Scientists have found a surprising explanation for unusually sudden global temperature rises: disappearing clouds.
The climate crisis is consistently setting terrifying new records – 2024 is set to be the hottest year yet and the first to have an average temperature of more than 1.5°C above pre-industrial levels.
Sea levels, glacier melt and heatwaves at sea have all hit their highest levels ever recently, caused by a sudden rise in global temperatures. But scientists have been struggling to explain why temperatures have shot up at such speed.
Calculating the effects of greenhouse gases, weather phenomena and natural events such as volcanic eruptions still leaves an unexplained gap of around 0.2°C.
But a team of researchers from Germany’s Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) believes they have identified another cause for the sudden rise in global temperatures: the Earth has become less able to reflect the sun’s rays –because certain types of clouds are disappearing.
2023 set a new record for ‘low planetary albedo’
Several causes of global warming are already accounted for: El Niño and the expected long-term warming from anthropogenic greenhouse gases with increased solar activity, large amounts of water vapour from a volcanic eruption and fewer aerosol particles in the atmosphere.
But there is still 0.2°C warming with no readily apparent cause, lead author of the study, Dr Helge Goessling, explains.
“The 0.2°C ‘explanation gap’ for 2023 is currently one of the most intensely discussed questions in climate research,” says Dr Goessling.
When climate modellers from the AWI and the European Centre for Medium-Range Weather Forecasts (ECMWF) examined their data, which goes back to the 1940s, alongside NASA’s statistics, they noticed something unusual.
“2023 stood out as the year with the lowest planetary albedo,” says study co-author Dr Thomas Rackow of the ECMWF.
‘Planetary albedo’ is science speak for the percentage of the sun’s rays that get reflected back into space. Low planetary albedo worsens global warming and could explain the ‘missing’ 0.2°C.
But what’s behind the reduction in planetary albedo?
The Earth’s albedo has been on a downward trajectory since the 1970s, partly due to a reduction in Arctic snow and sea ice – meaning there are fewer white areas of the planet to reflect back sunlight to space.
Since 2016, the albedo reduction has also been exacerbated by sea-ice decline in the Antarctic, but there’s more to it, as Dr Goessling explains.
“The decline in surface albedo in the polar regions only accounts for roughly 15 per cent of the most recent decline in planetary albedo,” he says. What’s more, the albedo has declined outside of the polar regions, too.
When the researchers calculated the effects of lowered albedo using complex climate models, they found that without reduced albedo, the mean temperature last year would have been around 0.23°C lower.
So, what is causing this significant effect on the planetary albedo? The AWI research team has identified one cause: a decline in low-altitude clouds in the northern mid-latitudes and the tropics.
There’s a marked decline in lower-altitude clouds over the Atlantic Ocean
The Atlantic stands out as one of the regions where the most unusual temperature records were set in 2023 – and it’s here that certain clouds are disappearing.
“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,” says Dr Goessling.
The NASA/ECMWF data examined by researchers shows that the cloud cover at low altitudes has declined – but that cloud cover has barely declined at moderate and high altitudes.
All clouds reflect sunlight to create a cooling effect on the Earth
Clouds in high, cold atmospheric layers also create warming because they act in a similar way to greenhouse gases, holding warmth emitted from the planet’s surface in the atmosphere.
Lower-altitude clouds don’t have the same effect: “If there are fewer low clouds, we only lose the cooling effect, making things warmer,” says Dr Goessling.
But why are there fewer low clouds in the sky?
One explanation for the decline in low-altitude clouds is that there are lower concentrations of anthropogenic aerosols in the atmosphere, which could be due to recent stricter regulations around marine fuel. Aerosols play an essential part in cloud formation and contribute to the albedo by reflecting sunlight.
Natural fluctuations and ocean feedbacks could also affect changes in cloud formation. Yet Dr Goessling suggests that global warming itself is reducing the number of low clouds.
“If a large part of the decline in albedo is indeed due to feedbacks between global warming and low clouds, as some climate models indicate, we should expect rather intense warming in the future,” he warns.
“We could see global long-term climate warming exceeding 1.5°C sooner than expected to date. The remaining carbon budgets defined in the Paris Agreement would have to be reduced and measures to adapt to future weather extremes would become even more urgent.”
