The Carbon Balance of the Terrestrial Biosphere: Ecosystem Models and Atmospheric Observations

Precise measurements in air are helping to clarify the fate of CO2 released by human activities. Oxygen-to-nitrogen ratios in firn (the transition state from snow to ice) and archived air samples indicate that the terrestrial biosphere was approximately carbon-neutral on average during the 1980s. CO2 release by forest clearance during this period must have been compensated for by CO2 sinks elsewhere on land. Direct atmospheric O2:N2 measurements became available during the 1990s. These measurements indicate net terrestrial CO2 uptake of ∼2 Pg C/yr. From the north–south O2:N2 gradient, it has been inferred that about this amount was taken up by terrestrial ecosystems in the northern nontropics while additional CO2 released by tropical-forest clearance must have been compensated for by additional, tropical, terrestrial CO2 sinks. These and other atmospheric observations provide independent tests of carbon-cycle reconstructions made with process-based terrestrial ecosystem models. Such models can account for major features of the atmospheric-CO2 record, including the amplitude and phase of the seasonal cycle of atmospheric-CO2 concentration at different latitudes, and much of the interannual variability in the rate of increase of atmospheric CO2. Models also predict direct effects of rising atmospheric-CO2 concentration on primary production, modified by feedbacks at the plant and ecosystem levels. These effects translate into a global carbon sink the right order of magnitude to compensate for forest clearance during the 1980s. The modeled sink depends on continuously increasing CO2 to maintain disequilibrium between primary production and carbon storage. There are still substantial differences among the carbon-balance estimates made by different models, reflecting limitations in current understanding of ecosystem-level responses to atmospheric-CO2 concentration, especially with regard to the interactions of C and N cycling and interactions with land-use change. Scenario calculations nevertheless agree that if atmospheric CO2 continues its rise unchecked then the terrestrial sink will start to decline by the middle of the next century, for reasons including saturation of the direct CO2 effect on photosynthesis.