The pandemic cut emissions, but did it help climate?

Lockdowns designed to slow the spread of COVID-19 caused a large decrease in vehicle traffic in 2020. As a result, global carbon dioxide emissions plummeted. Scientists wondered whether these short-term changes would be significant enough to affect the climate.

Now, a team led by John Fyfe of Environment and Climate Change Canada has an early look at the answer. The results show that the emission reductions caused by the pandemic led to undetectably small effects on global average temperatures (Sci. Adv. 2021, DOI: 10.1126/sciadv.abf7133).

In early 2020, as it became clear that the pandemic would cause large-scale emission changes, Fyfe and his colleagues used a powerful climate model to test a few projections for what might happen over 2 years of reduced emissions. They modeled scenarios in which CO₂ emissions and sulfate aerosol levels dropped as low as 25, 50, or 100% during that period, then looked for global average temperature changes over the next decade.

While CO₂ emissions disperse in the atmosphere and warm the climate, aerosols, which can be emitted directly or from secondary reactions in the atmosphere, rapidly reflect sunlight away from the planet, cooling things down, particularly in the region where they’re emitted. In the models, the team saw a slight rise in average global temperature, driven by decreased aerosol emissions, during the first 6 months after lowest emissions. This was followed by 6 months of slight global cooling due to lower CO₂ emissions.

In reality, emissions reached their lowest point in April 2020, when they were down about 20% from the previous April. The team’s scenarios that most closely matched this outcome showed that these reductions were too small and too short lived to have a detectable effect on average global temperature—and indeed, no such planetary-scale changes were observed.

Fyfe says the main takeaway from this research is that we have to be in it for the long haul if we want to achieve the global goals set out in the Paris Agreement and other climate plans. “We need emissions cuts year on year in order to stabilize the climate,” Fyfe says. Other modeling studies in progress corroborate this result, predicting a “small and undetectable signal” at the global level during the pandemic, Yang Yang of Nanjing University writes in an email. However, there were local effects. Yang says his and others’ research suggests that the pandemic led to significant regional warming tied to falling aerosol emissions, in places including eastern China, the eastern US, Europe, and South Asia.