Reducing Greenhouse Climate Proxy Uncertainty

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On current trajectories, the concentration of atmospheric carbon dioxide (CO2) will exceed 550 ppm by the middle of this century. Such high carbon dioxide concentrations last occurred over 25 million years ago during the "greenhouse" climates of the early Cenozoic. In particular, the early Eocene epoch (~55 to 48 million years ago) was characterized by the warmest climates of the past 65 million years, with no ice sheets on Antarctica, polar regions ~20-40 degrees C warmer and sea levels ~50 to 70m higher than present. These warm Eocene climates can be simulated using the same climate models that are used to predict future climate change, such as those used in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (2013-14). In this report, climate model simulations of the Eocene were compared against temperature estimates from the geological record to test the accuracy of modelled warming in Polar regions at greatly increased CO2.PI Dunkley Jones was responsible for collating the Eocene temperature estimates used and figured in the IPCC AR5 report. This work is now being substantially improved ahead of the next IPCC report within a collaborative international project to run IPCC-class climate models with a consistent set of boundary conditions and Eocene geographies, as part of the "Deep-time Model-data Intercomparison Project" (DeepMIP). Significant improvements in the accuracy of the critical geological data used to test these models - Eocene surface temperatures and atmospheric CO2 concentrations - are, however, more difficult to establish. Current moderately reliable estimates of ocean surface temperatures for the early Eocene are limited to only seven locations globally, and, at high latitudes, can diverge by up to 20 degrees C depending on the proxy method used. Current estimates of early Eocene CO2 concentrations are even more uncertain, ranging from ~300 ppm to in excess of 2000 ppm. There is only one sound early Eocene data point based on the CO2 proxy methods highlighted by the IPCC as having particular promise - those based on foraminiferal boron isotopes and alkenone carbon isotope compositions. Here we aim to make a step-change improvement in these "proxy" estimates by taking advantage of two new opportunities. The first, is the serendipitous discovery of a remarkable suite of very well preserved, unaltered marine microfossils, made of calcium carbonate, alongside similarly well-preserved organic molecular biomarkers produced by Eocene marine algae and bacteria. The chemistry of this fossil material is the basis for proxy temperature and/or atmospheric CO2 estimation. The quality of this material is so high that we propose to generate ~170 alkenone-based CO2 estimates for the early Eocene, where previously there were none, and 15 boron-isotope based estimates to test the single data point currently available. The rare co-occurrence of these substrates and their abundance also provides the opportunity to use multiple independent methods to estimate both ocean temperatures (4 methods) and atmospheric CO2 (2 methods) on the same sample set, and so directly compare estimates from different methodologies at the same time and place. The second key opportunity is a new collaboration between the PI Dunkley Jones and astrophysicists with advanced expertise in data analysis, statistical modelling and signal processing. With the generation of the largest ever dataset of proxy-to-proxy comparisons from any Greenhouse climate, this new collaboration will maximise our ability to draw robust conclusions about systematic errors within any given proxy method. This is vital for the reconstruction of warm climate states where there are persistent discrepancies between temperature reconstructions based on different proxy methods. Here, we will be able to directly compare methods from the same samples and with uniformly excellent preservation.