Using a Hierarchy of Models to Constrain the Temperature Dependence of Climate Sensitivity

Climate sensitivity is typically defined as the increase in global temperature that would result from a doubling of atmospheric carbon dioxide (CO2) starting from the pre-industrial level. Climate sensitivity is the most important factor in determining the risk posed by anthropogenic greenhouse gas emissions, and high values of sensitivity with severe consequences cannot be ruled out.

Climate sensitivity is generally assumed to be constant over the range of climate states relevant to global warming, but this assumption may not hold if climate sensitivity is high or if greenhouse gas concentrations rise to a level equivalent to several CO2 doublings (possibly over several centuries). Recent work by the PIs and others suggests that climate sensitivity could increase with global temperature, and this increase could influence the risk of large warming. Work performed here uses a simplified climate model, the Community Atmosphere Model version 5 (CAM5) coupled to a slab mixed layer ocean, to examine the temperature dependence of climate sensitivity over one, two, and three CO2 doublings. The model is used to generate a perturbed physics ensemble (PPE), in which parameters controlling the behavior of cloud and convection (for example the fractional mass entrainment rate for convection) are perturbed as a means of varying the strength of climate feedbacks. Further work uses a cloud resolving model (the System for Atmospheric Modeling, or SAM) on an idealized domain to identify reasonable choices of parameter values. The work also considers the possibility that the parameter values themselves could change as climate warms, thereby yielding a temperature dependence in sensitivity. Additional work looks at climate sensitivity in greenhouse warming simulations from the Coupled Model Intercomparison Project version 5 (CMIP5), to determine if the simulations exhibit temperature dependent climate sensitivity.

The work has broader impacts due to the substantial societal consequences by high climate sensitivity and the desirability of constraints on how much warming can result from anthropogenic emissions. Temperature-dependent sensitivity may also help to interpret proxy records of past warm climates and climate change, thus the work has scientific as well as societal broader impacts. In addition, the project supports a graduate student and a postdoctoral research associate, so that workforce development is ensured in this research area.