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African Droughts In Perspective–Part II

November 20, 2012
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By Paul Homewood

 

 

In Part I, we saw how mega droughts had gripped the Sahel many times in previous centuries. In Part II, we’ll take a closer look at droughts during the 20thC.

 

The Sustainable Development Dept at the UN published a paper by Gommes and Petrassi in 1996 entitled “Rainfall Variability and Drought in Sub-Saharan Africa” . Here are some of the main points:-

  • The continent has a long history of rainfall fluctuations of varying lengths and intensities. The worst droughts were those of the 1910s, which affected east and west Africa alike. They were generally followed by increasing rainfall amounts, but negative trends were observed again from 1950 onwards culminating, in West Africa, in 1984.
  • Since then, starting in 1988, the Sahel has recorded a series of good years (frequently accompanied by floods) which some interpret as the end of the Sahelian drought. The reality is that rainfall will continue fluctuating, and that good and bad years will continue occurring.

  • The years from 1960 to 1969 were among the wettest of the period, while the seventies and eighties mostly recorded lower rainfall. The downward trend from 1960 to 1970 affected the whole continent.

So again we see evidence of droughts becoming worse at the same time as the NH was growing colder, with a gradual return to previous conditions since the mid 1980’s, as graph from the CRU shows below.

 

http://www.eoearth.org/media/draft/f/f2/Greening_Sahel_Figure_1.jpg

 

Figure 1. Rainfall anomalies from the long term mean for meteorological stations in the Sahel. Red line shows the 7 year running mean, blue line shows the number of stations included in the calculation of z-score. (Source of data: Climatic Research Unit, University of East Anglia, and Global Historical Climatology Network, Oak Ridge National Laboratory)

 

 

Meanwhile, a paper by Rouault and Richard examined drought in South Africa and concluded :-

 

According to this index, the 8 most severe droughts at the 6-month time scale for the summer rainfall region of South Africa happened in 1926, 1933, 1945, 1949, 1952, 1970, 1983 and 1992. There is considerable decadal variability and an 18 to 20 year cycle is only found in the number of dry districts. 

 

Professor Alexander, of the University of Pretoria, goes further :-

 

With this objective and methodology in mind, I assembled the largest and most comprehensive set of meteorological and hydrological data yet analysed in South Africa. It consisted of a total of 11 804 years of data from 183 gauged sites and eight processes: open-water surface evaporation, concurrent rainfall, areal rainfall, dam inflow, river flow, flood peak maxima, ground water levels, and the southern oscillation index.

A surprising result, in that it had not previously been reported by others, was that the mean annual rainfall over South Africa has increased steadily from 497 mm at the beginning of the record in 1921 through to 543 mm at the end of the record in 1999. This is a substantial increase and is in close agreement with the 10% increase reported for the USA since 1910. There were corresponding increases in river flow, open-water surface evaporation and ground water levels. As open water surface evaporation is a function of solar radiation, air temperature and wind, all at water surface, this identifies global warming as the probable cause of the increases in evaporation, and consequently rainfall, river flow and groundwater levels as well. There were no indications of increases in the severity and magnitude of droughts and floods.

The conclusion must be that additional global warming will have a greater beneficial effect than detrimental effect on the natural environment. This is directly contrary to current views by South African climatologists and environmental scientists.

 

Climate has never been a static affair, and droughts come and go, but there is little evidence that they are becoming systematically worse in Africa because of global warming. As for the future? I’ll leave the final comments to this paper from Shongwe et al in 2010, “Projected changes in mean and extreme precipitation in Africa under global warming” :-

 

East Africa is shown to be a region in which a coherent projection of future precipitation change can be made, supported by physical arguments. Although the rate of change is still uncertain, almost all results point to a wetter climate with more intense wet seasons and less severe droughts.

5 Comments leave one →
  1. Coldish permalink
    November 20, 2012 5:37 pm

    I’d like to see temperature variations for the same set of stations and some period

  2. Coldish permalink
    November 20, 2012 5:38 pm

    same not some. Another good peice of work from you, Paul.

  3. miked1947 permalink
    November 21, 2012 1:40 am

    Paul:
    This points to the long term variability in weather patterns.
    Maybe Kathrine should go to the Sahel to do a few decades of research on this unprecedented phenomena. ;)

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