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The Planetary Society Blog

By Emily Lakdawalla

Climate change: Good for satellites in low Earth orbit?

Dec. 11, 2006 | 15:44 PST | 23:44 UTC

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Here's an interesting piece of good news / bad news that came out of the American Geophysical Union meeting taking place in San Francisco this week. The bad news: increased levels of carbon dioxide appear to be having a demonstrable effect on not only the lower atmosphere but also the upper atmosphere. The good news: the effect of carbon dioxide in the upper atmosphere is to lower rather than raise the temperature there, with the result that Earth's upper atmosphere drops in elevation, so that for a given altitude, the atmosphere is less dense. This turns out to be beneficial for spacecraft in low Earth orbit (at elevations between 200 and 800 kilometers), including Hubble and the International Space Station.

Here's how that works, according to a press release issued today about a study by Stan Solomon and Ray Roble (of the National Center for Atmospheric Research) and Liuing Qian (of Penn State).

Carbon dioxide cools the thermosphere, even though it acts to warm the atmosphere near the Earth's surface (the troposphere). This paradox occurs because the atmosphere thins with height. Near the Earth's surface, carbon dioxide absorbs radiation escaping Earth, but before the gas molecules can radiate the energy to space, frequent collisions with other molecules in the dense lower atmosphere force the carbon dioxide to release energy as heat, thus warming the air. In the much thinner thermosphere, a carbon dioxide molecule absorbs energy when it collides with an oxygen molecule, but there is ample time for it to radiate energy to space before another collision occurs. The result is a cooling effect. As it cools, the thermosphere settles, so that the density at a given height is reduced.

This isn't a newly discovered effect. Another natural phenomenon that affects the density of the thermosphere is the 11-year solar activity cycle. When the Sun is more active, ultraviolet radiation and energetic particles from the Sun act to heat and expand the thermosphere; when the Sun is less active, the thermosphere contracts.

If the thermosphere stays less dense due to the presence of higher levels of carbon dioxide, then it will cost less fuel to achieve and maintain low Earth orbit. But you can only save that fuel if you can predict accurately what your fuel costs are going to be, which requires good models of the future behavior of the thermosphere. Just such a successful model is the subject of Soloman, Qian, and Roble's presentation to the meeting. According to the abstract of their work, the solar cycle effects are much greater than the carbon dioxide effects at present.

This work is a reminder of why it's important to talk about "climate change" as opposed to "global warming." There are a wide variety of different anthropogenic effects on weather and climate. Some of them act to warm the air, ground, and oceans, and indeed the overall effect of human activity appears to be a net increase in global average temperatures of the part of the world we care most about -- the ground, sea surface, and lower hundred meters or so of atmosphere. But not everything gets warmer; the thermosphere gets cooler. It's an incredibly complex system, and it's really really hard to figure out how all of the different human inputs will change the climate. So it's good news when somebody can write down a mathematical model that works at predicting variability in any small part of our climate.

There are some pretty videos including an explanation by Solomon that you can download here.