Low interannual variability in recent oceanic uptake of atmospheric carbon dioxide

Abstract

An improved understanding of the partitioning of carbon between the atmosphere, terrestrial biosphere, and ocean allows for more accurate predictions of future atmospheric CO₂ concentrations under various fossil-fuel CO₂-emission scenarios. One of the more poorly quantified relevant processes is the interannual variability in the uptake of fossil-fuel CO₂ from the atmosphere by the terrestrial biosphere and ocean. Existing estimates, based on atmospheric measurements, indicate that the oceanic variability is large^1,2,3. Here we estimate the interannual variability in global net air–sea CO₂ flux using changes in the observed wind speeds and the partial pressure of CO₂ ($p_{{\text{CO}}_{\text{2}}}$) in surface sea water and the overlying air. Changes in seawater $p_{{\text{CO}}_{\text{2}}}$ are deduced from interannual anomalies in sea surface temperature and the regionally and seasonally varying temperature-dependence of seawater $p_{{\text{CO}}_{\text{2}}}$, assuming that variations in sea surface temperature reflect seawater $p_{{\text{CO}}_{\text{2}}}$ changes caused by thermodynamics, biological processes and water mixing. The calculated interannual variability in oceanic CO₂ uptake of 0.4 Gt C yr⁻¹ (2σ) is much less than that inferred from the analysis of atmospheric measurements^1,2,3. Our results suggest that variable sequestration of carbon by the terrestrial biosphere is the main cause of observed year-to-year variations in the rate of atmospheric CO₂ accumulation.