https://www.scientificamerican.com/article/covid-19-could-help-solve-climate-riddles1/ > As the world scrambles to contain the spread of COVID-19, many economic activities have ground to a halt, leading to marked reductions in air pollution. And with the skies clearing, researchers are getting an unprecedented chance to help answer one of climate science’s thorniest open questions: the impact of atmospheric aerosols. What they learn could improve predictions of the earth’s climatic future. “We hope that this situation—as tragic as it is—can have a positive side for our field,” says aerosol researcher Nicolas Bellouin of the University of Reading in England. > Aerosols are tiny particles and droplets that are emitted into the air by myriad sources—from fossil-fuel burning to fertilizer spraying and even natural phenomena such as sea spray. They alter cloud properties and intercept sunlight, with some scattering solar radiation and others absorbing it. All of these factors influence global temperature—sometimes in competing ways. Overall aerosols have a cooling effect on the climate, offsetting some of the warming caused by greenhouse gases—but just how much they have done so to date, or will do so in the future, remains unclear. The Intergovernmental Panel on Climate Change has estimated that a doubling of atmospheric carbon dioxide concentrations could increase temperatures by anywhere between 1.5 and 4.5 degrees Celsius, with the wide range linked, in part, to scientist’s incomplete understanding of the influence of aerosols. “The fact that the aerosol effect on climate, so far, is so uncertain has held us back,” says atmospheric scientist Trude Storelvmo of the University of Oslo.