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

热带半干旱地区是了解全球气候通信的关键，生活在这些地区的人口对气候变化高度敏感。干旱的增加将导致严重的饥荒、经济崩溃、冲突以及难民和经济移民。IPCC报告中使用的一般环流模式表明，敏感的干旱地区将在未来几个世纪经历这样的干旱。但是对这一预测的信心只被认为是“中等”，并且由于与古研究缺乏一致性而进一步削弱，古研究表明全球变暖期通常与北半球干旱地区的湿润期相关。因此，政策制定者缺乏规划未来所需的明确性。这个项目汇集了一个由古气候学家和建模者组成的多样化团队，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，应用尖端的分析方法，开发新的正演模拟和数据同化产品，我们将大大提高我们对热带干旱地区如何响应全球温度变化的认识。我们将为地球轨道变化引起的缓慢气候变化以及地球系统自身变化引起的快速气候变化提供新的见解。了解由自然变率引起的快速和缓慢的气候变化将极大地提高我们预测、理解和减轻未来问题的能力。《为什么沙漠会下雨？》是一个独特的石笋资源，在撒哈拉沙漠北缘靠近石笋洞穴的地方，有一堆厚厚的被风吹起的灰尘（黄土）。一层又一层的石笋，由雨水从洞穴顶部渗透而成，记录着何时在表面下雨，以及有多少水被供应。当没有水进入洞穴时，石笋就不会生长，它也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湿润的时间。当有水进来时，雨水的数量、水分的来源和生长在地表的植被的丰富程度也都记录在石笋的化学成分中。通过观察融入石笋方解石中的金属和同位素，以及晶体之间保存的小水滴，我们可以了解北非过去的不同之处。为了了解干旱时期，我们还将研究在陆地表面积累的风吹黄土沉积物。这些尘埃沉积物包含了从数百公里外吹来的粉粒，其中一些可以用来告诉我们运输的方式。锆石颗粒可以用它们的放射性同位素组成来测定年代，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，这可以像一个“指纹”一样用来追溯它们的运输到我们发现它们的尘埃沉积物中。洞穴和灰尘将共同告诉我们，过去北非中部的气候是何时以及如何变化的，以及沙漠中的雨水是如何影响全球气温的。一旦我们掌握了这些前所未有的知识，我们就可以检验这些变化发生的原因，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.