Testing the 'megadrought' hypothesis: the timing, cause and impacts of climate change in equatorial Africa (DeepCHALLA-UK)

Over the last 25,000 years, East African climate has responded to changes in the Earth's orbit (mainly precession) that influence long-term variations in monsoonal rainfall and the migration of the intertropical convergence zone. But climate of earlier periods, back to 250,000 years, is less well understood as few continuous high resolution terrestrial records exist. This is a major gap in our understanding of equatorial climate from a region critical to the evolution of our own species. The African megadrought hypothesis states that droughts lasting many thousands of years occurred during the last interglacial (130,000 - 80,000 years ago) across tropical Africa. Evidence from the few sites investigated indicate they were of a severity greater than any droughts of succeeding time periods, and had major implications for evolutionary processes, for example continent wide migrations in Homo sapiens. Such long and intensely dry events in the history of African climate are an unexpected phenomenon, and their precise timing, origin and extent, has yet to be established. We hypothesise that megadroughts arose due to monsoon failure caused by changes in the shape of earth's orbit around the sun (the ~100,000 year eccentricity cycle), amplifying changes in the seasonal distribution of solar radiation (the ~21,000 year precession cycle). In order to fully understand these extreme climatic events, we will explore climate changes in the East African equatorial region spanning two glacial-interglacial cycles (i.e. the last ~250,000 years) giving critical context to the megadroughts and their causes.

The project focuses on the sedimentary record from Lake Challa, a deep lake on the flank of Kilimanjaro. Unlike other studies from East Africa, the easterly position of Lake Challa places it beyond the direct influence of the Atlantic climate system, thus removing this aspect as a possible forcing and allowing us to isolate a record of monsoonal variation. The outstanding potential of these lake sediments to provide a long, sensitive repository of environmental change data has been established by a prior study of the last 25,000 years, that resulted in multi-disciplinary articles in 'Nature' on orbital forcing of climate, 'Science' on the laminated sediments and relation to ENSO, and 'Geology' concerning the seasonality of climate variations and the Kilimanjaro ice core. Additional data sets (published in other journals) confirm that the environmental proxies and dating methods proposed here will deliver a high quality record from this lake. The age of the deeper sediments has been estimated from seismic profiles of the sedimentary layers in the lake; new, absolute dates are required to identify the basal sediment age and rates of subsequent sedimentation. This project will lead in modelling of sediment ages from the new, deep cores through radiocarbon dates, palaeomagnetism to detect the presence of well-dated magnetic reversals and dating of volcanic ash layers through Ar/Ar dating and chemical correlation. Carbon and oxygen isotope data from diatom silica are excellent tools to reveal the megadroughts due to their sensitivity to humid/arid shifts as already demonstrated during the last 25,000 years.

This project is part of an international consortium, partly funded by the International Continental scientific Drilling Program to recover cores from the lake. Our international partners have already gained support from their own national funding councils and will contribute complimentary environmental proxies (e.g. pollen, organic biomarkers) as well as additional dating (Ar/Ar) and, finally, climate modelling (ranging from local hydrology to global climate modelling) used to understand the global significance of our results in terms of forcing factors. We have Kenyan and Tanzanian collaborators, who will also act as conduits to ensure our discoveries help inform ongoing and future conservation needs and development strategies.

The scientific and societal impact of our work will be both significant and diverse. Scientific outputs will be delivered in the very top journals; a credible ambition based on the past history of the team and the high impact of the previous CHALLACEA project on the lake (Nature, Science, Geology). This work is truly discovery-based, as fundamental climate changes which likely impact on our understanding of climate dynamics remain to be discovered in this region during the targeted time intervals. Broader implications for evolutionary processes, biodiversity and hominin dispersal are all expected from the objectives of this proposal.

International impact is ensured through our embedded international network within DeepCHALLA and the offices of the ICDP programme. We will also bring our findings to larger and broader international organisations such as Future Earth. Communications at top international conferences (AGU, EGU etc.) will further accelerate the impact.

As scientists working in developing countries we have a responsibility to communicate and engage with the local communities and national institutions. This has already begun through a two-centred meeting held in Nairobi and Taveta in 2012 hosted by the National Museum of Kenya and the University of Nairobi. This identified local priorities of water resources and capacity building. We will use the DeepCHALLA climate and hydrological modelling framework to deliver information about recent and future climate variability at Town Hall meetings. Secondly, we will work with regional development organisations and co-host a north-south conference in the area, likely Arusha, Tanzania. Thirdly we will build capacity through contributing to MSc programmes in Kenyan and Tanzanian universities, working with existing bilateral exchange programmes and using pathway to impact funds to invite an African PhD student to the UK to learn of the importance and practice of developing scientific impact.

A key tool to communicate our research beyond academia will be the writing of blogs and news stories in conjunction with our universities' media experts and promoted by Twitter, a communication tool that can be targeted at specific journalists. Outlets including regular articles in Climatica (http://climatica.org.uk/), the BGS GeoBlogy and university blog sites are proposed. Engagement with the general public and school children is to be achieved through talks at key popular science events such as the biennial BGS open day and local museum events. We will also use our institutional outreach networks to take an interactive demonstration into schools and museums to help inform the wider public and future generations about environmental change. Funds for developing high quality materials are requested.