Simulating Carbon Sequestration With Coupled Climate-Carbon Cycle Feedbacks

Terrestrial sequestration or other carbon management to mitigate anthropogenic greenhouse-gas emissions must be understood and implemented in the face of a variable and changing climate. Virtually all processes responsible for storage of carbon in the vegetation and soils of both managed and unmanaged ecosystems are significantly influenced by temperature, precipitation and other climate variables. Accordingly, forecasts of terrestrial carbon sequestration must include the influence of climate on the carbon dynamics of managed and unmanaged ecosystems as well as including purposeful human intervention in the carbon cycling of managed ecosystems. However, the future climates used in carbon management forecasts are often derived from coupled atmosphere-ocean general circulation models that do not include an interactive carbon cycle, and thus they miss important climate-carbon cycle feedbacks. In particular, such analyses miss the feedbacks between purposeful carbon management, its influence on atmospheric CO2 and thus climate, and the subsequent influence of climate on managed (and unmanaged) ecosystem carbon sequestration. This decoupling is a source of uncertainty in evaluating potential carbon management options. Here we describe modeling results that analyze components of this uncertainty. We describe a model of global carbon management that includes atmosphere, ocean and terrestrial (managed and unmanaged) carbon cycling. We describe aspects of coupling the carbon cycle model with energy-economic systems. We then present simulations using a future climate generated by an independent climate model. The projected changes in climate release CO2 from the terrestrial biosphere (primarily through soil response to warmer temperatures), resulting in higher future CO2 concentrations than in the absence of climate change (approximately 50ppm higher by 2100). These simulations indicate that future carbon management must compensate for carbon released in response to feedbacks from climate change as well as the emissions from energy production. We then describe the coupling of the carbon management model with a climate model for fully coupled climate-carbon simulations, and discuss how this coupling influences the analysis of terrestrial sequestration as an option for carbon management.