The declining uptake rate of atmospheric CO2 by land and ocean sinks

[摘要] Through 1959–2012, an airborne fraction (AF) of 0.44 of totalanthropogenic CO₂ emissions remained in the atmosphere, withthe rest being taken up by land and ocean CO₂sinks. Understanding of this uptake is critical because it greatlyalleviates 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 moredirectly than the AF is the CO₂ sink rate (k_S),the combined land–ocean CO₂ sink flux per unit excessatmospheric CO₂ above preindustrial levels. Here we show fromobservations that k_S declined over 1959–2012 by a factorof about 1 / 3, implying that CO₂ sinks increased more slowlythan excess CO₂.Using a carbon–climate model,we attribute the decline in k_Sto four mechanisms: slower-than-exponential CO₂ emissionsgrowth (~ 35% of the trend), volcanic eruptions (~ 25%),sink responses to climate change (~ 20%), andnonlinear responses to increasing CO₂, mainly oceanic (~ 20%).The first of these mechanisms is associated purely withthe trajectory of extrinsic forcing, and the last two with intrinsic, feedbackresponses of sink processes to changes in climate and atmosphericCO₂. Our results suggest that the effects of these intrinsic,nonlinear responses are already detectable in the global carboncycle. Although continuing future decreases in k_S willoccur under all plausible CO₂ emission scenarios, the rate ofdecline varies between scenarios in non-intuitive ways becauseextrinsic and intrinsic mechanisms respond in opposite ways to changesin emissions: extrinsic mechanisms cause k_S to declinemore strongly with increasing mitigation, while intrinsic mechanismscause k_S to decline more strongly under high-emission,low-mitigation scenarios as the carbon–climate system is perturbedfurther from a near-linear regime.

[学科分类] 地球化学与岩石