Methods and Tools 2 Q . K . AHMAD ( BANGLADESH ) AND RICHARD A . WARRICK

2. 6. Characterizing Uncertainty and " Levels of Confidence " in Climate A s s e s s m e n t 1 2 6 2. 6. 1. Treatments of Uncertainties in Previous IPCC A s s e s s m e n t s 1 2 7 2. 6. 2. " Objective " and " Subjective " Probabilities are not Always Explicitly Distinguished 1 2 9 2. 6. 3. Making Estimates 1 2 9 2.6.4. Aggregation and the Cascade of Uncertainty 1 3 0 2. 6. 5. The Debate over the Quality of Human Judgment 1 3 1 2. 6. 6. Building Experience with Subjective Methods in a " Science for Policy " A s s e s s m e n t 1 3 2 A d a p t a t i o n to Integrated Climate Change Decisions on Balancing Mitigation and A d a p t a t i o n 1 3 7 CONTENTS The purpose of this chapter is to address several overarching methodological issues that transcend individual sectoral and regional concerns. In so doing, this chapter focuses on five related questions: How can current effects of climate change be detected? How can future effects of climate change be anticipated, estimated, and integrated? How can impacts and adaptations be valued and costed? How can uncertainties be expressed and characterized? What frameworks are available for decisionmaking? In addressing these questions, each section of Chapter 2 seeks to identify methodological developments since the Second Assessment Report (SAR) and to identify gaps and needs for further development of methods and tools. Assessment of the impacts on human and natural systems that already have occurred as a result of recent climate change is an important complement to model projections of future impacts. Such detection is impeded by multiple, often inter-correlated, nonclimatic forces that are concurrently affecting those systems. Attempts to overcome this problem have involved the use of indicator species to detect responses to climate change and to infer more general impacts of climate change on natural systems. An important component of the detection process is the search for systematic patterns of change across many studies that are consistent with expectations, based on observed or predicted changes in climate. Confidence in attribution of these observed changes to climate change increases as studies are replicated across diverse systems and geographic regions. Since the SAR, approaches to analysing and …