Mohammed Umer

Temperature variability over Africa during the last 2000 years

A growing number of proxy, historical and instrumental data sets are now available from continental Africa through which past variations in temperature can be assessed. This paper, co-authored by members of the PAGES Africa2k Working Group, synthesises published material to produce a record of temperature variability for Africa as a whole spanning the last 2000 years. The paper focuses on temperature variability during the ‘Medieval Climate Anomaly’ (MCA), ‘Little Ice Age’ (LIA) and late 19th–early 21st centuries. Warmer conditions during the MCA are evident in records from Lake Tanganyika in central Africa, the Ethiopian Highlands in northeastern Africa, and Cango Cave, the Kuiseb River and Wonderkrater in southern Africa. Other records covering the MCA give ambiguous signals. Warming appears to have been greater during the early MCA (c. ad 1000) in parts of southern Africa and during the later MCA (from ad 1100) in Namibia, Ethiopia and at Lake Tanganyika. LIA cooling is evident in Ethiopian and southern African pollen records and in organic biomarker data from Lake Malawi in southeastern tropical Africa, while at Lake Tanganyika the temperature depression appears to have been less consistent. A warming trend in mean annual temperatures is clearly evident from historical and instrumental data covering the late 19th to early 21st centuries. General warming has occurred over Africa since the 1880s punctuated only by a period of cooling in the mid 20th century. The rate of temperature increase appears to have accelerated towards the end of the 20th century. The few long high-resolution proxy records that extend into the late 20th century indicate that average annual temperatures were 1–2°C higher in the last few decades than during the MCA.

The Mai Maikden sedimentary sequence: a reference point for the environmental evolution of the Highlands of Northern Ethiopia

Abstract This paper presents a geomorphologic–stratigraphic analysis of a travertine dammed lacustrine–swampy sedimentary sequence, composed of clay, peaty layers and phytoclastic travertine sands, deeply incised by the Mai Maikden river, on the Highlands of Tigray (Northern Ethiopia). Radiocarbon datings of peaty deposits allow us to establish that the travertine dams have developed at least between 7310±90 yr B.P. and 5160±80 yr B.P. In the upper part of the sequence, palaeochannels filled with travertine and limestone coarse gravels and blocks, indicate the activation of concentrated erosion on the surrounding slopes. Later, both the travertine dam and the lacustrine–swampy deposits were buried by alluvial and colluvial sediments, still mixed with large amounts of organic matter, testifying to generalized slope erosion processes. The end of travertine deposition may be connected with a progressive reduction of vegetation cover and the subsequent decrease of CO2 in groundwater. The occurrence of the latter phenomena may be related to the onset of drier climatic conditions even though the finding of a large prehistoric settlement in the area, also indicates some influence of human impact.

Lake — Groundwater Relationships, Oxygen Isotope Balance and Climate Sensitivity of the Bishoftu Crater Lakes, Ethiopia

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 δ A(isotopic composition of vapor) and e* (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 δ A) than to the isotopic composition of inflow (δ in). The isotopic composition of the lacustrine sedimentary carbonates should therefore provide a record of past rainfall variation.

Continental-Scale Temperature Variability during the Past Two Millennia: Supplementary Information

Past global climate changes had strong regional expression. To elucidate their spatio-temporal pattern, we reconstructed past temperatures for seven continental-scale regions during the past one to two millennia. The most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the nineteenth century. At multi-decadal to centennial scales, temperature variability shows distinctly different regional patterns, with more similarity within each hemisphere than between them. There were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age, but all reconstructions show generally cold conditions between ad 1580 and 1880, punctuated in some regions by warm decades during the eighteenth century. The transition to these colder conditions occurred earlier in the Arctic, Europe and Asia than in North America or the Southern Hemisphere regions. Recent warming reversed the long-term cooling; during the period ad 1971–2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years.

Environmental Monitoring in the Mechara caves, Southeastern Ethiopia: Implications for Speleothem Palaeoclimate Studies

Abstract The interpretation of palaeoclimate records in speleothems depends on the understanding of the modern climate of the region, the geology, the hydrology above the caves, and the within-cave climate. Monitoring within-cave climate variability, geochemistry of speleothem-forming drip waters, and associated surface and groundwater, provides a modern baseline for interpretation of speleothem palaeohydrological and palaeoclimate records. Here, we present results of such monitoring of the Mechara caves in southeastern Ethiopia, conducted between 2004-2007. Results show nearly constant within cave climate (temperature and humidity) in all caves, which generally reflects the surface climate; groundwater and surface water geochemistry is similar across the region (except slight modification by local lithological variations); and modern drip water isotope data fall close to regional Meteoric Water Line, but speleothems further from equilibrium. The Holocene and Modern speleothems from these caves give high-resolution climate records, implying that the Mechara caves provide a suitable setting for the deposition of annually laminated speleothems that could record surface climate variability in a region where rainfall is sensitive to both the strength of the intertropical convergence zone as well as Indian Monsoon variability.

Understanding paleoclimate and human evolution through the hominin sites and paleolakes drilling project

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Quaternary glacial activity on the Ethiopian mountains

Publisher Summary Central Ethiopia comprises a high plateau at 2000-3000m a.s.l., formed from successive lava flows and bisected by the eastern African rift. Ten mountains rise from this to altitudes of just over 4,000 m a.s.1 and though the reports of various workers are not in complete agreement it appears that five of these were glaciated at the last glacial maximum (LGM), but without clear evidence of earlier glaciations. The chapter explores that in moist central Ethiopia, on the Arsi mountains, there are large, clear terminal moraines on Mount Bada at 3200–3700m with the glaciated area estimated to be 85 km 2 with equilibrium line altitudes (ELAs) of 3700m a.s.1 on the east and 3900 m a.s.1 on the west. This appears to represent the LGM. Moraines are reported less authoritatively on Mounts Kecha and Chilalo, but are not visible on the air photographs. It also discusses that Kara Tuluke and other minor volcanic cones south of Tullu Dimtu and just outside the Big Boulder Moraine are the eroded remnants of small volcanic plugs. The boulder slopes around their bases display remarkable features dissimilar to any described elsewhere in the world. An ecological monograph on the Bale mountains has a useful review of climate history and photographs of the plateau.

Origin and Isotopic Composition of Aragonite Laminae in an Ethiopian Crater Lake

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 210Pb 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 δ18O 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 δ18O 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 ‰, which is comparable to the range of δ18O 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.

Connecting Scientific Drilling and Human Evolution: Scientific Drilling for Human Origins: Exploring the Application of Drill Core Records to Understanding Hominin Evolution; Addis Ababa, Ethiopia, 17–21 November 2008

How did environmental history, particularly climate, affect the evolution of our hominin ancestors and closely related species? The formulation of testable hypotheses about the climate-evolution connection is impeded by limited numbers of hominin specimens and the geographic and temporal gaps that characterize their fossil record. Additionally, knowledge of Earths environmental history close to these fossil finds remains limited. Scientists interested in the problem currently make use of temporally and geographically discontinuous outcrop exposures at the fossil sites, and/or deep-sea or lake paleoclimate records geographically distant from the hominin fossils, to address the Earth history side of this equation.