The Earth is absorbing more and more sunlight, which is contributing to its rising temperature. Scientists have long known that this trend is largely due to a decrease in the reflection of sunlight by clouds. A recent study, published in Geophysical Research Letters, brings a key missing piece to the puzzle that reveals the main culprit of this phenomenon in the 21st century. It turns out that the main reason is the contraction (shrinking) of the world's storm clouds.

What's going on with the clouds?

An analysis of satellite observations over the past 24 years (2001-2024) has revealed a worrying trend: storm cloud zones in tropical and mid-latitudes are are contracting at a rate of 1.5 % to 3 % per decade. Specifically, regions with high cloud cover (referred to as Large Total Cloud Cover, L-TCC) modes show statistically significant decreases from 1.84 % ± 0.38 % to 3.20 % ± 0.97 % per decade at high latitudes and 2.07 % ± 0.45 % per decade at low latitudes. Regions of strong shortwave cloud radiative effect (so-called S-SWCRE modes) also show similar decreases, at rates of 0.88 % ± 0.4 % to 1.32 % ± 0.3 % per decade.

This shrinkage, which manifests itself as a contraction of midlatitude storm regions and a narrowing of the Intertropical Convergence Zone (ITCZ), along with a decrease in low-latitude cloud cover, allows more sunlight to reach the Earth's surface.

Impact on solar radiation absorption:

The increased absorption of solar radiation by the Earth is significant, with the global trend for the period 2001–2024 being 0.45 W/m² per decadeFrom this value up to 0.37 W/m² per decade can be attributed to the contraction of the aforementioned global storm and tropical convective/stratocumulus regimesThis phenomenon is described as "cloud radiative warming".

Scientists, including authors studies George Tselioudis (corresponding author), Jasmine Remillard, Christian Jakob, and William B. Rossow analyzed data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Clouds and the Earth's Radiant Energy System (CERES) instruments on the Terra and Aqua satellites. In this way, they were able to decompose solar radiation budget trends into components related to the general atmospheric circulation and components influenced by local processes in the clouds.

Why does this happen? Connection with atmospheric circulation:

The contraction of storm clouds is closely related to observed shifts in the Earth's overall atmospheric circulation. The study by Tselioudis et al. (2024), referenced in this research, previously found:

• Poleward shift of mid-latitude storm clouds, similar to the shift of atmospheric jet streams.
• Poleward expansion of the subtropical low cloud region, which coincides with the expansion of the global tropics.
• Narrowing of the Intertropical Convergence Zone (ITCZ).

These changes in atmospheric circulation are well documented in observations and modeling studies, and indicate a significant impact of anthropogenic activity (human activities). The large differences in the shortwave cloud radiative effect (SWCRE) between defined regimes mean that even small changes in the relative coverage of these regimes can lead to significant changes in the radiation balance.

Other factors:

Although shifts in the general circulation are dominant, local processes affecting clouds also contribute to the trends. At low latitudes, scientists observed significant cloud radiative warming (0.21 W/m²/decade), which may be caused by indirect effects of aerosols (for example, reduced emissions from ships) or by a decrease in low cloud cover due to changes in boundary layer processes. Conversely, at high latitudes, there was a cooling effect of clouds (0.20 W/m²/decade), which may be caused by an increase in albedo and the proportion of low and middle clouds due to a "cloud phase feedback". However, the global impact of these local processes is minimized by their contradictory nature. in different latitudes.

Key puzzle piece:

This study offers a crucial missing piece to the puzzle of understanding the increase in solar radiation absorption in the 21st century and the great thermal anomaly of 2023He emphasizes that it is essential that climate models are able to accurately simulate the observed contraction of storm clouds and that the interactions between changes in atmospheric dynamics and cloud cover are further investigated.

We found that the contraction of storm zones is a major factor in the recent increase in solar radiation absorption, opening a new chapter in understanding climate change and its connection to the dynamics of our atmosphere. Spring