‘Pliocene Climates: The Nature of the Problem’ Revisited

Abstract:

In 1996, Tom Crowley published a paper with this title in the journal Marine Micropaleontology. He argues that “study of past warm periods is valuable because it gives insight into important processes that may be operating in the climate system during a warm regime.” The mid-Pliocene warm period (3.264 to 3.025 Ma) has been considered a possible analog for the future long-term consequences of anthropogenic greenhouse gas changes. Atmospheric carbon dioxide concentrations for this period have been estimated to be approximately 350 to 450 ppmv. We have now nearly surpassed globally the mid-point of this range and could surpass 450 ppmv by shortly after year 2030 in the most dire pathway of future emissions.

Since 2008, the Pliocene Model Intercomparison Project (PlioMIP) has taken on this challenge with simulations by coupled climate models of the mid-Pliocene warm period. These models could not reproduce the mid- and high-latitude Northern Hemisphere warmth indicated by the PRISM3 dataset. This is also the case for the Community Climate System Model (CCSM4) when applying the forcings and boundary conditions of PlioMIP1. To explore these mismatches, additional simulations with CCSM4 are being used to understand the sensitivity to the paleogeography, atmospheric carbon dioxide concentrations, and orbital conditions. We find that when the Bering Strait is closed, as in a new paleogeographic reconstruction for a time-slice centered at 3.205 Ma, the Atlantic Meridional Overturning Circulation and associated ocean heat transport increase and North Atlantic sea surface temperatures warm. Removing the West Antarctic Ice Sheet results in warming only in the Southern Ocean. Consideration of Northern Hemisphere warm summer orbital conditions also improves the correspondence with the PRISM3 reconstruction of North Atlantic peak annual warmth.