2023 was the warmest year on record – 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, climate physicist at the German Alfred Wegener Institute, decided to investigate the planet's albedo – its reflectivity – together with some colleagues.
Rebounding into space
When the sun's rays reach the planet, some of them bounce back into space.
If you imagine the Earth's surface, there are certain very reflective surfaces like ice, snow, and deserts, while the oceans, on the other hand, are quite dark, says Gössling.
The oceans absorb about 95 percent of the heat if the sun shines directly on them.
But if you have clouds instead of dark oceans, you get an albedo of 50 percent, if it's cloudy.
When the researchers examined the low-lying cloud cover for 2023 using satellite data, they found that it was unusually weak, about 4 percent less than a normal year, particularly over the tropics. This could mean that the Earth's reflective ability during the year was the lowest since the 1940s, and sufficient to explain the difference of 0.2 degrees.
The 0.2 degrees are significant, since it's actually about the current warming. If the long-term temperature increase is around 1.3-1.4, then it's more than a tenth of that, says Gössling.
Alarming theory
So where have the clouds gone? Gössling suggests natural variations or that shipping has reduced its emissions of sulfur dioxide, which could have led to reduced cloud formation. A third, more alarming theory, is that climate change is causing the decrease. If that's the case, a warmer climate leads to fewer clouds, which in turn leads to the Earth warming up faster, which could mean that climate models need to be adjusted.
I wouldn't say that this changes how worried you should be about global warming in the future, says Gössling and continues:
But you can 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's very interesting to see the 0.2 degrees linked to this, and it's very reasonable, he says.
Corrected: In an earlier version of the text, the wrong year was stated in some places.
2023 became a record warm year, nearly 1.5 degrees warmer than the average temperature in pre-industrial times, and 0.17 degrees warmer than the previous record.
Using satellite data and so-called reanalysis, the researchers found that the planet reflected a record low amount of the sun's heat radiation, which was largely due to an unusually weak cloud cover at low altitudes.
The study is published in the journal Science.
