Robust Increase in Effective Climate Sensitivity with Transient Warming in CMIP5 Simulations

Abstract:

Climate models diverge strongly in their projections of future global warming, in part due to different representations of how the effective global climate sensitivity (set by feedbacks linking surface warming to top-of-atmosphere radiative response) will vary in time as the Earth warms. Here we summarize the proposed mechanisms behind the time variation of effective climate sensitivity, and consider the implications for future climate prediction. CMIP5 simulations show a robust increase in effective climate sensitivity with transient global warming, indicating that estimates of equilibrium climate sensitivity based on modern observations of transient climate change may be biased low. Moreover, this increase in effective climate sensitivity leads to fundamental issues with a widely used method for diagnosing equilibrium climate sensitivity, radiative feedbacks and forcing within climate models -- the regression of the global top-of-atmosphere radiation flux on global surface temperature. We show that reported regression-based estimates of both equilibrium climate sensitivity and CO₂ forcing in CMIP5 models are systematically biased low.