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

Lead Research Organisation: University of Hull
Department Name: Geography, Environment and Earth Science

Abstract

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.

Planned Impact

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.
 
Description The title of this project is a question, "Do humid phases in costal Libya reflect an intensified Atlantic storm track?". The work we have performed during the project has provided an answer: "yes". We are as confident of this answer as we can be, because we have found water left overall from the rainfall deposited in Libya between 30 and 60 thousand years ago. This water is trapped inside and between crystals of calcite inside a stalagmite we recovered from a cave in northern Libya. The calcite is dated using the uranium and thorium isotopes, so we know exactly when the water was trapped. By cracking open the calcite, the water is let out and we can analyse the isotopes within it to ask our question.
Water is made of oxygen and hydrogen, and both elements exist in more than one isotope. The balance of 16- and 18-oxygen and 1- and 2-hydrogen provide a "fingerprint", recording the history of that water since it was evaporated from the global ocean. In arid regions in which a lot of rainwater is evaporated straight back off the ground after it lands, there is a strong tendency for the water to lose its light isotopes (16-O, 1-H). The Mediterranean basin is large, arid and (almost) closed from the world ocean - its water is therefore saltier than normal ocean water. It also has a different isotope fingerprint. This means we can tell whether water likely came from the Mediterranean or the Atlantic on the basis of its isotopic characteristics.
Water from our Libyan stalagmite scatters over both the isotope ranges expected for the Atlantic and the Mediterranean. This means that some of the water comes from the Mediterranean (as it mostly does today), but also that a substantial component comes from the Atlantic. Today, Atlantic water is supplied to North Africa by winter storms, so we conclude that these storms were also active in the past and made a significant contribution to higher humidity in Libya in the past.
This confirms a theoretical expectation of what would happen to this region when the North Atlantic is much warmer than the South Atlantic. Our new evidence is the first work to directly show that this expectation is correct, providing a new and important validation for theoretical and model representations of rainfall response to climate change. Both as a new validation for predictions of future changes in North Africa, and for predictions regionally (including for northwest Europe) and globally, this is an important step. Working together with modellers, we can now identify where the model is performing well or poorly, and move towards a new generation of predictions which are ever more precise and reliable.
Exploitation Route The primary user group will be palaeoclimatologists and climate modellers. The mid-latitude rainfall belts are poorly constrained in the empirical record, so model representation of the globally-important processes happening within them are largely unconstrained. Our findings are the start of a new generation of empirical constraints on the system, which will revolutionise our understanding of these systems.
Sectors Environment

 
Description Most of our findings are not yet published, although we are actively working towards the first of three manuscripts. Once these are published, we will seek to maximise the reach and impact of the results we describe.
 
Description A new speleothem-based record of past rainfall changes in central North Africa
Amount £43,153 (GBP)
Funding ID IN-2012-113 
Organisation The Leverhulme Trust 
Sector Charity/Non Profit
Country United Kingdom
Start 01/2013 
End 12/2015
 
Description Late Holocene Hydroclimate of Central Tunisia
Amount € 10,000 (EUR)
Organisation Austrian Academy of Sciences 
Sector Academic/University
Country Austria
Start 11/2017 
End 10/2019
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation Max Planck Society
Country Germany 
Sector Charity/Non Profit 
PI Contribution Modelling of rainfall in North Africa during MIS 5
Start Year 2012
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation Max Planck Society
Country Germany 
Sector Charity/Non Profit 
PI Contribution Modelling of rainfall in North Africa during MIS 5
Start Year 2012
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation Office National des Mines, Tunisie
Country Tunisia 
Sector Public 
PI Contribution Collaboration with colleagues at U. Tunis for speleothem research in Tunisia
Start Year 2013
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation Office National des Mines, Tunisie
Country Tunisia 
Sector Public 
PI Contribution Collaboration with colleagues at U. Tunis for speleothem research in Tunisia
Start Year 2013
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation University of Carthage
Country Tunisia 
Sector Academic/University 
PI Contribution Collaboration with colleagues at U. Bizerte for investigating impact of speleothem results on inference of landscape and archaeological change in the same region.
Start Year 2013
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation University of Carthage
Country Tunisia 
Sector Academic/University 
PI Contribution Collaboration with colleagues at U. Bizerte for investigating impact of speleothem results on inference of landscape and archaeological change in the same region.
Start Year 2013
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation University of Gabès
Country Tunisia 
Sector Academic/University 
PI Contribution Collaboration with colleagues at U Gabes for investigating impact of speleothem results on inference of landscape and archaeological change in the same region.
Start Year 2013
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation University of Gabès
Country Tunisia 
Sector Academic/University 
PI Contribution Collaboration with colleagues at U Gabes for investigating impact of speleothem results on inference of landscape and archaeological change in the same region.
Start Year 2013
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation University of Hull
Country United Kingdom 
Sector Academic/University 
PI Contribution Modelling of rainfall in North Africa during MIS 5
Start Year 2012
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation University of Hull
Country United Kingdom 
Sector Academic/University 
PI Contribution Modelling of rainfall in North Africa during MIS 5
Start Year 2012
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation University of Liverpool
Country United Kingdom 
Sector Academic/University 
PI Contribution Collaboration with colleagues at U. Liverpool, U. Sfax, U. Bizerte, U Gabes and U. Oxford for investigating impact of speleothem results on inference of landscape and archaeological change in the same region.
Start Year 2013
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation University of Liverpool
Country United Kingdom 
Sector Academic/University 
PI Contribution Collaboration with colleagues at U. Liverpool, U. Sfax, U. Bizerte, U Gabes and U. Oxford for investigating impact of speleothem results on inference of landscape and archaeological change in the same region.
Start Year 2013
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation University of Oxford
Country United Kingdom 
Sector Academic/University 
PI Contribution Collaboration with colleagues at U. Oxford for investigating impact of speleothem results on inference of landscape and archaeological change in the same region.
Start Year 2013
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation University of Oxford
Country United Kingdom 
Sector Academic/University 
PI Contribution Collaboration with colleagues at U. Oxford for investigating impact of speleothem results on inference of landscape and archaeological change in the same region.
Start Year 2013
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation University of Sfax
Country Tunisia 
Sector Academic/University 
PI Contribution Collaboration with colleagues at U. Sfax for investigating impact of speleothem results on inference of landscape and archaeological change in the same region.
Start Year 2013
 
