have surprising implications for climate. Ozone is a greenhouse gas, so having less of it in the lower atmosphere will tend to have a cooling effect. However, although some aerosols can warm the atmosphere, most of them reflect solar radiation back into space, preventing it from reaching the Earth's surface and subsequently warming the atmosphere. So the decreases in aerosol pollution that occurred during COVID-19 lockdowns could actually have had a warming effect on climate. But unlike carbon dioxide, ozone and aerosols only stay in the atmosphere for hours to weeks, and only have short-term effects on climate.

In order to understand how changes to greenhouse gas emissions and air pollution resulting from COVID-19 restrictions may be affecting climate in the near-term, researchers at the NASA Goddard Institute for Space Studies (GISS) and institutions in North America, Europe, Asia, and Australia launched the COVID Model Intercomparison Project (CovidMIP). It often requires years of effort to plan and conduct a model intercomparison, but CovidMIP came together in just a few months, and published its first results in Geophysical Research Letters this month. So far, 12 different Earth system models, including the GISS ModelE, have participated.

In the initial CovidMIP experiments, each model conducted two sets of climate simulations for 2020 through 2024. One set included emissions reductions due to COVID-19, in which emissions decrease during the first half of 2020, and then slowly return to normal levels by the beginning of 2022. The other set did not include such reductions.

Fig. 2: The above maps compare the column depth of nitrogen dioxide (NO2), an ozone precursor, in the atmosphere above the US Mid-Atlantic states during March 2015-2019 and in March 2020. The top image shows the average from 2015 to 2019. The bottom image shows measurements during March 2020, during which half of the period much the region was under COVID lockdown. (Credit: NASA/GSFC/SVS/Schindler et al. Also see the related NASA portal feature.)

As a result of these emission changes, all the models had lower aerosol concentrations, which allowed more solar radiation to reach the Earth's surface during 2020 and 2021. However, there was no detectable effect on the global surface climate — no model simulated significant changes in either global surface air temperature or precipitation. This may be in part because any warming caused by increased solar radiation may have been offset by reductions in ozone pollution, causing less of that heat to be trapped in the atmosphere.

When considered regionally instead of globally, COVID-19 may have caused some small climate changes. Most models simulated a very small warming — of only 0.1°C — over southern and eastern Asia. But when it comes to other regions or to precipitation, the models didn't show any consistent changes. In all likelihood, there is too much natural variability in the models for a consistent impact on precipitation and temperature to be detected in most regions.

The small effect of COVID-19 lockdowns on climate shouldn't be surprising. The emissions reductions simply weren't large enough and long enough — remember, it was assumed that they returned to normal levels in 2022 — to have a big impact. It's also important to remember that even if the amount of carbon dioxide in the atmosphere didn't grow as quickly in 2020 as expected, it still grew.

So far, researchers have only scratched the surface of the CovidMIP results, and future research will look in more detail at regional changes, how the changes in pollutants may affect human health, and other topics. These initial results suggest that economic and social changes brought on by the COVID-19 pandemic will not provide any real benefits for climate. Instead, the world will have to find opportunities that put it on the fast track to carbon neutrality as it recovers from the pandemic.

Reference

Jones, C.D., J.E. Hickman, S.T. Rumbold, J. Walton, R.D. Lamboll, R.B. Skeie, S. Fiedler, P. Forster, J. Rogelj, M. Abe, M. Botzet, K. Calvin, C. Cassou, J.N.S. Cole, P. Davini, M. Deushi, M. Dix, J.C. Fyfe, N.P. Gillett, T. Ilyina, M. Kawamiya, M. Kelley, S. Kharin, T. Koshiro, H. Li, C. Mackallah, W.A. Müller, P. Nabat, T. van Noije, P. Nolan, R. Ohgaito, D. Olivié, N. Oshima, J. Parodi, T.J. Reerink, L. Ren, A. Romanou, R. Séférian, Y. Tang, C. Timmreck, J. Tjiputra, E. Tourigny, K. Tsigaridis, H. Wang, M. Wu, K. Wyser, S. Yang, Y. Yang, and T. Ziehn, 2021: The climate response to emissions reductions due to COVID19: Initial results from CovidMIP. Geophys. Res Lett., 48, no. 8, e2020GL091883, doi:10.1029/2020GL091883. doi:10.1029/2020GL091883.

Contact

Please address all questions about this research to Dr. Jonathan Hickman.

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