In the area of the trade winds, extensive cumulus clouds cast shadows on large parts of the globe. Because they reflect the sun's rays, they mitigate global warming. However, previous climate models predicted a vicious circle: Global warming is causing the cooling clouds to dissipate faster than before, which in turn drives further heating. Researchers have now empirically examined this prediction for the first time - and have positive news: their observations and calculations suggest that the clouds will persist even in a warmer climate and can continue to help cushion the global warming.
North and south of the equator, in the trade wind zone, dense cumulus clouds shadow around 20 percent of the earth's surface. Because they reflect the sun's rays, they play a key role in mitigating global warming. However, previous climate models assumed that the clouds would soon no longer be able to perform this function. The hypothesis: Due to global warming, more water evaporates from the sea and ensures higher humidity on the underside of the clouds. But the air masses above the clouds remain very dry. Since the large difference in humidity is compensated by mixing, the water droplets could evaporate faster than before, so that the clouds would have to dissipate after a short time.
Measurement flights in the Caribbean
"However, the hypothesis of mixing and desiccation has never been verified on the basis of observations," writes a team led by Raphaela Vogel from the University of Hamburg in the journal Nature. "Since the trade wind cumulus clouds are geographically very widespread, even small errors in predicting their response to warming can have large effects on the global radiation budget."
To test the hypothesis, the scientists started a large-scale measurement campaign in January 2020. With two research aircraft, they undertook measurement flights in the Caribbean around Barbados for a month. "We chose this region because the clouds there are representative of the entire trade wind belt," says Vogel and her team. Hundreds of so-called drop probes were dropped from an airplane at an altitude of nine kilometers and collected data on temperature, humidity, pressure and wind as they fell. The other aircraft measured the clouds at the cloud base at an altitude of 800 meters.
Clouds react differently than expected
The results show that previous assumptions about the role of these clouds in climate change need to be corrected: "Trade wind clouds affect the climate system worldwide, but they obviously react differently than expected," says Vogel. "Therefore, a very extreme increase in the temperature of the earth's surface is less likely than previously thought. This is enormously important for an improved presentation of future climate scenarios, but it does not mean that climate protection is being given the all-clear.”
The empirically collected data is in contrast to the current climate models, according to which greater mixing leads to drying out of the lower cloud layer and thus to less cloud cover. Because even if this process can be proven, the previous climate models neglect another important factor: overturning processes on a medium-sized level, larger than the individual cloud, but smaller than large-scale circulations. These so-called mesoscale movements prevent drying out and, on the contrary, even ensure that more clouds form.
"This is good news because we were able to show that the trade wind clouds are far less sensitive to global warming than was long thought," says Vogel. "With our observations and findings, we can directly test how realistic climate calculation models represent the current and future occurrence of trade wind clouds. A new generation of high-resolution climate models that can depict the dynamics of clouds worldwide with an accuracy of up to one kilometer is promising. This will make future forecasts more accurate and meaningful.”
Source: Raphaela Vogel (University of Hamburg) et al., Nature, doi: 10.1038/s41586-022-05364-y