When and Why does it Rain in the Desert: Utilising unique speleothem and dust records on the northern edge of the Sahara

热带半干旱地区是理解全球气候通讯的关键，生活在这些地区的人口对气候变化高度敏感。干旱加剧将导致严重的饥荒、经济崩溃、冲突以及难民和经济移民。IPCC报告中使用的大气环流模型表明，敏感的干旱地区将在未来几个世纪经历这样的干旱。但对这一预测的信心只被认为是“中等”的，而且由于缺乏与古代学研究的一致，进一步削弱了人们对这一预测的信心，古代学研究表明，全球暖期往往与北半球干旱地区的湿润阶段相关。因此，政策制定者缺乏规划未来所需的明确性。该项目汇集了不同的古气候学家和模型师团队，以改变建立北非气候变化知识的经验基础。通过结合独特的洞穴和地表沉积物档案，应用尖端分析方法，以及开发新的正演模拟和数据同化产品，我们将显著提高我们对热带干旱地区如何应对全球气温变化的了解。我们将提供新的见解，既包括地球轨道变化引起的气候缓慢变化，也包括地球系统本身的变异性引起的快速变化。了解自然变化引起的快速和缓慢的气候变化将极大地提高我们预测、理解和缓解未来问题的能力。沙漠里为什么下雨？石笋是一种独特的石笋资源，在撒哈拉沙漠北缘靠近石笋出处的洞穴附近堆积了厚厚的风尘(黄土)。雨水渗入洞顶，石笋一层又一层地堆积起来，记录着地表何时下雨，以及补给了多少水。当没有水进入洞穴时，石笋就不会生长，它就不会记录该地区的气候。因此，通过测定石笋生长的时间，我们可以揭示该地区何时潮湿。当有水进来时，地表上生长的雨量、水分来源和植被的丰富度也都由石笋的化学物质记录下来。通过观察石笋方解石中的金属和同位素，以及晶体之间保存的微小水滴，我们可以了解北非在过去是多么不同。为了了解干旱时期，我们还将观察积累在陆地表面的风化黄土沉积。这些尘埃沉积物含有从数百公里外吹来的淤泥颗粒，其中一些可以用来告诉我们运输是以什么方式发生的。锆石颗粒可以用放射性同位素组成来测定年龄，它们的年龄告诉我们它们被侵蚀的母岩是什么时候形成的。结合它们化学的其他方面，这可以被用来像“指纹”一样追溯它们的运输到我们发现的尘埃堆积物。洞穴和尘埃将一起告诉我们过去北非中部的气候变化的时间和方式，以及沙漠中的雨水是如何与全球气温联系在一起的。一旦我们有了这种前所未有的知识，我们就可以测试为什么会发生变化的想法，并更有信心地预测未来。

Tropical semi-arid regions are key to understanding global climate telecommunications, and populations living in these areas are highly sensitive to climate change. An increase in aridity would result in serious famines, economic collapse, conflict and both refugee and economic migration. General Circulation Models used in IPCC reports indicate that sensitive arid regions will experience such a drying in the next few centuries. But confidence in this prediction is only regarded as "Medium", and is further undermined by a lack of agreement with palaeo-studies, which indicate that global warm periods often correlate to humid phases in northern hemisphere arid regions. Consequently, policy makers lack the clarity they need to plan for the future. This project brings together a diverse team of palaeoclimatologists and modellers to transform the empirical basis on which knowledge of northern African climate change is founded. By combining unique cave and surface sediment archives, applying cutting-edge analytical approaches and developing new forward modelling and data assimilation products, we will significantly improve our knowledge of how tropical arid regions respond to changes in the global temperature. We will provide new insights both for the slow changes in climate that are caused by changes in the Earths orbit, but also the fast changes that arise from variability in the Earth system itself. Understanding fast and slow climate changes arising from natural variability will hugely improve our ability to predict, understand and mitigate future problems. At the heart of Why does it Rain in the Desert? is a unique resource of stalagmites, and a thick pile of windblown dust (loess) which built up on the northern margin of the Sahara desert close to the caves the stalagmites came from. Layer after layer, built up by rainwater percolating through the cave roof, stalagmites record when it was raining on the surface, and how much water was being supplied. When there is no water coming into the cave, the stalagmite does not grow and it stops recording the regions climate. So, by dating when the stalagmite was growing we can reveal when the region was wet. When there is water coming in, the amount of rain, source of the moisture and the abundance of vegetation growing on the surface are also all recorded by the chemistry of the stalagmite. By looking at the metals and the isotopes incorporated into the stalagmite calcite, and also at tiny drops of water preserved between the crystals, we can understand how different North Africa was in the past. To understand dry times, we will also look at the deposits of wind-blown loess which have accumulated on the land surface. These dust deposits contain silt grains blown from hundreds of kilometers away, and some of them can be used to tell us which way the transport occurred in. Zircon grains can be dated using their radio-isotope composition, and their age tells us when the parent rock they were eroded from formed. Combined with other aspects of their chemistry, this can be used like a "fingerprint" to retrace their transport to the dust deposit we found them in.Together, the cave and the dust will tell us when and how climate changed in central North Africa in the past, and how rain in the desert is plugged into global temperature. Once we have this unprecedented knowledge, we can test ideas about why the changes happened, and predict the future with much more confidence.