In a warming climate, cloud patterns change in such a way that they exacerbate global warming. A research group led by Johannes Quaas from Leipzig University and Hao Luo and Yong Han from Sun Yat-sen University in China has discovered that the cloud cover is increasingly showing asymmetrical changes: it decreases more during the day than at night. This asymmetry means that the cooling effect of the clouds decreases during the day and the warming effect increases at night, which intensifies global warming. The researchers have just published their new findings in the renowned journal "Science Advances".
Clouds: more than just weather
During the day, clouds reflect sunlight back into space and thus cool the earth’s surface. At night, however, they act like a blanket that retains heat. This keeps the earth’s surface warm. For this reason, clouds have a decisive influence on the Earth’s climate," says meteorologist Quaas.
In their study, the scientists used satellite observations and data from the sixth phase of the Coupled Model Intercomparison Project (CMIP6), which provides comprehensive climate models and scenarios. These models cover historical data from 1970 to 2014 as well as projections up to the year 2100.
As cloud cover decreases more during the day than at night on a global scale, this leads to a reduction in the short-wave albedo effect during the day and an increase in the long-wave greenhouse effect at night," explains Hao Luo, the first author of the study.
Climate models and their importance
Climate models are essential for understanding and predicting the complex processes and interactions within the climate system. They help scientists to develop possible future scenarios and analyze the effects of various factors such as greenhouse gases, aerosols and clouds on the climate.
Johannes Quaas from the University of Leipzig emphasizes: "The asymmetry of the change in cloud cover is an important factor that was newly discovered here. Our study shows that this asymmetry leads to a positive feedback loop that amplifies global warming." According to the researcher, clouds are changing as a result of climate change. Overall, there are slightly fewer clouds, which means additional global warming.
The mechanisms behind the asymmetry
This daily asymmetry in cloud cover can be attributed to various factors. One main cause is the increasing stability in the lower troposphere caused by rising greenhouse gas concentrations. This stability means that clouds are less likely to form during the day, while they remain stable or even increase at night.
Yong Han, co-author of the study, explains: "The change in cloud cover is not evenly distributed throughout the day. During the day, when solar radiation is strongest, we observed a greater decrease in cloud cover. At night, when the Earth’s surface normally cools down, the cloud cover retains the heat and thus increases the greenhouse effect."
A look into the future
"Our results show that the reduction of greenhouse gases is even more urgent, as cloud cover not only simply reacts to warming, but also amplifies it via the new effect," warns Johannes Quaas. According to the scientists, further studies are needed to better understand changes in cloud cover. The ongoing studies at Leipzig University are also focusing on changes in vegetation and its biodiversity, for example, as well as the role of decreasing air pollution.
Original title of the publication in "Science Advances":
"Diurnally asymmetric cloud cover trends amplify greenhouse warming" , DOI: 10.1126/sciadv.ado5179
