BRACKETING THE WARM CLIMATE VARIABILITY OF THE MID-PLIOCENE

Estimates of sea surface temperatures (SSTs) from ocean cores reveal a warm phase of the Pliocene between about 3.15 and 2.85 million years ago. Pollen records from land-based cores, although not as well-dated, also show evidence for a warmer climate at about the same time. What caused the climate to be warmer is not known with certainty, but increased greenhouse forcing and altered ocean heat transports have been implicated in previous studies.

A number of climate simulations have been run to examine the middle Pliocene warming, all of which have employed the U.S. Geological Survey’s PRISM1 or PRISM2 digital data sets as boundary conditions. These data sets were constructed during the mid-1990s in order to analyze the average, global equilibrium climate of the warm peaks for this generally warm time slice. The “peak-to-peak averaging” scheme used by Dowsett and Poore (1991) was necessary because the GCMs require global, gridded data sets and it is simply not possible to correlate individual peaks from site to site on a global basis. Unfortunately, this process also smoothes the natural variability that is very apparent in the deep sea records of SST.

In a new study of the middle Pliocene we utilize two recently reconstructed SST data sets that are designed to provide a type of climatological error-bar for the warm peak phases of the middle Pliocene. Each new global data set includes a composite of the overall coolest PEAK (minimum warming) and the overall warmest PEAK (maximum warming) at each site. The new simulations should, therefore, bracket the minimum and maximum warm phases of the middle Pliocene, providing information on the variability within this important time slice.

Initial results point to the importance of understanding the variability that prior simulations smoothed over. In all experiments the pattern of temperature change is consistent, with greatest warming at high latitudes, diminishing toward the equator. However, global average temperatures in the warm peak phase are nearly 1.3 deg. C warmer than during the cool peak phase, a substantial difference given that “average” Pliocene climate is only estimated to be about 2.2 deg. C warmer than the today. Regional differences (e.g. North Atlantic) are even more pronounced and may have large implications for how we view the Pliocene climate relative to the future.