Researchers at the Alfred Wegener Institute (AWI) propose a critical contributing factor: a significant decline in planetary albedo, or the Earth's reflectivity. "In addition to El Nino and long-term warming driven by greenhouse gases, several other factors have been explored to explain the recent surge in global mean temperatures," said Dr. Helge Goessling, lead author of the study from AWI. "These include increased solar activity, elevated water vapor from volcanic eruptions, and reduced atmospheric aerosol particles. However, when combined, these factors still leave a 0.2-degree-Celsius gap unaccounted for."
This "explanation gap" has become a focal point for climate researchers, prompting AWI and the European Centre for Medium-Range Weather Forecasts (ECMWF) to delve into decades of satellite and reanalysis data. The findings point to 2023 as the year with the lowest recorded planetary albedo since at least 1940. "Planetary albedo, which measures how much solar radiation is reflected back into space, has shown a downward trend in recent years, with a significant drop in 2023," said co-author Dr. Thomas Rackow of ECMWF.
Lower clouds play a vital role in reflecting sunlight, providing a cooling effect. Unlike higher-altitude clouds, which can trap heat and contribute to warming, fewer low clouds result solely in reduced cooling. "If there are fewer low clouds, we only lose the cooling effect, making things warmer," explained Goessling.