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

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.