Cloudy skies have slowed global warming – but that may be changing

Stratocumulus clouds over the Pacific Ocean reflect solar radiation away from Earth

NASA/SCIENCE PHOTO LIBRARY

An increase in low-lying clouds has been shading the ocean and cooling the planet. But this feedback may now be reversing, which could accelerate global warming.

Clouds have been the biggest wild card in our efforts to predict future climate change, as they reflect radiation in ways that can both warm and cool the world.

Wispy cirrus clouds in the upper troposphere don’t reflect much incoming sunlight, and they trap outgoing heat. Thick blankets of stratocumulus clouds in the lower troposphere, on the other hand, reflect sunlight away from Earth, which is why geoengineering researchers have been spraying tiny droplets of seawater to try to make them even more reflective.

Climate models project that, as the world heats up, fewer low clouds will form to block sunlight. That could kick off a positive feedback loop towards more warming, leading to even fewer low clouds.

But satellite data hasn’t been consistent enough to tell if low clouds are increasing or decreasing, especially in the south-east Pacific and Atlantic oceans, where the largest, longest-lasting cloud decks tend to form.

Now, Gregory Cesana and Assia Arouf, both at Columbia University in New York, have estimated low cloud cover going back to the start of the satellite era in 1979 for the first time.

The pair essentially correlated data on sea surface temperature, wind speed and atmospheric stability with the low cloud cover measured by modern satellite instruments, then used this data to calculate low cloud cover in previous decades when satellite measurements were poorer.

They found that low cloud cover has been expanding, increasing the energy reflected from Earth’s surface by 0.32 watts per square metre since 1979.

For comparison, human-caused climate change is now heating Earth by about 2.72 watts per square metre. That means low clouds have shaved a fraction off global temperature rise, although it is difficult to say exactly how much.

“We get a negative feedback, so the clouds are actually opposing the warming,” says Cesana. “And this is completely different from what you get when you look at those idealised experiments using the climate models.”

In tropical areas like the eastern Pacific, low cloud cover has been increasing by 0.18 per cent per decade, the researchers found.

That is because the eastern Pacific has been warming less than many models expected, while the western Pacific has heated up strongly. As a consequence, the surface air over the eastern Pacific has remained cool, while atmospheric circulation has brought increasingly warm air from the western Pacific down on top of it. The resulting temperature inversion prevents the warm, dry, sinking air from mixing with low stratocumulus clouds.

“That’s going to favour the creation of low clouds or make them more stable so they can last longer,” says Cesana.

But scientists expect the eastern Pacific will soon start heating up, in part because of stricter controls on sulphur pollution, which forms clouds that cool the planet and influence atmospheric and ocean circulation. That would allow the warm, sinking air to dry up some of these low clouds.

This may already be happening. In March, Paulo Ceppi at Imperial College London and his colleagues found that low cloud cover has decreased globally since 2003, and a 2024 study said that the recent acceleration in global warming may be partly due to an emerging positive feedback effect of low clouds.

“There’s no reason to think that we are overestimating the impacts of future climate change,” says Ceppi. “If anything, we might be slightly underestimating the effects of future climate change because of this amplifying effect of clouds on global warming.”

The finding that low cloud cover had been expanding could actually be bad news, says Cesana, because if that trend reverses as expected, it could “unmask” warming and further accelerate climate change.

Planet-warming El Niño events like the one now developing in the eastern Pacific diminish low clouds, “so we’re going to have a taste of what could happen in the future,” he says.