More than a billion tons of carbon emitted into the atmosphere each year in the form of carbon dioxide cannot be accounted for but is probably being absorbed by land masses in the Northern Hemisphere, researchers from the National Oceanic and Atmospheric Administration reported last week in Science magazine.

Release of carbon dioxide into the atmosphere is believed to be contributing to a warming of the planet through the greenhouse effect, which traps heat at the Earth's surface.

Researchers had previously believed this carbon dioxide was being absorbed by oceans, but new measurements and computer modeling indicate it is not, according to climatologist Pieter Tans and his colleagues at the National Aeronautics and Space Administration and Columbia University.

Using data from satellites and ground-based measurements, the team is now trying to find out precisely where the excess carbon dioxide is being stored.