Could the Pliocene constrain the equilibrium climate sensitivity?

J C Hargreaves,J D Annan

doi:10.5194/cp-12-1591-2016

Abstract

Abstract. The mid-Pliocene Warm Period (mPWP) is the most recent interval in which atmospheric carbon dioxide was substantially higher than in modern pre-industrial times. It is, therefore, a potentially valuable target for testing the ability of climate models to simulate climates warmer than the pre-industrial state. The recent Pliocene Model Intercomparison Project (PlioMIP) presented boundary conditions for the mPWP and a protocol for climate model experiments. Here we analyse results from the PlioMIP and, for the first time, discuss the potential for this interval to usefully constrain the equilibrium climate sensitivity. We observe a correlation in the ensemble between their tropical temperature anomalies at the mPWP and their equilibrium sensitivities. If the real world is assumed to also obey this relationship, then the reconstructed tropical temperature anomaly at the mPWP can in principle generate a constraint on the true sensitivity. Directly applying this methodology using available data yields a range for the equilibrium sensitivity of 1.9–3.7 °C, but there are considerable additional uncertainties surrounding the analysis which are not included in this estimate. We consider the extent to which these uncertainties may be better quantified and perhaps lessened in the next few years.

Highlights

One important motivation for the study of palaeoclimates is that they may provide information as to how the climate will change in the future
We focus on the midPliocene Warm Period, 2.97–3.29 million years before the present, as this represents the most recent time that the atmospheric CO2 level was substantially higher than in pre-industrial times and substantial effort has been made to collect data from this interval (Dowsett et al, 2009), which suggests that the mPWP climate was warmer than the pre-industrial climate
As in Hargreaves et al (2007) and Hargreaves et al (2012), we anticipate that the relationship between S and palaeoclimate changes is likely to be stronger if we focus on the tropics for the palaeosimulations, since this will reduce the influence of ice sheet and vegetation changes

Summary

Introduction

One important motivation for the study of palaeoclimates is that they may provide information as to how the climate will change in the future. Increased attention has recently been given to warmer periods (Lunt et al, 2013) These are generally more distant in time, and data are less certain, but the inference from past to future is potentially more robust as the past climate is warmer than present and more similar to what we expect to see in the future, with for example changes in ice sheets being relatively small. It is this inference that the current paper explores. We focus on the midPliocene Warm Period (mPWP), 2.97–3.29 million years before the present, as this represents the most recent time that the atmospheric CO2 level was substantially higher than in pre-industrial times and substantial effort has been made to collect data from this interval (Dowsett et al, 2009), which suggests that the mPWP climate was warmer than the pre-industrial climate

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