Transient Climate Sensitivity of Simple Climate Models

Understanding anthropogenic climate change trends critically depends on estimates of climate sensitivity. Different than equilibrium sensitivity, transient climate sensitivity, or response (TCR), is the measure of the climate response to a 1% yr^-1 increase in [CO₂] until doubling is reached. This is useful for understanding the climate response on shorter time scales, as doubling takes place within 70 years. Here we describe the TCR of two simple climate models, Hector v1.0 and MAGICC 5.3. Simple climate models play an important role in the policy and scientific communities primarily due to their use in climate mitigation scenarios within integrated assessment models, among other applications. In order to analyze the reproducibility of climate trends in Hector and MAGICC, the TCR of both simple climate models is compared to CMIP5 models run under a 1% yr^-1 [CO₂] increase. These experiments are carried out, for example, in MAGICC by increasing the [CO₂] 1% yr^-1 starting in 2010, until doubling in 2080. The surface temperature response and feedback response to the perturbations are of particular interest in order to determine the general agreement between Hector, MAGICC, and the coupled ocean-atmosphere models of CMIP5. Thus, this initial study allows us to identify uncertainties within the simple climate models, with the aim to reduce such uncertainties with future model development.