The climate feedback parameter (λ, horizontal axis), adjusted radiative forcing (F_4×, vertical axis), and equilibrium temperature change (ΔT_4×, contours) for the 20 models submitting output to CMIP5, as calculated from abrupt4xCO2 simulations that posit an instantaneous four-fold increase in atmospheric CO₂ concentration

Figure 1. The climate feedback parameter (λ, horizontal axis), adjusted radiative forcing (F_4×, vertical axis), and equilibrium temperature change (ΔT_4×, contours) for the 20 models submitting output to CMIP5, as calculated from abrupt4xCO2 simulations that posit an instantaneous four-fold increase in atmospheric CO₂ concentration. Median and mean values represent the median and mean values of λ and F_4× determined by fits to each individual model. Crossed lines indicate 95% confidence intervals. Numeric values are presented in SOM table S1 (available at stacks.iop.org/ERL/8/034039/mmedia).

Abstract

The temperature response of atmosphere–ocean climate models is analyzed based on atmospheric CO₂ step-function-change simulations submitted to phase 5 of the Coupled Model Intercomparison Project (CMIP5). From these simulations and a control simulation, we estimate adjusted radiative forcing, the climate feedback parameter, and effective climate system thermal inertia, and we show that these results can be used to predict the temperature response to time-varying CO₂ concentrations. We evaluate several kinds of simple mathematical models for the CMIP5 simulation results, including single- and multiple-exponential models and a one-dimensional ocean-diffusion model. All of these functional forms, except the single-exponential model, can produce curves that fit most CMIP5 results quite well for both continuous and step-function CO₂-change pathways. Choice of model for any particular application would include consideration of factors such as the number of free parameters to be constrained and the conception of the underlying mechanistic model. Smooth curve fits to the CMIP5 simulation results realize approximately half (range 38%–61%) of equilibrium warming within the first decade after a CO₂ concentration increase, but approximately one quarter (range 14%–40%) of equilibrium warming occurs more than a century after the CO₂ increase. Following an instantaneous quadrupling of atmospheric CO₂, fits to four of the 20 simulation results reach 4 ° C of warming within the first decade, but fits to three of the 20 simulation results require more than a century to reach 4 ° C. These results indicate the need to reduce uncertainty in the temporal response of climate models and to consider this uncertainty when evaluating the risks posed by climate change.