Friday, January 26, 2007

Measuring Earthshine

Although emissions of greenhouse gases, especially those from industry and transportation, grab most of the headlines relating to climate change, they are not the only factor determining how much our planet is warming. One of the other key issues is the rate at which solar radiation is reflected back into space, by clouds, snow, or surface features. The technical term for this is "albedo", and it is central to the concerns about reforestation that I highlighted last Wednesday. It turns out that the albedo of the earth has not been constant over the last several decades. Because of the complex interactions involved, this could be a consequence of climate change, a factor compounding it, or both. In any case, measuring albedo as accurately as possible ought to be a high priority, if we're going to sharpen our predictions about the future course of global climate change. Surprisingly, a satellite built to do just that is sitting in a warehouse, with no launch date scheduled.

Space isn't the only place from which we can measure the earth's albedo. It has been done from right here on earth for decades by taking advantage of a phenomenon first discovered by Leonardo da Vinci. Scientists simply measure the amount of reflected "earthshine" illuminating the moon. From these readings, confirmed by instruments on existing satellites, we know that the albedo of the earth is roughly 0.30. That means that it reflects 30% of incident solar radiation back into space, compared to about 12% for the moon. Unfortunately, these measurements are apparently only accurate to within 2%. That doesn't harm their usefulness for discerning trends, such as the recent changes in albedo that have been reported. However, since the amount of extra "radiative forcing" responsible for climate change appears to be around 2 Watts/square meter over the last century--equivalent to 1% of the average solar radiation reaching the earth's surface--improving the accuracy of albedo measurement looks like a very useful thing to do.

NASA's DSCOVR satellite has already been built, but it has never been launched, and the whole mission appears to have fallen into a budgetary black hole. Reviving it would entail a bit more than just lining up a launch vehicle--supposedly France and Ukraine have offered that for free--but in light of the growing national interest in understanding and responding to climate change, perhaps we can drum up enough support to get it into its planned Lagrange Point 1 orbit before it's sold for scrap.

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