Quantitative Carbon cycle modelling to inform Climate Mitigation Policy

The global carbon cycle is a central part of the climate system which forms a direct link between human activity and climate change. This thesis presents my contribution to the field of research into the global carbon cycle with complex numerical models and its use to inform climate mitigation policy. Firstly, I present work I led to build, configure and apply the Hadley Centre Earth System Model, HadGEM2-ES, that successfully delivered the CMIP5 simulations. Then I present work that led to the design of the next generation of coupled carbon cycle intercomparison experiments. The aim of these experiments is to understand and quantify future centuryscale changes in land and ocean carbon storage and fluxes and their impact on climate projections. A set of ESM simulations was devised, with a common protocol, which all participating modelling centres should follow. A theoretical framework is commonly used to quantify carbon cycle feedbacks. I played an active role in its recommended use and definitions of terms. A feedback analysis I performed of future carbon cycle projections formed a central component of the IPCC’s Fifth Assessment Report. This is the first time that that the IPCC carbon cycle chapter had a section devoted to the feedbacks and future projections from coupled carbon cycle ESMs. Finally, I present three specific applications of my research and their relevance to climate mitigation policy. 1) I was the first to define the concept of committed ecosystem changes and demonstrate that ecosystems may continue to respond for many years or decades after climate is stabilised, leading to the recommendation that such committed change should be included in definitions of dangerous climate change. 2) I performed the first Earth System model analysis of the carbon emissions reductions required to follow the RCP pathways leading to the IPCC AR5 statement that, “For RCP2.6, an average 50% emission reduction is required by 2050 relative to 1990 levels”. 3) My research on carbon cycle feedbacks, especially the response of the carbon cycle to low CO2 pathways, found that models predict significant weakening, or even potential reversal, of natural carbon sinks in response to removal of CO2, which potentially hinders the effectiveness of the negative emissions. My research presented in this thesis has been influential in setting international research priorities in this field. It continues to inform global negotiations on climate mitigation policy.