Paris - A build-up of carbon dioxide in the upper levels of Earth's atmosphere risks causing a faster accumulation of man-made space junk and resulting in more collisions, scientists said on Sunday.While it causes warming on Earth, CO2 conversely cools down the atmosphere and contracts its outermost layer, the thermosphere, where many satellites including the International Space Station (ISS) operate, said a study published in the journal Nature Geoscience.A contracted thermosphere, in turn, reduces atmospheric "drag" on satellites - a similar force to that experienced when holding one's hand out the window of a moving car.This "drag" is what causes satellite orbits to change, drawing them closer to Earth, which means that orbiters like the ISS have to boost themselves back on course with on-board engines."The observed CO2 increase is expected to gradually result in a cooler, more contracted upper atmosphere and a consequent reduction in the atmospheric drag experienced by satellites," said a statement from the Naval Research Laboratory, which took part in the study.Earth riskCommenting on the paper, space expert Hugh Lewis said a cooler troposphere will extend the lifetime of space junk - staying farther out for longer instead of burning up in the lower layers of the atmosphere, closer to Earth."Consequently, space junk will accumulate at a faster rate and we will see more collisions between space objects as a result," he said."We will also see many more 'near-misses' and these have an important effect on spacecraft operators."Lewis said there would be no increased risk for us on Earth as the rate at which satellites re-enter would be reduced."However, we would see some effects on services provided from space if an important satellite was destroyed by a collision...," he said.On the positive side, satellites would no longer need to boost themselves back into orbit quite as often, meaning they can carry less fuel.