Fewer clouds, more heat: a new threat to the Earth

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For more than twenty years, NASA instruments have been carefully observing a worrying phenomenon: a growing imbalance between solar energy entering the Earth's atmosphere and that leaving it. While greenhouse gas emissions partly explain this anomaly, scientists have until now remained perplexed by the scale of the phenomenon. A study, published in May 2024 in the journal Surveys in Geophysics finally provides answers, putting an unexpected actor on stage: our clouds.

Cloud cover, protector of our climate

Data collected by NASA's Terra satellite, which has been scanning our planet for nearly a quarter century, reveals a slow but steady decrease in global cloud cover. George Tselioudis, a climatologist at NASA's Goddard Institute for Space Studies, puts it this way: Every decade, we lose 1.5% of our clouds globally.

This trend is particularly evident in two distinct zones: an equatorial band that encircles the planet, formed by the convergence of winds from the Northern and Southern hemispheres, and a second zone in the mid-latitudes, where jet streams “organize” the formation of cloud systems.

Analysis of 35 years of satellite imagery shows a gradual narrowing of these equatorial cloud bands as mid-latitude storm tracks move poleward. This movement limits their formation area and reduces their overall coverage. The persistence of these observations over several decades, confirmed by different instruments, rules out the hypothesis of a simple natural variation.

A less cloudy sky, a warmer climate

Satellite analyses prove that 80% of the variations in atmospheric reflectivity – this capacity of our planet to send sunlight back into space – come exclusively from the reduction of cloud surfaces. The transformation of their composition, the reduction of atmospheric pollution or the loss of reflective icy surfaces are therefore not the cause.

Researchers have long sought to explain this imbalance by two main factors. On the one hand, the decrease in industrial aerosols, these suspended particles that, paradoxically, partially protected the Earth from solar radiation. On the other hand, the melting of polar ice and glaciers which, by exposing darker surfaces, increases the absorption of heat by our planet. However, the most rigorous calculations demonstrate that these two phenomena are not sufficient to justify the extent of the observed warming.

George Tselioudis' team has demonstrated that the key lies in the profound modification of atmospheric circulation systems. These global air currents, the true architects of our climate, are undergoing transformations that directly alter the formation and distribution of clouds. This discovery not only changes our understanding of current warming – it will necessarily force us to reconsider our future climate projections. If clouds play a more important role than expected in cooling the planet, their reduction will mechanically accelerate global warming.

The mystery of the Pacific Ocean

The situation in the Pacific Ocean is very strange and presents scientists with a real enigma. Climate models predict an accelerated warming of its eastern part, likely to weaken a major branch of large-scale atmospheric circulation. However, Current observations show a cooling of this area, strengthening these winds on the contrary. A contradiction that is still poorly understood.

At the same time, other observations indicate a weakening of the rest of the atmospheric circulation. This divergence between different components of the climate system considerably complicates predictions on the future evolution of cloud cover. As Tiffany Shaw, a geophysicist at the University of Chicago, points out, only continued observation of the phenomena will make it possible to remove these uncertainties.

The implications of this discovery are deeply worrying the scientific community. Bjorn Stevens of the Max Planck Institute for Meteorology says the changes could indicatea climate feedback mechanism of unprecedented magnitude.

A climate feedback is like an infinite loop that feeds on itself. A positive feedback amplifies the initial change. For example, global warming melts ice. If there is less ice, that means there is less white surface to reflect sunlight. Our planet then absorbs more heat, which accelerates warming even more. It is therefore a perfect vicious circle that amplifies itself.

What Stevens suggests is that there may be a particularly powerful positive feedback mechanism linked to the changes observed in the Pacific Ocean. This could significantly amplify global warming, making it faster and more intense than current climate models predict. Scientists are currently continuing their research to untangle this web of causes and effects, knowing that understanding these mechanisms is essential to refine our climate models and anticipate future changes in the Earth's climate system. The consequences could be dramatic: accelerated sea level rise, increased extreme weather events, and destabilization of ecosystems. A painting now sadly known to all.

• NASA satellites show a gradual decrease in clouds, a major contributor to global warming.
• This phenomenon is linked to changes in atmospheric circulation rather than pollution or melting ice.
• A climate anomaly in the Pacific Ocean could reveal a feedback amplifying warming beyond current projections.

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