CO₂ emissions affect thermosphere

Carbon dioxide from fossil-fuel burning is expected to cause an additional 3% reduction in the density of Earth's outermost atmosphere by 2017, according to a team of scientists from the National Center for Atmospheric Research (NCAR) and Pennsylvania State University (Geophys. Res. Lett. 2006, 33, L23705). "We're seeing climate change manifest itself in the upper as well as lower atmosphere," notes coauthor Stanley C. Solomon, an atmospheric scientist at NCAR. Recent observations made by tracking satellites that orbit about 300 miles above Earth's surface show that the density of the thermosphere-a layer 60 to 400 miles above the surface-is declining. Although CO₂ warms the lower atmosphere, it has the opposite effect in the much less dense thermosphere. There, each time a CO₂ molecule collides with atomic oxygen, it radiates energy to space. These events cool the thermosphere, allowing it to contract and reducing its density at any given altitude, Solomon says. Between 1970 and 2000, the density of the thermosphere declined 5%.