Over the past two decades, scientists have observed a significant decline in low-level cloud cover, a shift that is playing a major role in accelerating global warming. These clouds, which typically form below 2,000 meters and include types such as stratus and stratocumulus, act as a natural cooling system for the planet by reflecting incoming sunlight back into space. When this reflective layer diminishes, more solar energy is absorbed by the Earth’s surface, intensifying warming. Recent research shows that this reduction in cloudiness has contributed substantially to the Earth’s growing energy imbalance—the difference between incoming solar radiation and the heat radiated back into space—which has more than doubled over the last 20 years.
The study finds that since 2003, decreasing cloud cover has accounted for roughly half of the increase in this energy imbalance. This highlights the critical role that even subtle atmospheric changes can play in shaping global climate trends. A less cloudy atmosphere allows more sunlight to penetrate, amplifying the warming already driven by greenhouse gas emissions. In this way, the decline of cloud cover is not just a secondary effect of climate change but an active contributor to it.
Researchers have identified several key human-driven factors behind this trend. One major cause is the reduction of aerosol emissions. Aerosols—tiny particles produced largely by burning fossil fuels—help form clouds by acting as nuclei for water droplets. As air pollution controls have reduced aerosol concentrations, fewer clouds are able to form. Another factor is the increase in greenhouse gases, which warm the atmosphere and reduce relative humidity, making conditions less favorable for cloud formation. Additionally, rising ocean temperatures influence atmospheric moisture and further suppress cloud development, creating a reinforcing cycle known as a “cloud feedback.”
Using a statistical method called cloud-controlling factor analysis, scientists determined that about 40% of the decline in low-level clouds is linked to ocean warming, while greenhouse gases and aerosol reductions contribute 21% and 14%, respectively. Natural climate variability, by contrast, plays only a minor role. This means that the vast majority of the observed changes in cloud patterns can be attributed directly or indirectly to human activity.
Despite these findings, climate models appear to capture these trends reasonably well. On average, simulations of low-level cloud changes align with real-world observations, suggesting that current projections of future warming already account for this effect. However, uncertainties remain, particularly regarding the precise strength of cloud feedbacks and other contributors to the energy imbalance, such as upper-level clouds or water vapor changes.
Ultimately, the decline in low-level clouds underscores the complexity of Earth’s climate system and the interconnected nature of its components. The behavior of cloud systems, once considered a major uncertainty, is now increasingly understood as both a driver and amplifier of warming. While current models provide some reassurance, the continued loss of reflective cloud cover reinforces the urgency of addressing greenhouse gas emissions and understanding feedback processes that could further intensify climate change.
www.carbonbrief.org/guest-post-how-declining-cloudiness-is-accelerating-global-warming