For the past few years, scientists have watched, aghast, as global temperatures have surged — with both 2023 and 2024 reaching around 1.5 degrees Celsius above the preindustrial average.
Two new studies offer a potential explanation: fewer clouds. And the decline in cloud cover, researchers say, could signal the start of a feedback loop that leads to more warming.
For years, scientists have struggled to incorporate clouds’ influence into the large-scale climate models that help them predict the planet’s future. Clouds can affect the climate system in two ways: First, their white surfaces reflect the sun’s light, cooling the planet. But clouds also act as a kind of blanket, reflecting infrared radiation back to the surface of the planet, just like greenhouse gases.
Which factor wins out depends on the type of cloud and its altitude. High, thin cirrus clouds tend to have more of a warming effect on the planet. Low, fluffy cumulus clouds have more of a cooling effect.
Researchers are beginning to pinpoint how clouds are changing as the world warms. In Goessling’s study, published in December in the journal Science, researchers analyzed how clouds have changed over the past decade. They found that low-altitude cloud cover has fallen dramatically — which has also reduced the reflectivity of the planet. The year 2023 — which was 1.48 degrees Celsius above the preindustrial average — had the lowest albedo since 1940.
In short, the Earth is getting darker.
That low albedo, Goessling and his co-authors calculated, contributed 0.2 degrees Celsius of warming to 2023’s record-high temperatures — an amount roughly equivalent to the warming that has so far been unexplained. “This number of about 0.2 degrees fairly well fits this ‘missing warming,’” Goessling said.
Researchers are still unsure exactly what accounts for this decrease. Some believe that it could be due to less air pollution: When particulates are in the air, it can make it easier for water droplets to stick to them and form clouds.
Another possibility, Goessling said, is a feedback loop from warming temperatures. Clouds require moisture to form, and moist stratocumulus clouds sit just underneath a dry layer of air about one mile high. If temperatures warm, hot air from below can disturb that dry layer, mixing with it and making it harder for wet clouds to form.