house gas. As such it is the source of a very strong positive feedback to the coupled climate system that amplifies any external forcing by a factor of approximately 1.6.

This section discusses trends in some extreme climate events over the past half century, science-based expectations of the futures of these types of events, and the prospects for using the science of climate change to estimate the changing likelihood of such events and to forecast their occurrence.

Trends in Extreme Climate Events

The frequency of extreme high-temperature events driven by global warming is increasing faster than would be the case if only the mean temperature were increasing because the variance of the temperature distribution is increasing as well (see Figure 1-1). Extreme weather and climate events have been responsible for a rapidly increasing loss of lives, well-being, and economic assets in recent decades because of the confluence of the events themselves with increases in the numbers of people and value of property exposed and vulnerable to the events. It has been difficult to determine conclusively whether damaging climate events themselves have yet been increasing in frequency or intensity enough to be detected in trends of damage, normalized for nonclimate factors. We discuss this issue further in Chapter 4.

Climate Change and Extreme Climate Events in the Coming Decade

Effects of Climate Change on Extreme Events

The frequency and intensity of extreme events are particularly hard to project because, among other things, there are by definition few of them, which makes it hard to validate predictive models against experience (see Appendix D for more detailed discussion of statistical issues and methods for assessing the probabilities of occurrence of extreme events). Analyses have, however, converged on a number of expectations for this century, as noted in a recent National Research Council review (2010a). In this century, it stated, “the frequency and intensity of heat waves is projected to continue to increase, both in the United States and around the world,” and “the frequency of cold extremes and the number of frost days will decline in the middle and high latitudes” (p. 223). It is also projected that “the fraction of rainfall falling in the form of heavy precipitation events will increase in many regions” (p. 224). Furthermore, “[r]ecent model projections indicate growing certainty that climate change could lead to increases in the strength of hurricanes, but how their overall frequency of occurrence might change is still an active area of research,” and “projections indicate that



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