Considerable attention has been devoted to the characteristics of systems (communities or regions) that influence their propensity or ability to adapt (as part of impact and vulnerability assessment) and/or their priority for adaptation measures (as a basis for policy development). These characteristics have been called determinants of adaptation. Generic concepts such as sensitivity, vulnerability, susceptibility, coping range, critical levels, adaptive capacity, stability, robustness, resilience, and flexibility have been used to differentiate systems according to their likelihood, need, or ability for adaptation (Sprengers et al., 1994; De Ruig, 1997; Klein and Tol, 1997; Smithers and Smit, 1997; Adger and Kelly, 1999; Kelly and Adger, 1999). These characteristics influence (promote, inhibit, stimulate, dampen, or exaggerate) the occurrence and nature of adaptations and thereby circumscribe the vulnerability of systems and their residual impacts. In the hazards literature, these characteristics are reflected in socially constructed or endogenous risks (Blaikie et al., 1994; Hewitt, 1997). Together (in whole or part), they represent the adaptive capacity of a system.

Table 18-5 lists terms that are commonly used to characterize the adaptive propensity of systems to climate stimuli. There is considerable overlap in the basic concepts captured in these terms. Particular terms have been employed to distinguish natural from socioeconomic systems or to differentiate the condition of a system before adaptation from its condition after adaptation (Klein and Nicholls, 1998). These distinctions are important and can be captured without narrowing the meaning of widely used terms. Thus, ecosystem vulnerability is different from socioeconomic vulnerability.

Table 18-5: Terms to describe characteristics of systems that are pertinent to adaptationa (from Smit et al., 1999).
Sensitivity	Degree to which a system is affected by or responsive to climate stimuli (note that sensitivity includes responsiveness to both problematic stimuli and beneficial stimuli)
Susceptibility	Degree to which a system is open, liable, or sensitive to climate stimuli (similar to sensitivity, with some connotations toward damage)
Vulnerability	Degree to which a system is susceptible to injury, damage, or harm (one part—the problematic or detrimental part—of sensitivity)
Impact Potential	Degree to which a system is sensitive or susceptible to climate stimuli (essentially synonymous with sensitivity)
Stability	Degree to which a system is not easily moved or modified
Robustness	Strength; degree to which a system is not given to influence
Resilience	Degree to which a system rebounds, recoups, or recovers from a stimulus
Resistance	Degree to which a system opposes or prevents an effect of a stimulus
Flexibility	Degree to which a system is pliable or compliant (similar to adaptability, but more absolute than relative)
Coping Ability	Degree to which a system can successfully grapple with a stimulus (similar to adaptability, but includes more than adaptive means of "grappling")
Responsiveness	Degree to which a system reacts to stimuli (broader than coping ability and adaptability because responses need not be "successful")
Adaptive Capacity	The potential or capability of a system to adapt to (to alter to better suit) climatic stimuli or their effects or impacts
Adaptability	The ability, competency, or capacity of a system to adapt to (to alter to better suit) climatic stimuli (essentially synonymous with adaptive capacity)
a These definitions of systems characteristics are based on widely (but not unanimously) held conventions. They focus on distinguishing generic properties and do not include factors that might influence the state of a property or the forms it might take. The terms "climate stimulus" and "system" are used as established earlier.

Adaptive capacity refers to the potential, capability, or ability of a system to adapt to climate change stimuli or their effects or impacts. Adaptive capacity greatly influences the vulnerability of communities and regions to climate change effects and hazards (Bohle et al., 1994; Downing et al., 1999; Kelly and Adger, 1999; Mileti, 1999; Kates, 2000). Vulnerability has been described as the "capacity to be wounded" (Kates et al., 1985). Human activities and groups are considered sensitive to climate to the degree that they can be affected by it and vulnerable to the degree that they can be harmed (Rayner and Malone, 1998). Because vulnerability and its causes play essential roles in determining impacts, understanding the dynamics of vulnerability is as important as understanding climate itself (Liverman, 1990; Handmer et al., 1999).

