Sedimentological and geochemical studies conducted on a 15.82-m long core collected from Lake Garba Guracha (Ethiopia) associated with a precise AMS-14C time-scale document a unique record of the sedimentary processes linked to the progressive retreat of a high-altitude glacier in the Bale Mountains since 17,000 yr cal BP. Lake sedimentation is interpreted as the result of discharges of meltwater and glaciogenic sediment which progressively filled the accommodation space generated by glacier retreat within the basin. Monogenic sediment originated from glacial erosion of the trachytic tuff forming the cirque floor. Ice melting ended progressively between 12,600 and 11,800 cal BP, as suggested by the decrease in sedimentation energy followed by a sharp change in sedimentary facies. From 11,800 cal BP, the lake reached its maximum development and clastic input was replaced by organic-rich sedimentation. This relates to a major increase of lake productivity, which lasted up to 4500 cal BP. From this period, a lowering in productivity reflects the widespread dryness which occurred throughout the East African tropics.

The oxygen and carbon isotope composition of sedimentary carbonates formed in Lake Hayq, a closed-basin lake in northern Ethiopia, suggests marked variation in the balance between precipitation and evaporation during the last 2000 years. Diatom stratigraphy shows that the lake remained deep enough to support planktonic taxa, and was never saline. The record suggests that rainfall was higher than that of the twentieth century during most of this time, especially during two short intervals centred on AD 700 and AD 1300. Climate was drier only around AD 800, and from AD 1750-1900. Similar, but slightly moister climate than today, with high interdecadal variability, prevailed from AID, 800 to AD 1200, equivalent to the European 'Mediaeval Warm Period'. A period of high effective precipitation followed, from AD 1200 to AD 1700, during the 'Little lee Age'. Pollen evidence suggests that increased forest cover from AD 1400 to AD 1750 was a response to higher rainfall; forest cover may have increased evaporative loss from the catchment, contributing to a subsequent trend of increasing lakewater delta O-18 values. The record shows similarities in the timing and direction of changes in proxy-climatic records from Lake Naivasha (Kenya) and from the Nile.

A sediment core recovered from Garba Guracha, a glacial lake at 3950m altitude in the Bale Mountains of Ethiopia, at the boundary of the Ericaceous and Afroalpine vegetation belts, provides a 16,700-year pollen record of vegetation response to climatic change. The earliest vegetation recorded was sparse and composed mainly of grasses, Amaranthaceae–Chenopodiaceae and Artemisia, indicating an arid climate. At 13,400cal BP, Amaranthaceae–Chenopodiaceae pollen declined sharply and Cyperaceae increased, suggesting a change to moister conditions. The Younger Dryas interval is represented by a small increase in Artemisia and reduced Cyperaceae, indicating aridity. Just after the start of the Holocene (11,200cal BP), the upper altitudinal limit of the Ericaceous belt rose, and woody Ericaceous vegetation extended across the Sanetti plateau, in response to increased moisture and temperature. The marked change from clastic to organic lake sedimentation at this time reflects the increase in woody vegetation cover in the lake catchment, accompanied by soil stabilisation, and increased leaf litter and soil humus content. From about 6000cal BP, and especially after 4500cal BP, mid-altitude dry Afromontane Juniper–Podocarpus forests developed on the northern slopes of the mountains in response to reduced rainfall in a shortened wet season. Erica shrub and forest decreased in area and altitude, and the Afroalpine ecosystem expanded on the plateau. Podocarpus declined from about 2000calBP, as Juniperus increased to its present dominance at 2500–3300m altitude. Human impact on the high-altitude Afroalpine and Ericaceous vegetation has been relatively minor, confirming that the endemic biodiversity of the Ethiopian mountains is a legacy of natural Holocene vegetation change, following repeated expansion and contraction of the upland ecosystems during the Quaternary.

Pollen and charcoal analysis of sediment cores from two lakes in the highlands of northern Ethiopia provide evidence that the vegetation has changed in response to human impact during the last 3000 years. The natural, pre-disturbance vegetation of the area was Podocarpus-Juniperus forest. At about 500 bc, following Semitic immigration to northern Ethiopia, the forests were cleared and replaced by a secondary vegetation of Dodonaea scrub and grassland that persisted for 1800 years. Grasslands were dominant from about ad 1200 to 1400, probably as a result of further intensification of grazing, perhaps exacerbated by drought. Juniperus forest, with Olea and Celtis, then expanded from ad 1400 to 1700, possibly because of drought-induceddepopu lation followed by increased rainfall. Deforestation and soil erosion has again intensified during the last three centuries. Since forest regrowth was possible after 1800 years of human impact, northern Ethiopia should again be capable of supporting forest under appropriate land management.

