Articles | Volume 12, issue 8
Research article
01 Aug 2016
Research article |  | 01 Aug 2016

Could the Pliocene constrain the equilibrium climate sensitivity?

J. C. Hargreaves and J. D. Annan

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.

Short summary
The mid-Pliocene Warm Period, 3 million years ago, was the most recent interval with high greenhouse gases. By modelling the period with the same models used for future projections, we can link the past and future climates. Here we use data from the mid-Pliocene to produce a tentative result for equilibrium climate sensitivity. We show that there are considerable uncertainties that strongly influence the result, but we are optimistic that these may be reduced in the next few years.