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More than a century of rainfall data in the Sahel show an unusually wet period from 1950 until 1970 (positive index values), followed by extremely dry years from 1970 to 1990 (negative index values). From 1990 until present rainfall returned to levels slightly below the 1898–1993 average, but year-to-year variability was high.
Recent "Greening" of the Sahel: The results of trend analyses of time series over the Sahel region of seasonally integrated NDVI using NOAA AVHRR NDVI-data from 1982 to 1999. Areas with trends of <95% probability in white.

The Sahel drought was a series of historic droughts, beginning in at least the 17th century (CE) affecting the Sahel region, a climate zone sandwiched between the African savanna grasslands to the south and the Sahara desert to the north, across West and Central Africa. While the frequency of drought in the region is thought to have increased from the end of the 19th century, three long droughts have had dramatic environmental and societal effects upon the Sahel nations. Famine followed severe droughts in the 1910s, the 1940s, and the 1960s, 70s and 80s, although a partial recovery occurred from 1975-80. While at least one particularly severe drought has been confirmed each century since the 1600s, the frequency and severity of recent Sahelian droughts stands out. Famine and dislocation on a massive scale—from 1968 to 1974 and again in the early and mid 1980s—was blamed on two spikes in the severity of the 1960-1980s drought period.[1] From the late 1960s to early 1980s famine killed a 100,000 people, left 750,000 dependent on food aid, and affected most of the Sahel's 50 million people.[2] The economies, agriculture, livestock and human populations of much of Mauritania, Mali, Chad, Niger and Burkina Faso (known as Upper Volta during the time of the drought) were severely impacted. As disruptive as the droughts of the late 20th century were, evidence of past droughts recorded in Ghanaian lake sediments suggest that multi-decadal droughts were common in West Africa over the past 3,000 years and that several droughts lasted far longer and were far more severe.[3][4]

Contents

Previous Sahel droughts

Because the Sahel's rainfall is heavily concentrated in a very small period of the year, the region has been prone to dislocation when droughts have occurred ever since agriculture developed around 5,000 years ago. The Sahel is marked by rainfalls of less than 100 mm a year, all of which occurs in a season which can run from several weeks to two months.

Despite this vulnerability, the history of drought and famine in the Sahel do not perfectly correlate. While modern scientific climate and rainfall studies have been able to identify trends and even specific periods of drought in the region, oral and written records over the last millennium do not record famine in all places at all times of drought. One 1997 study, in attempting to map long scale rainfall records to historical accounts of famine in Northern Nigeria, concluded that "the most disruptive historical famines occurred when the cumulative deficit of rainfall fell below 1.3 times the standard deviation of long-term mean annual rainfall for a particular place."[5] The 1982-84 period, for instance, was particularly destrutive to the pastoral Fula people of Senegal, Mali and Niger, and the Tuareg of northern Mali and Niger. The populations had not only suffered in the 1968-74 period, but the inability of many to rebuild herds destroyed a decade earlier, along with factors as various as the shift of political power to settled populations with independence in the 1960s, Senegalese-Mauritanian border relations, and Niger's dependence upon falling world uranium prices coinciding in a destructive famine.[6][7][8]

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600-700 CE

Surviving contemporary records of climate in the Sahel begin with early Muslim travellers in the early Medieval Warm Period. These suggest that Sahel rainfall was relatively low in the seventh and eighth centuries and then increased substantially from about 800 AD.[9] There was a decline in rainfall from about 1300 AD, but an increase again around 200 years later.

1400-1750

According to a study of West African drought based on Ghanaian lake sediments (not eyewitness historical accounts) published in the journal Science in April 2009:

"The most recent of these [multicentury droughts] occurred between 1400 and 1750 CE (550 to 200 yr B.P.), similar in timing to the Little Ice Age (LIA, 1400 to 1850 CE), a well-known interval when Northern Hemisphere temperatures were cooler than at present. In contrast with earlier studies, which reconstructed wetter conditions in East Africa during this period, evidence from Lake Bosumtwi supports more recent studies suggesting that this interval was dry. Evidence for LIA drought is not restricted to Africa, however. Records from throughout the tropics, including the western Pacific warm pool, the Arabian Sea, continental Asia, and tropical South America all show evidence for dry conditions during this time period.".[10]

1640

The first major historically recorded drought in the Sahel occurred around 1640, and a major drought after generally wet conditions occurred, based on the reports of European travellers,[11] during the 1680s.