Description A new speleothem-based record of past rainfall changes in central North Africa 
Organisation University of Sfax
Country Tunisia 
Sector Academic/University 
PI Contribution Collaboration with colleagues at U. Sfax for investigating impact of speleothem results on inference of landscape and archaeological change in the same region.
Start Year 2013
 
Description Quantitative intercomparison of speleothem and GCM isotope data fields 
Organisation University of Reading
Country United Kingdom 
Sector Academic/University 
PI Contribution Datasets and analysis of North African speleothems, from a region currently under-represented in PMIP analyses and allied intercomparison projects.
Collaborator Contribution GCM datasets and connection to wider isotope-enabled GCM communities, and model-empirical intercomparison specialists.
Impact Mutlidisciplinary - meteorology, climate modelling and geochemistry. Output in prep. are one research paper, and one research proposal
Start Year 2018
 
Description SISAL network 
Organisation PAGES (Past Global Changes)
Country Switzerland 
Sector Charity/Non Profit 
PI Contribution Responsibility for contributing dripwater database to the wider SISAL resource. Quantitative analysis of scaling relationships between climate data, GCM data, GNIP, dripwater and cave carbonate datasets.
Collaborator Contribution Access to the wider community and support of the PAGES-funded SISAL project.
Impact Multidisciplinary: meteorology and geochemistry.
Start Year 2017
 
Description Cafe Scientifique at Beverley, East Yorkshire 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact 30 members of the public came to cafe scientifique meeting in beverley, to learn about how speleothem studies in north africa can help inform about future climate changes

Very large interest in topic and in talk. Voted "favourite talk of the year" by Cafe regulars!
Year(s) Of Engagement Activity 2012
 
Description Seminar at Bristol University 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Professional Practitioners
Results and Impact Excitement at the new precision we have achieved

Many people asked about when we will be publishing results
Year(s) Of Engagement Activity 2014
 
Description Seminar at University of Derby 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Professional Practitioners
Results and Impact About a dozen academics and a small numbers of undergraduates attended a departmental seminar

INterest in developing speleothem work within the department at Derby.
Year(s) Of Engagement Activity 2013
 
Description Seminar at University of Sfax 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Undergraduate students
Results and Impact 40 undergraduate students in geology from U Sfax attended a seminar organised by Prof. Nejib Kallel, their programme director.

One subsequently joined us on fieldwork, and three have subsequently done research projects on the Tunisian Quaternary. We continue to work alongside Prof. Kallel.
Year(s) Of Engagement Activity 2012
 
Description Seminar at University of Sheffield 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Professional Practitioners
Results and Impact Seminar to UoS staff and masters students. Very well attended.

Considerable interest amongst masters students.
Year(s) Of Engagement Activity 2014
 
Description Seminar at University of Swansea 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Professional Practitioners
Results and Impact Departmenttal seminar at University of Swansea.

Significant debate about how confident we can be in our conclusions, and whether policy can be based on them. Changed aspects of my own thinking, in addition to audience (I hope).
Year(s) Of Engagement Activity 2014
 
Description University of Hull Science Cafe outreach event 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact About 60 members of the public (mostly sixth form students) attended a University of Hull Science Cafe event to discuss role of north african speleothem work in underpinning reconstruction of climate change

Significant interest from the audience, driving increase in requests to work with schools. Significant link created with Caedmon school to work with Gifted and talented youngsters in a deprived postcode region to raise their aspiration and study geoscience at University. Highlighted by education researchers at Sheffield Hallam as an outstanding activity, leading to general geology workshop organised at Rotunda geology Museum, Scarborough. Workshop not directly linked to funding.
Year(s) Of Engagement Activity 2012
 
Description Workshop at ONM, Tunis, Tunisia 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? Yes
Geographic Reach Local
Primary Audience Professional Practitioners
Results and Impact We were able to secure a research agreement, opening up access to caves throughout Tunisia. This research is generating publications (in prep) and will also underpin a proposal to NERC in 2015

Marked increase in speleothem and allied cave research in Tunisia.
Year(s) Of Engagement Activity 2012