Clouds: are we ignoring a crucial driver of recent global warming?

Typically, clouds are counted as contributors to planetary albedo – but are all clouds the same? Jed Thomas

Clouds play a crucial role in regulating Earth's climate by influencing both incoming solar radiation and outgoing infrared radiation.

Their effects, however, depend heavily on cloud type, altitude, and coverage. As global temperatures rise, changes in cloud behavior can either amplify or dampen warming—a process known as cloud feedback.

Most evidence suggests that cloud feedback is a net positive feedback, meaning it accelerates climate change rather than counteracting it.

The role of clouds in Earth's energy balance

Clouds impact Earth's energy balance in two key ways:

• Reflecting sunlight (shortwave cooling): Low-altitude clouds, especially stratocumulus and stratus, have high albedo (reflectivity) and efficiently reflect sunlight back into space, cooling the surface.
• Trapping heat (longwave warming): High-altitude clouds, such as cirrus, allow sunlight to pass through but trap outgoing infrared radiation, acting like a greenhouse gas and warming the Earth.

The balance between these effects determines whether clouds have a net cooling or warming effect. Climate change alters cloud cover and distribution, leading to feedback mechanisms that influence future warming.

How global warming undermines low-altitude cloud formation…

One of the most concerning cloud feedback mechanisms is the potential decrease in low-altitude clouds as temperatures rise. Several factors contribute to this reduction:

• Increased atmospheric stability: As greenhouse gases trap more heat, the upper troposphere warms more than the surface. This suppresses convective mixing, making it harder for low clouds to form.
• Higher lifting condensation level (LCL): A warmer atmosphere requires rising air to travel farther before it cools enough to condense into clouds. This reduces the formation of low clouds.
• Stronger subsidence in high-pressure zones: Warming intensifies large-scale atmospheric circulation patterns, increasing the downward movement of dry air, which suppresses low cloud formation, particularly over oceans.
• Infrared-induced cloud burn-off: More longwave radiation from the surface heats and evaporates low clouds, thinning them out and reducing their coverage.

Since low clouds are highly reflective, their loss means more sunlight reaches the surface, causing further warming—a classic positive feedback loop.

…and promotes high-altitude cloud formation

While low clouds tend to decrease, high clouds are expected to increase in a warming world. This also creates a positive feedback effect:

• More water vapor in a warmer atmosphere: Since warmer air holds more moisture, high-altitude cloud formation increases.
• Greater greenhouse effect from cirrus clouds: High clouds are poor at reflecting sunlight but excellent at trapping heat. Their increase strengthens the greenhouse effect.
• Delayed infrared emission to space: Because high clouds are cold, they radiate less energy to space, further enhancing warming.

How aerosols released by combusted fossil fuels form low-altitude clouds

Human activities have historically released aerosols (tiny particles) into the atmosphere, which help form low clouds by providing cloud condensation nuclei (CCN).

However, as air pollution controls reduce aerosol emissions, fewer CCN are available, leading to larger but fewer cloud droplets, which make clouds less reflective; faster cloud dissipation, reducing low cloud coverage; and a decrease in the overall cooling effect of low clouds, amplifying warming.

This means that as pollution declines, an unintended side effect may be less cloud cooling, allowing warming to accelerate.

Fewer aerosols in a warmer atmosphere means more heat-trapping clouds

Current climate models suggest that cloud feedback is predominantly positive, meaning it amplifies rather than mitigates climate change.

Observational studies, satellite data, and high-resolution models increasingly show low cloud coverage declining in key regions (e.g., subtropical oceans), reducing reflection of solar radiation, whilst high cloud coverage increases, enhancing the greenhouse effect.

Similarly, reductions in aerosols will decrease the formation of low-altitude clouds and there will be fewer reflective particulates in the atmosphere, too.

Some researchers, including James Hansen and colleagues, argue that climate sensitivity—how much warming results from a given CO₂ increase—may be higher than previously estimated due to underestimated cloud feedback strength.

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Uncertainties for estimates of climate sensitivity

Despite strong evidence for positive cloud feedback, significant uncertainties remain. Cloud formation and behavior are highly complex, and small-scale cloud processes are difficult to model with precision.

The key areas of uncertainty include the exact magnitude of low cloud reduction and its impact on future warming; regional variations in cloud feedback effects, particularly in the tropics and mid-latitudes; and how cloud-aerosol interactions will evolve as air pollution declines globally.

However, given the mounting observational evidence and improved climate models, it is increasingly clear that cloud feedback is not just a secondary factor but a major amplifier of climate change.

According to Hansen et al, cloud feedback is responsible for almost two thirds of the reduction in Earth’s albedo, which they believe has been occurring since 2000.¹

Cloud feedback is one of the most critical and uncertain aspects of climate change projections. The loss of reflective low clouds and the increase in heat-trapping high clouds both contribute to a net positive feedback loop, reinforcing global warming.

As scientists refine climate models and gather more observational data, there have been calls for higher climate sensitivity estimates, suggesting that future warming could be more severe than previously anticipated.

Understanding and quantifying these cloud-climate interactions remains a top priority for climate science and policy planning.

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¹ The Acid Test: Global Temperature in 2025. James Hansen and Pushker Kharecha. 2025.