Disappearing clouds may explain mysterious heat

 

Greenhouse gases
Disappearing clouds may be behind mysterious heat

TT

Updated 10.05 | Published 06.57

2023 offered unusually few clouds. It coincided with the planet's average temperature also being at a record high. German researchers believe there is a connection. Archive photo from India. Photo: Mukhtar Khan/AP/TT

The Earth was inexplicably warm the year before last.

The models have so far failed to explain why.

But now German researchers may be a worrying solution to the clues – some of the clouds have disappeared.

2023 was a record warm year on Earth – 1.5 degrees warmer than pre-industrial times. The weather phenomenon El Niño and human emissions of greenhouse gases account for a large part of the temperature increase – but not all of it, according to the researchers' models. 0.2 degrees of the increase could not be explained.

Helge Gössling, a climate physicist at the German Alfred Wegener Institute, decided together with some colleagues to look at the planet's albedo – its reflectivity.

Helge Gössling at the Alfred Wegener Institute. Press photo. Photo: Kerstin Rolfes/Awi

Bounces into space

When the sun's rays reach the planet, some of them bounce back into space.

- If you imagine the surface of the Earth, there are some very reflective surfaces such as ice, snow and desert, while the oceans, on the other hand, are quite dark, says Gössling.

The oceans absorb around 95 percent of the heat if the sun shines directly on them.

- But if you have clouds instead of dark ocean, you reach an albedo of 50 percent, if it is cloudy.

When the researchers looked at the low-lying cloud cover for 2023 using satellite data, they discovered that it was unusually weak, about four percent less than a normal year, including over the tropics. It may be that the Earth's reflective ability during the year was the lowest since the 1940s, and enough to explain the difference of 0.2 degrees.

- The 0.2 degrees is a lot, because it is actually the current warming. If the long-term temperature increase is somewhere around 1.3–1.4, then it is more than a tenth of it, says Gössling.

The dark oceans reflect almost no heat from the sun's rays, but instead the heat is absorbed. When the oceans are covered by clouds, however, nearly half of the sun's heat is reflected away. Photo: Noaa/AP/TT

Worrying theory

Where have the clouds gone?

Gössling highlights natural variations or that shipping has reduced its emissions of sulfur dioxide, which may have led to reduced cloud formation. A third, more worrying theory, is that climate change is causing the decrease.

If this is true, a warmer climate leads to smaller clouds, which in turn leads to the Earth warming faster, which may mean that climate models need to be adjusted.

- I wouldn't say that this changes how worried we should be about global warming in the future, says Gössling.

- But you could say that the best possible outcome has become worse.

Cloud researcher Vaughan Phillips at Lund University says that the study is in line with previous research that has shown a connection between low clouds and climate change.

- It is very interesting to see these 0.2 degrees linked to that, and it is very reasonable, he says.

FACTS

About the study

2023 was a record warm year, almost 1.5 degrees warmer than the average temperature in pre-industrial times, and 0.17 degrees warmer than previous records.

Using satellite data and so-called reanalysis, the researchers discovered that the planet reflected a record low amount of the sun's thermal radiation, which was largely due to unusually weak cloud cover at low altitudes.

The study is published in the journal Science.