Do humid phases in costal Libya reflect an intensified Atlantic storm track?

过去的气候变化并不是简单地以冷暖交替的顺序发生的。由自然发生的气候周期引起的一些最重要的变化与海洋和大气环流状态的改变有关。一个很好的例子是，由于墨西哥湾流/北大西洋暖流系统的减弱，使英国的气候相对温和，欧洲西北部经历了极端的降温。理解未来人为气候变化情景的一个关键问题是理解这些环流变化的物理“规则”。该项目旨在对地中海南部和北非地区降雨变化的物理机制产生新的认识。有令人信服的证据表明，在地球过去的特定时期，在整个北非地区发生了大规模和地理上广泛的降雨增加。在这些时期，北半球接收到的总入射太阳能的比例相对较高。额外的降雨在现在是沙漠的地区形成了新的湖泊和河流，并改变了包括早期人类在内的一系列动植物的分布。人们认为，额外的降雨是通过非洲季风向北移动而流向北非的，但这种变化很难在气候模型中模拟出来，而且似乎与所有的数据都不相符。因此，其他机制也需要研究。这个项目将测试撒哈拉沙漠绿化过程中的一些降雨是否来自来自大西洋的风暴，而不是来自非洲季风。我们将通过测量在石笋形成过程中被困在石笋中的水的性质来做到这一点。我们将使用的石笋来自利比亚东部的北海岸，它的位置非常适合接收和保留大西洋风暴路径带来的水分。石笋中的水是由氢和氧组成的，它们都有两种常见的同位素——1-H或2-H和16-O或18-O。地中海水域的2-H和18-O含量略高于大西洋水域。结合对石笋本身方解石的16 / 18- 0比的额外测量，我们期望能够使用简单的建模方法区分这两种来源。有一种说法认为，大西洋的湿气是在风暴事件中以降雨的形式提供给北非的，这为这种位于海岸几公里内的石笋提供了进一步的可能性。海水中锶的两种常见同位素（87和86）的特征比值与大多数淡水不同。由于海水在风暴期间以气溶胶的形式被输送到大气中，我们的石笋中的sr同位素比值很可能在大风暴发生频率较高的时期向海洋值偏移。因此，我们可以利用这一测量作为“风暴指数”来支持氧和氢同位素的工作。最后，我们将在现有证据的基础上，通过测量石笋方解石中的一系列微量元素，证明我们正在调查的时期比现在更潮湿。许多元素以各种方式对湿度作出反应，其中一些只在肥沃的土壤中可用（例如硫），而另一些则在干旱时期以大气尘埃的形式提供（例如铁）。如果我们的工作提供的测试表明我们对这个系统的理解是正确的，我们和其他国际研究小组可以在我们现有的范例中继续工作。如果我们的测试证明降雨事件发生在不同的地点和不同的时间，那么研究人员就可以调整他们的努力来研究更合适的系统表示，并为主要降雨系统的冰期-间冰期变化开发新的范例。

Past climate change did not simply occur as a sequence of alternating warm and cool periods. Some of the most important changes caused by naturally occurring climate cycles are related to alterations to the state of circulation in the ocean and atmosphere. A good example is the extreme cooling experienced by northwest Europe as a consequence of weakening in the Gulf Stream / North Atlantic Drift system that maintains Britains relatively mild climate. A crucial concern for understanding future, man-made climate change scenarios are the physical "rules" understanding these changes in circulation. This project aims to generate new understanding of the physical mechanism underlying changes in rainfall in the southern Mediterranean and North African regions.There is convincing evidence that large magnitude and geographically widespread increases in rainfall occurred throughout North Africa during particular periods of the Earths past. These are periods when the northern hemisphere is receiving a relatively high share of the total incoming solar energy. The additional rainfall caused formation of new lakes and rivers in regions that are now desert and changed the distribution of a range of plants and animals, including early humans. It is thought that the additional rainfall is being routed to North Africa via a northward movement of the African monsoon, but this change is difficult to simulate in climate models and does not seem to fit with all of the data. Other mechanisms therefore also need to be investigated. This project will test whether some of the rainfall involved in greening the Sahara was derived from storms coming in from the Atlantic, rather than the African monsoon. We will do this by measuring the properties of water trapped within a stalagmite during its formation. The stalagmite we will use came from the north coast of eastern Libya, and is perfectly positioned to receive and retain water from the Atlantic storm track. The water trapped in the stalagmite is made up of hydrogen and oxygen, both of which come in two common isotopes - 1-H or 2-H and 16-O or 18-O respectively. Mediterranean water is slightly more rich in 2-H and 18-O than Atlantic water. Combined with additional measurements of 16 / 18-O ratio made on the calcite of the stalagmite itself, we therefore expect to be able to differentiate between these two sources using a simple modelling approach. The suggestion that Atlantic moisture was supplied to North Africa as rainfall in storm events raises a further possibility for this stalagmite, which is positioned within a few kilometers of the coast. Seawater has a characteristic ratio of the two common isotopes of strontium (87 and 86) which is different to that of most freshwaters. As seawater is transported into the atmosphere as aerosols during storm events, it is highly likely that the Sr-isotope ratio in our stalagmite will be shifted towards marine values during periods with higher occurrence of major storms. We can therefore exploit this measurement as a "storm index" in support of the oxygen and hydrogen isotope work.Finally, we will build on our existing evidence that the time period we are investigating was more humid than today by measuring a suite of trace elements in the calcite of the stalagmite. Many elements respond to humidity in a variety of ways, with some only being available when a rich soil is in place (e.g. sulphur) and others being supplied in atmospheric dust during arid periods (e.g. iron). If the tests our work provides show that our understanding of this system is correct we and other international research groups can carry on working within our existing paradigms. If our test proves that rainfall events are occurring at different places at different times, then researchers can adjust their efforts to investigate more appropriate representations of the system and develop new paradigms for glacial-interglacial changes in major rainfall systems.