Knowing just how much of the sun’s light Earth absorbs, and how much it reflects back into space is vitally important for understanding human-caused global climate change. One way to do this is to observe the faint glow on the dark portion of the crescent moon caused by the reflected light of Earth using a ground-based telescope and a sensitive light measuring instrument.
In 2021 a team of astronomers reported results from such lunar observations over a period of twenty years between 1984 and 2017. They compared their data with satellite observations made by other scientists over the same period.
Detecting long term trends is hard because Earth’s brightness varies on a daily, monthly, and seasonal basis. Overall, the researchers claimed that Earth has progressively gotten slightly dimmer over the years they measured. They think this was due to reduced cloud and ice cover, which was, in turn, caused by a warming climate. However, the uncertainty in their data is large, and the amount of data is limited. There are important discrepancies with findings from satellites. More research will be needed to clarify these issues.
The issue is critically important. Any extra energy absorbed by a dimmer Earth would be re-radiated as heat, which can, in turn, be trapped by greenhouse gases in the atmosphere, producing more global warming. Even if only a small amount of extra energy is trapped, the impact on the climate could be significant. A warmer climate could cause more dimming, which would cause still more warming in a vicious circle. It is vital that scientists understand these complex interactions in order to formulate a prognosis for Earth’s changing climate.
