Ethiopian lake reveals history of African droughts

Adele Rackley in PlanetEarth Online: A new survey of Lake Tana in Ethiopia – the source of the Blue Nile – suggests that drought may have contributed to the demise of the Egyptian Old Kingdom, around 4200 years ago. A team led by the University of Aberystwyth used seismic surveys and sediment cores to work out how the lake's water levels has varied over the past 17,000 years and linked this to evidence for global climate change.

Understanding how and why rainfall patterns change is particularly important for sub-Saharan Africa, where prolonged droughts have such serious social and economic consequences. The climate here is dominated by the African-Asian monsoon and the movements of the Intertropical Convergence Zone (ITCZ). This is an area of erratic weather patterns, where winds from the northern and southern hemispheres meet close to the equator: sailors know it as the Doldrums.

Seasonal movements of the ITCZ can affect the strength of the monsoon. A strong monsoon leads to higher lake levels, and this can be traced in ancient lake sediments. Lake Tana is particularly good for this kind of research because it's close to the northern limit of the ITCZ so even slight a southward movement of the ITCZ is reflected in the lake's geological history.

Fleshing out the detail of the region's rainfall history and linking it to past climate change can improve predictions of future rainfall. The detail enables scientists to check the ability of their climate models to accurately 'predict' past climate change; this fine tuning means they can be more confident of the models' accuracy when predicting future events.

...'We were looking for evidence for long-term drought events to provide a historical context to data modellers,' says Dr Michael Marshall from Aberystwyth University, a lead author of the research paper published in Global and Planetary Change. 'We wanted to find out when and how quickly drought has come about in the past.'...

Delicately carved reliefs from the mastaba tomb of vizier Ankhmahor who served during the 6th dynasty of Egypt during the Old Kingdom, which might have ended because of drought. Shot by Andrew®, Wikimedia Commons via Flickr, under the Creative Commons Attribution 2.0 Generic license