Abstract
Abstract. Can we summarize uncertainties in global response to greenhouse gas forcing with a single number? Here, we assess the degree to which traditional metrics are related to future warming indices using an ensemble of simple climate models together with results from the Coupled Model Intercomparison Project phases 5 and 6 (CMIP5 and CMIP6). We consider effective climate sensitivity (EffCS), transient climate response (TCR) at CO2 quadrupling (T140) and a proposed simple metric of temperature change 140 years after a quadrupling of carbon dioxide (A140). In a perfectly equilibrated model, future temperatures under RCP8.5 (Representative Concentration Pathway 8.5) are almost perfectly described by T140, whereas in a mitigation scenario such as RCP2.6, both EffCS and T140 are found to be poor predictors of 21st century warming, and future temperatures are better correlated with A140. We show further that T140 and EffCS calculated in full CMIP simulations are subject to errors arising from control model drift and internal variability, with greater relative errors in estimation for T140. As such, if starting from a non-equilibrated state, measured values of effective climate sensitivity can be better correlated with true TCR than measured values of TCR itself. We propose that this could be an explanatory factor in the previously noted surprising result that EffCS is a better predictor than TCR of future transient warming under RCP8.5.
Highlights
Summarizing the response of the Earth system to anthropogenic forcers with metrics has long been practised as a way to illustrate uncertainty in Earth system response to greenhouse gases
The concept of the equilibrium climate sensitivity (ECS), the equilibrium global mean temperature increase which would be observed in response to a doubling of atmospheric carbon dioxide concentrations (Hansen et al, 1984), has existed for over 50 years (Charney et al, 1979) and a significant amount of literature has been devoted to constraining its value (Knutti et al, 2017)
The Earth system responds to a step change in forcing on timescales ranging from days to millennia (Knutti and Rugenstein, 2015), so an “effective climate sensitivity” (EffCS here on) is often used as a proxy for decadal to centennial feedbacks
Summary
Summarizing the response of the Earth system to anthropogenic forcers with metrics has long been practised as a way to illustrate uncertainty in Earth system response to greenhouse gases. EffCS is calculated by assuming that a model is associated with a single feedback parameter (i.e. a rate of change of top of atmosphere radiative flux per unit surface temperature increase), allowing the equilibrium temperature response to a step change forcing to be predicted by linear extrapolation. Another metric, the transient climate response (TCR) at the time of CO2 doubling or quadrupling (T140) is calculated from an “1pctCO2” idealized experiment in which CO2 concentrations are increased by 1 % each year, starting from a pre-industrial state, resulting in linearly increasing forcing
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