What can Pliocene tell us about Global Warming?

The early Pliocene warm period (dated around 4 million years ago) is an intriguing test case for Global Climate Models (GCMs) as it was the last time today's CO2 levels are seen in the paleorecord. However, proxy data of the early Pliocene sea surface temperatures (SSTs) suggest that the Earth was substantially warmer at that time than today. Here, we use the GFDL's AM2.1 atmospheric GCM, being forced with a Pliocene SST reconstruction, to study the roles of fast physical processes in sustaining such warming and consider the implications for the model's fitness in simulating anthropogenic global warming (AGW). At the top-of-the-atmosphere (TOA), outgoing longwave radiation (OLR) should be in approximate balance with shortwave absorption (SWABS) for the climate to remain extendedly steady (as in the Pliocene). It is found that with a global mean surface temperature increase of about 4.4K over today's conditions (control run) the Pliocene run shows an increase in both SWABS and OLR (9.7 and 6.1 W/m2, respectively). The 3.6 W/m2 imbalance introduced may indicate a difficulty in the model's ability to simulate the Pliocene steady conditions, most likely due to the model's cloud parameterizations. The Pliocene warming had many different climatic impacts than AGW is predicted to including redistribution of cloud cover and precipitation patterns, with a strong shift in both from ocean to land, as well as opposing changes in oceanic and atmospheric meridional heat transport.