Mr. Pretor-Pinney is an author in Somerset, Britain, and the founder of the Cloud Appreciation Society.
Sept. 9, 2025
I love the way clouds billow above your head, drift lazily across blue skies and cast fleeting shadows on the ground below. These ever-shifting sculptures of vapor and light are among nature’s least appreciated marvels.
That’s why 20 years ago, I started the Cloud Appreciation Society, to remind people to look up. Now climate science is catching up, revealing that clouds aren’t just poetic; they’re pivotal in helping to regulate Earth’s temperature. And their influence on the climate is evolving in ways we’re only beginning to understand.
How exactly cloud cover will shift in a warming world is anyone’s guess; it’s one of the largest sources of uncertainty in climate science. But it should also be everyone’s concern. What happens to our clouds as the planet warms is so important that we need a renaissance in the study of clouds.
Clouds are classified by their altitude and appearance — whether they look like solid clumps, diffuse layers or wispy streaks. You might remember some of their names from high school:
The 10 main types of cloud
Sources: Cloud Appreciation Society; World Meteorological Society; National Oceanic and Atmospheric Administration
These clouds don’t just look different from one another; they also have very different effects on temperatures at the surface of the land or water below. These differences have to do with how much of the sun’s heat they allow through and how much of Earth’s heat they trap in.
Low clouds such as these puffy cumulus typically have a cooling effect, as do the clumpy layers known as stratocumulus and the smooth layers known as stratus.
Animation of fluffy cumulus clouds slowly moving across a field.
Much of the sun’s rays are reflected off their white tops back up into space. And they are dense enough to cast shadows, cooling the surface below — the natural parasol effect you feel on a beach when one drifts overhead.
A yellow arrow for sunlight moves downwards then bounces off of the cumulus clouds and goes back upwards towards the sky.
Since their water droplets are warmer, they’re as good at radiating Earth’s heat up into space as they are at absorbing it from below. This means they don’t have a pronounced effect of trapping in the warmth below.
An orange arrow for heat moves upwards from Earth and passes through the clouds upwards towards the sky.
High clouds like cirrus and cirrocumulus do the opposite, warming Earth’s surface. Counterintuitively, this is because the ice crystals in these clouds are cold.
Animation of wispy cirrus clouds slowly moving across the same field.
These high clouds are often not as dense, and so they can let in more of the sun’s rays.
A yellow arrow for sunlight moves downwards and passes through the cirrus clouds towards the ground.
But since they are cold, they don’t radiate as much of Earth’s heat into space — more like blankets than umbrellas, resulting in a net warming effect.
An orange arrow for heat moves upwards from Earth and bounces off the cirrus clouds back towards the ground.
The mix of cloud types over our planet ensures they have an overall cooling effect because the shade from the low clouds outweighs the warming effect of the high ones.
But it’s unclear how clouds will change in a warming climate. With an atmosphere as vast as ours, there is scant empirical data. For now, climate scientists can only make informed guesses, based on ever more sophisticated computer models.
As global temperatures rise, scientists believe we may see fewer of the low, cooling clouds but not of the high, warming ones. It would be a classic feedback loop that accelerates the very warming that triggered it.
In a paper published in June in Geophysical Research Letters, scientists analyzed 24 years of satellite data on global cloud coverage and how much of the sun’s energy clouds reflect away. It reports a troubling decrease in highly reflective clouds in the regions of our planet where such clouds mostly form: the stormy mid-latitude zones of the Northern and Southern Hemispheres and the tropical stormy regions around the Equator. Cloud cover in these regions appears to be shrinking by about 0.9 percent to 1.3 percent per decade.
In a recent paper in the journal Science, reduced low cloud cover contributed about 0.2 degrees Celsius, or 0.32 degrees Fahrenheit, to 2023’s record-breaking average global temperatures. In other words, the reduction in Earth’s low clouds helped explain some of the extreme heat that year.
As low cloud cover decreased in recent years, Earth absorbed more heat from the sun
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’25
0
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2% increase in cloud cover
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2 watts per square meter of absorbed sunlight
The Intergovernmental Panel on Climate Change has identified climate feedback loops brought on by clouds as one of the largest unknowns in projections of warming. This uncertainty means we can’t say whether the changes we are beginning to observe in clouds are temporary or systemic shifts caused by human-driven climate change. This is why we urgently need greater investment in the study of clouds.
One thing we can do to avoid exacerbating the effect of high clouds is to stop adding to them with aircraft condensation trails, or contrails. When aircrafts fly through humid, cold air at cruising altitude — especially near weather fronts — their exhaust can seed the formation of ice-crystal clouds. When the air at cruising altitude is cool enough and moist enough, these contrails spread into high, thin layers that contribute to atmospheric warming.
It’s entirely possible for airlines to avoid flying at altitudes where the air is conducive to forming contrails. A 2020 study found that adjusting the cruising altitude of just 2 percent of flights could reduce contrail warming by nearly 60 percent, without using much more fuel.
A far riskier idea for influencing clouds to combat global warming is to try to brighten low clouds over oceans by spraying salt particles into them to increase their reflectivity and amplify their cooling effect.
Cumulus and cumulus congestus clouds
Kristina Barker
Cumulonimbus producing localized showers
Kristina Barker
I’m against manipulating clouds — especially given the possibility of unintended consequences for a system as chaotic and vast as Earth’s climate. And I’m not alone. In 2024, concerned local authorities in California shut down a University of Washington research project about the practicalities of brightening clouds with salt particles. A 2023 study and a 2024 study showed with climate model simulations that cloud brightening might cool one region while unintentionally exacerbating heat or leading to monsoon changes elsewhere.
But as we learn more about the potential tipping points of our climate — irreversible changes in the system — we may find ourselves approaching one too quickly for decarbonization efforts alone to prevent it. Some understanding of the likely effects of geoengineering projects such as cloud brightening will then be invaluable, no matter how misguided such interventions might seem. We need to understand the implications long before nations propose carrying out such projects on a large scale.
There’s also a role for citizen science to improve the analysis of clouds and their role in global warming. We at the Cloud Appreciation Society are partnering with Asterisk Labs, a worker-owned cooperative research lab, and will be inviting the public to use our CloudSpotter app to interpret the cloud types in satellite images. The data should enable us to train artificial intelligence models to trawl through the vast library of satellite cloud images to reveal with greater clarity and precision the gradual trends in Earth’s shifting cloud cover.
If clouds are changing, then so should the way we study them. This renaissance won’t happen on its own. It requires all of us to look up — and take notice.
