22.214.171.124 Sensitivity/vulnerability of ecosystems
Ecosystems are critical in Africa, contributing significantly to biodiversity and human well-being (Biggs et al., 2004; Muriuki et al., 2005). The rich biodiversity in Africa, which occurs principally outside formally conserved areas, is under threat from climate variability and change and other stresses (see Chapter 4, Section 4.2). Africa’s social and economic development is constrained by climate change, habitat loss, over-harvesting of selected species, the spread of alien species, and activities such as hunting and deforestation, which threaten to undermine the integrity of the continent’s rich but fragile ecosystems (UNEP/GRID-Arendal, 2002; Thomas et al., 2004).
Approximately half of the sub-humid and semi-arid parts of the southern African region are at moderate to high risk of desertification (e.g., Reich et al., 2001; Biggs et al., 2004). In West Africa, the long-term decline in rainfall from the 1970s to the 1990s caused a 25-35 km southward shift of the Sahelian, Sudanese and Guinean ecological zones in the second half of the 20th century (Gonzalez, 2001). This has resulted in a loss of grassland and acacia, the loss of flora/fauna, and shifting sand-dunes in the Sahel (ECF and Potsdam Institute, 2004).
The 1997/1998 coral bleaching episode observed in the Indian Ocean and Red Sea was coupled to a strong ENSO. In the western Indian Ocean region, a 30% loss of corals resulted in reduced tourism in Mombasa and Zanzibar, and caused financial losses of about US$12-18 million (Payet and Obura, 2004). Coral reefs are also exposed to other local anthropogenic threats, including sedimentation, pollution and over-fishing, particularly when they are close to important human settlements such as towns and tourist resorts (Nelleman and Corcoran, 2006). Recent outbreaks of the ‘crown-of-thorns’ starfish have occurred in Egypt, Djibouti and western Somalia, along with some local bleaching (Kotb et al., 2004).
Observed changes in ecosystems are not solely attributable to climate. Additional factors, such as fire, invasive species and land-use change, interact and also produce change in several African locations (Muriuki et al., 2005). Sensitive mountain environments (e.g., Mt. Kilimanjaro, Mt. Ruwenzori) demonstrate the complex interlinkages between various atmospheric processes including solar radiation micro-scale processes, glacier-climate interactions, and the role of vegetation changes and climate interactions (Kaser et al., 2004). For example, the drop in atmospheric moisture at the end of the 19th century, and the drying conditions that then occurred, have been used to explain some of the observed glacier retreat on Kilimanjaro (Kaser et al., 2004). Ecosystem change, also induced by complex land-use/climate interactions, including the migration of species and the interaction with fire (e.g., Hemp, 2005), produces a number of feedbacks or ‘knock-on’ impacts. Changes in the range of plant and animal species, for example, are already occurring because of forest fires on Kilimanjaro, and may place additional pressure on ecosystem services (Agrawala, 2005). The loss of ‘cloud forests’ through fire since 1976 has resulted in an estimated 25% annual reduction in ‘fog water’ (the equivalent of the annual drinking water demand of 1 million people living on Kilimanjaro) and is another critical impact in this region (see Chapter 4, Section 4.2; Box 9.1; Agrawala, 2005; Hemp, 2005).
Box 9.1. Environmental changes on Mt. Kilimanjaro
There is evidence that climate is modifying natural mountain ecosystems via complex interactions and feedbacks including, for example, solar radiation micro-scale processes on Mt. Kilimanjaro (Mölg and Hardy, 2004; Lemke et al., 2007). Other drivers of change are also modifying environments on the mountain, including fire, vegetation changes and human modifications (Hemp, 2005). During the 20th century, the areal extent of Mt. Kilimanjaro’s ice fields decreased by about 80% (Figure 9.2). It has been suggested that if current climatological conditions persist, the remaining ice fields are likely to disappear between 2015 and 2020 (Thompson et al., 2002).
Figure 9.2. Decrease in surface area of Mt. Kilimanjaro glaciers from 1912 to 2003 (modified from Cullen et al., 2006).