Low-Level Cloud Cover Limits Earth’s Ability To Reflect Solar Radiation – Study

Eddie Gonzales Jr. – MessageToEagle.com – AWI researchers find a possible explanation for the unusually high-temperature increase in 2023: Fewer low clouds impair the Earth’s ability to reflect solar rays.

Rising sea levels, melting glaciers, heat waves in the oceans – 2023 has set alarming records in many ways.

Clouds in Antarctica. Image credit: Photo: Alfred Wegener Institute / Jessica Helmschmidt

The global average temperature also reached a new high, rising to almost 1.5 degrees Celsius compared to pre-industrial levels. The search for the causes of this sudden increase posed a puzzle for researchers.

If you look at the effects of man-made influences such as the accumulation of greenhouse gases in the atmosphere, the El Niño weather phenomenon and natural events such as volcanic eruptions, much of the warming can be understood. However, there is a gap of around 0.2 degrees Celsius that has not yet been properly explained. A team led by the Alfred Wegener Institute has now described in the online edition of the journal Science what could have caused the unexpectedly high increase in the global average temperature:

Our planet is losing its reflective ability because it lacks certain clouds.

“In addition to the onset of El Niño and the expected long-term warming caused by anthropogenic greenhouse gases, several factors have already been discussed that could have contributed to the surprisingly high global average temperatures since 2023,” says Dr. Helge Gößling, lead author of the study from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI).

For example, stronger solar activity, larger amounts of water vapor from a volcanic eruption or fewer aerosol particles in the atmosphere. If all these factors are taken into account, there is still a warming of 0.2 degrees Celsius for which there is no obvious cause.

“The question of the ‘explanation gap’ of 0.2 degrees Celsius in 2023 is currently one of the most prominent questions in climate research,” says Helge Gößling. To close this gap, climate modelers at the AWI and the European Centre for Medium-Range Weather Forecasts (ECMWF) examined satellite data from NASA and reanalysis data from the ECMWF, which combines a wide range of observation data with a complex weather model. Some of the data go back to 1940 and enable a detailed analysis of the development of the global energy balance and cloud cover at different altitudes.

“It was striking that in both the NASA and ECMWF data sets, 2023 stands out as the year with the lowest planetary albedo,” says co-author Dr. Thomas Rackow of the ECMWF. The planetary albedo describes the proportion of solar radiation that is reflected back into space after any interaction with the atmosphere and the Earth’s surface. “We have already seen a certain decline in recent years. The data now suggest that the planetary albedo in 2023 could be lower than ever since at least 1940.” This fuels global warming and can explain the missing 0.2 degrees Celsius so far. But what caused the record-breaking decline in the planetary albedo?

Reduction of low clouds reduces the Earth’s reflective power

The albedo of the Earth’s surface has been declining since the 1970s. Initially, this was due to the fact that snow and sea ice in the Arctic have become increasingly scarce, and with it white areas that can reflect the sun’s rays. Since 2016, the decline in sea ice in the Antarctic has also been a factor.

“However, the analysis of the data sets shows that the decline in surface albedo in the polar regions has only contributed about 15 percent to the recent decline in planetary albedo,” explains Helge Gößling. Elsewhere, reflectivity has also noticeably decreased. To calculate what effect the reduced albedo might have had, the researchers used an established energy balance model that closely replicates the temperature curve of complex climate models. The result: Without the reduced reflection since December 2020, 2023 would have been about 0.23 degrees Celsius cooler on average.

One development appears to have had a significant impact on the decline in planetary reflectivity: the decline in low clouds in northern mid-latitudes and in the tropics. The Atlantic Ocean stands out in particular, precisely the region where the most unusual heat records were observed in 2023.

“It is striking that the eastern North Atlantic, which is one of the main drivers of the recent increase in global mean temperature, not only saw a significant decline in low clouds in 2023, but – like almost the entire Atlantic – over the past decade.” The data show that cloud cover has decreased at low altitudes, while it has declined only slightly, if at all, at high and mid-latitudes.

The fact that low clouds, rather than higher ones, are mainly responsible for the albedo decline has significant consequences.

Clouds at all altitudes reflect sunlight and thus have a cooling effect. However, clouds in high, cold layers of air also have a warming effect because they keep the heat that the earth’s surface radiates in the atmosphere. “This basically corresponds to the effect of greenhouse gases,” explains Helge Gößling. However, this effect is largely absent in lower clouds. “If there are fewer lower clouds, we just lose the cooling effect, so it gets warmer.”

But what has caused the decline in low clouds? Fewer man-made aerosols in the atmosphere, particularly due to stricter regulations on ship diesel, are likely to have contributed to this. As condensation nuclei, aerosols play a key role in cloud formation, and they also reflect sunlight themselves. Natural fluctuations and ocean interactions could also play a role. However, Helge Gößling believes it is unlikely that these factors are sufficient as an explanation and brings a third mechanism into play: it is global warming itself that is causing the low clouds to disappear.

“If the albedo decline is due to an amplifying feedback between global warming and clouds, as some climate models suggest, we must expect quite strong future warming,” he stresses.

“We could already be closer to global warming of over 1.5 degrees Celsius than previously thought. The remaining greenhouse gas emissions associated with these Paris Agreement limits would have to be revised downwards accordingly, and measures to counter the impacts of expected weather extremes would become even more urgent.”

Source

Paper

Written by Eddie Gonzales  Jr. – MessageToEagle.com Staff Writer