Things are not always clear and straightforward when it comes to global temperatures. Despite fewer emissions last year, temperatures were up between 0.1ºC and 0.3ºC during the spring, especially in the United States and Russia.

Emissions of greenhouse gases such as carbon dioxide (CO2) warm the planet over time, trapping heat in the atmosphere. But that’s not the case with aerosols. Aerosols are formed during combustion and make clouds brighter to reflect sunlight away from the earth. Fewer aerosols mean less of that cooling effect, leading to a warmer Earth.

“There was a big decline in emissions from the most polluting industries, and that had immediate, short-term effects on temperatures,” Andrew Gettelman, the study’s lead author from the National Center for Atmospheric Research (NCAR), said in a statement. “Pollution cools the planet, so it makes sense that pollution reductions would warm the planet.”

Looking at last year’s pollutants most closely, Gettelman and other researchers at NCAR found that the drop in aerosols last spring allowed more of the Sun’s warmth to reach the planet. This was especially the case in industrialized countries, which usually release a lot of aerosols into the atmosphere.

Temperatures over parts of the planet’s land surface last spring were between 0.1ºC and 0.3ºC warmer than expected, considering the prevailing weather at the time, the study showed. The effect was particularly significant in the United States and Russia, where temperatures were 0.3ºC above expected over much of the territory.

Scientists have long been able to quantify the climate impacts of CO2 and other greenhouse gases. This hasn’t been the case with aerosols such as sulfates, nitrates, black carbon, and dust. Projecting the extent of future climate change requires estimating the extent to which countries continue to emit aerosols in the future and the influence they have on clouds.

For their research, Gettelman and his team used two climate models: the NCAR-based Community Earth System Model and a model known as ECHAM-HAMMOZ, developed by a consortium of European nations. They did simulations on both models, adjusting emissions of aerosols and incorporating meteorological conditions in 2020.

Not a call for more pollution

While the study illustrates how aerosols can counter the warming influence of greenhouse gases, emitting more of them into the lower atmosphere isn’t a viable strategy to tackle climate change, the researchers argued. “Aerosol emissions have major health ramifications,” Gettelman said in a statement. “Saying we should pollute is not practical.”

Greenhouse gas emissions were down 6.4% in 2020 after rising steadily for decades, studies showed. This has been linked with the coronavirus pandemic, with limited economic and social activities worldwide. The US contributed to most of the global decline, with a nearly 13% drop in its emissions due to the limited use of cars.

Under the Paris Agreement on climate change, signed five years ago, countries need to cut one to two billion metric tons per year this decade to limit global warming from rising below 2ºC – the goal set in the climate deal. Still, since it was signed in 2015, emissions have only gone up, a trend only now interrupted by the pandemic.

The study was published in the journal Geophysical Research Letters.