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Biogeosciences, 11, 3453-3475, 2014
www.biogeosciences.net/11/3453/2014/
doi:10.5194/bg-11-3453-2014
© Author(s) 2014. This work is distributed
under the Creative Commons Attribution 3.0 License.
The declining uptake rate of atmospheric CO2 by land and ocean sinks
M. R. Raupach1,2,3, M. Gloor4, J. L. Sarmiento5, J. G. Canadell1,2, T. L. Frölicher5,6, T. Gasser7,8, R. A. Houghton9, C. Le Quéré10, and C. M. Trudinger11
1CSIRO, Centre for Australian Weather and Climate Research, Canberra, ACT 2601, Australia
2Global Carbon Project, CSIRO Marine and Atmospheric Research, Canberra, ACT 2601, Australia
3Present affiliation: Climate Change Institute, Australian National University, Canberra, ACT 2601, Australia
4School of Geography, University of Leeds, Woodhouse Lane LS9 2JT, UK
5Program in Atmospheric and Oceanic Sciences, Princeton University, Sayre Hall, Forrestal Campus, Princeton, NJ 08540-6654, USA
6Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, Switzerland
7Centre International de Recherche en Environnement et Développement, CNRS-CIRAD-EHESS-AgroParisTech-PontsParisTech, Campus du Jardin Tropical, 94736 Nogent-sur-Marne Cedex, France
8Laboratoire des Sciences du Climat et de l'Environnement, CEA-CNRS-UVSQ, CE l'Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France
9Woods Hole Research Center, Falmouth, MA 02540, USA
10Tyndall Centre for Climate Change Research, University of East Anglia, Norwich NR4 7TJ, UK
11CSIRO, Centre for Australian Weather and Climate Research, Aspendale, VIC 3195, Australia

Abstract. 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 (kS), the combined land–ocean CO2 sink flux per unit excess atmospheric CO2 above preindustrial levels. Here we show from observations that kS 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 kS to four mechanisms: slower-than-exponential CO2 emissions growth (~ 35% of the trend), volcanic eruptions (~ 25%), sink responses to climate change (~ 20%), and nonlinear responses to increasing CO2, 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 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 kS 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 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.

Citation: Raupach, M. R., Gloor, M., Sarmiento, J. L., Canadell, J. G., Frölicher, T. L., Gasser, T., Houghton, R. A., Le Quéré, C., and Trudinger, C. M.: The declining uptake rate of atmospheric CO2 by land and ocean sinks, Biogeosciences, 11, 3453-3475, doi:10.5194/bg-11-3453-2014, 2014.
 
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