1740s and 50s

This cycle of several wet decades followed by a drought was repeated during the 18th century. These droughts killed hundreds of thousands of people in the 1740's to 1750's. [12] The 1740s and 1750s was recorded in chronicles of what is today Northern Nigeria, Niger and Mali as the "Great Famine", the worst for at least 200 years prior. It caused massive dislocation of the Sahelian states of the time, but also disrupted the Trans Saharan trade routes to North Africa and Europe.[13]

1830s

Around 1790 dry conditions similar to those of the late twentieth century set in[9] and continued until around 1870. After that, a very wet period set in for around 25 years, followed by a return to drier conditions. While the drying begun around 1895 and caused its first large famine only in the early 20th century, the 1820s and 1830s saw a 12 to 15 year drought and regional instances of major famine from Senegal to Chad. Historical records suggest this drought caused a large-scale emigration from the Bornu Empire, contributing to its rapid decline in the 19th century.[13] It what is now northern Senegal, the Kingdom of Fouta Tooro was struck by a famine caused by the failure of 1833's rainy season, leading to waves of famine until 1837.[14]

Early 20th century droughts

The first rain gauges in the Sahel date from 1898 and they reveal that a major drought, accompanied by large-scale famine, in the 1910s, followed by wet conditions during the 1920s and 1930s reaching a peak with the very wet year of 1936. The 1940s saw several minor droughts — notably in 1949 — but the 1950s were consistently wet and expansion of agriculture to feed growing populations characterised this decade and many have thought it contributed to the severity of the subsequent Sahel droughts

Potential factors contributing to Sahel drought

Originally it was believed that the drought in the Sahel primarily was caused by humans over-using natural resources in the region through overgrazing, deforestation and poor land management.[15][16] In the late 1990s, climate model studies suggested that large scale climate changes were also triggers for the drought.

In the early 2000s, after the phenomenon of global dimming was discovered, some speculatively suggested that the drought was likely caused by air pollution generated in Eurasia and North America. The pollution changed the properties of clouds over the Atlantic ocean, disturbing the monsoons and shifting the tropical rains southwards.

In 2005, a series of climate modeling studies performed at NOAA / Geophysical Fluid Dynamics Laboratory indicated that the late 20th century Sahel drought was likely a climatic response to changing sea surface temperature patterns, and that it could be viewed as a combination of natural variability superimposed upon an anthropogenically forced regional drying trend.[17] Using GFDL CM2.X, these climate model simulations indicated that the general late 20th century Sahel drying trend was attributable to human-induced factors; largely due to an increase in greenhouse gases and partly due to an increase in atmospheric aerosols. In IPCC future scenario A2 (CO2 value of ≈860 ppm) Sahel rainfall could be reduced by up to 25% by year 2100, according to climate models.

A 2006 study by NOAA scientists Rong Zhang and Thomas L. Delworth suggests that the Atlantic Multidecadal Oscillation plays a leading role. An AMO warm phase strengthens the summer rainfall over Sahel, while a cold phase reduces it.[18] The AMO entered a warm phase in 1995 and, assuming a 70-year cycle (following peaks in ≈1880 and ≈1950), will peak around 2020.[19]

A 2009 study by Timothy Shanahan of the University of Texas at Austin (featuring work conducted as a graduate student at the University of Arizona) and his colleagues found further evidence for a link between the AMO and West African drought.[20]

United Nations Sahel drought response

In 1973, The United Nations Sahelian Office (UNSO) was created to address the problems of drought in the Sahel region following the West African Sahel drought of 1968-73. In the 1990s, the United Nations Convention to Combat Desertification (UNCCD) was adopted and UNSO became the United Nations Development Programme's Office to Combat Desertification and Drought, as its scope broadened to be global rather than only focused on Africa.[21]

