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Reflecting on the Night Side of the Moon


by Bill Chameides | February 18th, 2009
posted by Erica Rowell (Editor)

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Earthshine (usually not visible to the naked eye) is light from the unlit side of the moon. That light is sunlight reflected from Earth to the moon back to Earth. Moonshine (the crescent shown above) is sunlight reflected off the moon to Earth. (Dan Bush)

Could global warming be caused by a change in the Earth’s reflectivity? In other words, is the Earth absorbing more sunlight? Believe it or not, scientists have been spending their nights gazing at the moon to answer those questions.

Greenhouse Gases Are Only Factor Behind Climate Change

Global temperatures are influenced by a variety of factors, any or all of which could in principle be the cause of global warming. Over the past several decades scientists have been carefully monitoring these factors to see if they have varied in a way that could explain the warming trend. With the exception of greenhouse gas increases they have come up empty.

In an earlier post I discussed the data showing that changes in the energy from the sun could not be an explanation for the warming temperatures. Now, a new paper helps elucidate the role of another factor – changes in the Earth’s reflectivity.

In order for sunlight to heat our planet, that light must be absorbed. But not all the sunlight is absorbed. About 30 percent of the sunlight that reaches the Earth is reflected back to space. We call that fraction of reflected light the planetary reflectivity or albedo.

For the last few decades we’ve monitored the Earth’s albedo in two ways:

  1. Satellites recording the light bouncing off the planet and
  2. Scientists at night observing the light from the unlit portion of the moon (i.e., earthshine).

Cool.

Inconsistent Trends in Albedo Readings … Until Now

Earthshine is the sunlight that is reflected from Earth onto the lunar surface and then reflected back to the Earth. Long-term measurements of earthshine (as opposed to moonshine, which is sunlight reflected directly from the moon) can be used to infer changes in the Earth’s reflectivity. (A key assumption of the earthshine method is that the reflectivity from the lunar surface is not changing – a very good assumption since there does not appear to be much going on there.)

The data from these two approaches have not shown a consistent trend. They show periods of increasing and decreasing albedo since the 1980s and significant inconsistencies between various instruments and methods. To address this issue, Enric Palle of Instituto de Astrofisica de Canarias, Tenerife, Spain, and colleagues took a very careful look at the earthshine and satellite data collected from 1998 to 2007, a period with overlapping measurements.

The results from both data sets, as reported in the Journal of Geophysical Research, are consistent and show an increase in albedo from 1998 to mid-2000 and then a relatively flat albedo thereafter. Palle and colleagues conclude that the changes in the earthshine data set over this time period reflect changes in cloud properties.

Not Enough Evidence

It is difficult to interpret the role of these albedo changes vis-a-vis greenhouse gases causing global warming. In the first place it covers too short a time period to make inferences related to climate. Moreover, since changes in cloudiness could occur as a result of global warming from greenhouse gases, it is not clear if these albedo changes are independent of or caused by greenhouse gas pollution.

We will learn more as we see how the albedo varies in the coming years. In the meantime, I find it reassuring to know that some of my colleagues are spending their nights gazing at the moon.

Additional Resources

Project Earthshine – www.bbso.njit.edu/Research/EarthShine

Earthshine – “Don’t Gawk and Drive,” NASA – science.nasa.gov/headlines/y2002/12apr_earthshine.htm

Dan Bush Photos: Earthshine – www.pbase.com/missouri_skies/image/92706902

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