NSFGEO-NERC Pliocene sea level amplitudes (PLIOAMP)

据估计，世界上5%的人口生活在比当前海平面高出不到5米的土地上，这些社区容易受到海平面上升的影响，要么是土地的直接丧失，要么是洪水风险的增加。更广泛的社会可能受到沿海主要基础设施（如发电站）的破坏以及流离失所社区的迁移的影响。南极冰盖是未来海平面上升的最大潜在因素，对未来南极冰盖变化的预估也有最大的估计范围。这使得准确确定未来海平面上升的风险变得困难。由于南极冰盖融化导致的海平面上升并没有均匀地分布在整个海洋中，南极冰盖的退缩将不成比例地影响最远的海岸线，例如欧洲和北美的海岸线。在这个建议中，我们将通过重建上新世中期（大约300万年前）过去的温暖间隔期间冰盖的变化来改进南极冰盖变化的预估。上新世中期是大气中二氧化碳含量与现在相似的最后一个地质时期。该提案将重点重建上新世中期在较冷冰期和较暖间冰期之间的海平面变化幅度。我们将使用这些数据作为两类冰盖模型的约束条件。最近的研究使用上新世间冰期海平面最大值作为南极冰盖模式的约束条件，并对人为变暖下南极未来海平面上升的预估结果要高得多。然而，随后的工作表明，可能不可能准确地确定绝对的上新世海平面最大值，因此使用这些数据的价值受到质疑。这些估计值的不确定性主要来自于试图将它们相对于现代参考（即以上述公尺为单位）进行量化。我们将提出的另一种方法是关注上新世冰期-间冰期海平面振幅，这一方法可以大大改善过去的海平面估计。我们将利用海洋沉积物岩心钻探恢复的沉积物分析，重建上新世中期3个间隔的冰期-间冰期海平面振幅。具体来说，我们将测量方解石微生物（底栖有孔虫）的地球化学组成（氧、镁和钙的同位素）来重建过去的冰量。在没有现代参考资料的情况下，我们将模拟南极和北半球冰盖（主要是格陵兰冰盖）的上新世冰期（较冷的气候间隔）和间冰期（较暖的气候间隔）范围，并将其与我们将得到的海平面数据进行比较。这样我们就能确定过去南极冰盖融化的程度。联合英国和美国的两个小组，使用的冰盖模型将包括宾夕法尼亚州立冰盖模型（PSU-ISM）和BISICLES冰盖模型。这两种模型对接地线物理（接地冰成为浮冰架的点）的处理非常不同。PSU-ISM需要额外的过程（冰架水力破裂和冰崖破坏）来模拟与上新世海平面最大值相一致的南极退缩。通过使用BISICLES模型，它在接地线上具有更高的分辨率，我们将能够测试是否需要这些过程来模拟与我们测量的上新世海平面振幅一致的冰退缩。最后，我们将利用我们所学到的知识，在古气候数据的约束下，生成一套新的未来海平面估计值。这些将比以前的未来海平面预测有更严格的限制，从而能够更准确地估计未来南极洲海平面上升的风险。

It is estimated that 5% of the world's population lives on land which is less than 5 metres above current sea level, in communities that are vulnerable to the impacts of sea level rise, either from direct loss of land, or increased flood risk. Society more broadly may be impacted by disruption to key infrastructure which is located on the coast e.g. power stations, and by the movement of displaced communities. The Antarctic ice sheet is the largest potential contributor to future sea level rise and projections of Antarctic ice sheet change in the future also have the largest range of estimates. This makes it difficult to accurately determine the risks of future sea level rise. Because sea level rise from Antarctic ice loss is not evenly distributed across the oceans, retreat of the Antarctic Ice Sheet will disproportionately affect coastlines that are furthest away, such as those in Europe and North America. In this proposal we will improve projections of Antarctic ice sheet change by reconstructing how the ice sheet changed during past warm intervals during the mid-Pliocene (approximately 3 million years ago). The mid-Pliocene is the last geological interval when atmospheric CO2 was similar to present day. The proposal will focus on reconstructing the amplitudes of mid-Pliocene sea level change between colder glacial stages and warmer interglacial states. We will use these data as a constraint for two types of ice sheet models. Recent work has used Pliocene interglacial sea level maxima as a constraint for Antarctic ice sheet models and has led to much higher projections of future sea level rise from Antarctica under anthropogenic warming. However, subsequent work has suggested that it may not be possible to accurately determine absolute Pliocene sea level maxima, such that the value of using these data has been questioned. The main source of uncertainty on these estimates comes from attempts to quantify them relative to a modern-day reference (i.e. as metres above present). An alternative approach that we will propose and one that can greatly improve past sea level estimates is to focus on the Pliocene glacial-interglacial sea level amplitude. We will reconstruct the glacial-interglacial sea level amplitude for 3 intervals in the mid-Pliocene using analysis of sediments recovered from the drilling of ocean sediment cores. Specifically, we will measure the geochemical composition (the isotopes of oxygen, magnesium and calcium) of calcite microorganisms (benthic foraminifera) to reconstruct past ice volume. In the absence of a modern-day reference we will simulate both the Pliocene glacial (cooler climate intervals) and interglacial (warmer climate intervals) extent of the Antarctic and Northern Hemisphere Ice Sheets (principally the Greenland Ice Sheet) and compare this with the sea level data that we will produce. We will then be able to determine what was the magnitude of Antarctic ice sheet melt during the past. Combining two groups based in the UK and US, the ice sheet models used will include the Penn State Ice Sheet Model (PSU-ISM) and the BISICLES ice sheet model. The treatment of the grounding line physics (the point at which grounded ice becomes floating ice shelf) is very different in these two models. The PSU-ISM requires additional processes (ice shelf hydrofracture and ice cliff failure) to simulate Antarctic retreat that was consistent with Pliocene sea level maxima. By using the BISICLES model, which has much higher resolution at the grounding line, we will be able to test whether these processes are needed to simulate ice retreat consistent with our measured Pliocene sea level amplitudes. Finally, we will use what we learn to produce a new set of future sea level estimates that are constrained using the palaeoclimate data. These will have tighter constraints than previous future sea level projections, enabling a more accurate estimate of the risk of future sea level rise from Antarctica.