Rainfall and drought in equatorial east Africa during the past 1,100 years

Abstract

Knowledge of natural long-term rainfall variability is essential for water-resource and land-use management in sub-humid regions of the world. In tropical Africa, data relevant to determining this variability are scarce because of the lack of long instrumental climate records and the limited potential of standard high-resolution proxy records such as tree rings and ice cores. Here we present a decade-scale reconstruction of rainfall and drought in equatorial east Africa over the past 1,100 years, based on lake-level and salinity fluctuations of Lake Naivasha (Kenya) inferred from three different palaeolimnological proxies: sediment stratigraphy and the species compositions of fossil diatom and midge assemblages. Our data indicate that, over the past millennium, equatorial east Africa has alternated between contrasting climate conditions, with significantly drier climate than today during the 'Medieval Warm Period' (approximately AD 1000-1270) and a relatively wet climate during the 'Little Ice Age' (approximately AD 1270-1850) which was interrupted by three prolonged dry episodes. We also find strong chronological links between the reconstructed history of natural long-term rainfall variation and the pre-colonial cultural history of east Africa, highlighting the importance of a detailed knowledge of natural long-term rainfall fluctuations for sustainable socio-economic development.

Observations: Cryosphere

“…Citing "historical accounts of lake levels (Hastenrath, 1984;Nicholson and Yin, 2001), wind and current observations in the Indian Ocean and their relationship to East African rainfall (Hastenrath, 2001), water balance models of lakes (Nicholson and Yin, 2001), and paleolimnological data (Verschuren et al, 2000)," Molg et al (2003a, b) say "all data indicate that modern East African climate experienced an abrupt and marked drop in air humidity around 1880," and the resultant "strong reduction in precipitation at the end of the 19th century is the main reason for modern glacier recession in East Africa," as it considerably reduces glacier mass balance accumulation, as demonstrated for the region by Kruss (1983) and Hastenrath (1984). In addition, they note "increased incoming shortwave radiation due to decreases in cloudiness-both effects of the drier climatic conditions-plays a decisive role for glacier retreat by increasing ablation, as demonstrated for Mount Kenya and Rwenzori (Kruss and Hastenrath, 1987;Molg et al, 2003a)."…”

Quantifying and specifying the solar influence on terrestrial surface temperature

“…Holzhauser et al (2005) compared glacier and lake level fluctuations in westcentral Europe over the last 3500 years and demonstrated synchronicity between glacier advances, periods of higher lake levels and maxima of atmospheric radiocarbon. There are numerous other examples of sediment and lake level data that point to a solar link (Baker et al, 2005;Morrill et al, 2006;Patterson et al, 2004;Stager et al, 2005;Verschuren et al, 2000;Wu et al, 2006;Haltia-Hovi et al, 2007).…”

Solar forcing as an important trigger for West Greenland sea-ice variability over the last millennium

“…Solar activity has been shown to influence key components of the climate system, such as temperature, winds, precipitation, ocean circulation and iceberg transport (e.g. Verschuren et al, 2000;Bond et al, 2001;Hodell et al, 2001;Andrews et al, 2003;Jiang et al, 2005;Sejrup et al, 2010;Martin-Puertas et al, 2012;Knudsen et al, 2014;MoffaSanchez et al, 2014a;Jiang et al, 2015). However, only a few studies have investigated the link between variations in sea-ice proxies from the North Atlantic and solar variability on multidecadal to centennial time scales (e.g.…”