Middle Holocene Climate Change in Northern Africa: The Drying of the Sahara

by Irene Nester
ES 767 Quaternary Geology
10 December 2008

 


Table of ContentsIntroduction
Abrupt or Gradual Climate Change?
Drivers of Holocene Climate Change in Northern Africa
The Case for Abrupt Climate Change
Paleoclimate Data from Central Sahara Supports Gradual Change
Conclusion
References


Introduction

The Sahara in northern Africa is the largest warm desert on Earth at approximately 9 million km2, slightly larger than the contiguous United States. It is the second largest desert after Antarctica, a cold desert (Wikipedia, 2008). Areas receiving less than 25 cm of rainfall annually are considered arid. Arid conditions have existed in region since the middle Holocene, about 4300 years ago. Prior to that time, during the early Holocene, the Sahara region had a far moister climate and supported tropical vegetation. This interval was termed the "African Humid Period" (Kröpelin et al., 2008a).

Map of World Non-polar Deserts from the USGS (USGS, 1997).


Abrupt or Gradual Climate Change?

Some climate modeling results and paleoclimate data have indicated that the change from a moister climate with about 250 mm of rainfall to a hyperarid climate with less than 50 mm per year of rainfall occurred over a relatively short period of time, on the order of hundreds of years. However, recent evidence from lake sediment cores in the east-central Sahara indicates that a slower transition over several thousand years took place, at least in that location. The rate that the process of desertification occurred is important to our understanding of the feedback mechanisms involved and potential future climate changes. Relatively abrupt near continental-scale climate changes over a period of a few hundred years would have had a much greater impact on human culture than changes occurring over thousands of years.

The Sahara Desert covers most of Northern Africa.

NASA World Wind Image, retrieved by I. Nester on 8 December 2008.

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Drivers of Holocene Climate Change in Northern Africa

The mid-Holocene drying trend in northern Africa is linked to weakening of the summer monsoon rains due to a slow decrease in summer solar radiation as a result of orbital changes, i. e., primarily the Milankovitch cycle of precession (Alley et al., 2003). Milankovitch cycles affect the amount of solar radiation reaching the Earth's surface in a given location, and therefore the climate. During the early Holocene, perihelion occurred during late summer in the northern hemisphere, and currently it occurs during the winter. Precession since the early Holocene has steadily decreased the solar insolation during summer in the northern hemisphere. The relative reduction in solar radiation absorbed by the Atlantic Ocean in the middle and latter Holocene may have lead to a decreased relative temperature difference between the surface of the ocean and the adjacent landmass, as shown by model results (COHMAP, 1988, and Kutzbach et al., 1997). The monsoon winds and resulting precipitation presumably were weakened as a result.



© National Geographic Society 2008
Sandstone pinnacles eroded by rain indicate that the Saharan climate was much wetter in the past.

Photograph by George Steinmetz. From National Geographic, http://travel.nationalgeographic.com/places/photos/photo_chad_chad.html.


The Case for Abrupt Climate Change

The change in climate from moist and vegetated to arid desert was thought to have occurred rapidly based on the sudden appearance of fine dust in cores drilled in the seafloor of the eastern Atlantic Ocean (Ocean Drilling Program Site 658) adjacent to the coast of Mauritania. The proportion of dust derived from the Sahara in the cores increased significantly at a depth corresponding to about 5500 years ago. The increase in dust is thought to be due to a corresponding decrease of vegetation in the Sahara, but could also reflect an increase in dry lakebeds as a source of dust (Brovkin and Claussen, 2008).

Biogeophysical feedback from surface vegetation may have accelerated a decline in precipitation. Climate modeling results from atmospheric models that did not account for the effect of changes in the areal extent of vegetation cover tended to underestimate the climatic effects of changes in solar insolation. Models that included vegetation changes indicated that a positive feedback response from a reduction in vegetation may have been an important factor in Holocene climate shifts such as the desertification of the Sahara (Ganopolski et al., 1998).

