Africa2k is part of the 2k Network under PAGES Focus 2 Regional Climate Dynamics for the last two millennia. The network was established in Corvallis, Oregon, USA at the PAGES YSM/OSM meeting on 7 July 2009.
It is known that Africa is vulnerable to climate change, mainly through impacts on precipitation and through changes in the frequency and magnitude of climate extremes. This vulnerability is linked to widespread poverty, mismanagement of natural resources, conflicts and dependence on rain-fed agriculture.
Assessments of climate variability in Africa are difficult due to:
1. to the sparse spatial coverage and often short time span of available instrumental records, and
2. over the last two millennia as a result of the prevalence of semi- to hyper-arid environments in the extra-equatorial regions. These dry climates have generally precluded the long-term existence of lakes and wetlands, which in temperate and tropical regions typically preserve long records of environmental change. The result is a regional dataset characterized by spatially disjunctive, temporally discontinuous records, often with unreliable chronological control and ambiguous palaeoenvironmental significance.
The Africa2k Working Group, considering the facts and the potential impacts of future climate change, found it necessary and timely to:
1. conduct an audit of existing palaeoenvironmental archives, compile and evaluate the significance of published material
2. collate these data to construct a pan-African synthesis of climate change spanning the last 2 kyr, and
3. use this synthesis to identify key areas for future work, including new archives that can provide reliable palaeoenvironmental information at annual to multi-decadal time scales. This will provide a basis for direct comparisons with general circulation model simulations (GCMs), and for assessing the range of natural climate variability within which humans and ecosystems have been operating effectively, and the extent to which climates may exceed this envelope under potential future scenarios.
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A rock hyrax
The aim here is to bring together experts who work on the various high-resolution archives: lake sediments, shell and hyrax middens, tree rings, stalagmites, ice cores, corals and documentary evidences in Africa to exchange information with each other with the aim of producing a synthesis of the state of the art.
It is intended that this task will also encourage other scientists to recover and analyze further high resolution data over the last 2000 years. This sub-theme reflects the PAGES Focus 2 science agenda.
The information will also have the potential to be compared with processes that govern the modes of rainfall variability in the region such as ENSO. A meta-database will also be established.
The classic evidence of climate variability in Africa over the past 2000 years comes from the Nile flood records, providing information on floods and droughts in the source areas of the Blue and White Nile rivers. Moreover historical lake level flections have been reconstructed in several sites of the continent (Nicholson, 1998; 1999; 2000; Nicholson and Yin, 1998).
Over the last few decades, however, similar high resolution paleo-data are gradually being recovered from other types of archives. For example, a record of rainfall and drought in equatorial East Africa was obtained from a 1100 yr core from Lake Naivasha (Verschuren et al., 2000).
Similarly, a 3000 yr record from Lake Hayk in Ethiopia (Darbyshire et al., 2002; Lamb et al. 2007) has provided new information on the roles of both human impact and climate change. These two studies have shown that East Africa experienced a dry Medieval Warm Period (MWP) and a wet Little Ice Age (LIA).
However it is apparent that the signal of centennial scale climate variability in northern Africa during the last few millennia is complex, with other high resolution records from more central portions of East Africa (e.g. Russell and Johnson, 2005; Russell et al. 2007), West Africa (Shanahan et al. 2009) and West Central Africa (Ngomanda et al., 2007), indicating a dry LIA and a wet MWP. These spatial heterogeneities in the response of the climate system demonstrate the significance of regional climatological differences in the response to external forcing and highlight the need for better spatial data coverage when interpreting paleoclimatological data sets from widely separated sites.
