Reducing Greenhouse Climate Proxy Uncertainty

按照目前的轨迹，到本世纪中叶，大气中二氧化碳(CO2)的浓度将超过550ppm。如此高的二氧化碳浓度上一次出现在2500万年前的早期新生代的“温室”气候中。特别是，始新世早期(约5500万至4800万年前)的特点是过去6500万年来最温暖的气候，南极洲没有冰盖，极地地区温度升高约20-40摄氏度，海平面比现在高约50至70米。这些温暖的始新世气候可以使用与预测未来气候变化相同的气候模型来模拟，例如政府间气候变化专门委员会(IPCC)第五次评估报告(2013-14)中使用的模型。在这份报告中，对始新世的气候模型模拟与地质记录中的温度估计值进行了比较，以测试在二氧化碳大幅增加的情况下极地地区变暖模型的准确性。在下一份IPCC报告之前，这项工作正在大幅改进，这是一个合作的国际项目，作为“深度时间模式-数据对比项目”(DeepMIP)的一部分，该项目运行具有一致的边界条件和始新世地理位置的IPCC级气候模型。然而，用来测试这些模型的关键地质数据--始新世地表温度和大气二氧化碳浓度--的准确性有了显著的改善，但更难确定。目前对始新世早期海洋表面温度的中等可靠估计仅限于全球七个地点，在高纬度地区，根据所使用的替代方法，可能会出现高达20摄氏度的偏差。目前对始新世早期二氧化碳浓度的估计更加不确定，从大约300ppm到超过2000ppm不等。只有一个可靠的早期始新世数据点是基于气专委强调的具有特殊前景的二氧化碳替代方法--基于有孔虫的硼同位素和烯烃碳同位素组成的方法。在这里，我们的目标是通过利用两个新的机会，对这些“代理”估计进行阶段性的改进。第一，偶然发现了一套保存完好、原封不动的海洋微化石，这些化石由碳酸钙制成，还有由始新世海洋藻类和细菌产生的类似保存完好的有机分子生物标记物。这种化石物质的化学成分是估算温度和/或大气二氧化碳的基础。这种材料的质量如此之高，以至于我们建议为始新世早期生成约170个基于烯酮的二氧化碳估计值，而以前没有任何估计值，以及15个基于硼同位素的估计值来测试现有的单一数据点。这些底物及其丰度的罕见共存也提供了在同一样本集上使用多种独立方法估计海洋温度(4种方法)和大气二氧化碳(2种方法)的机会，从而在同一时间和地点直接比较不同方法的估计。第二个关键机会是皮邓克利·琼斯与在数据分析、统计建模和信号处理方面拥有先进专业知识的天体物理学家之间的新合作。随着从任何温室气候中产生有史以来最大的代理对代理比较数据集，这种新的合作将最大限度地提高我们对任何给定代理方法中的系统误差得出可靠结论的能力。这对于重建温暖的气候状态至关重要，在这些国家，基于不同替代方法的温度重建之间存在持久的差异。在这里，我们将能够直接比较来自相同样本的方法，并具有一致的良好保存性。

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.