Conditions for instability in the climate–carbon cycle system

Abstract

The climate and carbon cycle interact in multiple ways. An increase in carbon dioxide in the atmosphere warms the climate through the greenhouse effect, but also leads to uptake of CO 2 by the land and ocean sink, a negative feedback. However, the warming associated with a CO 2  increase is also expected to suppress carbon uptake, a positive feedback. This study addresses the question: “under what circumstances could the climate–carbon cycle system become unstable?” It uses both a reduced form model of the climate–carbon cycle system as well as the complex land model JULES, combined with linear stability theory, to show that: (i) the key destabilising loop involves the increase in soil respiration with temperature; (ii) the climate–carbon system can become unstable if either the climate sensitivity to CO 2 or the sensitivity of soil respiration to temperature is large, and (iii) the climate–carbon system is stabilized by land and ocean carbon sinks that increase with atmospheric CO 2 , with CO 2 -fertilization of plant photosynthesis playing a key role. For central estimates of key parameters, the critical equilibrium climate sensitivity (ECS) that would lead to instability at current atmospheric CO 2 lies between about 11 K (for large CO 2  fertilization) and 6 K (for no CO 2  fertilization). Given the apparent stability of the climate–carbon cycle, we can view these parameter combinations as implausible. The latter value is close to the highest ECS values amongst the latest Earth Systems Models. Contrary to a previous study that did not include an interactive ocean carbon cycle sink, we find that the stability of the climate–carbon system increases with atmospheric CO 2 , such that the glacial CO 2 concentration of 190 ppmv would be unstable even for ECS greater than around 4.5 K in the absence of CO 2  fertilization of land photosynthesis.