Today new data, at the scale of inter-annual to annual variability, are emerging from tree ring (Whils and Zewdu 2007; Zwdu et al. 2009; Zewdu, 2009; Whils et al. 2009), stalagmite, varved lake and documentary sequences (Baker et al. 2007; Asrat et al., 2008; Nash & Grab; 2009; Shanahan et al. 2009). Varved sediments from Lake Bosumtwi, for example show that the West African monsoon is particularly susceptible to long-lasting, intense droughts unlike any seen in recent history (Shanahan et al. 2009). Multidecadal droughts in this region appear to be linked to changes in the Atlantic sea surface temperatures, as predicted by coupled ocean-atmosphere models (Knight et al., 2005) indicating that this persistence may be external to the system and linked to the forcing rather than land-surface processes as previously hypothesized (Shanahan et al., 2009).
In southern Africa, recent work has established and refined the potential of rock hyrax middens (accumulations of urine and faecal pellets) as stratified, continuous palaeoenvironmental archives containing a range of climate and vegetation proxies (Chase et al., 2009; Gil-Romera et al., 2007; Meadows et al., 2010; Scott and Woodborne, 2007). Most interestingly in terms of Africa2k, new stable isotope data from hyrax middens from the Namib Desert have revealed the potential for the recovery of reliable, well-dated decadal records of climate change spanning thousands of years (Chase et al., 2009). These data allow the first broad-scale inter-hemispheric comparisons of climate change from the African subtropics. This is particularly important because 1) low-latitude insolation forcing of monsoon systems does not adequately account for South African climate in recent millennia, and 2) contrary to the East African evidence, the southern African data indicate a wetter MWP and a drier LIA, highlighting the need for a broad regional synthesis of data and forcing mechanisms.
Asrat, A., Baker, A., Leng, M.J., Gunn, J.and Umer, M., 2008. Environmental monitoring in the Mechara Caves, Southeastern Ethiopia: implications for speleothem paleoclimate studies. International Journal of Speleology, 37, 207-220.
Baker, A., Asrat, A., Fairchild, I.J., Leng, M.J., Wynn, P.M., Bryant, C., Genty, D. and Umer, M., 2007. Analysis of the climate signal contained within delta-18O and growth rate parameters in two Ethiopian stalagmites. Geochimica et Cosmochimica Acta, 71, 2975-2988.
Chase, B. M., Meadows, M. E., Scott, L., Thomas, D. S. G., Marais, E., Sealy, J., andReimer, P. J. (2009). A record of rapid Holocene climate change preserved in hyrax middens from southwestern Africa. Geology 37, 703-706.
Darbyshire, I., Lamb, H.F., and Umer, M. (2003) Forest clearance and regrowth in northern Ethiopia during the last 3000 years. The Holocene 13 (3), 553-562.
Gil-Romera, G., Scott, L., Marais, E., and Brook, G. A. (2007). Late Holocene environmentalchange in the northwestern Namib Desert margin: new fossil pollen evidence from hyrax middens. Palaeogeography, Palaeoclimatology, Palaeoecology 249, 1-17.
Holmgren, K., Lee-Thorp, J. A., Cooper, G. R. J., Lundblad, K., Partridge, T. C., Scott, L., Sithaldeen, R., Talma, A. S., and Tyson, P. D. (2003). Persistent millennial-scale climatic variability over the past 25,000 years in Southern Africa. Quaternary Science Reviews 22, 2311-2326.
Knight, J. R., Allan, R. J., Folland, C. K., Vellinga, M., and Mann, M. E. (2005). A signature of persistent natural thermohaline circulation cycles in observed climate. Geophysical Research Letters 32, L20708.
Lamb HF, Leng MJ, Telford RJ, Ayenew T, Umer M. (2007). Oxygen and carbon isotope, composition of authigenic carbonate from an Ethiopian lake: a climate record of the last 2000 years. The Holocene, 17: 517-526.
Meadows, M. E., Seliane, M., and Chase, B. M. (2010). Holocene palaeoenvironments of theCederberg and Swartruggens mountains, Western Cape, South Africa: pollen and stable isotope evidence from hyrax dung middens. Journal of Arid Environments 74, 786-793.
Nicholson, S.E (2000): The Nature of Rainfall Variability over Africa on Time-Scales of Decades to Millenia. Global and Planetary Change, 26, 137-138.
