Less reflection Lack of clouds could have contributed to heat records
Clouds have an influence on how warm the Earth's climate is. (archive image)
Image: dpa
Last year there were fewer low clouds than in previous years. (archive image)
Image: dpa
Clouds have an influence on how warm the Earth's climate is. (archive image)
Image: dpa
Last year there were fewer low clouds than in previous years. (archive image)
Image: dpa
Because low clouds reflect sunlight, they cool the earth. Data from satellites show: There were particularly few of them last year. This could contribute to the climate crisis.
05.12.2024, 23:12
dpa
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- Satellite images show that there were fewer low clouds than usual last year.
- This could have contributed to the heat records, as clouds reflect sunlight and thus cool the earth.
- 2023 was an extreme manifestation of a multi-year trend.
- Last year alone, the average temperature rose by 0.3 percent - that is exceptional.
Last year broke all records: the Earth was warmer than ever before. German scientists now have an idea of what could have led to the big jump in temperature: There were fewer clouds at low altitudes. This has become clear from the analysis of satellite data and the application of computer models, writes the team from Bremerhaven, Bonn and Bremen in the journal "Science".
Man-made greenhouse gases are considered to be the main reason for the steady rise. There have also been other effects recently: the current increase in solar activity, the El Niño weather phenomenon, volcanic activity and less particulate matter over the oceans. However, the researchers believe that all of this together cannot fully explain the jump in temperature.
Last year, the global mean surface temperature was almost 1.5 degrees Celsius higher than the comparable period from 1850 to 1900. In the previous year, the temperature had been 0.3 degrees lower.
Earth has absorbed more solar radiation
Helge Gössling from the Alfred Wegener Institute in Bremerhaven and his colleagues determined an unusually high value for the solar radiation absorbed from the analyzed data. This is synonymous with a low reflectivity of the planet, the so-called albedo. It indicates the proportion of solar radiation that is reflected back into space.
For many years, researchers have been studying the albedo in the polar regions in particular. If there is less light-colored ice there and more dark ocean surface, less solar radiation is reflected. However, the change in the polar regions only explains a small part of the decline in surface albedo, explains Gössling. That's why the researchers looked at the clouds.
Fewer low clouds = less cooling
High clouds cool the earth because they reflect sunlight at the top. However, they also generate a warming effect because they retain heat radiated from the earth in the atmosphere.
The warming effect is largely absent in low clouds. "If there are fewer low clouds, we only lose the cooling effect, so it gets warmer," explains Gössling. Last year, satellite records showed the lowest value for low clouds since 2000.
The decline in low-hanging clouds was particularly pronounced in the tropics and in the mid-latitudes of the northern hemisphere. "It is striking that the eastern North Atlantic, which is one of the main drivers of the recent rise in global mean temperature, not only recorded a significant decline in low clouds in 2023, but - like almost the entire Atlantic - already in the last ten years," says Gössling. 2023 thus showed an extreme manifestation of a multi-year trend.
Cause unclear
According to the scientists, it is not yet clear what is causing the reduction in low clouds. Last year, weaker winds carried particularly little Saharan dust out into the Atlantic. In addition, stricter regulations for marine diesel could have led to lower emissions of fine particles. Both developments mean fewer suspended particles in the air on which water could precipitate, causing clouds to form.
However, the authors of the study also suspect another phenomenon: climate change itself could contribute significantly to the reduction of low clouds. The researchers refer to studies that have shown that the warming of the sea surface can reduce cloud cover.
"If there is an amplifying feedback between global warming and clouds behind the albedo decline, as some climate models also suggest, we must expect quite strong warming in the future," emphasizes Gössling. The Earth could therefore already be closer to global warming of more than 1.5 degrees than previously thought.