Modelling Climate-Carbon Cycle Interactions

Kirsten Zickfeld


Abstract:With the development of coupled climate-carbon cycle or "Earth System" models new insights are emerging about the response of the climate system to anthropogenic carbon emissions. Particularly, it has been shown that feedbacks between climate and the carbon cycle have the potential to significantly amplify the climate changes resulting from anthropogenic carbon dioxide (CO2) emissions, and that these feedbacks render CO2-induced climate change largely irreversible on timescales of centuries to millennia. The goal of this project is to use and further develop the University of Victoria Earth system climate model to better constrain future climate-carbon cycle feedbacks, to explore the reversibility of anthropogenic climate change, and to investigate the role of climate-carbon cycle feedbacks for the attainability of climate stabilization targets.Introduction:With the development of coupled climate-carbon cycle models over the last decade it has become possible to explore the interactions between carbon sinks, atmospheric carbon dioxide (CO2) and climate change. A number of studies with these models suggest that climate change could reduce CO2 uptake and accelerate accumulation of CO2 in the atmosphere, providing for a positive feedback (Cox et al. 2000). Recent studies have also shown that the climate response to carbon dioxide emissions is largely irreversible on centennial to millennial timescales even after CO2 emissions are completely halted (Eby et al., 2009). Furthermore, coupled climate-carbon cycle models have been instrumental in demonstrating that the cumulative CO2 emissions compatible with long-term temperature stabilization targets are independent of the timing of those emissions (Zickfeld et al., 2009). These results can be generalized to show that the instantaneous global mean temperature response is proportional to cumulative carbon emissions (Matthews et al., 2009).These findings are of high societal relevance, as they indicate (i) the long legacy (millennia) of anthropogenic emissions, and (ii) the challenge of reversing climate change, should the actual level of atmospheric CO2 turn out to be "dangerous". Furthermore, it has been suggested that a policy framework aimed at avoiding dangerous climate change based on cumulative carbon emissions could help overcome obstacles currently obstructing international climate negotiations.Despite significant advances in our understanding of the climate response to CO2 emissions over recent years, many open questions remain. These pertain to the persistence of anomalies in climate variables other than global mean temperature, the feasibility of CO2 concentration "overshoot" scenarios, the physical mechanisms responsible for the quasi-linear relationship between global mean temperature and cumulative CO2 emissions, and the applicability of the cumulative emissions framework to non-CO2 greenhouse gases and regional climate targets. The proposed research program aims at elucidating these open questions.Objectives:The goals of this project are:1.To further explore the irreversibility of CO2-induced climate change at global and regional scales.2.To test the applicability of the cumulative emissions framework to regional climate targets, such as limiting temperature change above the major ice sheets, or constraining surface ocean pH changes in sensitive ocean regions.3.To explore the emissions reduction required to revert from potentially "dangerous" levels of warming.4.To investigate to climate response to anthropogenic emissions of non-CO2 greenhouse gases and aerosols, and assess their potential role in climate change mitigation.Research ToolsTo achieve these objectives we plan to use simulations with the University of Victoria Earth System Climate Model (UVic ESCM), a model of intermediate complexity. The latest version (2.9) of the UVic ESCM consists of a 3-D ocean general circulation model coupled to a dynamic-thermodynamic sea-ice model and an energy-moisture balance model of the atmosphere with dynamical feedbacks (Weaver et al. 2001). The land surface and terrestrial vegetation are represented by a land-surface scheme coupled to a dynamic vegetation model. Ocean carbon is simulated by means of an inorganic carbon-cycle model and a marine ecosystem/biogeochemistry model solving prognostic equations for nutrients, phytoplankton, zooplankton and detritus. Version 2.9 of the UVic ESCM also includes a marine sediment component. The UVic-ESCM has been extensively and successfully validated against observational and paeoclimatic data. It has been used as an assessment tool in both the third and fourth assessment reports of the Intergovernmental Panel on Climate Change (IPCC), and has participated in numerous international model intercomparison projects including the Coupled Carbon Cycle Climate Model Intercomparison Project (C4MIP; Friedlingstein et al., 2006).