See also

References

  1. ^ The Sahel region; assessing progress twenty-five years after the great drought. Simon Batterbury, republished paper from 1998 RGS-IBG conference. Global Environmental Change (2001) v11, no 1, 1-95.
  2. ^ AFRICA ENVIRONMENT OUTLOOK. Past, present and future perspectives. United Nations Environmental Programme (2002). Retrieved 2009-02-13.
  3. ^ Timothy Shanahan; et al. (2009). Atlantic Forcing of Persistent Drought in West Africa. Science, Vol. 324. no. 5925, pp. 377 - 380.
  4. ^ Severity, Length of Past Megadroughts Dwarf Recent Drought in West Africa. Jackson School of Geosciences Online, April 16, 2009.
  5. ^ Aondover Tarhule1 and Ming-Ko Woo. Towards an Interpretation of Historical Droughts in Northern Nigeria. Climatic Change, no 37, 1997. pp.601-613
  6. ^ David Tenenbaum. Traditional drought and uncommon famine in the Sahel. Whole Earth Review , Summer, 1986.
  7. ^ J Swift. Sahelian Pastoralists: Underdevelopment, Desertification, and Famine. Annual Review of Anthropology Vol. 6: 457-478
  8. ^ Timberlake L. The Sahel: drought, desertification and famine. Draper Fund Report, 1985 Sept(14):17-9.
  9. ^ a b Rain, David (1999). Eaters of the Dry Season: Circular Labor Migration in the West African Sahel. Boulder, CO: Westview Press. p. 77. ISBN 0813338727.  
  10. ^ Timothy Shanahan; et al. (2009). Atlantic Forcing of Persistent Drought in West Africa. Science, Vol. 324. no. 5925, pp. 377 - 380.
  11. ^ "Climate and Man in the Sahel during the Historical Period". World Environmental Library. http://www.greenstone.org/greenstone3/nzdl;jsessionid=1AB0DFF58D047EC9E8B55C523A65856F?a=d&d=HASH99771026b305b974da2200.7&c=envl&sib=1&dt=&ec=&et=&p.a=b&p.s=ClassifierBrowse&p.sa=. Retrieved 2008-06-19.  
  12. ^ Len Milich: Anthropogenic Desertification vs ‘Natural’ Climate Trends
  13. ^ a b Paul E. Lovejoy and Stephen Baier. The Desert-Side Economy of the Central Sudan. The International Journal of African Historical Studies, Vol. 8, No. 4 (1975), pp. 551-581
  14. ^ Philip D. Curtin, Economic Change in Precolonial Africa: Senegambia in the Era of the Slave Trade, 2 vols. University of Wisconsin Press (1975)
  15. ^ "Desertification - a threat to the Sahel", August 1994
  16. ^ Hunger is spreading in Africa
  17. ^ Held, I. M.; et al. (2005). "Simulation of Sahel drought in the 20th and 21st centuries". PNAS 102 (50): 17891–17896. doi:10.1073/pnas.0509057102. PMID 16322101.  
  18. ^ Zhang, Rong; Delworth, Thomas L. (2006). "Impact of Atlantic multidecadal oscillations on India/Sahel rainfall and Atlantic hurricanes". Geophysical Research Letters 33: L17712. doi:10.1029/2006GL026267.  
  19. ^ Enfield, David B.; Cid-Serrano, Luis (2009). "Secular and multidecadal warmings in the North Atlantic and their relationships with major hurricane activity". International Journal of Climatology Forthcoming. doi:10.1002/joc.1881.  
  20. ^ Timothy Shanahan; et al. (2009). Atlantic Forcing of Persistent Drought in West Africa. Science, Vol. 324. no. 5925, pp. 377 - 380.
  21. ^ UNDP | Drylands Development Centre

Further reading

  • Folland, C. K.; Palmer, T. N.; Parker, D. E. (1986), "Sahel rainfall and worldwide sea temperatures, 1901−85", Nature 320 (6063): 602–607, doi:10.1038/320602a0  
  • Giannini, A.; Saravanan, R.; Chang, P. (2003), "Oceanic Forcing of Sahel Rainfall on Interannual to Interdecadal Time Scales", Science 302 (5647): 1027–1030, doi:10.1126/science.1089357, PMID 14551320  
  • Glantz, Michael H., ed. (1976), The Politics of Natural Disasters: The Case of the Sahel Drought, New York: Praeger  

External links


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