New AMS dates, stratigraphic correlations and decadal climatic cycles for the past 4 ka at Lake Turkana, Kenya

Halfman, John D.; Johnson, Thomas C.; Finney, Bruce P.

doi:10.1016/0031-0182(94)90349-2

Cited by 79 publications

(46 citation statements)

References 23 publications

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“…3J; see also Halfman et al 1994; Appendix 1). As noted earlier, the decrease in model precipitation in the Highlands reflects an inhibited northward migration of the ITCZ into the eastern and central Sahel with the warmer Indian Ocean SSTs, suggesting the possibility of reduced precipitation in that region during the MCA, though currently available regional proxy records are not adequate to address this question (Verschuren 2004).…”

Section: Relation To Other Modeling Studies Using Warmer Indian Oceanmentioning

confidence: 99%

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Support for global climate reorganization during the “Medieval Climate Anomaly”

et al. 2010

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Widely distributed proxy records indicate that the Medieval Climate Anomaly (MCA; *900-1350 AD) was characterized by coherent shifts in large-scale Northern Hemisphere atmospheric circulation patterns. Although cooler sea surface temperatures in the central and eastern equatorial Pacific can explain some aspects of medieval circulation changes, they are not sufficient to account for other notable features, including widespread aridity through the Eurasian sub-tropics, stronger winter westerlies across the North Atlantic and Western Europe, and shifts in monsoon rainfall patterns across Africa and South Asia. We present results from a full-physics coupled climate model showing that a slight warming of the tropical Indian and western Pacific Oceans relative to the other tropical ocean basins can induce a broad range of the medieval circulation and climate changes indicated by proxy data, including many of those not explained by a cooler tropical Pacific alone. Important aspects of the results resemble those from previous simulations examining the climatic response to the rapid Indian Ocean warming during the late twentieth century, and to results from climate warming simulations-especially in indicating an expansion of the Northern Hemisphere Hadley circulation. Notably, the pattern of tropical Indo-Pacific sea surface temperature (SST) change responsible for producing the proxy-model similarity in our results agrees well with MCA-LIA SST differences obtained in a recent proxy-based climate field reconstruction. Though much remains unclear, our results indicate that the MCA was characterized by an enhanced zonal Indo-Pacific SST gradient with resulting changes in Northern Hemisphere tropical and extra-tropical circulation patterns and hydroclimate regimes, linkages that may explain the coherent regional climate shifts indicated by proxy records from across the planet. The findings provide new perspectives on the nature and possible causes of the MCA-a remarkable, yet incompletely understood episode of Late Holocene climatic change.

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Section: Relation To Other Modeling Studies Using Warmer Indian Oceanmentioning

confidence: 99%

“…Letter indices correspond to time series in Fig. 3; numbers correspond to entries in Table 3 Seager et al (2007Seager et al ( , 2008 Kenya; Halfman et al 1994;Verschuren 2004) and from southern Oman (Fleitmann et al 2003; Fig 3L). (Thompson et al 1995;Wang et al 2007; Fig.…”

Section: Proxy Recordsmentioning

confidence: 99%

Support for global climate reorganization during the “Medieval Climate Anomaly”

et al. 2010

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“…Lakes Naivasha, Victoria, Turkana, Edward, Tanganyika, and Malawi all experienced pronounced and prolonged drought during the A.D. 1050-1400 period (Halfman and Johnson 1988;Owen et al 1990;Mohammed et al 1996;Verschuren et al 2000;Vershcuren 2000;Johnson et al 2001;Alin and Cohen 2003;Lamb et al 2001;Russell et al 2003;Stager et al 2005). Close by, Mt.…”

