Global environmental change and sustainability science increasingly recognize the need to address the consequences of changes taking place in the structure and function of the biosphere. These changes raise questions such as: Who and what are vulnerable to the multiple environmental changes underway, and where? Research demonstrates that vulnerability is registered not by exposure to hazards (perturbations and stresses) alone but also resides in the sensitivity and resilience of the system experiencing such hazards. This recognition requires revisions and enlargements in the basic design of vulnerability assessments, including the capacity to treat coupled human-environment systems and those linkages within and without the systems that affect their vulnerability. A vulnerability framework for the assessment of coupled humanenvironment systems is presented. R esearch on global environmental change has significantly improved our understanding of the structure and function of the biosphere and the human impress on both (1). The emergence of ''sustainability science'' (2-4) builds toward an understanding of the human-environment condition with the dual objectives of meeting the needs of society while sustaining the life support systems of the planet. These objectives, in turn, require improved dialogue between science and decision making (5-8). The vulnerability of coupled human-environment systems is one of the central elements of this dialogue and sustainability research (6, 9-11). It directs attention to such questions as: Who and what are vulnerable to the multiple environmental and human changes underway, and where? How are these changes and their consequences attenuated or amplified by different human and environmental conditions? What can be done to reduce vulnerability to change? How may more resilient and adaptive communities and societies be built?Answers to these and related questions require conceptual frameworks that account for the vulnerability of coupled human-environment systems with diverse and complex linkages. Various expert communities have made considerable progress in pointing the way toward the design of these frameworks (10, 11). These advances are briefly reviewed here and, drawing on them, we present a conceptual framework of vulnerability developed by the Research and Assessment Systems for Sustainability Program (http:͞͞sust.harvard.edu) that produced the set of works in this Special Feature of PNAS. The framework aims to make vulnerability analysis consistent with the concerns of sustainability and global environmental change science. The case study by Turner et al. (12) in this issue of PNAS illustrates how the framework informs vulnerability assessments. The Emergence of Vulnerability AnalysisApproaches to and Composition of Vulnerability. Vulnerability is the degree to which a system, subsystem, or system component is likely to experience harm due to exposure to a hazard, either a perturbation or stress͞stressor. i This definition and the concept it addresses are not new (13); they ...

In this review, we highlight new insights into the conceptualization of the vulnerability of social-environmental systems and identify critical points of convergence of what otherwise might be characterized as disparate fields of research. We argue that a diversity of approaches to studying vulnerability is necessary in order to address the full complexity of the concept and that the approaches are in large part complementary. An emerging consensus on the issues of critical importance to vulnerability reduction-including concerns of equity and social justice-and growing synergy among conceptual frameworks promise even greater relevancy and utility for decision makers in the near future. We synthesize the current literature with an outline of core assessment components and key questions to guide the trajectory of future research.

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The vulnerability framework of the Research and Assessment Systems for Sustainability Program explicitly recognizes the coupled human-environment system and accounts for interactions in the coupling affecting the system's responses to hazards and its vulnerability. This paper illustrates the usefulness of the vulnerability framework through three case studies: the tropical southern Yucatá n, the arid Yaqui Valley of northwest Mexico, and the pan-Arctic. Together, these examples illustrate the role of external forces in reshaping the systems in question and their vulnerability to environmental hazards, as well as the different capacities of stakeholders, based on their access to social and biophysical capital, to respond to the changes and hazards. The framework proves useful in directing attention to the interacting parts of the coupled system and helps identify gaps in information and understanding relevant to reducing vulnerability in the systems as a whole.