Nicholson, S.E. (1999): Historical and Modern Fluctuations of Lakes Tanganyika and Rukwa and their Relationship to Rainfall Variability. Climatic Change 41, 53-71.
Nicholson, S.E. (1998): Fluctuations of Rift Valley Lakes Malawi and Chilwa During Historical times: A Synthesis of Geological, Archaeological and Historical Information. Environmental Change and Response in East African Lakes, 207-231.
Nicholson, S. E., and X. Yin 1998: Variations of African lakes during the last two centuries . In Water resources variability in Africa during the XXth Century (E. Servat, D. Hughes, J.-M;. Fritsch and M. Hulme, eds.), IAHS Press,Wallingford, UK, 181-188.
Ngomanda A., Jolly D., Bentaleb I., Chepstow-Lusty A., M’voubou M., Maley J., Fontugne M., Oslisly R., Rabenkogo N., 2007. Lowland rainforest response to hydrological changes during the last 1500 years in Gabon, Western Equatorial Africa. Quaternary Research, 67, 411-425.
Russell, J. M., Verschuren, D., and Eggermont, H. (2007). Spatial complexity of "Little Ice Age" climate in East Africa: sedimentary records from two crater lake basins in western Uganda. The Holocene 17, 183-193.
Russell, J. M., and Johnson, T. C. (2005). A high-resolution geochemical record from Lake Edward, Uganda Congo and the timing and causes of tropical African drought during the late Holocene. Quaternary Science Reviews 24, 1375-1389.
Scott, L., and Woodborne, S. (2007). Vegetation history inferred from pollen in late Quaternary faecal deposits (hyraceum) in the Cape winter-rain region and its bearing on past climates in South Africa. Quaternary Science Reviews 26, 941-953.
Shanahan, T.M., Overpeck, J.T., Anchukaitis, K.J., Beck, J.W., Cole, J.E., Dettman, D.L., Peck, J.A., Scholz, C.A., and King, J.W. (2009) Atlantic Forcing of Persistent Drought in West Africa: Science 324, p. 377-380.
Verschuren, D., Laird, K. R. and Cumming, B. F., 2000. Rainfall and drought in equatorialEast Africa during the past 1100 years. Nature 403: 410-414.
Zewdu Eshetu, 2009. Forests as Natural Proxies of Climate Variability in the Highlands of Ethiopia 2009. In 2nd East African Quaternary Research Association Workshop, 22-15 May 2009, Addis Ababa, Ethiopia.
Zewdu Eshetu; Iain Robertson; Steven W Leavitt, Neil J. Loader. 2009. Rainfall Variability and Drought History in Ethiopia, and Societal Response to Climate Change. In Association of American Geographers (AAG) Annual Meeting Conference, 22-28 March 2009, Las Vegas, USA.
Zewdu Eshetu, 2006. Rainfall variability in Ethiopia from the ring-width characteristics of Juniperus procera at the 7th International Conference on Dendrochronology, June 11-17, 2006 Beijing, China.
Wils, T.H.G., Eshetu, Z., 2007. Reconstructing the flow of the River Nile from Juniperus procera and Prunus africana tree rings (Ethiopia) - an explorative study on cross-dating and climate signal, in: Tree Rings in Archaeology, Climatology and Ecology (TRACE), Volume 5, Proc. dendrosymposium 2006. P. 277-284.
Wils THG, Robertson I, Eshetu Z, Sass-Klaassen UGW, Koprowski M (2009) Periodicity of growth rings in Juniperus procera from Ethiopia inferred from crossdating and radiocarbon dating. Dendrochronologia 27: 45-58
David Nash (University of Brighton, UK)
Dirk Verschuren (Ghent University, Belgium)
Anne-Marie Leziné (LSCE UMR, France)
Mohammed Umer (1959-2011): Former Africa2k leader and PAGES SSC member. It is with great sadness that PAGES notes the demise of the charismatic Africa2k Group Leader Mohammed Umer (PAGES SSC), Addis Ababa University, Ethiopia. PAGES tribute.