Moonlighting in the Earthshine
In writing about items of interest from scientists, sometimes I want to borrow from funnyman Dave Barry to assure you of my good faith: I am not making this up. This is one of those columns. Trust me; I am not making this up.
It turns out that one of the best, least expensive tools in the hands of scientists trying to study global climate change is earthshine.
OK, now, I warned you that trust was necessary. Here's the story. If, on a clear evening, you can see the crescent moon, often you can also see the ghostly image of the rest of the moon's disc. The brilliant crescent is the edge of the moon; it's reflecting sunshine. The dim remainder of the moon is visible because it's reflecting earthshine.
About 30 percent of the solar radiation that strikes Earth is reflected back into space; that's earthshine. Its intensity is a measure of Earth's reflectivity, or albedo. And our planet's albedo plays a significant part in its climate. According to the journal Science, where I found the report of this work, if the earth's albedo decreases by as little as one percent, its temperature will rise by two degrees Fahrenheit.
This should come as no surprise to any Alaskan gardener, for we all quickly learn that scraping the snow off the garden in spring gets the soil ready for planting far earlier than if we let nature take its course. The white snow reflects away the spring sun's heat; the dark soil absorbs it. The garden dirt has a lower albedo than the snow does.
The standard way climatologists have monitored global albedo is by satellite, but that method has had several drawbacks. Satellites are pricey technology, typically costing hundreds of millions from drawing board through launch. They also measure albedo across fairly small patches of the planet, and errors can occur when those patchy measurements are integrated to come up with a whole-earth albedo. They're also not useful for historical comparisons, because satellites have been available only for a few decades.
It was trying to find a way to extend the satellite record into the presatellite past that led University of Arizona physicist Donald Huffman to work previously done on Earth's natural satellite, the moon. In 1989, a colleague of Huffman's, atmospheric scientist Sean Twomey, told him of the earthshine record made by French astronomer Andre Danjon. Beginning in 1925, Danjon used a telescope in which a prism split the moon's image into two identical side-by-side images. He could adjust a diaphragm to dim one of the images until the sunlit portion had the same apparent brightness as the earthlit portion on the unadjusted image. He could quantify the diaphragm adjustment, and thus had a real measurement for the brightness of earthshine--and a good record of Earth's albedo until the 1950s.
Huffman and Twomey received a small grant from the National Science Foundation and reconstructed Danjon's equipment. Since then, Huffman has taken morning and evening earthshine measurements of every crescent moon.
Recently the earthshine work has received encouragement from another scientist who had originally set out to prove it was all, well, moonshine. Physicist Steven Koonin of the California Institute of Technology doubted the usefulness of the technique because the moon catches light reflected essentially from Earth's equatorial regions. The high-albedo polar icecaps can scatter light out into space that never reaches the moon at all. But Koonin had no doubts about his ability to construct computer models, and his models told him that earthshine was indeed a good indicator of the earth's albedo, just about as good as the satellite data.
Thus, a few physicists moonlighting in another field have established the effectiveness of a 70-year-old technique in gathering information to help solve modern problems--inexpensively, to boot.
