Evolution and variation of atmospheric carbon dioxide concentration over terrestrial ecosystems as derived from eddy covariance measurements

Carbon dioxide (CO₂) is the most important anthropogenic greenhouse gas contributing to global climate change. Understanding the temporal and spatial variations of CO₂ concentration over terrestrial ecosystems provides additional insight into global atmospheric variability of CO₂ concentration. Using 355 site-years of CO₂ concentration observations at 104 eddy-covariance flux tower sites in Northern Hemisphere, we presented a comprehensive analysis of evolution and variation of atmospheric CO₂ concentration over terrestrial ecosystem (ACTE) for the period of 1997-2006. Our results showed that ACTE exhibited a strong seasonal variations, with an average seaonsal amplitude (peak-trough difference) of 14.8ppm, which was approximately threefold that global mean CO₂ observed in Mauna Loa in the United States (MLO). The seasonal variation of CO₂ were mostly dominant by terrestrial carbon fluxes, i.e., net ecosystem procution (NEP) and gross primary produciton (GPP), with correlation coefficient(r) were-0.55 and-0.60 for NEP and GPP, respectively. However, the influence of carbon fluxes on CO₂ were not significant at interannual scale, which implyed that the inter-annual changing trends of atmospheric CO₂ in Northern Hemisphere were likely to depend more on anthropogenic CO₂ emissions sources than on ecosystem change. It was estimated, by fitting a harmonic model to monthly-mean ACTE, that both annual mean and seasonal amplitude of ACTE increased over the 10-year period at rates of 2.04 and 0.60ppmyr^-1, respectively. The uptrend of annual ACTE could be attributed to the dramatic global increase of CO₂ emissions during the study period, whereas the increasing amplitude could be related to the increases in Northern Hemisphere biospheric activity. This study also found that the annual CO₂ concentration showed large variation among ecosystems, with the high value appeared in deciduous broadleaf forest, evergreen broadleaf forest and cropland. We attribute these discrepancies to both differential local anthropogenic impacts and carbon sequestration abilities across ecosystem types.