In autumn 2014, Iceland’s Holuhraun volcano erupted, spewing daily about 120,000 tonnes of sulphur dioxide into the air at its peak. A thick belt of tiny particles, also known as aerosols - a notorious air pollutant, formed in the atmosphere above this otherwise virtually unspoiled region. This volcanic eruption served as a very good natural experiment that allowed climate researchers to study how the sudden upwelling of particulate matter affected clouds. “Since aerosols can promote the formation of cloud droplets, they are an important factor in projecting climate change but we still know very little about it,” Wang explains. Since September 2021, the 30-year-old environmental scientist has been an ETH Zurich Fellow at the university’s Institute for Atmospheric and Climate Science, working as a member of the group run by Ulrike Lohmann, Professor of Atmospheric Physics.
Wang talks enthusiastically about a study – published recently in Nature Geoscience – that she, her husband Ying Chen and Ulrike Lohmann co-authored along with other researchers from the British Met office, the Universities of Exeter, Cambridge, Leeds (UK), and Munich (Germany) and NASA (US). She laughs warmly from time to time, clearly delighted by the interest being shown in her results. “I’m really excited about my work,” she says. “Emissions that affect the climate essentially fall into two groups: greenhouse gases and aerosols.” Greenhouse gases heat up the planet, while aerosols counteract this effect mainly through cloud formation.
“Clouds act like an umbrella for the Earth, cooling it down,” Wang says, spreading her arms wide to illustrate her point. But the problem, she adds, is that we are unable to quantify with precision how aerosols and the cooling effect of clouds work. According to the Intergovernmental Panel on Climate Change (IPCC), aerosols are the primary source of uncertainty when it comes to understanding how humanity has impacted the current climate.
So when Iceland had its volcanic eruption, climate researchers jumped at the chance to study the effects of the aerosols released during this event: they compared the clouds over the North Atlantic in autumn 2014 with the situation in the years before and after. But this comparison proved inconclusive because cloud formation also depends largely on the weather, which was different during the eruption from that in the other years.
Machine-made meteorologists
“We also used the volcanic eruption in our work,” Wang says. “But what we did was to apply a machine learning method that can tell us what the clouds are like under certain weather conditions.” This makes it possible to use data from the “clean” years to determine what the cloud situation would have been in 2014 had there been no eruption. “It’s like having a weather forecast,” Wang says. By comparing the machine learning forecast for the cloud situation minus the Holuhraun eruption with data of clouds in the same months in years before and after the eruption, it’s possible to say that the difference is due entirely to the aerosols.
The result of this study surprised the researchers because it contradicts previous notions. “It’s also important to know,” Wang says, “that interactions between aerosols and clouds produce two different effects.” An increase in emissions results in a higher number of cloud droplets, but these are smaller. This makes the clouds brighter, which means they reflect more sunlight away from the Earth. A higher number of smaller droplets also means that the clouds can retain more water before it rains, meaning the clouds last longer. “People used to think that it was cloud brightness that dominated the cooling effect, but we discovered that a cloud’s lifespan or the formation of new clouds is more important,” Wang says. Overall, the aerosols released by the volcanic eruption increased cloud cover by around 10 percent.
