Fewer Low-Altitude Clouds Helped Fuel 2023’s Record-Breaking Temperatures

A new study might have discovered the reason behind the unexpected level of global temperature rise that was recorded over the course of 2023: a decrease in cloud cover, especially over the ocean, has been allowing more sunlight to reach the surface, contributing a fifth of a degree Celsius (0.36°F) to the year’s record-breaking 1.45°C (2.61°F) above the pre-industrial average. This decrease in the amount of reflective clouds itself appears to be a result of a warming climate, yet another feedback cycle that may contribute to the acceleration of climate change.

As the hottest year on record, 2023 handily beat the previous record holder, a double-El Niño-fueled 2016, by 0.16°C (0.29°F); however, when climatologists conducted an autopsy of the factors that contributed to such an escalation in the warming of our planet, including taking into account the effects of human-based greenhouse gas accumulation, El Niño, an increase in solar output and other natural phenomena, there was still about 0.2°C (0.36°F) of warming that couldn’t be accounted for.

But a research team, coordinated by Germany’s Alfred Wegener Institute for Polar and Marine Research (AWI), may have found the answer: a decrease in low-altitude clouds appears to have lowered the overall albedo, or reflectiveness, of the planet, allowing more solar radiation to reach the surface.

“What caught our eye was that, in both the NASA and ECMWF datasets, 2023 stood out as the year with the lowest planetary albedo,” study co-author European Center for Medium-Range Weather Forecasts (ECMWF) climatologist Dr. Thomas Rackow said in regards to their investigation into the discrepancy.

“We had already observed a slight decline in recent years,” Rackow continued. “The data indicates that in 2023, the planetary albedo may have been at its lowest since at least 1940.”

Earth’s albedo has been in decline since the 1970s, due mostly to a reduction in Arctic sea ice, with the Antarctic following suit in more recent years, however “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,” explained AWI study lead Dr. Helge Goessling: if sea ice loss was the sole culprit for a lack of Earthly reflectiveness, the climate would be about 0.23°C (0.41°F) cooler.

But when the team looked at Earth’s cloud cover, particularly over the North Atlantic, they found a marked drop in the rate of cloud formation, a factor that could account for the missing planetary albedo.

“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 not just in 2023, but also—like almost all of the Atlantic—in the past 10 years,” Goessling pointed out, finding that the northern mid-latitudes and the tropics were particularly affected.

Regardless of what regions have been affected, most of this reduction in cloud cover has occurred at lower altitudes, with mid-and high-altitudes actually seeing little change. This is an important factor in how clouds either reflect or retain energy: while clouds at all altitudes reflect solar radiation back into space, it’s the denser, brighter formations closer to the surface that are the most reflective, preventing a portion of the light from reaching the surface. Conversely, high-altitude clouds, being somewhat darker than their lower-level counterparts, tend to absorb the infrared radiation being emitted from the surface before it can reach space.

“Essentially, it’s the same effect as greenhouse gases,” if any given cloud layer absorbs energy instead of reflecting it, according to Goessling. “If there are fewer low clouds, we only lose the cooling effect, making things warmer.”

While determining the cause of this decrease in cloud cover was outside of the scope of the study, the team did speculate on why this is happening; although there could be a variety of causes, ranging from natural variabilities in global climate to the more recent discovery that a reduction in sulfur-based pollution from global marine traffic has been reducing cloud formation, they believe the culprit may be global warming itself.

While global warming increases the amount of moisture in the atmosphere—warmer air can hold more water vapor—clouds form when that moisture condenses on dust particles, a process that requires cool conditions for the droplets to form; think of dew forming on the ground on a cool early morning, just much higher up. But if conditions are too warm, as is increasingly becoming the case in the lower atmosphere, this doesn’t happen.

“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 warns.

“We could see global long-term climate warming exceeding 1.5 degrees Celsius sooner than expected to date. The remaining carbon budgets connected to the limits defined in the Paris Agreement would have to be reduced accordingly, and the need to implement measures to adapt to the effects of future weather extremes would become even more urgent.”