Why has it been even hotter than expected recently? Research points to diminishing cloud cover

Temperatures around the world have risen far higher over the past two years than scientists expected. The trend has given rise to a puzzle: Are hidden climate change dynamics behind the sudden shift?

Last year was the hottest in recorded history, and through summer, 2024 was on pace to be hotter. Even after factoring in the expected effects of greenhouse gas pollution and El Niño — a natural pattern that typically boosts temperatures — researchers couldn’t account for roughly 0.2 degrees Celsius (0.36 degrees Fahrenheit) of the warming observed in 2023.

A new study offers a possible explanation: Cloud cover has decreased over the past two years, it found, allowing more light to reach and heat the Earth’s surface, rather than being reflected back to space. 

The research, published Thursday in the journal Science, suggests that an overall decline in the planet’s albedo, as that dynamic is called, is a likely cause of the temperature anomaly observed in 2023.

“This pretty much fits this most recent additional increase of observed solar radiation,” said Helge Goessling, an author of the study and a climate physicist at the Alfred Wegener Institute in Germany.

The expected behavior of clouds in a warmer world has been one of the toughest aspects of the climate system to study and model. Answering questions about it will help scientists better determine how sensitive the planet is to greenhouse gas emissions. 

If the reductions in low cloud cover aren’t a result of chance, it likely means Earth is warming even faster than scientists thought. 

“It’s not really clear yet to what extent some of this might be variability that goes away again,” Goessling said. “It shifts the odds towards a higher-than-to-be-expected warming.”

The new research is based on an analysis of climate models and NASA satellite data about the Earth’s reflectivity. It outlines three possible reasons why fewer low clouds are developing, but doesn’t offer conclusions about how much each factor is contributing.

One option is that a natural process is temporarily diverging from normal, causing a reduction in cloud cover. It’s possible, for example, that natural variability is causing the ocean’s surface to warm more than expected, and that is shifting the physics of how clouds form. 

A second possibility is changes to maritime shipping regulations: The International Maritime Organization in 2020 imposed limits on the sulphur content allowed in maritime fuels. Some scientists think that reducing the number of sulphur particles that pollute the atmosphere could have the unintended effect of dampening marine cloud formation. 

“Because these act as condensation nuclei for clouds, they can make clouds brighter and also more long-lived,” Goessling said of the sulphur particles.

The third option is that unidentified feedback loops in the climate system are causing clouds to decrease because of global warming. 

If the latter two possibilities turn out to be primary contributors, that would mean the climate is more sensitive to human pollution than many scientists had thought — and, therefore, that humanity is closer to exceeding the targets world leaders set for limiting emissions than previously realized. (The term “climate sensitivity” refers to how warm the planet would be if the concentration of carbon dioxide and other greenhouse gases in the atmosphere doubled.) 

Still, many questions remain, said Zeke Hausfather, the climate research lead at the finance company Stripe and a research scientist at Berkeley Earth.  

“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,” he said in an email.

According to the National Oceanic and Atmospheric Administration, average land and ocean surface temperatures in 2023 were about 2.12 degrees Fahrenheit above 20th-century averages. 

World leaders’ efforts to reduce greenhouse gas emissions remain insufficient. Global temperatures are on pace to rise more than 3 degrees Celsius (5.4 degrees Fahrenheit) on average — well above the goal of 1.5 degrees Celsius (2.7 degrees Fahrenheit) set in the Paris Agreement.