Stratigraphic analysis of alluvial/colluvial sequences and 14C dating have been used as proxies for Holocene climate changes in the highlands of Tigray (northern Ethiopia). The studied records show alternations of buried soils and peaty–clayey sediments, pointing to wet, stabilization phases, and organic-free colluvium layers resulting from the abrupt occurrence of dry-climate episodes. The 14C dates, mostly unpublished, cluster in the 11,090–9915, 9465–9135, 8450–7330, 6720–3635, 2710–2345, and 1265–790 cal yr B.P. time spans. Evidence of subsequent pedogenesis is lacking in the area, apart from a buried humified horizon dated at 300 ± 60 14C yr B.P. (460–295 cal yr B.P.). Both the timing and the pattern of Tigray paleoclimatic events fit the corresponding framework, based on lake level changes, previously implemented for the Main Rift Valley. These findings give further support for arguing that the forcing mechanisms of the wet/dry fluctuations during the Holocene were effective over a large scale.

Millimetre-scale white aragonite laminations alternating with dark diatom-rich organic layers are present in the uppermost sediments of the crater lakes Hora and Babogaya, at Debre Zeit, Ethiopia. The sediment accumulation rate calculated from lamina counts matches that estimated from a Pb-210 chronology, indicating that the laminations were deposited annually. The oxygen-isotope composition of the surface white layer is equivalent to that of surface water, which shows that white-layer aragonite is formed in isotopic equilibrium with the lake water, and suggests that isotopic analyses of these layers may provide valid paleoclimatic information. Because the aragonite is probably precipitated during dry-season mixing, aragonite delta(18)O values for individual laminae reflect the composition of the entire lake, integrated over its water-residence time of about 10 years. The sedimentary record of oxygen-isotope variations should therefore be interpreted as a proxy-climate record with decadal rather than annual resolution. Comparisons between delta(18)O values for the laminae and climate data for equivalent years show no clear relationships, so calibration of the sedimentary record requires a more detailed understanding of the climatic controls on the isotopic composition of these groundwater-fed lakes. An isotopic mass-balance model of the lake's response to rainfall variation shows (1) that the oxygen isotope composition of the lake waters varies by about 1 parts per thousand, which is comparable to the range of delta(18)O values determined from the individual laminae; and (2) that modelled lake level is a reasonable match to observed levels, confirming that climate changes can interpreted from the oxygen-isotope record.

Oxygen isotope data from groundwater, streams and crater lakes in central Ethiopia provide a basis for modelling lake hydrological and isotopic budgets. The environmental parameters delta(A) (isotopic composition of vapor) and epsilon* (equilibrium fractionation factor) were determined by a semi-empirical approach using one lake as a terminal index lake. The models show that lake oxygen-isotope composition is more sensitive to rainfall-related parameters (humidity and (5,1) than to the isotopic composition of inflow (delta(in)). The isotopic composition of the lacustrine sedimentary carbonates should therefore provide a record of past rainfall variation.

Biome reconstruction from pollen and plant macrofossil data provides an objective method to reconstruct past vegetation. Biomes for Africa and the Arabian peninsula have been mapped for 6000 years bp and provide a new standard for the evaluation of simulated palaeovegetation distributions. A test using modern pollen data shows the robustness of the biomization method, which is able to predict the major vegetation types with a high confidence level. The application of the procedure to the 6000 years data set (pollen and plant macrofossil analyses) shows systematic differences from the present that are consistent with the numerous previous regional and continental interpretations, while providing a more extensive and more objective basis for such interpretations. Madagascar, eastern, southern and central Africa show only minor changes in terms of biomes, compared to present. Major changes in biome distributions occur north of 15°N, with steppe in many low‐elevation sites that are now desert, and temperate xerophytic woods/scrub and warm mixed forest in the Saharan mountains. These shifts in biome distributions imply significant changes in climate, especially precipitation, between 6000 years and present, reflecting a change in monsoon extent combined with a southward expansion of Mediterranean influence.