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 CO 2 emissions remained in the atmosphere, with the rest being taken up by land and ocean CO 2 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 CO 2 sink rate ( kS), the combined land–ocean CO 2 sink flux per unit excess atmospheric CO 2 above preindustrial levels. Here we show from observations that kS declined over 1959–2012 by a factor of about 1 / 3, implying that CO 2 sinks increased more slowly than excess CO 2. Using a carbon–climate model, we attribute the decline in kS to four mechanisms: slower-than-exponential CO 2 emissions growth (~ 35% of the trend), volcanic eruptions (~ 25%), sink responses to climate change (~ 20%), and nonlinear responses to increasing CO 2, mainly oceanic (~ 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 CO 2. Our results suggest that the effects of these intrinsic, nonlinear responses are already detectable in the global carbon cycle. Although continuing future decreases in kS will occur under all plausible CO 2 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 kS to decline more strongly with increasing mitigation, while intrinsic mechanisms cause kS to decline more strongly under high-emission, low-mitigation scenarios as the carbon–climate system is perturbed further from a near-linear regime.