In 2023, we witnessed numerous distressing records: rising sea levels, melting glaciers, and extreme sea heatwaves. The global average temperature soared to almost 1.5 degrees higher than the levels before industrialization, marking yet another unprecedented record. Understanding the reasons behind this sudden temperature increase has been a complex task for scientists. While factors such as human-made greenhouse gas emissions, the El Niño weather phenomenon, and natural occurrences like volcanic eruptions explain a significant portion of this warming, there remains an unexplained difference of roughly 0.2 degrees Celsius. A research team has proposed a potential explanation for this increase in global average temperature: the Earth has become less reflective due to a reduction in certain types of clouds.
“Beyond the impact of El Niño and the anticipated long-term warming from human-driven greenhouse gases, various other elements that could have played a role in the unexpectedly high global average temperatures since 2023 have been discussed,” states Dr. Helge Goessling, the lead author from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). These factors include heightened solar activity, substantial water vapor released from volcanic eruptions, or a drop in aerosol particles in the atmosphere. Nevertheless, even when considering all these elements, there remains a warming of 0.2 degrees Celsius that lacks a clear explanation.
“This 0.2-degree Celsius ‘explanation gap’ for 2023 is currently a hot topic in climate research,” Dr. Goessling adds. To address this gap, climate modelers from AWI and the European Centre for Medium-Range Weather Forecasts (ECMWF) closely examined satellite data from NASA and ECMWF’s reanalysis data, which combines a variety of observational data with advanced weather modeling. Some datasets date back to 1940, allowing for an in-depth analysis of changes in the global energy budget and cloud cover over different altitudes.
“What stood out to us was that 2023 marked the lowest planetary albedo in both NASA and ECMWF datasets,” says co-author Dr. Thomas Rackow from ECMWF. Planetary albedo refers to the percentage of incoming solar radiation that gets reflected back into space after interacting with the atmosphere and the Earth’s surface. “We had previously noticed a slight decrease in recent years, and our findings suggest that 2023’s albedo may have reached its lowest point since at least 1940.” This decline could amplify global warming and may clarify the unexplained 0.2 degrees Celsius increase. But what led to this near-record decline in planetary albedo?
Decrease in low-altitude clouds impacts Earth’s albedo
Since the 1970s, the albedo of Earth’s surface has been diminishing, partly due to a decrease in Arctic snow and sea ice, resulting in fewer reflective white areas. This decline has worsened since 2016 due to the loss of sea ice in Antarctica. “However, our examination of the datasets reveals that the decrease in surface albedo in polar regions only accounts for around 15 percent of the recent drop in planetary albedo,” explains Helge Goessling. A notable decline in albedo has also occurred in other regions. Using an established energy budget model that can simulate the temperature response of intricate climate models, researchers assessed the potential effects of this lower albedo. Their results indicated that without the decline in albedo since December 2020, the average temperature in 2023 would have been approximately 0.23 degrees Celsius lower.
A significant factor contributing to the reduced planetary albedo is the decline in low-altitude clouds in northern mid-latitudes and tropical regions. Notably, the Atlantic region exhibited a substantial drop in low-altitude clouds, correlating with the unusual temperature highs recorded in 2023. “It’s notable that the eastern North Atlantic, a primary contributor to the recent spike in global average temperature, experienced a significant decrease in low-altitude clouds not just in 2023, but also over the past decade,” explains Goessling. Data indicates that while low-altitude cloud cover has decreased, cloud coverage at moderate and high altitudes has either diminished slightly or remained unchanged.
The observation that mainly low clouds, and not higher-altitude clouds, are accountable for the lower albedo has crucial implications. All types of clouds reflect sunlight, providing a cooling effect, but high-altitude clouds also generate a warming effect by trapping heat that escapes from the surface. “This phenomenon is similar to the effect of greenhouse gases,” states Helge Goessling. In contrast, fewer low clouds only reduce the cooling effect, resulting in warmer temperatures.
But what is causing the reduction in low clouds? Decreased levels of human-made aerosols in the atmosphere, largely due to stricter marine fuel regulations, are likely contributing factors. Aerosols are important for cloud formation as condensation nuclei and also reflect sunlight. Additionally, natural variations and ocean feedback may play a part. However, Helge Goessling believes these factors alone are unlikely to be sufficient and proposes a third explanation: that global warming itself is leading to fewer low clouds. “If a large portion of the decline in albedo is indeed linked to feedback loops between warming and low clouds, as suggested by some climate models, we might face more intense warming in the future,” he warns. “There is a possibility that global long-term climate warming could exceed 1.5 degrees Celsius sooner than previously anticipated. This would necessitate a reduction in the remaining carbon budgets associated with the objectives established in the Paris Agreement and intensify the urgency for implementing measures to adapt to future extreme weather impacts.”