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confidence: 99%

North American Droughts of the Last Millennium from a Gridded Network of Tree-Ring Data

et al. 2007

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Drought is the most economically expensive recurring natural disaster to strike North America in modern times. Recently available gridded drought reconstructions have been developed for most of North America from a network of drought-sensitive tree-ring chronologies, many of which span the last 1000 yr. These reconstructions enable the authors to put the famous droughts of the instrumental record (i.e., the 1930s Dust Bowl and the 1950s Southwest droughts) into the context of 1000 yr of natural drought variability on the continent. We can now, with this remarkable new record, examine the severity, persistence, spatial signatures, and frequencies of drought variability over the past milllennium, and how these have changed with time.The gridded drought reconstructions reveal the existence of successive "megadroughts," unprecedented in persistence (20-40 yr), yet similar in year-to-year severity and spatial distribution to the major droughts experienced in today's North America. These megadroughts occurred during a 400-yr-long period in the early to middle second millennium A.D., with a climate varying as today's, but around a drier mean. The implication is that the mechanism forcing persistent drought in the West and the Plains in the instrumental era is analagous to that underlying the megadroughts of the medieval period. The leading spatial mode of drought variability in the recontructions resembles the North American ENSO pattern: widespread drought across the United States, centered on the Southwest, with a hint of the opposite phase in the Pacific Northwest.Recently, climate models forced by the observed history of tropical Pacific SSTs have been able to successfully simulate all of the major North American droughts of the last 150 yr. In each case, cool "La Niña-like" conditions in the tropical Pacific are consistent with North American drought. With ENSO showing a pronounced signal in the gridded drought recontructions of the last millennium, both in terms of its link to the leading spatial mode, and the leading time scales of drought variability (revealed by multitaper spectral analysis and wavelet analysis), it is postulated that, as for the modern day, the medieval megadroughts were forced by protracted La Niña-like tropical Pacific SSTs. Further evidence for this comes from the global hydroclimatic "footprint" of the medieval era revealed by existing paleoclimatic archives from the tropical Pacific and ENSO-sensitive tropical and extratropical land regions. In general, this global pattern matches that observed for modern-day persistent North American drought, whereby a La Niña-like tropical Pacific is accompanied by hemispheric, and in the midlatitudes, zonal, symmetry of hydroclimatic anomalies.

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“…Variation in the carbonate content of sediment cores from the northern part of Lake Turkana, reflecting Omo River inflow and thus precipitation over the Ethiopian highlands, indicates that several arid events occurred between c. 200 BCE and 300 CE (Halfman et al, 1994). A lake-level reconstruction for Lake Tanganyika based on fossil ostracod assemblages suggests that its lowest level of the past 2500 years was reached between 200 BCE and 0 CE (Alin and Cohen, 2003).…”

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confidence: 99%

African hydroclimatic variability during the last 2000 years

Nash

Cort

Chase

et al. 2016

Quaternary Science Reviews

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The African continent is characterised by a wide range of hydroclimate regimes, ranging from humid equatorial West Africa to the arid deserts in the northern and southern subtropics. The livelihoods of much of its population are also vulnerable to future climate change, mainly through variability in rainfall affecting water resource availability. A growing number of data sources indicate that such hydroclimatic variability is an intrinsic component of Africa's natural environment. This paper, co-authored by members of the PAGES Africa 2k Working Group, presents an extensive assessment and discussion of proxy, historical and instrumental evidence for hydroclimatic variability across the African continent, spanning the last two millennia. While the African palaeoenvironmental record is characterised by spatially disjunctive datasets, with often less-than-optimal temporal resolution and chronological control, the available evidence allows the assessment of prominent spatial patterns of palaeomoisture variability through time. In this study, we focus sequentially on data for six major time windows: the first millennium AD, the Medieval Climate Anomaly (900-1250 CE), the Little Ice Age (1250-1750 CE), the end of the LIA (1750-1850 CE), the Early Modern Period , and the period of recent warming (1950 onwards). This results in a continentwide synthesis of regional moisture-balance trends through history, allowing consideration of possible driving mechanisms, and suggestions for future research.

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New AMS dates, stratigraphic correlations and decadal climatic cycles for the past 4 ka at Lake Turkana, Kenya

Cited by 79 publications

References 23 publications

Support for global climate reorganization during the “Medieval Climate Anomaly”

Support for global climate reorganization during the “Medieval Climate Anomaly”

North American Droughts of the Last Millennium from a Gridded Network of Tree-Ring Data

African hydroclimatic variability during the last 2000 years

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