Cooperative Institute for Climate Science

The Cooperative Institute for Climate Science at Princeton University hosted a workshop on “The Pliocene Paradox: An Ice-Age Perspective on Global Warming Workshop” on October 21-23, 2004 at NOAA’s Geophysical Fluid Dynamics Laboratory and Princeton’s Geosciences Department. The workshop was chaired by CICS Fellow S. George H. Philander, Knox Taylor Professor of Geosciences and Director of the Atmospheric and Oceanic Sciences Program.

The workshop’s twenty participants discussed conditions during the early Pliocene (3 to 5 million years ago approximately) which amounts to a paradox: the world was much warmer then than it is now even though the same sunlight was incident on essentially the same global geography, and even though the atmospheric concentration of carbon dioxide was essentially the same as today. This paradox implies that climatic conditions then, and those of today, are two very different responses to essentially the same external forcing. The transition from that state (which included perennial El Nino conditions) to the present one involved recurrent Ice Ages induced by Milankovitch forcing. What disturbances can cause a return to the earlier warmer world? Can the current rise in atmospheric CO2 cause such a transition?

From the observational results it is evident that, in the equatorial Pacific, sea surface temperatures were as warm in the east as the west up to 3 Ma. El Niño was in effect perennial up to that time. The appearance of cold surface waters introduced feedbacks associated with tropical ocean-atmosphere interactions, significantly enhancing climate sensitivity to perturbations such as the Milankovitch forcing. A solution to the Pliocene paradox could be the following important difference between the early Pliocene and today: atmospheric CO2 has been high for merely a few decades before the present, but had been high for many millennia leading up to 3 Ma. It is entirely possible that the prolonged persistence of high CO2 levels will cause a return of perennial El Nino conditions.

These results were presented during the final half day of the workshop to a public audience at Princeton University.