Abstract
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.
Highlights
The properties of natural land and ocean CO2 sinks have major implications both for climate mitigation goals and for adaptive responses
The CO2 airborne fraction (AF, the fraction of total anthropogenic CO2 emissions from fossil fuels and net land use change that accumulates in the atmosphere) determines the fraction of emissions that contribute to rising atmospheric CO2 concentrations, with the remainder being absorbed by land and ocean sinks
The natural CO2 sinks that absorb more than half of all anthropogenically emitted CO2 represent a massive ecosystem service to humankind (Millennium Ecosystem Assessment, 2005), with implications that are directly quantified by the AF (Raupach et al, 2008)
Summary
The properties of natural land and ocean CO2 sinks have major implications both for climate mitigation goals and for adaptive responses. Since the commencement of high-quality atmospheric CO2 measurements in 1958, the AF has averaged about 0.44 (Canadell et al, 2007; Knorr, 2009; Le Quéré et al, 2009; Tans, 2009; Ballantyne et al, 2012), with significant interannual variability (Keeling and Revelle, 1985) This fact is one of the most important attributes of the contemporary carbon cycle, with major policy implications both for the climate mitigation challenge and for adaptation to climate change. The basic reason for the approximate past constancy of the AF is well known: a constant or zero-trend AF would be expected under a “LinExp” idealisation of the carbon cycle, in which land and ocean CO2 sinks increase linearly with excess CO2 above preindustrial concentrations (assumption “Lin”) and total anthropogenic CO2 emissions increase exponentially (“Exp”) (Bacastow and Keeling, 1979; Hofmann et al, 2009; Tans, 2009; Gloor et al, 2010; Raupach, 2013). We compare the observed past behaviours, likely future trajectories and diagnostic properties of the sink rate and the AF
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