GEO-6 Regional Assessment for Africa
2.1.5
Climate variability and change
The global concentration of carbon dioxide increased from 280 parts per million in 1850 to 390 ppm in 2011, making greenhouse gases the most likely cause of the observed increase of around 0.5°C in global average surface temperature over the past 50 years (WMO 2015). The increase in temperature also affected other climatic systems, including rainfall patterns.
Rainfall patterns Most areas of Africa lack sufficient observational data to draw conclusions about trends in annual precipitation over the past century (Niang et al. 2014). In addition, in many areas of the continent there are discrepancies between different observed precipitation data sets (Kalognomou et al. 2013; Kim et al. 2013; Nikulin et al. 2012; Sylla et al. 2011). Areas where there are sufficient data show very probable decreases in annual precipitation over the past century over parts of the western and eastern Sahel region in northern Africa. Over the past few decades, the northern regions of North Africa (north of the Atlas Mountains and along the Mediterranean coast of Algeria and Tunisia) have experienced a strong decrease in the amount of precipitation in winter and early spring (Barkhordarian et al. 2013). The Sahara Desert, which receives less than 25 mm per year, shows little seasonal change (Liebmann et al. 2012).
Rainfall over the Sahel has experienced an overall reduction over the course of the 20th century, with a recovery towards the last 20 years of the century (Biasutti 2013; Ackerley et al. 2011; Nicholson et al. 2000). There were a large number of droughts in the Sahel during the 1970s (Greene et al. 2009; Biasutti et al. 2008; Biasutti and Giannini 2006). The recovery of the rains may be due to natural variability (Mohino et al. 2011) or a forced response to increased greenhouse gas concentrations (Biasutti 2013; Haarsma et al. 2005), or to reduced aerosols (Ackerley et al. 2011).
Precipitation in eastern Africa shows a high degree of temporal and spatial variability dominated by a variety of physical processes (Hession and Moore 2011; Rosell and Holmer 2007). Funk et al. (2008) indicate that over the past
30
three decades rainfall has decreased over eastern Africa between March and May/June. The suggested physical link to the decrease in rainfall is rapid warming of the Indian Ocean, which causes an increase in convection and precipitation over the tropical Indian Ocean and thus contributes to increased subsidence over eastern Africa and a decrease in rainfall during March to May/June (Funk et al. 2008). Similarly, Lyon and DeWitt (2012) show a decline in the March-May seasonal rainfall over eastern Africa. Summer (June–September) monsoonal precipitation has declined throughout much of the Great Horn of Africa over the past 60 years (during the 1948–2009 period; Williams et al. 2012) as a result of the changing sea level pressure (SLP) gradient between Sudan; the southern coast of the Mediterranean Sea and the southern tropical Indian Ocean region (Williams et al. 2012).
Over southern Africa a reduction in late summer precipitation has been reported over the western parts, extending from Namibia, through Angola, and towards the Congo, during the second half of the 20th century (Hoerling et al. 2006; New et al. 2006). The drying is associated with an upward trend in tropical Indian Ocean sea surface temperatures (SSTs). Modest reduction trends in rainfall are found in Botswana, Zimbabwe, and western South Africa. Apart from changes in total or mean summer rainfall, certain intra-seasonal characteristics of seasonal rainfall have changed, such as onset, duration, dry spell frequencies, rainfall intensity, and delay of rainfall onset (Kniveton et al. 2009; Tadross et al. 2009).
Changes in the distribution and magnitude of extreme rainfall events observed in many parts of Africa are associated with both climate change and variability (Williams et al. 2010), and these changes vary between sub-regions (Omondi et al. 2013; van de Giesen et al. 2010; Muthama et al. 2008). Desertification, desert encroachment and an alteration of hydrological regimes have been observed in several African ecosystems and regions (Odjugo 2010; Descroix et al. 2009; IPCC 2007). A shortage of water can have a damaging impact on vegetation, agricultural production and livelihoods, as
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