Interaction of ocean and biosphere in their transient responses to increasing atmospheric CO2

Increasing atmospheric CO₂ induces a net uptake of carbon in the ocean by a shift in chemical equilibrium in seawater, and in the terrestrial biosphere by a stimulated photosynthesis and productivity. The fractions absorbed in both biosphere and ocean decline with increasing dynamics of the release rate of CO₂ into the atmosphere. However, the relative portion of ocean absorption descends much faster with annual growth rate of CO₂ release than biospheric absorption does, due to a difference in dynamics. The equilibrium absorption capacity of the biosphere is estimated to be only one quarter of that of the ocean, but the current sink size of the biosphere is about half of that of the ocean.Apart from CO₂-stimulated carbon fixation, the biosphere releases CO₂ as a result of land use changes, in particular after deforestation. Both of these fluxes are of the order of 1–1.5 Pg of carbon per year. The CO₂-fertilization effect and regrowth together have turned the terrestrial biosphere as a whole from a source into a sink.