Satellite measurements show that the Earth has reflected sunlight less and less over the past 20 years. The “darkening of our planet” leads to a further disruption of the energy balance, which in turn affects climate change. The reasons for the decrease in sunlight reflectivity are varied, but the decrease in low clouds over the sea is likely to be the main reason.
Satellite measurements from NASA’s Clouds Project and the Earth’s Radiant Energy System (CERES) show that the Earth has been reflecting sunlight less and less in recent years. The measure of this is “albedo,” which refers to how much sunlight is reflected. On average, the Earth reflects about 30% of incoming solar radiation. This percentage has now decreased relatively by about 2%.
Reduced sunlight reflection can be the result of two reasons: either due to less incident sunlight, or because sunlight is less reflected and therefore more absorbed. Measurements show that the amount of incoming sunlight has remained roughly constant, meaning that it is reflectance that has decreased. But what factors play a role in this?
The factors that play a role in Earth’s declining albedo are multifaceted. One often cited cause is the melting of ice and snow on the ground. Snow can reflect up to 90% of incoming sunlight. In the case of ice, this percentage is lower, but still higher than on Earth’s surface or ocean water, for example. As the amount of sea ice decreases, the glaciers retreat, and the snow-covered surface becomes smaller, the area with a high reflectance value is shrinking. This leads to a decrease in the total albedo of the Earth’s surface.
However, most of the decrease in ground reflectivity is caused by cloud feedback. Clouds, especially low clouds, reflect most of the incoming sunlight. Due to global warming, the amount of clouds, especially low clouds above sea level, is decreasing. This means that less sunlight is reflected. Observations show the greatest impact of this over the eastern Pacific.
Lower reflectivity of the Earth leads to more warming, which in turn leads to lower reflectivity
Finally, there is also a contribution from aerosols, which are particles in the atmosphere that reflect sunlight. As the atmosphere becomes cleaner (due to stricter regulations and reduced pollution), the amount of aerosols and thus their reflectance is decreasing.
Reduced ground reflectivity leads to more warming. The Earth currently reflects approximately 0.5 watts less per square meter than it did 20 years ago. This additional energy is used to further warm the atmosphere, exacerbating climate change. A vicious circle, or positive feedback, because the Earth’s reflectivity decreases further due to climate change.
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