Extreme Weather, Climate Change, and Attribution

When extreme weather occurs, the question often arises whether the event was produced by climate change. Two types of errors are possible when attempting to answer this question. One type of error involves underestimating the role of climate change and thereby failing to properly alert the public. The second type of error arises from overestimating the role of climate change and thereby desensitizing the public with false alarms. Long before public concerns about global warming became widespread, meteorologists have been addressing essentially the same tradeoff when faced with a binary decision of whether to issue a warning for hazardous weather. Here we review forecast verification statistics such as the probability of detection (POD) and the false alarm rate (FAR) in the context of tornado warnings. Empirical and theoretical evidence suggests that adjusting warning thresholds in an attempt to reduce FAR produces even larger reductions in POD. Similar tradeoffs between improving FAR and degrading of POD are shown to apply to the attribution of extreme high temperatures to climate change. Moreover, conventional forecast scores like FAR can be directly related to metrics commonly used in attribution of extreme events under climate change, such as the fraction of attributable risk (which is also abbreviated as FAR). The experiences of the weather forecasting community can thereby help inform those attempting to set practical thresholds for the attribution of extreme events to climate change.