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    By Atreyee Bhattacharya | October 15th 2009 07:52 PM | 1 comment | Print | E-mail | Track Comments

    A devastating drought, possibly the worst of the last century, is sweeping through Kenya-- killing children, spreading malnutrition, crippling economies and resulting in the death of livestock and wildlife reserve. Rewind 30 years. Reports on the worst famine in West Africa—Mali, Mauritania, Chad and Sudan— during the 1970’s and early 1980’s made  headlines across the world. Same situation. Different country. From the Atlantic Ocean in the west to the Indian Ocean in the east, along the southern border of the Sahara, straddling roughly the 13-north latitude (approximately 2500 miles in length and 500 miles in breadth) is a narrow, semi arid stretch of land that is called the Sahel. Sahel means ‘shore’. This swath of land includes countries like Senegal, Mauritania, Mali, Burkina Faso, Niger, Nigeria, Chad, Sudan, Eritrea, Kenya —all drought  ravaged, their economies and societies torn by all engulfing famine over the last half a century. Kenya is the easternmost country of this stretch.

    Why do droughts of such severity strike the Sahel?  The answer lies in what causes rains to fall in the Sahel in the first place.

    Rainfall in the Sahel is governed by the location of the Intertropical Convergence Zone (ITCZ), also called the ‘rain belt’. The ITCZ is not static in time and space. It moves north and south between 2 to 10 N, annually. When in its northerly location, the Sahel, that straddles the ITCZ –  and perhaps the only place on earth to do so— receives rainfall.  However, instrumental data over the last 60 years [1] along with a plethora of paleoclimatic archive (also known as ‘ proxy’ data)—primarily of lake level fluctuations from all over the Sahel and other parts of north Africa [2,3] have shown that movement and location of the ITCZ is erratic, causing the monsoon in the Sahel to be similarly erratic in its incidence, duration and distribution. The unique geographical location of the Sahel is to blame.  Research has suggested that variation of sea surface temperature, particularly in the North Atlantic ocean exerts maximum influence on the movement of the ITCZ and hence rainfall in the Sahel [2]. This behavior of the monsoon or in some cases, the complete lack of monsoon has been a regular feature of life over the last 3000 years [2, 3]. 
     Droughts are more frequent when there is a shift in climatic regime [3]. Warm to cold or vice versa [3]. Even little changes in sea surface temperature can lead to severe impacts on rainfall through complicated, interrelated phenomena of the global climate system [2,3]. In fact, reconstructions of the drought history  of the Sahel have suggested that the ongoing drought in the Sahel is low in its severity when compared to those of the 17th and the 18th century [2,4]. The Sahelians had evolved a way, over the last few millennia,  adapt to such vagaries of climatic system—a nomadic lifestyle. Moving their livestock wherever the rainfall made the pastures green. 

    Over the last few decades things, however, things have been a bit different. The last major drought was in the mid 19th century (1840-1860)- during an interval when sea surface temperatures increased by about 1C. However, this was at a time before, the beginning of industrial revolution. This was before, anthropogenic carbon dioxide interfered with natural proceedings of the climate system. This was before, agriculture was tried on a semi desert like patch of land—a system that affects the climate system in major ways by emitting huge quantities of dust—even more so during dry years—into the atmosphere  [1,3]. Before, the new system of government decided to promote agriculture and tie the Sahelians to a strip of land-- where rainfall has failed regularly over the last 3000 years. .


    So, no, its not the same Sahel any longer. The dynamics between human population and nature has undergone drastic changes in a span of half a century in  several ways , mostly due to anthropogenic modification of the natural system-- both of climate and of human society.

    Although, paleoclimatic research shows that nature is being kinder to Sahel today in terms of duration of dry conditions than it has been in the past [4], modeling studies, on the other hand have indicated that given the complex nature of anthropogenic impact on climate, drier areas would become drier in the future and wetter areas wet as carbon dioxide begins to keep building in the atmosphere, contributing at least in part, to the current warming trend [5]. Whichever set of studies we pin our hopes to, one thing is for certain-- it is a very bleak future for the Sahel. Still to factor in is the poor and fragile economic condition, ill health, tribal wars – all stemming from the fact that the Sahelian society is ill equipped and still under developed to sustain agriculture based economy in a swath if land where failure of rainfall is more of a norm than anomaly. All of these and much more, severely impact on a group of nation who are in the frontlines of climate change, on the possible brink of desertification and strangely may not have been even remotely responsible for the climatic conundrum they are facing right now.

    It is not always a fair dice.


     

    Referances : [1]Prospero et al, Science 2003 ; [2] Shanahan et al, Science, 2009 ; [3] Harrison et al, Earth Science Reviews, 2001; [4] Bhattacharya and Mukhopadhyay, AGU abstract 2008 [5] Held et al, PNAS, 2005









    Comments

    rholley

    This is grim news.  I understand that the Sahara has been getting drier for thousands of years, and there are remains of lost societies that used to flourish there.  More insight on this topic would be welcome.

    Robert H. Olley / Quondam Physics Department / University of Reading / England