Paleoclimatic research in Africa has relied heavily upon analysis of lake sediment cores and has provided abundant evidence of dramatic climate fluctuations during the late Quaternary. However, much of that work has suffered from several important limitations, including (1) coarse temporal resolution of time series, (2) gaps due to discontinuous sedimentation or loss of soft upper sediments during coring, (3) complications in sediment dating, (4) a scarcity of continuous lake records from the southern hemisphere, and (5) difficulty in teasing out the relative contributions of rainfall and temperature to inferred environmental changes. These limitations have reduced the clarity of climate reconstructions and hindered the development of robust regional and global-scale models of rainfall dynamics, especially for the late Holocene.
Under this RUI grant, the PI and undergraduate students analyze diatoms (algae with glassy shells) and geochemical records in lake sediment cores representing the last 2000 years that were recently collected in South Africa in collaboration with American and African students and colleagues. These cores represent key sites in the southern summer and winter rainfall regions of the continent and they contain clear evidence of paleohydrological fluctuations. The central goal is to clarify the nature and causes of African rainfall variability during the late Holocene.
Intellectual merit: The fine temporal resolution of these analyses, conducted at annual to decadal increments and dated with 14C, 210Pb, 137Cs, and pollen indicators is unusually high for African cores. These detailed time series help to refine the nature and timing of key paleoclimate events in southern Africa in relation to records from equatorial and polar latitudes. They also provide continuous paleohydrological records leading into modern times, and help to clarify the interpretations of pollen- and isotope-based studies in the region. These findings are used to evaluate models of rainy season responses to past global warmings and coolings, of African-Antarctic teleconnections, and of solar influences on African rainfall during the last two millennia. Transfer functions are used to convert diatom data into the longest and most detailed reconstructions of Holocene water chemistry parameters that have ever been obtained in South Africa. Results are compared to other records ranging from equatorial to polar latitudes, putting the climatic history of South Africa into a global context.
Broader impacts: Rainfall has much greater ecological and societal impacts in Africa than temperature alone does. By clarifying the recent hydrological history of South Africa, this study supports efforts to model and predict rainfall dynamics, as well as the roles of Antarctic and solar influences (and, indirectly, greenhouse warming) in relation to past and future African climate changes. Two undergraduate students are helping to collect and analyze cores for their senior capstone projects at Paul Smith's, a small college in an under-served rural region of upstate New York. Collaborations with paleoecologists in South Africa and Britain enhances PI and student access to research facilities, professional networks, and expertise, and the data is archived at the NOAA World Data Center. The internationally syndicated "Natural Selections" radio program broadcasts the results of this work widely and also makes podcasts available from the North Country Public Radio website.
As the world warms, the associated effects on precipitation in Africa could have severe consequences for people and ecosystems. Climate models conflict over what the most likely changes in precipitation will be, but preliminary hypotheses suggested that the tropical rain belt might widen and bring more moisture to the northern portion of South Africa and that a poleward shift of westerly storm tracks might reduce rainfall on the southernmost tip of the continent. Looking to the past for examples of what happened during natural warm/cool episodes of the last millennium can help to refine projections for the future. In this study, we examined lake sediment core records from two distinct climatic subregions of South Africa that allowed us to infer precipitation history during the last millennium. We found no clear evidence of tropical rain expansions in the northerly "summer rainfall zone" during a warm period roughly 1200-700 years ago. However, we did find a strong, semi-cyclic increase in rainfall in the southerly "winter rainfall zone" during the last 7 centuries. Synchronous changes in ice core records of atmospheric circulation over Antarctica show that the most likely cause of the rainfall pulses was widespread cooling and expansion of westerly wind belts outwards from the south polar region. This evidence of a major cool/wet, warm/arid relationship in the winter rainfall zone supports models that anticipate a poleward drift of the austral westerlies as the world warms. It therefore appears likely that winter storms, which are the primary source of rainfall in this water-stressed, populous region, will strike the tip of Africa less frequently as temperatures continue to rise through this century. In this RUI (Research in Undergraduate Institutions) project, four undergraduate students from a small college in a rural, underserved region of upstate New York participated in the South African coring operations and core analyses, and they co-authored articles that have been published in peer-reviewed journals.
