Projecting Future Climate Change: Implications of Carbon Cycle Model Intercomparisons

Models simulate responses of future climate to scenarios for anthropogenic emissions. While the uncertainty of these simulated responses is not established, perspective on uncertainty is provided by the intercomparison of the responses of alternative models. The envelope of alternative model simulations, if plausible responses, must be contained within the uncertainty range. In this way model intercomparisons form a lower-bound estimate of uncertainty range of future response. In this study, a parameterized Earth system model is used as a meta-model to simulate responses of alternative detailed models for the ocean and biosphere components of the global carbon cycle. The metamodel allows the integration of these components of the carbon cycle with an energy balance climate model for a prescribed range of climate sensitivity. The meta-model, parameterized in this way, is then used to construct ranges of: 1) past carbon budgets given past CO2 concentrations, fossil carbon emissions, and temperature records, 2) future CO2 concentrations and temperature for given emission scenarios, and 3) CO2 emissions and temperature for given trajectories of future CO2 concentrations leading to constant levels within the next several centuries. Uncertainty in projections of future CO2 concentration contributes to the range of future global temperature projected in this way. Carbon cycle is an additional contributor to uncertainty in climate projections that further expands the range of climate projections beyond that assessed by the Intergovernmental Panel on Climate Change.