LONDON (Reuters) - More satellites and orbiting debris could collide in the upper atmosphere because a buildup of carbon dioxide (CO2) has reduced the "drag effect" which can eventually send some space junk back down to Earth, a study shows.
Over the past eight years CO2 concentrations in the upper atmosphere have risen from burning fossil fuels that have warmed the Earth's surface and caused temperatures to increase, the study in the journal Nature Geoscience said.
This can result in a cooler, less dense atmosphere above a 90-km (55-mile) altitude, the study said, adding that this "will reduce atmospheric drag on satellites and may have adverse consequences for the orbital debris environment that is already unstable".
Less drag, or friction, in the upper atmosphere means space debris such as redundant satellites and defunct rocket bodies will stay at a certain altitude for longer, increasing the risk of collisions.
Global temperatures are now about 0.8 degree C (1.4 F) above pre-industrial times. Two degrees is viewed as a threshold to dangerous change including more powerful storms like Sandy that struck the United States this month, more heatwaves, droughts and rising sea levels.
The scientists, from the Naval Research Laboratory in Washington, Old Dominion University in Virginia, University of Waterloo in Ontario and the University of York in Britain, used satellite data to study changes in CO2 concentrations at a 101-km altitude between 2004 and 2012 and found that CO2 rose significantly over that time.
So far, CO2 trends have been measured only up to a 35-km altitude because balloons and aircraft do not reach high altitudes, and ground measurements and rockets only provide limited coverage.
Debris is always a danger to spacecraft and collisions can prove costly for spacecraft manufacturers.
There are 21,000 bits of debris larger than 10 cm (4 inches) in orbit, but collisions occur infrequently - about once a year on average, according to NASA, the U.S. space agency.
However, a U.S. National Research Council report in 2011 warned NASA that the amount of space debris orbiting the Earth was at critical level and the United States has been trying to develop technologies to remove debris and reduce hazards.
Editing by Michael Roddy