Two decades of ocean CO₂ sink and variability

Atmospheric CO₂ has increased at a nearly identical average rate of 3.3 and 3.2 Pg C yr⁻¹ for the decades of the 1980s and the 1990s, in spite of a large increase in fossil fuel emissions from 5.4 to 6.3 Pg C yr⁻¹. Thus, the sum of the ocean and land CO₂ sinks was 1 Pg C yr⁻¹ larger in the 1990s than in to the 1980s. Here we quantify the ocean and land sinks for these two decades using recent atmospheric inversions and ocean models. The ocean and land sinks are estimated to be, respectively, 0.3 (0.1 to 0.6) and 0.7 (0.4 to 0.9) Pg C yr⁻¹ larger in the 1990s than in the 1980s. When variability less than 5 yr is removed, all estimates show a global oceanic sink more or less steadily increasing with time, and a large anomaly in the land sink during 1990–1994. For year-to-year variability, all estimates show 1/3 to 1/2 less variability in the ocean than on land, but the amplitude and phase of the oceanic variability remain poorly determined. A mean oceanic sink of 1.9 Pg C yr⁻¹ for the 1990s based on O₂ observations corrected for ocean outgassing is supported by these estimates, but an uncertainty on the mean value of the order of ±0.7 Pg C yr⁻¹ remains. The difference between the two decades appears to be more robust than the absolute value of either of the two decades.