According to NASA, the hottest years on record are the ten most recent years, and 2023 was the warmest on record since people started keeping records in 1880. But there have been rapid temperature increases in the past few years that scientists are rushing to try to understand. While we know that greenhouse gases like carbon dioxide and methane are to blame for climate change, there are other factors that may be causing the pace of change to accelerate.
The weather pattern known as El Niño, which refers to sea surface temperatures in the equatorial Pacific Ocean that are unusually warm, can affect weather around the globe. Other incidents like massive volcanic eruptions can have an effect on worldwide temperature averages as well. But a new study has determined that cloud loss is also having a significant effect. The planet seems to be reflecting less sunlight due to the loss of certain types of clouds, according to work reported in Science.
“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 first study author Dr. Helge Goessling of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI)
So many factors, such as high levels of solar activity, a volcanic eruption that throws large amounts of water vapor into the atmosphere, or reduced atmospheric aerosol particles can have an effect on global temperatures, but they cannot explain all of the warming that was recorded in 2023. Researchers calculated that a gap of about 0.2 degrees Celsius could not be sufficiently explained by all of these events.
“The '0.2-degree-Celsius explanation gap’ for 2023 is currently one of the most intensely discussed questions in climate research,” added Goessling.
The investigators took another look at data from NASA, and the ECMWF for 2023. In both datasets, the researchers found that 2023 had the least planetary albedo, which refers to the level of solar radiation that gets reflected back to space after it has interacted with the Earth's surface, explained study co-author Dr Thomas Rackow of the ECMWF.
While there was a recent decline already, planetary albedo was the lowest in 2023 that it had been since 1940, at least. This could explain the 0.2 degrees Celsius gap. Earth's surface albedo has been declining since the 1970s, in part because of Arctic sea ice loss, and since 2016, in the Antarctic too. This can account for about 15 percent of the decline.
But albedo has also been lost from other places. Some significant loss has occurred in the clouds; there was a reduction in low-altitude clouds in the northern mid-latitudes as well as the tropics. Moderate and high altitude clouds did not seem to be as affected. This low-level cloud loss also coincides with an area of major warming; The Atlantic had a lot fewer clouds and was much warmer in 2023.
The atmosheric level of cloud loss is also important, because clouds that are at upper altitudes can reflect sunlight, but they are have a warming effect by trapping heat in the atmosphere.
“Essentially it’s the same effect as greenhouse gases,” said Goessling. But lower clouds don’t have the same effect. “If there are fewer low clouds, we only lose the cooling effect, making things warmer.”
The researchers suspect that a reduction in atmospheric aerosols may be part of the reason why there are fewer low-altitude clouds. But Goessling is concerned that it is a consequence of global warming.
“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,” he noted.
Sources: Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research; Science