Climate change is making Earth dimmer

A 2011 image from the International Space Station shows Earthshine on a moon.
New research indicates that Earth's light is reflecting less as it changes its climate.

Clouds are a beautiful connection between climate and brightness. Clouds are an important piece of climate puzzle. Scientists struggle to model how clouds might respond to climate change, and how that will affect the future climate. The new study's scientists believe that reflectivity is dependent on the dynamics of clouds above the Pacific Ocean.

This research is based on over two decades of observations of "earthshine", which is light that Earth reflects onto a dark side of the Moon's surface. Satellite observations of Earth's reflectivity or albedo and sun's brightness are also part of the data.

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Different Earth features reflect different amounts light. The oceans reflect very little while the land reflects twice as much. Clouds reflect half of the sunlight they receive, while snow and ice reflect most of the light that hits them.

Since 1998, scientists at Big Bear Solar Observatory in Southern California are studying the earthshine's fluctuations. They have looked for changes on time scales ranging from daily to decadal. These measurements are not absolute and require more reliable observations from cubesats, or a lunar observatory.

Scientists combined the data from NASAs Clouds and Earths Radiant Energy System projects (CERES), which have been operating since 1997, with instruments on several NASA and National Oceanic and Atmospheric Administration satellites, to create the new research.

Researchers gathered the data from both datasets in order to determine if and how Earth's brightness has changed. The amount of light Earth reflected decreased by 0.5% over the entire two-decade period. This is about half a Watt less light per square meter. One square meter is about 11 square feet. The majority of the changes occurred in the three most recent years of the earthshine dataset, which researchers analysed through 2017. CERES data continues to analyze until 2019, and shows a much more stark decline at its conclusion.

Researchers found that the decrease in reflectance was not related to the brightness of sun, which had two periods of maximum activity during the study. The scientists concluded that a change of Earth's reflectance must be caused by a change within Earth.

The CERES data showed that there was a decrease in bright low-altitude cloud activity over the eastern Pacific Ocean off the west coast the Americas. Scientists are also reporting a sharp rise in temperature at the ocean's surface.

Because light is not reflected into space, but trapped in the Earth's system, any change in brightness can have implications for climate and possibly increase the rate of human-caused climate changes.

The research was described in a paper published in Geophysical Research Letters on Aug. 29, 2009.