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

地球正在经历自石器时代以来人类从未见过的明显变暖的气候状态。适应这些变化将是具有挑战性的。旱地(缺水地区)覆盖了地球近一半的陆地表面，养活了世界近40%的人口。国际气候变化专门委员会预测，到本世纪末，干旱地区将进一步干旱，使已经经历水资源短缺的人口陷入危机。然而，全球气候模型在未来水气候变化的方向和规模上都存在分歧，仅表明对预测结果的中等信心。此外，地球地质史上以前的温暖和高二氧化碳时期，如上新世晚期(~3 Ma)，几乎一致地表明，在全球变暖的条件下，干旱地区的湿度更大。这一“旱地水文气候悖论”必须得到紧急解决，以改进我们对未来气候变化对全球旱地地区气候影响的预测。过去的地质历史可能是理解地球过程在以前较暖和较冷气候状态下如何运行的最佳工具，实际上是唯一的工具，使我们能够确定控制全球亚热带地区干旱的关键驱动因素。这项拟议的研究将开发旱地洞穴沉积物(例如石笋)的新兴潜力，将其用作“古代雨量计”，在降雨量足够时启动，并记录过去雨季的准确时间。测年石笋使我们能够准确地确定亚热带地区过去什么时候更潮湿，以及这种情况发生在什么条件下(全球气候变暖或变冷)。Hyrax项目将利用辐射测年和统计评估技术的独特组合，提供过去约600万年来亚热带旱地正水平衡变化的史无前例的数据集。随着洞穴的形成，它们还捕获了其他有用的信息，这些信息可以用来替代形成时的环境条件。利用数百万年来被捕获和保存在洞穴中的这些线索的组合：化石花粉、化石水和化石微生物，将重建石笋生长时的环境。这将提供关于过去雨季当地植被、降雨量和降雨量以及地表温度的独特量化信息。这个项目结合了独特的记录、尖端技术、最先进的分析方法和代理系统建模技术，将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老环境方面的熟练程度。这将是我们理解旱地如何应对全球气温和大气二氧化碳浓度变化的重大飞跃。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，以更好地理解未来旱地气候变化的物理基础。如果没有这一点，利益攸关方在制定缓解目标和政府政策方面仍将面临挑战。

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.