Africa2k scientific goals
The aim was 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 was intended that this task would also encourage other scientists to recover and analyze further high resolution data over the last 2000 years. This sub-theme reflected the PAGES Focus 2 science agenda.
The information also had the potential to be compared with processes that govern the modes of rainfall variability in the region such as ENSO. A meta-database was also 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) 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, 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.