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ABSTRACT: A faunal record of sea-surface temperature (SST) variations off West Africa documents a series of abrupt, millennial-scale cooling events, which punctuated the Holocene warm period. These events evidently resulted from increased southward advection of cooler temperate or subpolar waters to this subtropical location or from enhanced regional upwelling. The most recent of these events was the Little Ice Age, which occurred between 1300 to 1850 A.D., when subtropical SSTs were reduced by 3° to 4°C. These events were synchronous with Holocene changes in subpolar North Atlantic SSTs, documenting a strong, in-phase link between millennial-scale variations in high- and low-latitude climate during the Holocene. |
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To read or view the full study, please visit the
Science website. It was published in Science, Vol. 288 (5474), pp. 2198, 23 June 2000. |
| DATA: Download the Data from the WDC Paleo Archive. |
| DISCUSSION: From the end of the last ice age until 6,000 BP, the Sahara and adjacent Sahel regions were much greener than today. Using pollen data and a biome model, researchers in the BIOME 6000 project have reconstructed both vegetation and climatic conditions around 6,000 BP. Hoelzmann et al. 1998 found that much of what is desert today was covered in steppe vegetation, and that many small lakes and streams existed above 23° North latitude.
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The rapid transition from a green Sahara to today's desert conditions began around 5,700 BP.
This paper by deMenocal et al. (2000) provided records of ocean temperature and terrigenous
dust in marine sediment cores off western Africa. DeMenocal and colleagues found that ocean
temperatures off western Africa dropped around 5 °C between 5,700-5,000 BP, and that
there was a dramatic increase in the amount of dust. The dust increase relates directly
to the changes in vegetation as the Sahara expanded across northern Africa.
During that same period, Oppo et al. (2003) found a downward trend in NADW production around 6,500 BP and ending around 5,000 BP. This also corresponds to greater freshwater influx to the North Atlantic (Bond et al. 2001) and more winter-like conditions in Greenland (Alley et al. 1997). It is hypothesized that the climate and vegetation changes in Africa were related to changes in NADW formation. Further back in time, the deMenocal et al. data support the vegetation reconstructions from pollen. From 6,000 BP to the end of the last glacial around 14,500, terrigenous sediments are low in the marine sediments. In fact, the terrigenous sediment content also reflects the Younger Dryas cooling with a move to more arid conditions. The transition from the last glacial is not pronounced in the sea surface temperature data. However, there is evidence of the abrupt cooling at 8,200 BP in addition to the abrupt warming at 5,700 BP.
References Cited: Bond, G., B. Kromer, J. Beer, R. Muscheler, M.N. Evans, W. Showers, S. Hoffmann, R. Lotti-Bond, I. Hajdas, and G. Bonani. 2001. Persistent Solar Influence on North Atlantic Climate During the Holocene. Science 294: 2130-2136. Hoelzmann, P., D. Jolly, S.P. Harrison, F. Laarif, R. Bonnefille, and H.-J. Pachur. 1998. Mid-Holocene land-surface conditions in northern Africa and the Arabian Peninsula: A data set for the analysis of biogeophysical feedbacks in the climate system. Global Biogeochem. Cycles Vol. 12, No. 1, p. 35-52. Oppo, D.W., J.F. McManus and J.L. Cullen. 2003. Palaeo-oceanography: Deepwater variability in the Holocene epoch. Nature, 422 (6929): 277-278 and 400 (correction). |
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Contact Us National Oceanic and Atmospheric Administration 6 May 2004
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