The role of oceans and vegetation in greening the Sahara


Contact Prof Alan Haywood to discuss this project further informally.

Project description

Many climate archives across the Sahara point to significantly wetter conditions than today around 6,500 years before present, as reflected for example by expansion of lakes, calcite deposits in flowing rivers and a reduction in desert area in favour of shrubland/woodlands and grasses. Work in the 1970s and 1980s led to the hypothesis that this wet phase was due to a northward progression of the West African monsoon, but more recent geological data suggest two Saharan rainy seasons per year. It is unclear to what degree changes in monsoon dynamics or mid-latitude influences contributed to this seasonality.

Attempts to model this African humid period have centred on the response of the West African monsoon to seasonal variations in the amount of incoming solar radiation (a result of cyclical fluctuations of the Earth’s orbit around the sun). Whilst many models simulate changes in North African precipitation in agreement with geological data, no model has yet been able to increase precipitation sufficiently to ‘green the Sahara’. More sophisticated modelling approaches and an improved process understanding is urgently needed to reconcile the geological evidence with climate models.

Aim and objectives of the project

The overarching aim of the project is an improved understanding of interactions within the Earth system that can support the assessment of potential future climate change. The main objectives are:

  1. Conduct climate simulations with a state-of-the-art climate model (HadCM3/HadGEM2) for the African Humid period about 6,500 years ago.
  2. Couple the model with a dynamic land surface vegetation model (JULES) to investigate interactions between the large-scale atmospheric circulation, rainfall and vegetation.
  3. Vary sea-surface temperatures (SSTs) within the range of uncertainties based on climate archives and explore the response in the climate system over Africa.
  4. Run the climate model with perturbed physics to assess potential consequences of model errors.
  5. Analyse the seasonality of rainfall and try to separate contributions from changes in the West African monsoon and in the extratropical circulation.

Potential for high-impact outcomes

The conundrum of the green Sahara has fascinated scientists and the public for many years. The fact that climate models still struggle to reproduce the precipitation patterns suggested by climate archives is worrying and challenges our confidence in projections for the future. Any progress in our understanding and modelling of the African humid period is likely to lead to high-impact publications. The usage of the latest generation of climate models coupled to a land-surface-vegetation model together with sensitivity studies with respect to SSTs and model physics is a promising approach to make progress on this important scientific question. The issue of extratropical influences and seasonality has been disregarded by the research community so far and has therefore potential for a scientific breakthrough.


The successful PhD student will have access to a broad spectrum of training workshops put on by the Faculty that include an extensive range of training workshops in numerical modelling, through to managing your degree, to preparing for your viva.   A full listing is available through The student will be based within the Palaeo@Leeds research group ( the PhD studentship will provide a broad spectrum of training in analytical methods used in climate modelling and palaeoceanographic reconstruction. This will equip the student with the skills necessary to become the next generation of climate scientist, capable of contributing to understanding and predicting past and future global change. Specifically, the student will be trained in coupled ocean-atmosphere modelling gaining considerable IT and programming skills. The student will attend the NERC Earth System Science and Urbino Summer School (in Italy) to broaden his/her understanding of Earth Systems Science and palaeoclimatology, and will have the opportunity to undertake a variety of postgraduate training workshops at the University of Leeds.  The student will also benefit from a close link with the NERC Centre for Ecology and Hydrology (CEH) and will visit CEH each summer to facilitate the integration of their modelling with the research priorities of this world-renowned institute. The co-supervision of (Jean Maley and Gil Mahé) will bring profound expertise in Saharan climate archives to the project.  

Further Reading

Coe, M. and Harrison, S. (2002). The water balance of northern Africa during the mid-Holocene: an evaluation of the 6 ka BP PMIP simulations. Climate Dynamics 19, 2, 155-166, DOI: 10.1007/s00382-001-0219-3.

Joussaume, S., et al. (1999). Monsoon changes for 6000 years ago: Results of 18 simulations from the Paleoclimate Modeling Intercomparison Project (PMIP). Geophysical Research Letters 26 (7), 859-862.

Knippertz, P, Fink, A.H. (2009). Prediction of dry-season precipitation in tropical West Africa and its relation to forcing from the extratropics. Weather Forecast, 24, 1064-1084. doi:10.1175/2009WAF2222221.1.

Maley, J. (2010). Climate and Palaeoenvironment evolution in north tropical Africa from the end of the Tertiary to the Upper Quaternary. Palaeoecology of Africa, 30: 227-278.

Renssen, H., Brovkin, V., Fichefet, T., Goosse, H. (2006). Simulation of the Holocene climate evolution in Northern Africa: The termination of the African Humid Period. Quaternary International 150, 95-102.



Entry requirements

Applications are invited from candidates with or expecting a minimum of a UK upper second class honours degree (2:1), and/or a Master's degree in a physical or mathematical discipline, such as mathematics, physics, geophysics, engineering or meteorology. Experience with computing under a Linux operating system is of advantage.

If English is not your first language, you must provide evidence that you meet the University’s minimum English Language requirements.

Additional staff contact

Prof Peter Knippertz

How to apply

Formal applications for research degree study should be made online through the university's website. Please state clearly in the research information section that the PhD you wish to be considered for is the ‘The role of oceans and vegetation in greening the Sahara' as well as Prof Alan Haywood your proposed supervisor.

We welcome scholarship applications from all suitably-qualified candidates, but UK black and minority ethnic (BME) researchers are currently under-represented in our Postgraduate Research community, and we would therefore particularly encourage applications from UK BME candidates. All scholarships will be awarded on the basis of merit.

If you require any further information please contact the Graduate School Office e:, t: +44 (0)113 343 1634.