9.2 Current sensitivity/vulnerability
9.2.1 Current sensitivity to climate and weather
The climate of the continent is controlled by complex maritime and terrestrial interactions that produce a variety of climates across a range of regions, e.g., from the humid tropics to the hyper-arid Sahara (see Christensen et al., 2007). Climate exerts a significant control on the day-to-day economic development of Africa, particularly for the agricultural and water-resources sectors, at regional, local and household scales. Since the TAR, observed temperatures have indicated a greater warming trend since the 1960s. Although these trends seem to be consistent over the continent, the changes are not always uniform. For instance, decadal warming rates of 0.29°C in the African tropical forests (Malhi and Wright, 2004) and 0.1 to 0.3°C in South Africa (Kruger and Shongwe, 2004) have been observed. In South Africa and Ethiopia, minimum temperatures have increased slightly faster than maximum or mean temperatures (Conway et al., 2004; Kruger and Shongwe, 2004). Between 1961 and 2000, there was an increase in the number of warm spells over southern and western Africa, and a decrease in the number of extremely cold days (New et al., 2006). In eastern Africa, decreasing trends in temperature from weather stations located close to the coast or to major inland lakes have been observed (King’uyu et al., 2000).
For precipitation, the situation is more complicated. Rainfall exhibits notable spatial and temporal variability (e.g., Hulme et al., 2005). Interannual rainfall variability is large over most of Africa and, for some regions, multi-decadal variability is also substantial. In West Africa (4°-20°N; 20°W-40°E), a decline in annual rainfall has been observed since the end of the 1960s, with a decrease of 20 to 40% noted between the periods 1931-1960 and 1968-1990 (Nicholson et al., 2000; Chappell and Agnew, 2004; Dai et al., 2004). In the tropical rain-forest zone, declines in mean annual precipitation of around 4% in West Africa, 3% in North Congo and 2% in South Congo for the period 1960 to 1998 have been noted (e.g., Malhi and Wright, 2004). A 10% increase in annual rainfall along the Guinean coast during the last 30 years has, however, also been observed (Nicholson et al., 2000). In other regions, such as southern Africa, no long-term trend has been noted. Increased interannual variability has, however, been observed in the post-1970 period, with higher rainfall anomalies and more intense and widespread droughts reported (e.g., Richard et al., 2001; Fauchereau et al., 2003). In different parts of southern Africa (e.g., Angola, Namibia, Mozambique, Malawi, Zambia), a significant increase in heavy rainfall events has also been observed (Usman and Reason, 2004), including evidence for changes in seasonality and weather extremes (Tadross et al., 2005a; New et al., 2006). During recent decades, eastern Africa has been experiencing an intensifying dipole rainfall pattern on the decadal time-scale. The dipole is characterised by increasing rainfall over the northern sector and declining amounts over the southern sector (Schreck and Semazzi, 2004).
Advances in our understanding of the complex mechanisms responsible for rainfall variability have been made (see Reason et al., 2005; Warren et al., 2006; Washington and Preston, 2006; Christensen et al., 2007). Understanding how possible climate-regime changes (e.g., in El Niño-Southern Oscillation (ENSO) events) may influence future climate variability is critical in Africa and requires further research. The drying of the Sahel region since the 1970s has, for example, been linked to a positive trend in equatorial Indian Ocean sea-surface temperature (SST), while ENSO is a significant influence on rainfall at interannual scales (Giannini et al., 2003; Christensen et al., 2007). In the same region, the intensity and localisation of the African Easterly Jet (AEJ) and the Tropical Easterly Jet (TEJ) also influence rainfall variability (Nicholson and Grist, 2003), as well as SSTs in the Gulf of Guinea (Vizy and Cook, 2001), and a relationship has also been identified between the warm Mediterranean Sea and abundant rainfall (Rowell, 2003). The influence of ENSO decadal variations has also been recognised in south-west Africa, influenced in part by the North Atlantic Oscillation (NAO) (Nicholson and Selato, 2000). Changes in the ways these mechanisms influence regional weather patterns have been identified in southern Africa, where severe droughts have been linked to regional atmospheric-oceanic anomalies before the 1970s but to ENSO in more recent decades (Fauchereau et al., 2003).
Several studies also have highlighted the importance of terrestrial vegetation cover and the associated dynamic feedbacks on the physical climate (see Christensen et al., 2007). An increase in vegetation density, for example, has been suggested to result in a year-round cooling of 0.8°C in the tropics, including tropical areas of Africa (Bounoua et al., 2000). Complex feedback mechanisms, mainly due to deforestation/land-cover change and changes in atmospheric dust loadings, also play a role in climate variability, particularly for drought persistence in the Sahel and its surrounding areas (Wang and Eltahir, 2000, 2002; Nicholson, 2001; Semazzi and Song, 2001; Prospero and Lamb, 2003; Zeng, 2003). The complexity of the interactions precludes ‘simple interpretations’; for instance, the role of human-induced factors (e.g., migration), together with climate, can contribute to changes in vegetation in the Sahel that feed back into the overall physical system in complex ways (see, e.g., Eklundh and Olsson, 2003; Held et al., 2005; Herrmann et al., 2005; Olsson et al., 2005). Mineral dust is the largest cause of uncertainty in the radiative forcing of the planet and the key role of the Sahara has long been known. Better quantitative estimates of Saharan dust loadings and controls on emissions have now emerged from both satellite and field campaigns (e.g., Washington and Todd, 2005; Washington et al., 2006).
Finally, changes in extreme events, such as droughts and floods, have major implications for numerous Africans and require further attention. Droughts, notwithstanding current limitations in modelling capabilities and understanding of atmospheric system complexity, have attracted much interest over the past 30 years (AMCEN/UNEP, 2002), particularly with reference to impacts on both ecological systems and on society. Droughts have long contributed to human migration, cultural separation, population dislocation and the collapse of prehistoric and early historic societies (Pandey et al., 2003). One-third of the people in Africa live in drought-prone areas and are vulnerable to the impacts of droughts (World Water Forum, 2000). In Africa, for example, several million people regularly suffer impacts from droughts and floods. These impacts are often further exacerbated by health problems, particularly diarrhoea, cholera and malaria (Few et al., 2004). During the mid-1980s the economic losses from droughts totalled several hundred million U.S. dollars (Tarhule and Lamb, 2003). Droughts have mainly affected the Sahel, the Horn of Africa and southern Africa, particularly since the end of the 1960s (see Section 9.6.2; Richard et al., 2001; L’Hôte et al., 2002; Brooks, 2004; Christensen et al., 2007; Trenberth et al., 2007). Floods are also critical and impact on African development. Recurrent floods in some countries are linked, in some cases, with ENSO events. When such events occur, important economic and human losses result (e.g., in Mozambique – see Mirza, 2003; Obasi, 2005). Even countries located in dry areas (Algeria, Tunisia, Egypt, Somalia) have not been flood-free (Kabat et al., 2002).