The planet's lower reflectivity is causing a sharp increase in global warming

In 2023, the global average temperature reached a new high, rising to almost 1.5 degrees Celsius above pre-industrial levels. The search for the causes of this sharp increase presented researchers with a puzzle. If you consider 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, a large proportion of the warming can be explained. However, there is a gap of about 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 global average temperature: the Earth's reflectivity is decreasing because certain clouds are missing.

The so-called planetary albedo plays a crucial role in the Earth's heat balance. The term describes the proportion of solar radiation that is reflected back into space after interacting with the atmosphere and the Earth's surface. The data now suggest that in 2023, the planetary albedo may have been lower than at any time since at least 1940. This fuels global warming and can explain the missing 0.2 degrees Celsius. The albedo of the Earth's surface has been trending downward since the 1970s. Initially, this was partly due to the fact that snow and sea ice in the Arctic were decreasing, and with it the white surfaces that can reflect solar radiation. Since 2016, the decline in sea ice in the Antarctic has added to this. “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, lead author of the study from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research. Reflectivity has also decreased noticeably elsewhere. Using an energy balance model, the researchers calculated that without the reduced reflection, 2023 would have been on average about 0.23 degrees Celsius cooler.

One development has apparently had a significant influence on the decline in planetary reflectivity: the decline of low clouds at mid-latitudes and in the tropics. The fact that low clouds, and not higher clouds, are primarily responsible for the decrease in albedo has significant consequences. Clouds at all altitudes reflect sunlight and thus have a cooling effect. However, clouds in high, cold air layers also have a warming effect because they trap the heat radiated by the Earth's surface in the atmosphere. “This is basically the same effect as that of greenhouse gases,” explains Helge Gößling. However, this effect is largely absent in the case of lower clouds. ”If there are fewer lower clouds, we only lose the cooling effect, so it gets warmer.”

But what has caused the decline in low clouds? Fewer man-made aerosols in the atmosphere, in particular due to stricter requirements for marine diesel, may have contributed to this. As condensation nuclei, aerosols play a significant role in cloud formation; in addition, they also reflect sunlight themselves. Furthermore, natural fluctuations and ocean interactions could play a role.

Helge Gößling, however, considers it unlikely that these factors alone can explain the phenomenon 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 intensifying feedback between global warming and clouds, as some climate models suggest, we have to expect quite a strong warming in the future,” he emphasizes. “We could already be closer to global warming of more than 1.5 degrees Celsius than previously thought. The remaining greenhouse gas emissions associated with these Paris Agreement 'stop lines' would have to be revised downwards accordingly, and action to counter the consequences of expected extreme weather events would become even more urgent.”

Rapid surge in global warming mainly due to reduced planetary albedo (AWI)

Original-Publikation

H. F. Goessling, T. Rackow, T. Jung, Recent global temperature surge intensified by record-low planetary albedo, Science (2024). doi: 10.1126/science.adq7280