Manmade greenhouse gas emissions aren't just causing global warming, according to a new research report, but they might aggravate dangers from a growing population of space junk.

File photo of Earth's atmosphere from the International Space Station. Credit: NASA
 
A team of U.S. and Canadian scientists found carbon dioxide caused by human activity reaches the upper atmosphere, according to findings published in the journal Nature Geoscience on Nov. 11.

Unlike the warming effect caused at the surface, higher carbon dioxide levels in the upper atmosphere lead to cooler temperatures, which contract the thermosphere, a rarefied layer of gas at the edge of space.

Many satellites, including the International Space Station, fly through the thermosphere, along with thousands of chunks of space debris.

The thermosphere always cools and contracts when the solar activity reaches the low point of its 11-year cycle, but the rise in carbon dioxide levels could reduce the average density of the atmosphere in regions where satellites operate.

Atmospheric drag is the only current way to get rid of space junk. Slight pressure from the thin upper atmosphere gradually pushes against objects in low Earth orbit, causing satellites and debris to lose altitude and eventually drop from orbit and burn up during re-entry.

But if the atmosphere shrinks, as postulated by scientists studying greenhouse gas emissions, there might be less drag to push against objects in space, meaning space junk could stay in orbit longer.

Experts and satellite operators are concerned the number of pieces of space junk could skyrocket as collisions between objects beget more debris.

The same properties of carbon dioxide that cause it to trap heat in the dense lower atmosphere cause cooler conditions at high altitudes, according to scientists.

Artist's concept of the SciSat satellite. Credit: Bristol Aerospace
 
"In the upper atmosphere, thermal energy is transferred via collisions from other atmospheric constituents to carbon dioxide, which then emits the energy as heat that escapes to outer space," said John Emmert of the Naval Research Laboratory.

In the lower atmosphere, the heat emitted in particle collisions is trapped, causing rising temperatures.

Scientists used a spectrometer on the Canadian SciSat satellite to measure abundances of carbon dioxide and other gases as sunlight passes through the atmosphere.

The measurements show a steady upward trend in carbon dioxide concentrations in the upper atmosphere since the satellite's launch in 2003.

The findings support other evidence showing the thermosphere is contracting, according to scientists. Data from satellites indicate they are experiencing less drag.

Emmert was joined in the research by other scientists from NRL, Old Dominion University in Norfolk, Va., and the University of Waterloo in Ontario.