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

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.

The primary aim of this project is to shed new light on the mechanisms underlining the changes in rainfall in North Africa and in circulation within the Mediterranean that are known to have occurred periodically in the past. In particular, it aims to generate the basic evidence to test whether rainfall on the Libyan coast is synchronous with freshening in the eastern Mediterranean, which is thought to be a referred impact of changes in the position of the African monsoon.

The user-groups for the knowledge created during this work are therefore widespread, spanning palaeoceanographers and terrestrial palaeoclimatologists but also climate modellers and archaeologists. We will make use of our extensive personal connections throughout these communities to disseminate our findings as widely as possible, and we anticipate that the test of the "Green Sahara" paradigm our work constitutes will be of significant interest to these groups regardless of whether the hypotheses are confirmed or refuted.
In addition to normal dissemination of science via publication in internationally significant journals and attendance at major meetings and congresses, we will seek to engage with specific user groups within the climate modelling community, with which we have significant involvement. We expect that the outcome of this engagement would be a new generation of modelling experiments testing improved "Green Sahara" scenarios.

Outside of the scientific community, we will seek to engage with the general public via communication with the written and broadcast media (we have costed for the PI to attend NERC media training to support this) and via a project website which we will set up to disseminate our work as widely as possible.

In parallel with this effort, we will seek to make use of our excellent connections with schools within the Hull University outreach programs. This is exceptionally important in the current financial climate, as research requires effective advocates in wider society if it is to maintain its ability to draw on general taxation. Engagement with schools is an exceptionally efficient way to achieve this goal.
We will integrate the findings of our work with the ongoing outreach activities at our two Universities, for instance incorporating our findings into the annual "Masterclass in Geography" for Year 12 students at Hull. We also seek financial support to offer an event to National Science Week, which is a very large annual event with national impact.