Pliocene climate variability on glacial-interglacial timescales (PlioVAR): lessons learned from multi-proxy reconstructions of sea-surface temperatures and data-model comparisons

The Pliocene epoch (~2.6-5.3 million years ago) offers an opportunity to study a climate state in long-term equilibrium with current or predicted near-future atmospheric CO2 concentrations. Compared to today, the late Pliocene was characterised by a globally warmer climate, with reduced continental ice volume and reduced ocean/atmosphere circulation intensity. Towards the end of the Pliocene, there was a marked increase in glaciation in the northern hemisphere and atmospheric CO2 concentrations declined.

The Past Global Changes (PAGES) PlioVAR working group co-ordinated a synthesis of marine data to characterise spatial and temporal variability of Pliocene climate, underpinned by high quality data sets and robust stratigraphies. Here we present some of the main findings of this synthesis effort, including new assessments of sea surface temperatures (SSTs) during the KM5c interglacial (~3.2 million years ago) and Pliocene-Pleistocene intensification of northern hemisphere glaciation. We outline our approaches to integrating multi-proxy reconstructions of sea-surface temperatures from a globally distributed suite of marine sediment cores, which included a review and assessment of the impacts of SST calibration choice and interpretation. We show that an improved relationship between proxy data and climate models could be generated by focussing on a single interglacial. Differences between proxies, and between proxies and models, tended to be associated with surface ocean fronts or currents, although seasonality may also be important. The transition towards enhanced northern hemisphere glaciation at the end of the Pliocene had asynchronous trends and patterns in SST as well as benthic stable isotope records. We consider how these results might inform our understanding of past climate forcings and feedbacks during both warm intervals of the past and the development of larger ice sheets in the northern hemisphere. Additional proxy data is required from high-latitude regions of both hemispheres, to assess polar amplification and meridional temperature gradients. Co-ordinated multiproxy SST analyses will also significantly enhance our understanding and interpretation of the signals they record, and provide additional detail for comprehensive data-model comparisons.