Scientists confounded by the rapid onset of extreme temperatures last year believe they have identified a previously unidentified driver.

One expert says the problem suggests the planet may exceed the 1.5°C Paris Agreement target sooner than forecast.

In 2023 the global average temperature soared to almost 1.5°C above the preindustrial level, setting several new records. And deeply concerning atmospheric scientists.

Now, new climate modelling research has identified a previously unknown driver of this sudden increase: a record low “planetary albedo” – the percentage of incoming solar radiation that is reflected out to space.

They found it was due to reduced low cloud cover.

This effect accounts for 0.2°C of 2023’s warming, which has not been satisfactorily explained before.

“The 0.2-degree-Celsius ‘explanation gap’ for 2023 is currently one of the most intensely discussed questions in climate research,” says Dr Helge Goessling, lead author of the study from the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research in Germany.

“In addition to the influence of El Niño and the expected long-term warming from anthropogenic greenhouse gases, several other factors have already been discussed that could have contributed to the surprisingly high global mean temperatures since 2023,” says Goessling.

The factors include increased solar activity, large amounts of water vapour from the Hunga Tonga-Hunga Ha’apai volcanic eruption, and fewer aerosol particles in the atmosphere.

To identify the cause of the remaining 0.2°C of warming, Goessling and colleagues looked at satellite data from NASA and data from the European Centre for Medium-Range Weather Forecasts (ECMWF).

“What caught our eye was that, in both datasets, 2023 stood out as the year with the lowest planetary albedo,” says co-author Dr Thomas Rackow from the ECMWF.

“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.”

The albedo of the surface of the Earth has been declining since the 1970s. At first this was mainly due to the decline of Arctic sea-ice and snow, resulting in less white area to reflect sunlight back out to space. Since 2016, this his has been exacerbated by sea-ice decline in the Antarctic.

“However, our analysis of the datasets shows that the decline in surface albedo in the polar regions only accounts for roughly 15% of the most recent decline in planetary albedo,” says Goessling.

So, what caused the near-record drop in planetary albedo in 2023? The researchers say it can be attributed to a decline in low-altitude clouds in the northern mid-latitudes and the tropics. 

“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 10 years,” says Goessling.

The data shows that cloud cover at low altitudes has declined, while declining only slightly, if at all, at moderate and high altitudes.

This distinction is important because while clouds at all altitudes reflect sunlight back to space to produce a cooling effect, those at high altitudes also trap heat emitted from the Earth’s surface to produce a warming effect.

“Essentially it’s the same effect as greenhouse gases,” says Goessling. “If there are fewer low clouds, we only lose the cooling effect, making things warmer.”

The researchers suggest several factors are contributing to the problem.  Lower concentrations of aerosols in the atmosphere, which act as “condensation nuclei” essential to cloud formation and also reflect sunlight, may play a role.

Global warming itself may also be 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,” says Goessling.

“We could see global long-term climate warming exceeding 1.5°C sooner than expected to date.

“The remaining carbon budgets connected to the limits defined in the Paris Agreement would have to be reduced accordingly, and the need to implement measures to adapt to the effects of future weather extremes would become even more urgent.”  

The research appears in the journal Science.