Contents

18 January 1991
Vol 251, Issue 4991

About The Cover

Lakes and ponds on the arctic tundra with Itigaknit Mountain in the background, North Slope, Alaska. These aquatic ecosystems are continuously releasing carbon dioxide to the atmosphere. Much of the carbon originates in terrestrial environments, and accounting for this release substantially lowers the estimate of the worldwide arctic sink for atmospheric carbon dioxide. See page 298. [Photograph by George W. Kling]