It is a curious fact of climate science that clouds can both cool the Earth’s surface and keep it warm. This apparent paradox occurs because during the day, clouds reflect shortwave sunlight back into space thanks to the albedo effect, whereas at night they act like blankets, trapping longwave radiation close to the surface.
Researchers from Sun Yat-sen University in China and Leipzig University in Germany have now found that as the climate changes, cloud cover – especially in the lower atmosphere – decreases more during the day than at night. This asymmetry, they say, could contribute to a feedback spiral that makes planet warmer still.
“Our findings show that there is an even greater need to reduce greenhouse gases,” explains Johannes Quaas, the meteorologist who led this study, “because not only does cloud cover respond to warming, it also amplifies warming through this new effect.”
Daily variations
Other studies have previously shown that nighttime temperatures are increasing faster than daytime ones. The reason for this is not yet clear, however, as several feedback processes (including changes in cloud cover, atmospheric humidity, soil moisture and aerosol emissions) may be contributing to it.
In their work, which they detail in Science Advances, Quaas and colleagues studied how daily variations in cloud cover affect climate. The diurnal asymmetry in cloud cover they identified could stem from several factors, but the main one is that rising concentrations of greenhouse gases have made the lowest layer of the atmosphere (known as the lower troposphere) more stable. “This enhanced stability is having a negative effect overall,” says Quaas. “Less clouds are forming during day (so reducing their sunlight-reflecting effect), but they are remaining more stable at night (so increasing their blanket effect, in relative terms).”
The researchers obtained their results by analysing satellite observations and data from the sixth phase of the Coupled Model Intercomparison Project (CMIP6), which incorporates both historical data collected between 1970 and 2014 and projections up to the year 2100. “We found some day-night differences and started to wonder whether there might be a systematic effect in a warming climate,” Quaas says.
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The fact that these differences had not been studied before, he adds, may be due to the way climate researchers tend to analyse data. “When looking at global climate, one typically uses one time of day, or averages over time and analyses the geographical distribution,” Quaas explains. “Climate models for their output, and satellite observations, thus choose one time zone for the entire globe – typically UTC (Greenwich time). It took us the extra step to convert this to local time to see our result. Not a very elaborate idea, admittedly, but one that turned out to be revealing.”
While the asymmetry effect is not huge, it is systematic, he tells Physics World. “Effects such as those on solar energy are not going to be overwhelming, but are an important supplement to global warming,” he says.
The researchers now want to explore other factors – beyond the changes in atmospheric stability that they have identified – that may also be driving changes to cloud cover. “One such reason could be deforestation, for example,” Quaas suggests.
