Earth is dimming, according to new study looking at how much sunlight the planet reflects
Earth is seen in this file photo.
TORONTO -- Earth’s bright blue glow is dimming, according to a new study, and it may be due to climate change.
With warming oceans potentially fuelling less cloud cover, the planet is simply reflecting less light back into space. And if the trend continues, it could contribute to the overall warming of the planet.
A new study published in the journal Geophysical Research Letters at the end of August looked at decades of data on something called the “earthshine” — the level of light reflected off of Earth that illuminates the dark face of the Moon — in order to judge whether there’s been a drop in Earth’s terrestrial albedo.
Albedo is a thermodynamics term that measures, essentially, the whiteness of a surface and thus how much solar energy it can reflect or absorb. A value of zero means a black so dark it absorbs all incoming energy, while a value of one means the surface is a “perfect reflector.”
For instance, according to the National Snow and Ice Data Center, bare sea ice has a higher albedo than open water, at around 0.07 versus 0.06, which means that sea ice absorbs less solar energy.
In this new study, researchers found that the Earth is now reflecting “about half a watt less light per square meter than it was 20 years ago,” according to a press release.
The Earth generally reflects around 30 per cent of the sunlight that shines on us, but this is a 0.5 per cent decrease in the Earth’s reflectance. It may sound small, but it is “climatologically significant,” according to the study.
In the mid-1990s, the Big Bear Solar Observatory (BBSO) in southern California, began to take measurements of the Earth’s albedo using observations of the Moon. The largest chunk of data looked at in this study is from 1998 to 2017.
“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, a researcher at New Jersey Institute of Technology and the lead author of the new study, in the release.
Researchers found that when recent data was added to this legacy data, the dimming trend was clear.
And most of the dimming has occurred in the last three years of earthshine data.
So why is this happening, and what does it mean?
Researchers found that there was no correlation between changes in the brightness of the Sun itself and changes in Earth’s albedo, which means that whatever is dimming the Earth is occurring here at home.
When researchers cross-referenced their data with satellite measurements taken as part of NASA’s Clouds and the Earth’s Radiant Energy System (CERES) project, they noticed that there was a reduction in low-lying clouds over the eastern Pacific Ocean in recent years, clouds that regularly reflect a lot of sunlight.
The affected area is the same region where a climatic condition called the Pacific Decadal Oscillation (PDO) has led to increases in sea surface temperatures. And while the PDO is a naturally occurring phenomenon, scientists have noted that it has become less predictable due to global warming.
If the Earth is dimming, that means it is also absorbing more solar energy than before, something that could have serious implications in the future.
“It’s actually quite concerning,” Edward Schwieterman, a planetary scientist at the University of California at Riverside, said in the release. Schwieterman was not involved in the new study.
He explained that scientists had hoped that as the Earth warmed, it might lead to more clouds and a higher albedo that could help balance the Earth’s warming.
“But this shows the opposite is true.”