As global warming continues, a big unknown is what will happen to the carbon balance between the atmosphere and the land, especially in the far north. Will Arctic and near-Arctic regions continue to take more carbon dioxide from the atmosphere through plant activity than they release, or will they release more than they store?

A new study suggests that Alaska, with its huge stretches of tundra and forest, may be shifting from a net sink, or storehouse, of carbon to a net source. The study focused on one possible cause: warmer temperatures that keep the Arctic tundra from freezing until later in the fall, allowing plant respiration and microbial decomposition — processes that release carbon dioxide — to continue longer.

Roisin Commane, a researcher at Harvard, and others studied atmospheric carbon dioxide in the state, using measurements from aircraft and a 40-year record from sensors operated by the National Oceanic and Atmospheric Administration in Barrow, in the North Slope Borough.