Unexpected source to blame for rapid surge in global warming

Rising sea levels, melting glaciers, and ocean heatwaves — 2023 brought alarming new climate warming records in just about every measurable category.

One of the most concerning was the rise in global mean temperature, reaching almost 1.5 degrees Celsius (2.7 degrees Fahrenheit) above pre-industrial levels.

Scientists have been working to determine what triggered this sudden surge in global warming.

They considered anthropogenic influences like greenhouse gases, the El Niño effect, and natural events like volcanic eruptions.

Yet, these factors leave an unexplained warming gap of 0.2 degrees Celsius.

New theory: Changes in cloud cover

A research team from the Alfred Wegener Institute (AWI) proposes that changes in Earth’s cloud cover may explain this warming gap.

“In addition to the influence of El Niño and the expected long-term warming from anthropogenic greenhouse gases, several other factors have already been discussed that could have contributed to the surprisingly high global mean temperatures since 2023,” said Dr. Helge Goessling, lead author from AWI.

Dr. Goessling mentioned factors like increased solar activity, excess water vapor from volcanic eruptions, and fewer atmospheric aerosol particles.

But adding these elements still leaves 0.2 degrees Celsius (0.36 degrees Fahrenheit) unexplained.

The 0.2-degree gap mystery

“The “0.2-degree-Celsius ‘explanation gap’ for 2023 is currently one of the most intensely discussed questions in climate research,” noted Dr. Goessling.

To investigate further, climate modelers from AWI and the European Centre for Medium-Range Weather Forecasts (ECMWF) analyzed satellite data from NASA.

They also used the ECMWF’s reanalysis data, which combines observational data with weather models. The data revealed an alarming trend – 2023 recorded the lowest planetary albedo since at least 1940.

Understanding the albedo effect

Planetary albedo is all about how much sunlight a planet reflects back into space. Think of it as the planet’s “shine factor.”

Earth, for example, reflects about 30% of the sunlight that hits it, which means its albedo is 0.3 (on a scale from 0 to 1). Different surfaces on a planet contribute to this reflection.

Bright areas like ice caps and clouds bounce a lot of light back, while darker areas like oceans and forests absorb more sunlight.

The balance of reflected and absorbed sunlight plays a huge role in shaping a planet’s climate and energy budget.

On Earth, changes in albedo can have a big impact. When ice melts due to warming, it exposes darker water or land underneath, which absorbs more sunlight and speeds up the warming process — a feedback loop known as the albedo effect.

“What caught our eye was that, in both the NASA and ECMWF datasets, 2023 stood out as the year with the lowest planetary albedo,” said study co-author Dr. Thomas Rackow from ECMWF.

Lower albedo means less sunlight reflected, contributing to increased global temperatures. But what caused this drop in albedo?

Low-altitude clouds and warming surge

The answer lies in declining low-altitude clouds, particularly in the northern mid-latitudes and tropics. This trend was most pronounced over the Atlantic, which coincidentally saw unusual heat records in 2023.

“It’s conspicuous that the eastern North Atlantic, which is one of the main drivers of the latest jump in global mean temperature, was characterized by a substantial decline in low-altitude clouds,” said Dr. Goessling.

The decline in low cloud cover means a reduction in Earth’s albedo, intensifying global warming.

Role of clouds in climate regulation

Clouds play a critical role in moderating Earth’s temperature. They reflect sunlight, providing a cooling effect.

Low clouds are especially important since they lack the greenhouse effect seen in higher clouds, which trap heat.

As lower cloud cover declines, the cooling effect diminishes, contributing to warmer temperatures. “If there are fewer low clouds, we only lose the cooling effect, making things warmer,” Goessling explained.

Factors contributing to fewer low clouds

But what’s causing fewer low-altitude clouds? A combination of factors could be at play.

Stricter marine fuel regulations have reduced atmospheric aerosols — tiny particles that serve as condensation nuclei for cloud formation.

Aerosols also reflect sunlight, and fewer aerosols mean less cloud formation and less sunlight being reflected.

Natural climate fluctuations and ocean feedbacks might also contribute, but Goessling believes these aren’t the whole story. “Global warming itself may be reducing low cloud cover,” he said.

Warming surge and cloud loss

This feedback loop between warming and cloud loss could indicate a worrying trend.

“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,” noted Dr. Goessling.

This means we might cross the critical 1.5-degree threshold of global warming sooner than anticipated.

If that happens, the remaining carbon budgets under the Paris Agreement would shrink, making adaptation measures even more urgent.

Cloud cover and global warming

The study sheds light on why temperatures surged so suddenly and highlights the importance of Earth’s cloud cover in regulating climate.

Reduced cloud cover, particularly at low altitudes, lessens Earth’s ability to reflect solar radiation, adding to the warming already driven by greenhouse gases.

With climate models pointing to this feedback as a significant factor, urgent action becomes even more vital to manage future warming and minimize extreme weather events.

The study is published in the journal Science.

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