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Different structures on Earth reflect varying amounts of light: oceans reflect very little, while continents reflect twice as much. Clouds reflect about half, and snow reflects much of the sunlight that strikes them.
Scientists from the Big Bear Solar Observatory in Southern California, USA, studied Earthshine variability from 1998, investigating changes ranging from daily to decadal timescales. The researchers noted that these measurements are relative and require stronger observations, possibly from satellites or lunar observation posts.
In this new study, scientists merged the data with observations from NASA’s Clouds project and the Earth Radiant Energy System (CERES) project. CERES, active since 1997, involves instruments aboard NASA’s satellite fleet and the US National Oceanic and Atmospheric Administration (NOAA).
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The researchers combined the two datasets to observe changes in Earth’s brightness. Over two decades, Earth’s reflected light has decreased by around 0.5%—less than half a watt per square meter. Most changes occurred in the last three years of Earthshine data analysis, leading up to 2017. CERES data continued to be analyzed until 2019, indicating an even more significant decline towards the end.
During this time, researchers tracked solar irradiance through two periods of maximum activity and two periods of lower activity, which did not significantly correlate with the decrease in Earth’s reflectance. Therefore, scientists explained that the changes in Earth’s reflected light must come from the planet itself.
Notably, CERES data recorded the loss of bright clouds at low altitudes over the eastern Pacific Ocean, off the western coast of the Americas. This is the area where scientists are observing serious increases in sea surface temperatures.
Due to light not being reflected into space but rather being retained within Earth’s system, changes in brightness also impact the planet’s climate future, potentially accelerating the effects of human-caused climate change. The study has been published in the Geophysical Research Letters journal.