17.4.2.1 Physical and ecological limits

There is increasing evidence from ecological studies that the resilience of coupled socio-ecological systems to climate change will depend on the rate and magnitude of climate change, and that there may be critical thresholds beyond which some systems may not be able to adapt to changing climate conditions without radically altering their functional state and system integrity (see examples in Chapter 1). Scheffer et al. (2001) and Steneck et al. (2002), for instance, find thresholds in the resilience of kelp forest ecosystems, coral reefs, rangelands and lakes affected both by climate change and other pollutants. Dramatic climatic changes may lead to transformations of the physical environment of a region that limit the possibilities for adaptation (Nicholls and Tol, 2006; Tol et al., 2006). For example, rapid sea-level rise that inundates islands and coastal settlements is likely to limit adaptation possibilities, with potential options being limited to migration (see Chapter 15, Barnett and Adger, 2003; Barnett, 2005). Tol et al. (2006) argue that it is technically possible to adapt to five metres of sea-level rise but that the resources required are so unevenly distributed that in reality this risk is outside the scope of adaptation. In the Sudano-Sahel region of Africa, persistent below-average rainfall and recurrent droughts in the late 20th century have constricted physical and ecological limits by contributing to land degradation, diminished livelihood opportunities, food insecurity, internal displacement of people, cross-border migrations and civil strife (Mortimore and Adams, 2001; Leary et al., 2006; Osman-Elasha et al., 2006). The loss of Arctic sea ice threatens the survival of polar bears, even if hunting of bears were to be reduced (Derocher et al., 2004). The loss of keystone species may cascade through the socio-ecological system, eventually influencing ecosystems services that humans rely on, including provisioning, regulating, cultural, and supporting services (Millennium Ecosystem Assessment, 2006).

The ecological literature has documented regime shifts in ecosystems associated with climatic changes and other drivers (Noss, 2001; Scheffer et al., 2001). These regime shifts are argued to impose limits on economic and social adaptation (van Vliet and Leemans, 2006). Economies and communities that are directly dependent on ecosystems such as fisheries and agricultural systems are likely to be more affected by sudden and dramatic switches and flips in ecosystems. In a review of social change and ecosystem shifts, Folke et al. (2005) show that there are significant challenges to resource management from ecosystem shifts and that these are often outside the experience of institutions. The loss of local knowledge associated with thresholds in ecological systems is a limit to the effectiveness of adaptation (Folke et al., 2005).