Two decades of ocean CO₂ sink and variability

Abstract

Atmospheric CO 2 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 CO 2 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 O 2 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.

Multiple constraints on regional CO2 flux variations over land and oceans

“…The magnitude of this effect is similar in amplitude and in phase with the contribution of DIC to the total sea-air CO 2 flux (Figure 9). In comparison, the trend in the ocean sink caused by the increase in atmospheric CO 2 is estimated at $0.3 PgC per decade [Le Quéré et al, 2003b], about half of the trend produced by the OPA model and by the inversion in the Southern Ocean only. Thus this trend appears to be caused by changes in ocean stratification.…”

“…[5] ''Bottom-up'' studies used process-based biogeochemical models in climate models [Jones et al, 2001] or forced by climate fields to quantify regional IAV of carbon fluxes over land [Kindermann et al, 1996;Tian et al, 1998;Knorr, 2000;Gerard et al, 1999;Botta et al, 2002] and over the ocean [Le Quéré et al, 2003b;McKinley et al, 2004;Winguth et al, 1994]. Although those studies all agree in suggesting that the land IAV is larger than the ocean IAV, they may not attribute the IAV phenomenon to the same processes, and different models exhibit large discrepancies in the IAV regional signal [McGuire et al, 2001].…”

“…These relationships are found to hold in the North Atlantic for the MIT model, but not in OPA. In the North Pacific, SST and DIC variations compete in driving the interannual variability of pCO 2 in both models [Le Quéré et al, 2003b;McKinley et al, 2004].…”

Earth’s Climate System

“…1.6d. There is considerable year-to-year variability, which has been widely studied and is attributed mainly to terrestrial biosphere (Bousquet et al 2000;Le Quéré et al 2003). Averaged over the entire data record, 56 % of the CO 2 released to the atmosphere by humans by the combustion of fossil fuels and land use change has been absorbed by land and ocean sinks.…”

Monitoring and interpreting the ocean uptake of atmospheric CO 2

“…Some of these are strongly model-based, using biogeochemical ocean carbon models (e.g. [5,6]). Others combine models and observations, such as calculations of ocean 'anthropogenic' carbon using observations of oxygen, nutrients and carbon in the ocean [7,8] and inversion modelling of ocean or atmosphere [9,10].…”