Long-term changes in amplitudes of atmospheric CO₂ concentrations at Ocean Station P and Alert, Canada

Abstract

Atmospheric CO₂ concentrations sampled at Ocean Weather Station P and Alert, Canada, were analyzed for possible increase in the seasonal amplitudes. The sampled data sets were reduced by smoothing spline method to obtain mean monthly values. Each set of the monthly means was detrended with a 21-point moving-average filter, and then was fitted with a mixed additive-multiplicative model. The changing amplitudes of the seasonal components were adjusted with the local amplitude scaling factors which were estimated from seven-month moving regressions. Linear regression was used to estimate the rates of change of the amplitudes. Trends of the seasonal components were also approximated by regressing the scaling factors of the same calendar months on time increments. Our results suggest that the growth rate of CO₂ amplitudes was ˜ 1.4% yr^-1 from 1971 to 1979 for Station P. However, the growth rate for Alert between 1977 and 1985 was about - 0.02% yr^-1, which is not statistically significant above zero. The different trends obtained are attributed mainly to the geographical differences in CO₂ sources and sinks, one of which may be the biosphere. Factors such as the winter-season photosynthesis, which probably is increasing and may be altering the CO₂ amplitudes, are discussed.