The declining uptake rate of atmospheric CO 2 by land and ocean sinks

Through 1959-2012, an airborne fraction (AF) of 0.44 of total anthropogenic CO2 emissions remained in the atmosphere, with the rest being taken up by land and ocean CO2 sinks. Understanding of this uptake is critical because it greatly alleviates the emissions reductions required for climate mitigation, and also reduces the risks and damages that adaptation has to embrace. An observable quantity that reflects sink properties more directly than the AF is the CO2 sink rate (k(S)), the combined land-ocean CO2 sink flux per unit excess atmospheric CO2 above preindustrial levels. Here we show from observations that k(S) declined over 1959-2012 by a factor of about 1/3, implying that CO2 sinks increased more slowly than excess CO2. Using a carbon-climate model, we attribute the decline in k(S) to four mechanisms: slower-than-exponential CO2 emissions growth (similar to 35% of the trend), volcanic eruptions (similar to 25 %), sink responses to climate change (similar to 20 %), and nonlinear responses to increasing CO2, mainly oceanic (similar to 20 %). The first of these mechanisms is associated purely with the trajectory of extrinsic forcing, and the last two with intrinsic, feedback responses of sink processes to changes in climate and atmospheric CO2. Our results suggest that the effects of these intrinsic, nonlinear responses are already detectable in the global carbon cycle. Although continuing future decreases in k(S) will occur under all plausible CO2 emission scenarios, the rate of decline varies between scenarios in non-intuitive ways because extrinsic and intrinsic mechanisms respond in opposite ways to changes in emissions: extrinsic mechanisms cause k(S) to decline more strongly with increasing mitigation, while intrinsic mechanisms cause k(S) to decline more strongly under high-emission, low-mitigation scenarios as the carbon-climate system is perturbed further from a near-linear regime.