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Global mean temperatures in 2023 reached nearly 1.5 degrees Celsius above pre-industrial levels, with new research suggesting that 0.2°C of this rise may be attributed to declining low-altitude cloud cover. The cloud cover — particularly over northern mid-latitude and tropical oceans like the Atlantic — showed a sharp decline, the study, published in the journal Science, highlighted.
Low-altitude clouds play a crucial role in cooling the planet by reflecting sunlight back into space. However, their global cover fell by 1.5 per cent in 2023 compared to average levels, continuing a decade-long decline of 1.27 per cent.
This reduction has caused a measurable drop in planetary albedo — the proportion of solar radiation reflected into space after interacting with Earth’s atmosphere and surface.
Approximately 15 per cent of the albedo decline is linked to the loss of Arctic snow and sea ice, which are vital for reflecting sunlight.
“We had already observed a slight decline in recent years. The data indicates that in 2023, the planetary albedo may have been at its lowest since at least 1940,” Thomas Rackow from the European Centre for Medium-Range Weather Forecasts (ECMWF), said in a statement.
The El Niño phenomenon (the warm phase of a natural climate pattern across the tropical Pacific known as the El Niño-Southern Oscillation) and the expected long-term anthropogenic greenhouse gas emissions remain primary drivers of the 2023 temperature surge, the study identified additional contributors.
Helge Goessling, one of the authors of the study from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, explained that other factors like increased solar activity, large amounts of water vapour from a volcanic eruption, or fewer aerosol particles in the atmosphere could also explain this rise.
But if all these factors are combined, there is still 0.2 degrees Celsius of warming with no readily apparent cause, the expert added.
So the researchers analysed satellite data from NASA and ECMWF data and used a complex weather model to get a detailed understanding of how the global energy budget and cloud cover at different altitudes have evolved.
Though the decline in cloud cover was more pronounced in low-altitude clouds in the northern mid-latitudes and the tropics, the Atlantic — which saw the most unusual temperature record in 2023 — was distinct.
“It’s conspicuous that the eastern North Atlantic, which is one of the main drivers of the latest jump in global mean temperature, was characterised by a substantial decline in low-altitude clouds not just in 2023, but also — like almost all of the Atlantic — in the past ten years,” Goessling explained.
The researchers also observed a slight decline in moderate- and high-altitude clouds, which typically trap heat emitted from the surface in the atmosphere. “Essentially it’s the same effect as greenhouse gases,” the expert explained. However, the pronounced loss of cooling low-altitude clouds appears to be the more significant factor.
The decline in cloud cover could be likely explained by lower concentrations of anthropogenic aerosols in the atmosphere, especially due to stricter regulations on marine fuel. Aerosols are small particles suspended in the atmosphere and are known to influence climate, weather, health, and ecology. These aerosols play an essential part in cloud formation, while also reflecting sunlight themselves.
Other natural fluctuations and ocean feedbacks may have contributed and global warming itself could have reduced the number of low clouds, the authors speculate.
The paper also warned that if the decline in albedo was largely due to interactions between global warming and low clouds, more intense warming can be expected in the future.
