Studying past geologic warm periods can give valuable insight into important processes that may operate in the climate system during a warm regime. It also allows testing of our climate models ‘out-of-sample’. That is, climate models are most often developed and tested for the period of instrumental measurements, before being used to project the future. But the likely future long-term consequences of anthropogenic greenhouse gas changes is outside the range of this testing.
The mid-Pliocene warm period (~ 3 million years ago) has been considered a possible analog for the future. 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. In this talk, I will review the geological reconstructions for the mid-Pliocene that indicate strong warming at polar latitudes and much higher sea level than today, and the various hypotheses that have been proposed to explain this warmth. These mechanisms and feedbacks are tested with climate models that simulate the responses of the atmosphere, ocean, sea ice, and land to the greenhouse gas forcing. I will conclude with a discussion of the possible reasons for the mismatches between the data and models.