Assessing confidence in Pliocene sea surface temperatures to evaluate predictive models

Abstract

In light of mounting empirical evidence that planetary warming is well underway, the climate research community looks to palaeoclimate research for a ground-truthing measure with which to test the accuracy of future climate simulations. Model experiments that attempt to simulate climates of the past serve to identify both similarities and differences between two climate states and, when compared with simulations run by other models and with geological data, to identify model-specific biases. Uncertainties associated with both the data and the models must be considered in such an exercise. The most recent period of sustained global warmth similar to what is projected for the near future occurred about 3.3-3.0 million years ago, during the Pliocene epoch. Here, we present Pliocene sea surface temperature data, newly characterized in terms of level of confidence, along with initial experimental results from four climate models. We conclude that, in terms of sea surface temperature, models are in good agreement with estimates of Pliocene sea surface temperature in most regions except the North Atlantic. Our analysis indicates that the discrepancy between the Pliocene proxy data and model simulations in the mid-latitudes of the North Atlantic, where models underestimate warming shown by our highest-confidence data, may provide a new perspective and insight into the predictive abilities of these models in simulating a past warm interval in Earth history. This is important because the Pliocene has a number of parallels to present predictions of late twenty-first century climate.

Diatom evidence for the onset of Pliocene cooling from AND-1B, McMurdo Sound, Antarctica

“…Pliocene cooling in the Ross Sea precedes the onset of Northern Hemisphere glaciation and was accompanied by a radiation of the diatom genus Fragilariopsis. Although diatombased estimates from AND-1B (this work) and other circum-Antarctic sites (Barron, 1996a,b;Dowsett et al, 2012) indicate that Antarctic SSTs were only modestly warmer than present during the interval from 3.3 to 3.0 Ma, AND-1B sediments provide evidence for orbital oscillations of the Ross Ice Shelf and WAIS during this best-analog window into our planet's future.…”

“…Existing paleoclimate reconstructions identify the middle portion of the late Pliocene, ~3 Ma, as a promising analog in the recent geologic past for our anticipated planetary future (Dowsett et al, 2012). (This interval is hereafter referred to as the "late Pliocene" for simplicity.)…”

“…The three-dimensional PRISM reconstruction of global ocean temperature (Dowsett et al, 2009(Dowsett et al, , 2012 identifies dramatic warm departures from modern temperatures in the North Atlantic surface ocean and in upwelling zones, moderate warm anomalies at north and south mid-latitudes, and slight warm anomalies at the equator and in the highest southern latitudes around the Antarctic continent. The minimally warmer-than-present SSTs we propose at AND-1B at 3.2 and 3.03 Ma are in line with this global reconstruction and provide an ice-proximal reference point in the growing body of evidence that the Southern Ocean began to cool even as Pliocene warmth lingered in the Northern Hemisphere (Fig.…”

Fossil proxies of near-shore sea surface temperatures and seasonality from the late Neogene Antarctic shelf

“…The peak-averaged SST warming in this interval has been quantified from multi-proxy studies by the PRISM project (Dowsett et al 2010), providing a large number of marine SST estimates. These are globally correlated, if non-contemporaneous, as a target for GCMs (Dowsett et al 2012;Haywood et al in press).…”

Stratigraphic and Paleoenvironmental Reconstruction of a Mid-Pliocene Fossil Site in the High Arctic (Ellesmere Island, Nunavut): Evidence of an Ancient Peatland with Beaver Activity + Online Appendix Figures S1 and S2 (See Article Tools)

“…Consequently, the Mid Pliocene has been considered to be a historical analogue for current warming, providing an example of climate system dynamics for planet Earth with atmospheric CO 2 of ~400 ppm (Dowsett et al, 2012). Climate estimates from the Beaver Pond site and the nearby Fyles Leaf Bed site, using multiple proxies, show evidence of a climate Mean Annual Temperature (MAT) that was 14˚ to 22˚C warmer than today (Table 1).…”