adopted, including expert judgment, backcasting, and various modeling methods.

Scenarios can produce a number of benefits. One impact is to overcome cognitive barriers (e.g., optimism biases, strategic use of uncertainty, ambiguity aversion, status quo bias). Scenarios use various mechanisms to overcome the barriers. For example, they can focus on possibilities rather than predictions. There is some evidence that scenarios can actually reduce overconfidence and increase the coherence of beliefs, and in one study with firms, the use of scenarios was correlated with future profits.

Challenges in the use of scenarios for climate analysis lie in (a) the potential for divergent views on what scenarios are, potentially leading to an illusion of communication; (b) the tension between the desire for consistency and the need to consider surprises (e.g., formal models tend to leave out the discontinuities); (c) the need to include context in scenarios (the trade-off between simplicity and utility, the tendency to ignore scenarios when they can’t deal with the projected futures); and (d) the need to emphasize process over product in decision support (National Research Council, 2009a).

Scenarios for decision support can be framed as a way to analyze vulnerability under existing plans and response options. Stakeholders in a decision may disagree on much but still agree on the need to think through how and when an option may not work. A database of many model runs can help identify the key drivers of failure and the scenarios leading to failure, thus helping in the consideration of response options. For example, a group at RAND looked for climate scenarios that failed to reach a concentration target of 450 ppm and found that, in most of these cases, carbon capture and storage and transportation systems failed to meet their targets.

The literature indicates that vast arrays of scenario methods are used for many different purposes. Some empirical evidence exists on the factors affecting scenario effectiveness in various applications, and these studies emphasize the importance of process (rather than products) and of close coupling with decision makers as determinants of effectiveness. Lempert concluded that, for some purposes, people may want to think less about developing standard scenarios and narratives and more about developing tools that particular decision makers can use to identify multi-stressor vulnerabilities and to consider their decision options.



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