Records of the annual variation of the atmospheric carbon dioxide concentration at Mauna Loa, Point Barrow, and Weathership P are examined for secular changes. The amplitude of the annual variation appears to have increased in recent years with a best estimate of the rate of change, based on the Mauna Loa data, of 0.45¿0.42% yr-1. This change is discussed in terms of changes in biospheric respiration and photosynthesis and the use of fossil fuels. The analysis does not allow for the separation of several possible causes of amplitude change. However, if the change is interpreted as reflecting enhanced biospheric growth, the effect is equivalent to a 8% change in the net summer uptake of carbon over the years 1959-1978 and to a growth of the northern hemisphere seasonal biosphere of 0.5¿1012 kg of carbon per year. Such a conclusion is consistent with recent inventory studies, which indicate that temperature zone forests have acted as a net sink of about 1012 kg of carbon per year in recent decades.