Fewer clouds at low altitude could explain the 'missing' 0.2 degrees of warming from the hottest year on Earth: study

Fewer clouds at low altitude could explain the ‘missing’ 0.2 degrees of warming from the hottest year on Earth: study

A drop in the number of low-altitude clouds was behind the unexplained warming that contributed to the world’s hottest year on record, a new study suggests.

The record-breaking heat of 2023, which saw the planet warm an average of 1.45 degrees Celsius above pre-industrial surface temperatures, took many climate scientists by surprise.

Their closest predictions, which simulated the effects of human-induced warming and other known factors, were about 0.2 degrees Celsius lower than observed temperatures.

Experts could not fully explain the additional heat source at the time.

Now research published in Science today suggests that the missing warming mechanism from the forecast models was low “planetary albedo”.

That is the amount of solar radiation, including heat, that is reflected into space. The less radiation is reflected, the more it reaches the Earth and warms the atmosphere.

A young balding man in a blue denim shirt with collar and close-up portrait. — Climate physicist Helge Goessling. (Supplied: Alfred Wegener Institute)

Lead author Helge Goeslling, a climate physicist at the Alfred Wegener Institute in Germany, said 2023 appears to have the lowest albedo level since 1940.

Earth’s albedo, a Latin word meaning “whiteness,” has been declining for decades, with much of that decline linked to a decline in large reflective areas such as sea ice and snow cover in polar regions.

But Dr. Goeslling said the study suggested that a decrease in another white substance was largely responsible for the 0.22 degrees Celsius warming in 2023.

“What we find is that it is very clearly related to changes in the cloud and in particular to changes in low cloud levels,” he said.

But what this decrease in low cloud cover means for the future is uncertain.

What do clouds have to do with it?

All clouds have some degree of cooling effect on the planet, as they reflect the sun’s rays away from the planet.

But high-level clouds, which form in cold atmospheric layers, also act like a blanket, trapping heat from below.

Low clouds, which form within two kilometers of the Earth’s surface, reflect radiation but do not trap as much heat.

Three panels show the reflection of solar radiation and energy retention for high, no and low clouds. — Clouds at low altitudes allow more energy to escape into space, compared to when there are high or no clouds which cause more warming of the atmosphere. (Supplied: Alfred-Wegener-Institut/Yves Nowak)

To assess how low cloud cover changed in 2023, the new study used data from the European Center for Medium-Range Weather Forecasts and radiation measurements from NASA’s Clouds and Earth’s Radiant Energy System.

The analysis suggested that there was much less low cloud cover over the northern and tropical oceans in 2023, especially in the Atlantic Ocean. and this could explain the warming of 0.2 degrees Celsius.

But the reasons for this drop in low cloud cover are unclear.

Dr. Goeslling said changes in aerosol use by humans could affect low cloud cover. Climate change can also alter the formation of low clouds.

A third reason could be natural regional variability, which is variations in the climate system beyond human influence.

“We dare not give figures that contribute to the extent to which we can do this,” said Dr. Goeslling.

“I view our research as just one piece of the puzzle.”

An animated gif showing how monthly temperatures are getting hotter above the pre-industrial average. — A climate spiral showing monthly global temperature anomalies from 1880 through October 2024. (Provided: NASA/Scientific Visualization Studio)

Gavin Schmidt, director and climatologist at NASA’s Goddard Institute for Space Studies, who was not involved in the study, said the study “explains to some extent the process of recent warming.”

“But we still can’t say why the albedo has changed so much, and so more needs to be done before we can say what this means in the future.”

A white man in a dark sweater, standing with his arms crossed in front of a globe. — NASA Senior Climate Advisor Gavin Schmidt. (Provided: NASA)

Climate researcher Zeke Hausfather of the nonprofit data science group Berkeley Earth, and who was also not involved in the study, found the study provided a useful assessment of changes in cloud cover.

“Although it raises as many questions as it provides answers,” he said.

‘We are still unsure whether these changes in cloud behavior are not due to short-term variability – which would return to more normal conditions over time – or whether they represent a new ongoing change in the climate system.

“If they represent ongoing change, it remains difficult to disentangle how much could be due to changing human aerosol emissions versus a feedback from human greenhouse gas emissions.”

Dr. Hausfather said both cases were not good news because they both suggested a warmer future.

What can we expect?

If global warming produces fewer low clouds, the world can more quickly exceed its commitment to try to keep long-term annual warming averages below 1.5 degrees Celsius.

But further research is needed to find out what exactly is behind the decline of the cloud.

Dr. Goeslling and his colleagues would also like to analyze what impact albedo will have had in 2024.

According to Dr. Goeslling, climate scientists should have sufficient information in the coming years to make better climate predictions and reduce uncertainty.

In the meantime, it looks like 2024 is almost guaranteed to take the crown from 2023 as the warmest year on record.