THROUGHOUT the Northern Hemisphere the concentration of atmospheric carbon dioxide rises in winter and declines in summer, mainly in response 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 southwards to about 3 p.p.m. near the Equator, owing to the diminishing seasonality of plant activity towards the tropics. In spite of atmospheric mixing processes, enough spatial variability is retained in the seasonal cycle of CO2 to reveal considerable regional detail in seasonal plant activity(6). Here we report that the annual amplitude of the seasonal 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 cycle, suggesting a lengthening of the growing season. In addition, the annual amplitudes show maxima which appear to reflect a sensitivity to global warming episodes that peaked in 1981 and 1990. We propose that the amplitude increases reflect increasing assimilation of CO2 by land plants in response to climate changes accompanying recent rapid increases in temperature.