Fine particles from ship exhaust act as seeds for clouds, as shown in this satellite image. Stricter rules on marine pollution mean there are fewer tracks for ships to reflect sunlight back into space. This may be one reason why the Earth is warming faster than models predict, but not the only one, a new study says. Credit: NASA/GSFC/Jeff Schmaltz/MODIS Land Rapid Response Team
In a study published today on Scienceresearchers say they have solved a climate puzzle: the inexplicable rise in global temperatures in 2023, which will rise faster than climate models predict.
By analyzing satellite data and weather records, a team of climate scientists in Germany found that the culprit is likely fewer clouds at low altitudes, below about 10,000 feet (3,000 meters). Clouds play a crucial role in keeping the Earth cool by reflecting sunlight back into space, and clearer skies mean more sunlight reaches the Earth.
The lack of low-level clouds had previously gone unnoticed because studies based on satellite images had been unable to distinguish low-level clouds from higher ones.
What is worrying is that this trend for clearer skies at low altitudes could be the result of global warming itself, meaning the Earth could be entering a feedback loop that could further accelerate warming.
Balancing the Earth’s energy balance
The surprising warming trend in 2023 was first noticed in the North Atlantic, but “the warming turned out to be more widespread,” says Helge Goessling, lead author of the study and a climate physicist at the Alfred Wegener Institute in Bremerhaven, in Germany. His team also noticed anomalies related to high temperatures in the North Pacific and near the equator.
To understand Earth’s changing climate, scientists need to understand how much energy is absorbed by the Earth, how much is trapped in the atmosphere by greenhouse gases, and how much sunlight is reflected back into space before it reaches the ground. Clouds are crucial as they reflect about 50% of the sunlight that reaches them. In contrast, the oceans reflect only 5%.
But climate scientists have been unable to explain all of last year’s anomalous temperature rise. To be precise, 0.2 degrees Celsius (0.36 degrees Fahrenheit) of warming could not be accounted for even after including factors such as the Sun’s peak activity, losses of polar ice, and decreases in fine particles (aerosols). ) in the atmosphere.
In other words, Earth’s overall reflectivity – what scientists call albedo – had decreased, and scientists didn’t know why.
“What happened couldn’t easily be explained by El Niño or other contributors,” Goessling says. “That’s where these decreases in low clouds come in.”
Goessling’s team began to focus on low-level clouds and how they affected Earth’s energy balance. In particular, they used satellite images from NASA to monitor cloud cover and meteorological records compiled by the European Center for Medium-Range Weather Forecasts (ECMWF) to monitor cloud density at different altitudes.
NASA’s Clouds and the Earth’s Radiant Energy System (CERES) project compiles satellite data over extended periods of time to take stock of Earth’s radiation budget, tracking the amount of incoming sunlight our planet absorbs versus the amount of infrared energy emitting into space. Meanwhile, ECMWF’s ERA5 project compiles and analyzes various data from satellites, weather balloons, and atmospheric instruments on an hourly basis from sea level to an altitude of 50 miles (80 kilometers), and has done so since 1940.
Since CERES only indicates total cloud cover, ERA5 was needed to determine cloud density at different atmospheric levels. Using the ERA5 data, Goessling’s team was able to refine their interpretations of satellite imagery, which indicated a deficit of lower-level clouds while upper-level clouds remained stable.
Captured in a feedback loop
So what is causing the lack of low-level clouds? It could be the warm atmosphere itself.
“When we talk about warming induced by greenhouse gases, many climate models show us that this also has an effect on clouds, and in particular on low-altitude clouds,” says Goessling.
Goessling says the decrease in low-level clouds may also be due in part to a decline in coal burning and tighter controls on discharges from marine vessels. The fine particles present in such pollution act as seeds for the formation of clouds. The irony is that as we clean up the air, we may also be unleashing further climate change. Fewer clouds reflecting less sunlight means more warming.
Another feedback effect, Goessling says, is that as the Earth warms, “you also tend to see upper-level clouds moving into the colder parts of the upper troposphere.” Colder clouds mean they radiate less energy into space, leaving more energy – and heat – in the atmosphere.
“It is also possible that ocean feedbacks have led to strong warming of the sea surface, which could reduce low-level cloud cover,” adds co-author Thomas Rackow. “When the upper layers of the ocean become shallower, they warm more easily.”
Long-term ocean cycles may also be at play. The ocean circulations of the Atlantic and Pacific, in particular, are known to vary over decades. These can reduce low-level cloud cover and exacerbate global warming at one time, but produce opposite effects at another. According to Goessling, it is difficult to say to what extent such ups and downs may confound current trends.
Overall, the work shows that small variations in low-level clouds are more important than previously imagined – and Goessling’s team believes this could mean the 2023 wave will not be an isolated event. “If much of the albedo decline is indeed due to feedbacks between global warming and low clouds, as some climate models indicate, we should expect quite intense warming in the future,” he said in a statement.
