In 2023, the global average temperature rose to almost 1.5 degrees Celsius above pre-industrial levels. Climate science has so far only been able to partially explain this new temperature record – a gap of around 0.2 degrees Celsius remains a mystery. A study now attributes this increase to the fact that there are fewer and fewer low-hanging clouds that would reflect sunlight. As a result, our planet is warming more than previously thought. From the researchers’ perspective, the results could also have an impact on global CO2 budgets.
Since March 2023, sea surface temperatures have significantly broken previous temperature records. Researchers recorded strong heat waves, particularly in the North Atlantic. Sea ice at the poles also declined significantly – including Antarctic sea ice after many years of stability. At the same time, the global average temperature reached a value of almost 1.5 degrees Celsius above pre-industrial levels for the first time. Climate science can explain some of the warming with a number of known factors. In addition to human greenhouse gases, the El Niño weather phenomenon in particular contributed to the warming during this time. In addition, solar activity, which runs in eleven-year cycles, is approaching a maximum.
Decline in planetary albedo
But even together with other factors such as volcanic eruptions, there remained a gap of around 0.2 degrees Celsius, which has not yet been conclusively explained. “The 0.2 degree Celsius ‘explanation gap’ for 2023 is currently one of the most intensively discussed questions in climate research,” says Helge Goessling from the Alfred Wegener Institute in Bremerhaven. In search of causes, he and his team have now taken a closer look at satellite data from NASA and analysis data from the European Center for Medium-Range Weather Forecasts (ECMWF) in Bonn. The researchers combined the observation data with a complex weather model.
“What we noticed was that in both the NASA and ECMWF data, 2023 stood out as the year with the lowest planetary albedo,” says co-author Thomas Rackow from ECMWF. Planetary albedo describes the percentage of incoming solar radiation that is reflected back into space after all interactions with the atmosphere and the Earth’s surface. “We have already observed a slight decline in recent years. The data suggests that by 2023, planetary albedo may have reached its lowest level since at least 1940.” Reduced reflection causes land and sea to absorb more solar heat. This would exacerbate global warming and could explain the “missing” 0.2 degrees Celsius.
Fewer low-hanging clouds
But why has planetary albedo dropped so much? The team explains that the decline in snow and sea ice at the poles plays a role. Since the water of the ice-free sea and the un-snowy surface reflect less sunlight than snow and ice, more heat remains in the atmosphere. “However, our analysis of the data sets shows that the decline in surface albedo in the polar regions only accounts for about 15 percent of the recent decline in planetary albedo,” says Goessling. According to the analyses, the clouds had a greater influence.
“It is striking that the eastern North Atlantic, which is one of the main drivers of the recent increase in global mean temperature, has been characterized by a significant decrease in low-altitude clouds – not only in 2023, but also over the past decade,” reports Goessling . However, the low-hanging clouds are particularly important for the albedo effect, as the researchers explain. Clouds in all layers of the atmosphere reflect solar radiation back into space. The high clouds also reflect heat radiated from the earth’s surface back downwards, thereby keeping it in the atmosphere. “If there are fewer low clouds, we just lose the cooling effect and it gets warmer,” says Goessling.
Warming up faster than expected?
According to the researchers, one reason why the low clouds have decreased could be that fewer aerosols are released into the atmosphere thanks to stricter regulations for ship fuels. Exhaust gases from ships have caused dense, low-hanging clouds over the world’s oceans for decades, as the aerosols form condensation nuclei for water vapor. Natural feedback mechanisms in the ocean and global warming itself can also contribute to fewer low-level clouds forming.
“If a large part of the albedo decline is indeed due to feedbacks between global warming and low-level clouds, as some climate models suggest, we should expect quite a bit of warming in the future,” says Goessling. “Long-term global warming could exceed 1.5 degrees Celsius faster than expected. The remaining carbon budgets tied to the limits set in the Paris Agreement would need to be reduced accordingly, and the need to take measures to adapt to the impacts of future weather extremes would become even more urgent.”
Source: Helge Goessling (Alfred Wegener Institute, Bremerhaven) et al., Science, doi: 10.1126/science.adq7280