Scientists Find Another Reason Why 2023 Was So Hot: a Decline in Low-Lying Clouds

With an average temperature of almost 1.5 degrees Celsius warmer than pre-industrial levels, 2023 was the hottest year ever registered. But after breaking the record for the hottest day twice in one week this past summer—and setting other heat records throughout—2024 is now “effectively certain” to take its place. While factors including greenhouse gases, El Niño and volcanic eruptions likely played a role in the heat last year, researchers say they still don’t account for an unexplainable 0.2 degrees Celsius of warming.

Now, scientists in Germany have suggested a surprising explanation: declining cloud cover. Their findings were published last week in the journal Science.

“2023 [took] us by surprise, and 2024 continues to do so,” study co-author Thomas Jung, a climate scientist at the University of Bremen, tells CBC News’ Nicole Mortillaro. “And you know, we had some explanations for some of the warming … [but] there was this gap, this 0.2 or so degree of global warming that was unexplained. So, the idea was to find out where that was coming from.”

The team analyzed observational data and weather models from NASA satellites and the European Center for Medium-Range Weather Forecasts and discovered that 2023 had surprisingly low albedo—the amount of solar radiation a planet reflects back into space. Pale-colored surfaces, such as snow and ice, reflect more light. Albedo, appropriately, means “whiteness” in Latin, as Peter de Kruijff notes for the Australian Broadcasting Corporation.

But if Earth is reflecting less radiation, that means it’s absorbing more of it, heating the planet.

“We had already observed a slight decline [in albedo] in recent years,” Thomas Rackow, a scientist and ocean modeler at the European Center for Medium-Range Weather Forecasts, says in a statement. But “the data indicates that in 2023, the planetary albedo may have been at its lowest since at least 1940.”

When the researchers modeled what the average temperature in 2023 would have been without the recent albedo decrease seen since 2020, the result fit like a puzzle piece: The modeled temperature was approximately 0.23 degrees Celsius lower than the real-world one.

One trend appears to have mainly affected the planetary #albedo: the decline in low-altitude #clouds in the northern mid-latitudes and the tropics. The Atlantic Ocean particularly stands out - the region where the most unusual temperature records were observed in 2023. (5/8) pic.twitter.com/xCpGEGw80K

— Thomas Rackow (@thomas_rackow) December 6, 2024

The researchers then wondered what could have caused the sudden drop in albedo. Though highly reflective snow and ice is declining in the Arctic, “our analysis of the datasets shows that the decline in surface albedo in the polar regions only accounts for roughly 15 percent of the most recent decline in planetary albedo,” Helge Goessling, a climate physicist at the Alfred Wegener Institute and lead author of the study, explains in the statement.

They identified a decline in low-altitude clouds, specifically over parts of the Atlantic Ocean. This trend is consistent with the extreme 2023 temperatures seen in those same regions. Low-altitude clouds, in particular, play a strong role in cooling the planet. While all clouds reflect solar radiation back into space, those at higher altitudes also trap some radiation within the atmosphere. Lower altitude clouds, on the other hand, don’t cause this warming effect.

“It really is quite striking that this, this decrease of the cloud cover, is mostly happening in the lower levels,” Goessling tells New Scientist’s Madeleine Cuff.

Now, the new question is, what’s driving the decrease in low-lying clouds? The researchers considered several options. For example, due to recent shipping regulations, the world has seen a decrease in the use of aerosols, which reflect sunlight and sustain cloud formation. Global warming and climate change might also be partially responsible, potentially affecting how low clouds form. The true answer, however, is still unclear.

“We still do not know for sure that these changes in cloud behavior are not due to short-term variability—which would return to more normal conditions with time—or if they represent a new ongoing change to the climate system,” Zeke Hausfather, a climate scientist at Berkeley Earth who was not involved in the study, tells NBC News’ Evan Bush.

Either way, “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,” Goessling says in the statement.

Margherita Bassi | READ MORE

Margherita Bassi is a freelance journalist and trilingual storyteller. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.