Observational constraints on parameter estimates for a simple climate model

Abstract

Observations of ocean temperature and ocean heat content changes are investigated in order to better estimate the primary ocean parameters in a simple climate model, namely MAGICC (Model for the Assessment of Greenhouse gas Induced Climate Change). A re-examination of the simulated 20th century ocean temperature changes has been made possible by the release of new observational data, which indicates that complex climate models and MAGICC are mixing too much heat into the deeper ocean. Goodness-of-fit testing between the simulated and observed 1960-2008 world ocean temperature change leads to a revised set of parameters for the simple climate model that include a value for the ocean effective vertical diffusivity that is nearly a quarter of that used in the IPCC Third and Fourth Assessment Report versions of the model. Testing an independent constraint based on the ratio of changes in sea surface temperature to 700 m ocean heat content changes produces similar results. The lower ocean diffusivity affects the surface temperature results and alters the best estimate for the climate sensitivity parameter. The projected temperature changes for a high-growth emissions scenario show a larger increase in temperature by 2100 even with a reduced climate sensitivity.