HYdroclimate Reconstruction in Arid eXtremes (HYRAX): understanding the mechanisms of global desertification

地球正在经历明显的变暖，朝着人类自石器时代以来从未见过的气候状态发展。适应这些变化将是一项挑战。旱地（水资源有限的地区）几乎覆盖了地球陆地表面的一半，养活了世界上近40%的人口。国际气候变化专门委员会预测，到本世纪末，旱地将进一步干涸，使已经缺水的人口陷入危机。然而，全球气候模式在未来水文气候变化的方向和规模上都存在分歧，对预测结果的置信度仅为中等。此外，以前地球地质历史上的温暖和高二氧化碳时期，如上新世晚期（~3 Ma），几乎一致表明，在全球变暖的条件下，干旱地区的湿度更高。必须紧急解决这一“旱地水文气候悖论”，以改善我们对未来全球旱地气候变化影响的预测。过去的地质历史可能是最好的，实际上也是唯一的工具，可以帮助我们了解地球在以前温暖和寒冷的气候状态下是如何运作的，使我们能够确定控制全球亚热带干旱的关键驱动因素。拟议中的研究将利用旱地洞穴沉积物（如石笋）的新兴潜力，将其用作“古代雨量计”，在降雨充足时开启，并记录过去潮湿时期的确切时间。测定石笋的年代可以让我们准确地确定过去亚热带地区何时更潮湿，以及在什么条件下发生（全球气候变暖或变冷）。HYRAX项目将提供一个前所未有的数据集，通过使用辐射测年和统计评估技术的独特组合，在过去的~ 600万年里，亚热带旱地的正水平衡变化。当洞穴形成时，它们也捕获了其他有用的信息，这些信息可以作为形成时环境条件的代表。结合这些在洞穴中被困和保存了数百万年的线索，如：化石花粉、化石水和化石微生物，石笋生长时的环境将被重建。这将提供关于过去潮湿时期当地植被、降雨组成、数量和地表温度的独特定量信息。结合独特的记录、尖端技术、最先进的分析方法和代理系统建模技术，该项目将把这些信息转化为现实世界的气候数据，使我们能够测试模型在预测这些古代环境方面的技巧。这将是我们对旱地如何应对全球温度和大气二氧化碳浓度变化的理解的重大飞跃。我的中心目标是为陆地水文气候模拟和数据模型比较提供急需的基准，以更好地了解未来旱地气候变化的物理基础。如果没有这一点，利益攸关方制定缓解目标和政府政策将仍然具有挑战性。

Earth is experiencing clear warming toward climate states humans have not seen since the stone age. Adaptation to these changes will be challenging. Drylands (water-limited regions) cover almost half of Earth's land surfaces, supporting almost 40% of the world's population. The International Panel on Climate Change predicts further dryland aridification by the end of this century, plunging populations already experiencing water shortages into crisis. However, global climate models disagree in both the direction and scale of future hydroclimate change and only suggest medium confidence for predicted outcomes. Further, previously warm and high-CO2 periods in Earth's geological history, such as the late Pliocene (~3 Ma), almost uniformly suggest more humidity in dryland regions during globally warmer conditions. This "Dryland Hydroclimate Paradox" must be urgently addressed to improve our forecast on the climate impacts of future climate change in global dryland regions.The geological past may be the best, indeed only, tool to understand how Earth's processes operate under previously warmer and colder climate states, allowing us to identify the key drivers controlling aridity in global subtropical zones. The proposed research will exploit the emerging potential of dryland cave deposits (e.g. stalagmites) to be used as 'ancient-rain gauges', switching on when there is enough rainfall and recording the exact timing of past wet periods. Dating stalagmites allows us to identify exactly when subtropical zones were more humid in the past and under what conditions this occurred (globally warmer or cooler climate states). Project HYRAX will deliver an unprecedented dataset of changes in positive hydro-balance in subtropical drylands over the last ~6 million years by using a unique combination of radiometric dating and statistical evaluation techniques. As speleothems form they also trap other useful information that can be used as a proxy for the environmental conditions at the time of formation. Using a combination of these clues trapped and preserved in speleothems for millions of years such as: fossil pollen, fossil water and fossil micro-organisms, the environment at the time the stalagmite grew will be reconstructed. This will deliver unique quantitative information about the local vegetation, rainfall composition and amount and surface temperature during past wet periods. Combining unique records, cutting edge techniques, state of the art analytical approaches and proxy system modelling techniques, this project will translate this information into real world climate data, allowing us to test how skilful models are in predicting these ancient environments. This will be a significant leap forward in our understanding of how drylands respond to changes in global temperature and atmospheric CO2 concentrations. My central aim is to provide much needed benchmarks for terrestrial hydroclimate simulations and data-model comparison to better understand the physics underlying change in dryland climate in the future. Without this, it will remain challenging for stakeholders to develop mitigation targets and government policies.