With regard to climate change, the vulnerability of a given system or society is a function of its physical exposure to climate change effects and its ability to adapt to these conditions. Chambers (1989) distinguishes between these two aspects of differential vulnerability: physical exposure to the hazardous agent and the ability to cope with its impacts. Thus, vulnerability recognizes the role of socioeconomic systems in amplifying or moderating the impacts of climate change and "emphasizes the degree to which the risks of climate catastrophe can be cushioned or ameliorated by adaptive actions that or can be brought within the reach of populations at risk" (Downing, 1991).

The significance of climate variation or change depends on the change itself and the characteristics of the society exposed to it (Ausubel, 1991a; Rayner and Malone, 1998; Munasinghe, 2000). These characteristics of society determine its adaptive capacity and its adaptability. Adaptive capacity refers to the ability to prepare for hazards and opportunities in advance (as in anticipatory adaptation) and to respond or cope with the effects (as in reactive adaptation).

Studies of similar hazardous events recurring at different times in a given region show vastly different consequences because of societal transformations that occurred between the events. For example, rainfall and temperature fluctuations in western Europe have far milder effects on human well-being today (society generally is less vulnerable) than they did in the medieval and early modern periods, essentially as a result of enhanced adaptive capacity that reflects changes in practices, economics, and government programs (Abel, 1976; De Vries, 1977; Rayner and Malone, 1998). Similarly, particular climate events or hazards can have "vastly different consequences for those on whom they infringe because of differences in coping ability" (Rayner and Malone, 1998). An extreme climatic event will result in higher losses of life in a developing country than in a developed country because of differential adaptive capacity (Burton, et al., 1993; Blaikie et al., 1994; Kundzewicz and Takeuchi, 1999). Martens et al. (1999) describe potential adaptations to deal with increases in disease incidence associated with climate change but note that in most poor developing countries, socioeconomic, technical, and political barriers will mean that the changed health risks will not be addressed.

" In developing countries overall social, environmental, and economic vulnerability enhances the effects of droughts and other climatic events. Overpopulation (relative to current productivity, income, and natural resources), poverty, and land degradation translate into a poor capacity to face any kind of crisis. Poor people have no insurance against loss of income. Weak economic structures mean difficulties in maintaining jobs during an economic failure. Degraded marginal lands become totally unproductive when precipitation decreases. As a result, these regions have difficulty in facing climatic crises, although such crises are recurrent. Any extreme climatic event can become a social catastrophe when combined with the social-political characteristics of the region. For example, the droughts and internecine wars in Ethiopia interact to increase the adverse effects of both. Although developing regions are more vulnerable to climate changes than are developed countries, the degree of vulnerability varies in each specific region." (Magalhães, 1996)

Research on comparative adaptive capacity and vulnerability is evolving, and its difficulties are well recognized (Bohle et al., 1994; Downing, 1996; Handmer et al., 1999; Kelly and Adger, 1999). Estimates of adaptive capacity tend to be based on premises such as the position that highly managed systems (such as agriculture), given sufficient resources, are likely to be more adaptable (and at a lower cost) than less managed ecosystems (Strzepek and Smith, 1995; Burton, 1996; Toman and Bierbaum, 1996). It is also widely accepted that systems with high levels of capacity to cope with historical and/or existing stresses can be expected to have high adaptive capacity for stresses associated with climatic change (Ausubel, 1991a). Such premises have formed the basis for broad assessments of sensitivity and adaptability (USNAS, 1992). Of course, sensitivity and adaptive capacity vary according to the climate change-related stress being considered. Thus, adaptive capacity to gradual changes in mean temperature may be high (or not much needed), but adaptive capacity to changes in the magnitude or frequency of extreme climatic conditions may not be so high (Appendi and Liverman, 1996).

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