GRL - Earth's Brightness, Measured On The Moon, Dropping In Another Confirmation Of Global Warming
For 20 years, researchers stared at the dark side of the moon to measure its faint but visible “earthshine,” a glow created by sunlight reflecting off Earth and onto the lunar surface. Their new analysis, published recently in Geophysical Research Letters, revealed that this ghostly light has darkened slightly, confirming satellite measurements that our planet is getting dimmer. As the planet reflects less light, the incoming heat gets absorbed by the seas and skies. This lingering warmth probably intensifies the rate of global warming, scientists believe.
Typically, about 30 percent of the light streaming from the sun gets redirected by Earth back to space, mostly from bright white clouds. But that percentage can vary over time. In 1998, a team from the Big Bear Solar Observatory in southern California set out to track Earth’s reflectivity, or albedo, by monitoring earthshine during the days each month the telescope could see the moon’s dark side. “It is just so naturally appealing,” said lead author Philip Goode, a physicist at the New Jersey Institute of Technology, which operates the observatory. “We’re using the moon as a mirror for the Earth.” The study ran for a full solar cycle—about 20 years—to account for variations in the sun’s activity.
Three years after Goode started Project Earthshine, NASA also began to measure Earth’s albedo with a string of satellites called Clouds and the Earth’s Radiant Energy System, or CERES. Data from both projects has matched up neatly. Since the year 2000, the planet has reflected less energy back into space: about one-half a watt per square meter. That’s similar to the dimming effect from turning off one lightbulb on a panel of 200. When these experiments began two decades ago, many scientists expected that water in warmer seas would evaporate more quickly and create thicker clouds—thus reflecting more sunlight back into space. But the satellite and earthshine results show just the opposite: “Somehow, the warm ocean burned a hole in the clouds and let in more sunlight,” Goode told Mongabay.
This graph shows the downward trend in Earth’s albedo, as measured by earthshine (black) and CERES data (blue) in watts per square meter. Image credit: Goode et al. (2021), Geophysical Research Letters.
Other researchers analyzing these patterns have pinned the gradual dimming of Earth on human influences. One team at Princeton University managed to model the satellite data with near-perfect accuracy by considering the impacts of pollution, greenhouse gases and sea ice levels. “This is on us,” said Shiv Priyam Raghuraman, a Ph.D. student who led the Princeton study and was not involved in the Earthshine Project. “We should be aware that we’re driving these changes.” The earthshine data revealed the planet dimmed noticeably during its final three years of data collection from 2015 to 2017. That pattern may have different roots, Goode noted. “I would have bet my house that those three years were going to look just like the previous 17,” he said. “Now we have this really cool mystery.”
EDIT
https://news.mongabay.com/2021/11/earthshine-from-the-moon-shows-our-planet-is-dimming-intensifying-global-warming/