A rapid climate change could have been brought about by positive feedback mechanisms that increased the rate and degree of drying. Surface vegetation cover has a lower albedo than bare ground and retains heat, which tends to increase the temperature differential between the land and ocean, and therefore monsoon precipitation, in a positive feedback loop (Brovkin and Claussen, 2008). Loss of vegetation has the opposite effect, increasing albedo and decreasing precipitation. In addition, evapotranspiration from vegetation supplies atmospheric water vapor that contributes to precipitation in many areas. When plants die due to insufficient water, this source of moisture is reduced, exacerbating the drought. A reduction in vegetation also allows rain to run off in streams and rivers rather than being retained by plant roots (Kröpelin et al., 2008a).

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Paleoclimate Data from Central Sahara Supports Gradual Change

Recent evidence from sediment in a lake in northeastern Chad supports a more gradual transition. Lake bottom sediment cores obtained from Lake Yoa, a hypersaline lake in the east-central Sahara, have provided a continuous, accurately dated record of climate indicators for the last 6000 years. Lake Yoa is one of several small lakes located in the valley between the Tibesti and Ennedi mountain ranges. The current climate is subtropical desert with dry trade winds that blow almost continuously through the valley, high diurnal temperatures and almost no rainfall (less than 21 mm/year). The lake has survived for millennia even though it is in an area of severe evaporation because it is fed by an aquifer that was recharged during the moister early Holocene. It is unlikely that another continuous record of climatic indicators has been preserved in the Sahara for this time period.

Click Photos to EnlargeLocation of Lake Yoa

© Google Earth 2008
Location of Lake Yoa in northeastern Chad (upper center of map).

Image from Google Earth, retrieved by I. Nester on 22 November 2008.


© Google Earth 2008
Aerial (Satellite) View of Lake Yoa. The northeast-southwest trending elongated structures along the northern shore are sand dunes that are encroaching on the lake.

Image from Google Earth, retrieved by I. Nester on 22 November 2008.

 

Kröpelin et al. collected sediment cores from two locations in Lake Yoa which span a continuous period of deposition of over 6000 years. Although the lake is presumed to contain a complete sedimentary record of the Holocene, only the upper 7.47 m of the sequence could be retrieved with the coring equipment used by the researchers (Kröpelin et al., 2008b). In this sequence, which represents the middle to late Holocene, dates obtained by radiocarbon dating of organic material agree with the average sedimentation rates indicated by sequences in the cores which contain annual varves, about 1.3 mm per year. The radiocarbon dates also correlate with the horizon of Cesium from nuclear bomb testing in 1963/64.

Paleoclimate indicators contained in the cores were used to reconstruct the history of the Holocene climate transition in the region. Pollen, spores, and other plant and animal remains found in the cores, as well as sedimentological and geochemical evidence, indicate the paleo-ecosystems that surrounded Lake Yoa and their response to the drying climate over time. The sequence of sediments and associated organic materials in the cores record progressive changes in the salinity of the lake, the vegetation surrounding the lake, and changing regional wind patterns.


© National Geographic Society 2008

Lake Yoa in northeastern Chad, a lake that has persisted since the middle Holocene.

Photograph courtesy S. Kröpelin/University of Cologne. From National Geographic News, http://news.nationalgeographic.com/news/bigphotos/18230993.html.

Prior to about 4200 years ago, Lake Yoa was a freshwater lake. Over the next several hundred years, the lake became rapidly more saline until becoming a salt lake about 3900 years ago. This dramatic change is thought to be the result of a site-specific shift from surface or subsurface outflow from the lake to water loss only through evaporation in response to gradual regional drying. According to the researchers, "the exact timing of this transition depended on a site-specific threshold in the evolving balance between summed inputs (rain, local runoff, groundwater, and river inflow) and outputs (evaporation and subsurface outflow), rather than the timing and rate of regional climate change." This is in agreement with the indicators of vegetation type in the cores which show a progressive change over several millennia from a humid climate suite of plants including populations of tropical trees, ferns, shrubs and herbs, to an arid plant assemblage. The association of tropical trees with ferns prior to 4300 years B.P. indicates that they probably grew in river valleys that flooded periodically. Based on the data from Lake Yoa, the transition from a humid to an arid climate occurred over a period of about 2500 years (Kröpelin et al., 2008a).

