Moon's Dark Side Gives Climate Hints

If he were alive today, Leonardo da Vinci could spend most of his time saying "I told you so." Four centuries ago he postulated that the light that makes the dark part of the crescent moon visible to us some of the time comes from the Earth, not the moon.

And now, all these years later, scientists have found a way to use Leonardo's discovery to help us understand the Earth’s incredibly complex climate system. They have figured out how to use "Earthshine," the sunlight reflected from the Earth to the moon and back again, to shed a little light on how the planet’s climate is changing.

When you think about it, Leonardo's finding was quite extraordinary. In those days, no one knew the moon didn’t generate its own light, and didn’t even have an atmosphere that could scatter light into areas that would otherwise be dark.

He figured out that when the Earth reflects enough light, we can see the entire moon, not just the crescent.

Measuring the Light

Early last century the French astronomer Andre-Louis Danjon made the first measurements of the intensity of earthshine, but no one really knew what to do with them. Then in 1991, Steven E. Koonin, professor of theoretical physics at the California Institute of Technology, co-authored a paper in which he speculated that if earthshine could be measured very precisely, it might tell much about the health of the Earth’s climate.

The amount of radiation that the Earth reflects back into space, called its "albedo," depends on a number of factors, including cloud cover and seasonal changes in the amount of snow and ice. When the sun rises on a region covered with winter snow, for example, more solar rays are reflected back into space.

If you were somewhere in space, in the path of that radiation, the Earth would appear much brighter because the planet’s albedo would be very high.

Koonin theorized that it should be possible to monitor earthshine for a long period of time, thus providing a constant record of changes in the Earth’s climate. If the Earth is getting warmer because of the greenhouse effect, as many experts believe, then that should reduce the amount of snow and ice and produce a gradual decrease in the planet’s albedo.

… A Surprisingly Simple Experiment …

So Cal Tech teamed up with the New Jersey Institute of Technology and in 1994 a surprisingly simple experiment was launched. In these days of ultra-sophisticated gadgets costing millions and millions of dollars, the two institutions began taking measurements with a 6-inch telescope at the Big Bear Solar Observatory in the mountains northeast of Los Angeles.

The small telescope was equipped with a charge-coupled device, a very precise version of the electronic gizmo in video cameras that captures the light to produce an image.

"We measure the intensity of the dark part of the moon," Koonin says, which is no small accomplishment because the adjacent crescent is about 10,000 times brighter.

Measurements were taken on 70 nights in 1994-95, and the scientists recently completed a two-year observation during which measurements were taken on about 200 nights.

…And Surprising Results

Right out of the barrel, the experiment produced some surprising results.

"We have found surprisingly large — up to 20 percent — seasonal variations in Earth’s reflectance," says Philip R. Goode, leader of the New Jersey team and director of the Big Bear Solar Observatory. Everyone expected some seasonal variation, because the amount of ice, snow and cloud cover changes with the seasons, but Koonin says the variation was four or five times larger than had been expected.

"That is a bit of a mystery right now that we want to understand," Koonin says.

But that’s only the tip of the iceberg, so to speak. The data also reveals what Goode calls a "hint" of a 2.5 percent decrease in the Earth’s albedo over the past five years.

The researchers are a little cautious on this point, because their equipment is not quite precise enough to be definitive, but Koonin points out that if the decrease is only one percent, that’s a major change.

In fact, it’s too much of a change to be the result of greenhouse warming.

It would take all of the greenhouse gases released since pre-industrial times to push the Earth’s albedo down by 2.5 percent, and their data indicates that the drop occurred over just five years.

But What's the Reason?

Clearly, there must be another explanation. The scientists note that the drop occurred during a period when the magnetic activity on the sun that produces sun spots rose from minimum to maximum.

That’s fascinating to solar physicists, because it has long been hypothesized that the sun’s magnetic field plays a role in the Earth’s climate. Nobody has been able to prove that, but now scientists have a new tool.

It would be nice if Earthshine could settle the debate over how much damage we are doing to our planet by burning fossil fuels, but that’s not likely to be the case. The subject is just too complicated for any one approach. Still, it is a "good diagnostic" tool that could help fill in a lot of blanks, Koonin says.

"Greenhouse gases are one perturbation of the climate system, and hence will cause climate change," he says. "In order to assess whether they are significant or not, we need to understand all perturbations of the climate system."

It’s a little like trying to assess your health just by taking your blood pressure, he says. You also need to look at the level of sugar in your blood, your pulse and respiration, as well as other things that can reflect the condition of the entire system.

So toward that end, the scientists are expanding their project into a global network of earthshine telescopes. Each system will cost about $20,000, and three are expected to begin operations in Asia in the coming months.

Together, the telescopes will provide a continual and precise record of the intensity of the light reflected back from the moon, an ongoing measurement of how much solar energy the Earth is keeping to itself.

"It’s really amazing, if you think about it," Koonin says, "that you can look at this ghostly reflection on the moon and measure what Earth’s climate is doing."

Yeah, but if Leonardo were still here, he probably would wonder why it took us so long.

Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.

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