For more than twenty years, NASA instruments carefully observe a worrying phenomenon : a growing imbalance between the solar energy which enters the earth’s atmosphere and that which leaves it. If 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 magazine Surveys in Geophysics finally comes to provide answers, bringing on stage an unexpected actor: our clouds.
Cloud cover, protective of our climate
Data collected by NASA’s Terra satellite, which has been analyzing our planet for nearly a quarter of a century, reveals a slow but steady decrease in global cloud cover. George Tselioudis, climatologist at NASA’s Goddard Institute for Space Studies, puts forward these figures: every decade, we lose 1.5% of our clouds on a global scale.
This development is particularly evident in two distinct areas : an equatorial band which 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 satellite images over 35 years shows a gradual shrinking of these equatorial cloud bands, while the tracks of mid-latitude storms shift toward the poles. This move limits their training 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.
Less cloudy skies, a warmer climate
Satellite analyzes prove that 80% of variations in atmospheric reflectivity – our planet’s ability to reflect 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 reduction in industrial aerosols, these suspended particles which, 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 demonstrated that the key lies in the profound modification of atmospheric circulation systems. These planetary air currents, the true architects of our climate, undergo transformations which 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 accelerated warming of its eastern part, likely to weaken a major branch of large-scale atmospheric circulation. Yet, current observations show a cooling of this areaon the contrary strengthening these winds. A contradiction 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 about the future evolution of cloud cover. As Tiffany Shaw, geophysicist at the University of Chicago, points out, only continued observation of phenomena will allow these uncertainties to be resolved.
The implications of this discovery deeply concern the scientific community. Bjorn Stevens of the Max Planck Institute for Meteorology explains that these changes could indicate a climate feedback mechanism of unprecedented magnitude.
A climate feedback is a bit like an infinite loop that feeds itself. Positive feedback amplifies the initial change. For example, global warming is melting ice. If there is less ice, that means there is less white surface area to reflect sunlight. Our planet then absorbs more heat, which further accelerates warming. It is therefore a perfect vicious circle that amplifies itself.
What Stevens therefore suggests is that there may be a particularly powerful positive feedback mechanism linked to the changes observed in the Pacific Ocean. The latter could considerably amplify global warming, making it faster and more intense than what current climate models predict. Scientists are currently continuing their research to untangle this tangle of causes and effects, aware that understanding these mechanisms is essential to refining our climate models and anticipate future developments in the Earth’s climate system. The consequences could be dramatic: accelerated rise in water levels, increase in extreme weather events, and destabilization of ecosystems. A painting now sadly known to all.
- NASA satellites show a gradual decrease in clouds, contributing largely 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 predictions.
