INCREASED ACTIVITY OF NORTHERN VEGETATION INFERRED FROM ATMOSPHERIC CO2 MEASUREMENTS

THROUGHOUT the Northern Hemisphere the concentration of atmospheric ca rbon dioxide rises in winter and declines in summer, mainly in respons e to the seasonal growth in land vegetation(1-4). In the far north the amplitude of the seasonal cycle, peak to trough, is between 15 and 20 parts per million by volume(5). The annual amplitude diminishes south wards to about 3 p.p.m. near the Equator, owing to the diminishing sea sonality of plant activity towards the tropics. In spite of atmospheri c mixing processes, enough spatial variability is retained in the seas onal cycle of CO2 to reveal considerable regional detail in seasonal p lant activity(6). Here we report that the annual amplitude of the seas onal CO2 cycle has increased by 20%, as measured in Hawaii, and by 40% in the Arctic, since the early 1960s. These increases are accompanied by phase advances of about 7 days during the declining phase of the c ycle, suggesting a lengthening of the growing season. In addition, the annual amplitudes show maxima which appear to reflect a sensitivity t o global warming episodes that peaked in 1981 and 1990. We propose tha t the amplitude increases reflect increasing assimilation of CO2 by la nd plants in response to climate changes accompanying recent rapid inc reases in temperature.