Adaptations occur in something (i.e., who or what adapts?), which is called the "system of interest," "unit of analysis," "exposure unit," "activity of interest," or "sensitive system" (Carter et al., 1994; Smithers and Smit, 1997; UNEP, 1998; Reilly and Schimmelpfennig, 2000). In unmanaged natural systems, adaptation is autonomous and reactive and is the means by which species and communities respond to changed conditions. In these situations, adaptation assessment is essentially equivalent to natural system impact assessment (addressed in other WGII chapters). This chapter focuses on adaptations consciously undertaken by humans, including those in economic sectors, settlements, communities, regions, and managed ecosystems.

Human system adaptation can be motivated by private or public interest (i.e., who adapts?). Private decisionmakers include individuals, households, businesses, and corporations; public interests are served by governments at all levels. The roles of public and private participants are distinct but not unrelated. Figure 18-5 shows examples of types of adaptation differentiated according to timing, natural or human systems, and public or private decisionmakers.

Planned adaptation often is interpreted as the result of a deliberate policy decision on the part of a public agency, based on an awareness that conditions are about to change or have changed and that action is required to minimize losses or benefit from opportunities (Pittock and Jones, 2000). Autonomous adaptations are widely interpreted as initiatives by private actors rather than by governments, usually triggered by market or welfare changes induced by actual or anticipated climate change (Leary, 1999). Smith et al. (1996) describe autonomous adaptations as those that occur "naturally," without interventions by public agencies, whereas planned adaptations are called "intervention strategies." Thus defined, autonomous and planned adaptation largely correspond with private and public adaptation, respectively (see Figure 18-5).

The extent to which society can rely on autonomous, private, or market adaptation to reduce the costs of climate change impacts to an acceptable or nondangerous level is an issue of great interest. Autonomous adaptation forms a baseline against which the need for planned anticipatory adaptation can be evaluated.
Distinguishing among the various decisionmakers involved in adaptation is important. The case of African agriculture and water resources illustrates that stakeholders and potential adapters range from vulnerable consumers to international organizations charged with relief and research (Eele, 1996; Magadza, 1996; Downing et al., 1997). Poor and landless households have limited resources, yet failure to adapt can lead to significant deprivation, displacement, morbidity, and mortality. Subsistence farmers do not have the same adaptation options as commercial producers. Water supply adaptations may involve landowners, private traders, local authorities, water-dependent businesses, national governments, and international organizations. Each stakeholder has distinct interests, information, risks, and resources and hence would consider distinct types of adaptive responses (Downing et al., 1997).

In order to predict autonomous adaptations and provide input to adaptation policies, there is a need for improved knowledge about processes involved in adaptation decisions. This knowledge includes information on steps in the process, decision rationales, handling of uncertainties, choices of adaptation types and timing, conditions that stimulate or dampen adaptation, and the consequences or performance of adaptation strategies or measures (Burton, 1997; Rayner and Malone, 1998; Tol et al., 1998; Basher, 1999; Klein et al., 1999; Pittock, 1999; Smit et al., 1999).

Decisions regarding adaptations can be undertaken at any of several scales, by private individuals, local communities or institutions, national governments, and international organizations. Where these adaptations are consciously planned activities, whether by public agencies or individuals, there is an interest in assessing the performance or relative merits of alternative measures and strategies (see Figure 18-4). This evaluation (i.e., how good is the adaptation?) can be based on criteria such as costs, benefits, equity, efficiency, and implementability (see Sections 18.3.5 and 18.4.3).

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