Over the last two decades, Earth has lost a little bit of its shine -- and no, not just because of the world-changing pandemic. By studying the glow of our humble space orb, scientists have discovered a surprising dimming. They hypothesize the underlying cause could be related to climate change.
In a study, published in the journal Geophysical Research Letters on Aug. 29, researchers examined the Earth's "albedo" by analyzing earthshine at the Big Bear Solar Observatory in California between 1998 and 2017 -- equivalent to around 1,500 nights of data. This analysis allowed them to assess how much light is reflected by the planet.
The data revealed the Earth has dimmed by about half a percent since the late 1990s.
"The albedo drop was such a surprise to us when we analyzed the last three years of data after 17 years of nearly flat albedo," said Philip Goode, an astronomer at New Jersey Institute of Technology and lead author of the study.
Earthshine is the reason you can see the dark face of the moon during its waxing phase. Usually about 30% of sunlight that reaches the planet bounces off things like clouds, snow and oceans and back into space, giving our natural satellite a gentle light that can be measured. "Moonshine" is the bright, yellow-white light we can see from down here on Earth (, but that's not what we're talking about here).
By watching the moon for two decades, the research team were able to detect small changes in the albedo. Particularly in the last few years, the shine has really come off our Earth.
Clouds appear to be the major in influence of Earth's reflectiveness, but the sun's brightness is another reason for how much sunlight reaches Earth (and thus bounces off). The team also assessed the sun's periodic changes in brightness to see if it was having any effect on what they were seeing.
The dimming of Earth didn't correlate with changes in the sun's brightness which, the team suggest, means something on Earth must be causing it. One suggestion is ocean warming.
Comparing their findings with those of NASA's Clouds and the Earth's Radiant Energy System (CERES) project, which features five satellites measuring reflectiveness, the team suggest the reduction in low-lying clouds over the eastern Pacific may have resulted in the precipitous drop off in brightness. These reductions in cloud cover may be caused by increases in sea surface temperatures, with "likely connections" to the climate crisis.
Scientists have previously thought a warmer planet could result in higher albedo, due to increasing cloud coverage. Thus, there'd be more reflection of sunlight and less trapped by greenhouse gases -- a good thing. But that might not be the case. "It's actually quite concerning," said Edward Schwieterman, a planetary scientist at the University of California at Riverside not involved in the study, in an AGU press release.
The team note in their conclusions that albedo analysis from earthshine can be prone to a lack of sensitivity because of long-term calibration issues and encourage further analysis of the Earth's brightness.