1. ^* Department of Geography, Boston University, Commonwealth Avenue, Boston, Massachusetts 02215, USA
2. ^† Scripps Institution of Oceanography, La Jolla, California 92093-0220, USA
3. ^‡ NASA Goddard Space Flight Center, Code 923, Greenbelt, Maryland 20771, USA
4. ^§ Office of Mission to Planet Earth, NASA Headquarters, Washington DC 20546, USA
5. School of Forestry, University of Montana, Missoula, Montana 59812, USA

Abstract

Variations in the amplitude and timing of the seasonal cycle of atmospheric CO₂ have shown an association with surface air temperature consistent with the hypothesis that warmer temperatures have promoted increases in plant growth during summer¹ and/or plant respiration during winter² in the northern high latitudes. Here we present evidence from satellite data that the photosynthetic activity of terrestrial vegetation increased from 1981 to 1991 in a manner that is suggestive of an increase in plant growth associated with a lengthening of the active growing season. The regions exhibiting the greatest increase lie between 45°N and 70°N, where marked warming has occurred in the spring time³ due to an early disappearance of snow⁴. The satellite data are concordant with an increase in the amplitude of the seasonal cycle of atmospheric carbon dioxide exceeding 20% since the early 1970s, and an advance of up to seven days in the timing of the drawdown of CO₂ in spring and early summer¹. Thus, both the satellite data and the CO₂ record indicate that the global carbon cycle has responded to interannual fluctuations in surface air temperature which, although small at the global scale, are regionally highly significant.