Satellite measurements show that the Earth has reflected less and less sunlight over the past 20 years. The “darkening of our planet” leads to an additional disruption of the energy balance, which in turn affects climate change. The reasons for the decrease in sunlight reflection are diverse, although the decrease in low clouds over the sea is probably the main cause.
Satellite measurements from NASA’s Clouds and the Earth’s Radiant Energy System (CERES) project show that the Earth has been reflecting less and less sunlight in recent years. A measure of this is the ‘albedo’, which indicates how much sunlight is reflected. On average, the Earth reflects approximately 30% of the incoming solar radiation. This percentage has now fallen relatively by about 2%.
The decrease in reflection of sunlight can be due to two reasons: either due to less incident sunlight, or due to less reflection and therefore more absorption of sunlight. Measurements show that the amount of incoming sunlight has remained almost constant, which means that it is the reflection that has decreased. But which factors play a role in this?
The factors that play a role in the decrease in Earth’s albedo are multifaceted. An often cited reason is the melting of ice and snow on Earth. Snow can reflect up to 90% of incoming sunlight. With ice this percentage is lower, but still higher than, for example, land surface or ocean water. Due to the decreasing amount of sea ice, retreating glaciers and a smaller snow-covered surface, 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 Earth’s reflectance is caused by the cloud feedback. Clouds, especially low clouds, reflect most of the incoming sunlight. Due to global warming, the amount of clouds, especially the low clouds above the sea, is decreasing. This means that less sunlight is reflected. Observations show the greatest effect of this over the eastern Pacific.
The reduced reflection of the Earth leads to additional warming, which in turn causes reduced reflection
Finally, there is also a contribution from aerosols, particles in the atmosphere that reflect sunlight. Because the atmosphere has become cleaner (due to stricter regulations and less pollution), the amount of aerosols and therefore their reflection is decreasing.
The reduced reflection of the Earth leads to additional warming. Currently the Earth reflects approximately 0.5 watts less per square meter than 20 years ago. This extra energy is used to further warm the atmosphere, exacerbating climate change. A vicious circle, or a positive feedback, because the reflection of the earth is further decreasing due to climate change.
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