Scientists are watching the dark side of the moon for clues about climate change on Earth.
New Jersey Institute of Technology physics professor Philip Goode has teamed up with Caltech physicist and Provost Steven Koonin to establish a global network to monitor "earthshine" from the moon's dark side. The goal is to track changes in the solar radiation that Earth reflects back to space.
The fraction of sunlight reflected spaceward, known as Earth's albedo, is a key indicator of how much solar radiation the planet is retaining to drive its climate system. The researchers say measuring earthshine is an inexpensive way to augment satellite-based albedo measurements.
"Studies of climate change require well calibrated, long-term measurements of large regions of the globe," Dr. Koonin says. "Earthshine observations are ideally suited for this."
Earthshine is visible to the naked eye on a clear night during a crescent moon, but "shine" is a bit of a misnomer. The glow appears slightly brighter than the dark sky.
But a sensitive detector on the back end of a 6-inch telescope at the Big Bear Solar Observatory in southern California's San Bernardino Mountains has tracked subtle variations in that glow, the researchers say. Those changes correlate to seasonal and even day-to-day shifts in albedo.
Now, the team is building the second of two telescopes dedicated to a long-term monitoring effort. One will remain at Big Bear, while the other will be shipped to an observatory in Crimea. Ultimately, Dr. Koonin says, the team hopes to plant another scope in Republic of China, and is working with collaborators in Taiwan to develop automated "earthshine telescopes" that would be deployed in other regions of the world.
Building on a technique used by French astronomer André-Louis Danjon early in the 20th century, Goode, Koonin, and their colleagues used a sensitive CCD to measure the intensity of the earthshine. Beginning in December 1998, they took data for a total of 200 nights. They also included data from more-limited measurements taken in 1994 and '95.
Based on those data, reported in the April 17 edition of Geophysical Research Letters, the team calculated that Earth is returning 29.7 percent of the sun's incoming radiation back into space (give or take half a percent). They also found an intriguing hint that the albedo may have dropped by 2.5 percent during the past five years - a quantity significant to climatologists.
"More-interesting results may come from spectroscopic studies of the atmosphere" using earthshine as a light source, says Goode. For the past two years, the team has been using the 60-inch telescope on Mt. Palomar for such studies.
These studies, he says, would be useful for tracking features ranging from cloud-top heights to changes in water-vapor and greenhouse-gas concentrations. Such globally averaged data, he adds, could help put real-world limits on results from global climate models.