Since the IPCC's Fourth Assessment Report (AR4), demand for knowledge regarding the planning and implementation of adaptation as a strategy for climate risk management has increased significantly (Preston et al., 2011a; Park et al., 2012). This chapter assesses recent literature on the opportunities that create enabling conditions for adaptation as well as the ancillary benefits that may arise from adaptive responses. It also assesses the literature on biophysical and socioeconomic constraints on adaptation and the potential for such constraints to pose limits to adaptation. Given the available evidence of observed and anticipated limits to adaptation, the chapter also discusses the ethical implications of adaptation limits and the literature on system transformational adaptation as a response to adaptation limits.

To facilitate this assessment, this chapter provides an explicit framework for conceptualizing opportunities, constraints, and limits (Section 16.2). In this framework, the core concepts including definitions of adaptation, vulnerability, and adaptive capacity are consistent with those used previously in the AR4 (Adger et al., 2007). However, the material in this chapter should be considered in conjunction with that of complementary WGII AR5 chapters. These include Chapter 14 (Adaptation Needs and Options), Chapter 15 (Adaptation Planning and Implementation), and Chapter 17 (Economics of Adaptation). Material from other WGII AR5 chapters is also relevant to informing adaptation opportunities, constraints, and limits, particularly Chapter 2 (Foundations for Decision Making) and Chapter 19 (Emergent Risks and Key Vulnerabilities). This chapter also synthesizes relevant material from each of the sectoral and regional chapters (Section 16.5).

To enhance its policy relevance, this chapter takes as its entry point the perspective of actors as they consider adaptation response strategies over near, medium, and longer terms (Eisenack and Stecker, 2012; Dow et al., 2013a,b). Actors may be individuals, communities, organizations, corporations, non-governmental organizations (NGOs), governmental agencies, or other entities responding to real or perceived climate-related stresses or opportunities as they pursue their objectives (Patt and Schröter, 2008; Blennow and Persson, 2009; Frank et al., 2011).