The major conclusion of the study by Kröpelin et al. is that the continuous record of climate indicators in Lake Yoa sediments show no indication of rapid climate change, but instead show progressive and threshold-driven ecosystem responses to gradual aridification. Consequently, the abrupt increase in Saharan dust in Atlantic ocean sediment cores does not represent rapid climate change across all of the Sahara region. In addition, these findings suggest that the biogeophysical feedbacks were weaker than predicted by some climate modeling studies, at least in the eastern and central portions of the Sahara.

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Conclusion

Understanding the rate of Holocene climate change and the feedback mechanisms that may have played a role is important in order to predict possible climate responses to future warming trends in Africa and other continents. Data from sediment cores from the eastern Atlantic ocean seem to indicate that the change from moist to arid occured rapidly based on terrestrial dust content. Studies that have used climate models to estimate the positive biogeophysical feedback from vegetation cover have also shown that the change may have been abrupt. However, recent detailed paleoclimate evidence from a continuous sediment record in a lake in the east-central Sahara indicates a more gradual transition, over several thousand years, at least in the eastern and central Sahara region. As some modelling studies indicate, middle Holocene climate change may have been more abrupt in the west, but a continuous empirical climate record is unlikely to exist in the western Sahara to verify the model results.


References

Alley, R. B., Marotzke, J., Nordhaus, W. D., Overpeck, J. T., Peteet, D. M., Pielke, R. A. Jr., Pierrehumbert, R. T., Rhines, P. B., Stocker, T. F., Talley, L. D., and Wallace, J. M. 2003. Abrupt Climate Change. Science 299:2005-2010.

Brovkin, V., and Claussen, M. 2008. Comment on "Climate-Driven Ecosystem Succession in the Sahara: The Past 6000 Years". Science 322:1326.

Cooperative Holocene Mapping Project (COHMAP) Members. 1988. Climatic Changes of the Last 18,000 Years: Observations and Model Simulations. Science 241:1043-1052.

Ganopolski, A., Kubatzki, C., Claussen, M., Brovkin, V., and Petoukhov, V. 1998. The Influence of Vegetation-Atmosphere-Ocean Interaction on Climate During the Mid-Holocene. Science 280:1916-1919.

Kröpelin, S., Verschuren, D., Lézine, A.-M., Eggermont, H., Cocquyt, C., Francus, P., Cazet, J.-P., Fagot, M., Rumes, B., Russell, J. M., Darius, F., Conley, D. J., Schuster, M., von Suchodoletz, H., and Engstrom, D. R. 2008a. Climate-Driven Ecosystem Succession in the Sahara: The Past 6000 Years. Science 320:765-768.

Kröpelin, S., Verschuren, D., Lézine, A.-M., Eggermont, H., Cocquyt, C., Francus, P., Cazet, J.-P., Fagot, M., Rumes, B., Russell, J. M., Darius, F., Conley, D. J., Schuster, M., von Suchodoletz, H., and Engstrom, D. R. 2008b. Supporting Online Material for Climate-Driven Ecosystem Succession in the Sahara: The Past 6000 Years. Science 320:765.

Kutzbach, J. E., and Liu, Z. 1997. Response of the African Monsoon to Orbital Forcing and Ocean Feedbacks in the Middle Holocene. Science 278:440-443.

National Geographic News web site. May 2008. Once Lush Sahara Dried Up Over Millennia, Study Says. http://news.nationalgeographic.com/news/bigphotos/18230993.html. Retrieved 22 November 2008.

National Geographic Travel and Cultures web site. Chad Photo: Sahara Sandstone. http://travel.nationalgeographic.com/places/photos/photo_chad_chad.html. Retrieved 22 November 2008.

United States Geological Survey (USGS). 1997. Distribution of Non-Polar Arid Land (map). http://pubs.usgs.gov/gip/deserts/what/world.html. Retrieved 22 November 2008.

Wikipedia web site. Sahara. http://en.wikipedia.org/wiki/Sahara. Retrieved 22 November 2008.


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