Greenhouse Gases Cooling Edge of Space

Nov. 29, 2006 — The same greenhouse gases guilty of warming the surface of the Earth are doing the opposite in the outer reaches of the atmosphere — cooling and contracting the air at the edge of space, which plays a big role in the operations of satellites, according to research in the latest issue of the journal Science.

The two-faced, high-altitude cooling effect of greenhouse gases has been seen before on the planet Venus, where a natural runaway greenhouse effect roasts the surface and makes its upper atmosphere four to five times colder than Earth’s.

Models and decades of data on space junk, which feels the drag of Earth’s outermost atmosphere, show the same effect is underway in Earth’s more modest greenhouse warming.

"Probably the most important output of the paper is clear demonstration of existence of the greenhouse effect at higher altitudes," said Jan Lastovicka of the Czech Republic’s Institute of Atmospheric Physics in Prague.

Lastovicka, along with colleagues from the United States, India and Germany reviewed the data on how the well-charted low-Earth orbits of space junk have changed over the decades. They concluded that the outer limits of Earth’s atmosphere is dropping. This shows up as less air drag on the space junk, and so less change in their orbits.

The space junk data matches climate model predictions about what ought to happen in the upper reaches of the atmosphere, says Stanley Solomon, deputy director of the High Altitude Observatory at the National Center for Atmospheric Research.

"It has been borne out," said Solomon, of predictions by researchers in 1989 of this cooling and shrinkage at highest levels of the atmosphere. "Three separate groups have measured the long-term decrease in density with long-term satellite data."

By lower density, Solomon is referring to the fact there is simply less and less tenuous air in low-Earth orbit. That’s because, like most things do when they cool off, the upper atmosphere is shrinking.

As for how greenhouse gases can switch from heaters to coolers at high altitude, it’s all about collisions, says Solomon.

Down in the dense, warm lower part of the atmosphere greenhouse gas molecules like carbon dioxide suck up solar energy and are constantly colliding with other gas molecules. As they collide, they pass that energy around and make things warmer.

In the upper atmosphere, on the other hand, there are far fewer air molecules to collide with. So when a carbon dioxide molecule absorbs solar energy it has a lot more time before it collides with another air molecule. In that interim, the carbon dioxide tends to radiate the heat into space as infrared light.

Few other gases have the ability to both absorb and radiate energy in infrared, said Solomon. So if you add lots of carbon dioxide to the upper atmosphere, it can cause the temperature to drop. Meanwhile, lower down where we live, the same gas ramps up the heat and causes global climate warming.

"This is evidence that our models work," said Solomon of the space junk data matching model predictions.

He added that it’s just one more reason everyone ought to have confidence in global climate models and heed their results.