Key finding: Cloud decline is fueling warming

A new study by an international team of researchers published in Atmospheric Chemistry and Physics Letters shows that the decline in low-level clouds over the past two decades accounts for half of the increase in global energy imbalance.

Earth's energy imbalance refers to the difference between the energy absorbed from the sun and the heat emitted into space, and this imbalance becomes the driving force behind global warming. As a result of analyzing data from 2003 to 2024, the research team confirmed that 0.22 watts per square meter (W/m²) of additional solar heat per decade is being absorbed by the Earth due to a decrease in low-level clouds.

Why cloud reduction is important

Low-level clouds that form below 2,000 meters include stratus, stratocumulus, and cumulus clouds and mainly occur over the ocean. These clouds act as the Earth's 'sunscreen' and have played a key role in lowering the Earth's temperature by reflecting a significant portion of the incident sunlight back into space.

However, satellite observations since the new millennium show that these low-level clouds are continuously decreasing globally. As clouds decrease, more sunlight reaches the earth's surface, creating the dual effect of increasing greenhouse gases and increasing warming.

Why are clouds disappearing: three causes?

The research team identified three main causes of cloud decline in the 21st century.

First, it is a paradoxical effect of air pollution reduction policy. Aerosols (fine particles generated from burning fossil fuels) serve as 'seeds' for cloud formation. As part of recent efforts to improve air quality, regulations on ship fuel have been strengthened, resulting in a decrease in aerosol emissions, which paradoxically has the effect of suppressing cloud formation.

Second, there are changes in the atmosphere due to the increase in greenhouse gases. As the concentration of greenhouse gases in the atmosphere increases, the atmosphere becomes warmer and drier. These conditions act in the direction of dissipating clouds.

Third, there is a feedback effect within the climate system. As warming progresses, unfavorable conditions for cloud formation are created, which in turn creates a vicious cycle that accelerates warming.

Human activity is the key driver

One of the study's most important findings is that cloud decline over the past two decades has been driven by human activities rather than natural climate fluctuations. This strengthens the scientific basis for more reliably attributing recent record high temperatures to human activities.

Global energy imbalance has more than doubled over the past 20 years, and the fact that half of this is due to cloud decline presents new challenges in responding to climate change. This is because the positive goal of reducing air pollution is unintentionally producing the side effect of accelerating warming.

Future outlook [AI analysis]

The results of this study have important implications for establishing climate policy. The complex task of finding a balance between the two goals of improving air quality and responding to climate change is likely to be highlighted.

In the short term, the cloud reduction effect due to aerosol reduction is expected to continue, which could further steepen the ongoing warming trend. The scientific community is expected to intensify efforts to reflect this feedback effect more precisely in climate models.

In the long term, there is a possibility that discussions on climate engineering approaches that artificially promote cloud formation will become active. However, the side effects and ethical issues of these interventions still remain to be resolved.