As a result of the global COVID-19 pandemic, unprecedented lockdown measures were imposed worldwide to reduce the spread of the disease, causing huge reductions in economic activity and corresponding reductions in emissions from automobiles (40-75%), industry (~35%), and aircraft (50-90%). As well as lowering emissions of greenhouse gases, such as carbon dioxide (CO2) by a total of 11-25%, lockdown caused a dramatic reduction of many air pollutants. This improved air quality in many regions, but reducing these pollutants also affects climate in a multitude of different ways.
Some pollutants produce aerosols, tiny particles suspended in the Earth’s atmosphere. Sulphur dioxide (SO2), mainly produced from industrial processes, forms sulphate aerosol which reflects heat from the sun, cooling the planet. If SO2 were the only pollutant being reduced, one would expect the Earth’s temperature to increase. However, other aerosols like black carbon (also known as soot), which is produced by combustion, absorb sunlight, heating the planet and so reducing them leads to a cooling. Nitrogen oxides (NOx), which are produced by transport and saw massive reductions during the lockdown, make ozone, which is toxic to life and a greenhouse gas and therefore warms the planet. In our study, we used a state-of-the-art computer model to simulate the atmosphere under five scenarios: a “business as usual” case with no lockdown, and four scenarios featuring reductions in emissions of SO2, black carbon, and NOx from industry, transport, and aircraft for 3 months, spanning the range of best estimates.
As emissions decreased during lockdown, we found that reductions of these different pollutants only had a small impact on climate, in part due to their opposing impacts. While this may sound like an uneventful conclusion, it has important consequences. Which sectors were impacted was hugely important – the large reduction in NOx and black carbon from the decrease in transport offset the SO2 lost from industry. Understanding how different pollutants effect climate is essential to improving predictions of future climate change, and the lockdown period provides a unique testbed for our theories.
Overall, we found the effect on climate to be small and temporary. CO2 emissions may have dropped, but levels in the atmosphere are still rising, just slightly slower than they would have otherwise. Climate change has not stopped during the COVID-19 pandemic and a decisive shift away from fossil fuels must therefore be at the heart of the post-pandemic recovery.
Read more in our published findings: Minimal Climate Impacts From Short‐Lived Climate Forcers Following Emission Reductions Related to the COVID‐19 Pandemic
James Weber is a PhD Student at the Centre for Atmospheric Science, Department of Chemistry in Pembroke College, University of Cambridge. Scott Archer-Nicholls is a Postdoctoral Research Associate also at the Centre for Atmospheric Science, University of Cambridge.