the area affected by drought will probably increase in the decades ahead and that the number of dry days annually will also increase” (p. 265). The models are not highly specific about the timing of these changes, however.

A subsequent IPCC special report on extreme events (Intergovernmental Panel on Climate Change, 2012) summarized available evidence on observed and projected extremes of a variety of important types of climate events globally and at the regional level. Table 3-1 summarizes observed trends in several of these.1 The level of confidence in an observed trend neither implies nor excludes the possibility of changes in an extreme at other geographic scales. The report also offers projections of trends in these events, along with statements about the levels of confidence that the scientific community places in the projections.

Considering the difficulty of validating projections of extreme climate events, it would be a mistake to conclude from the lack of confidence in a projection of change for any type of extreme event that one can prudently act as if there will be no change. When there is good fundamental science behind an expectation of change—for example, in the frequency of extreme high-temperature and high-precipitation events or the likelihood of droughts—combined with noisy data or a small number of events for model validation, there may be sufficient reason for the intelligence community to develop and consider the security implications of scenarios in which the extreme event parameter changes in the direction suggested by the fundamental science.

Effects of Predictable Climate Variation on Extreme Events

Earth’s climate includes various regular cycles that may make it possible to anticipate an increased likelihood of climate events of concern months or longer in advance. Other than the succession of the seasons, the best understood of these is the ENSO (see Box 4-1). ENSO affects weather on most of Earth’s surface in cycles that last two to seven years. The largest and most predictable impacts are in the tropics. The warm phase of ENSO “is usually accompanied by drought in southeastern Asia, India, Australia, southeastern Africa, Amazonia, and northeast Brazil, with fewer than normal tropical cyclones around Australia and in the North Atlantic. Wetter than normal conditions during El Niño episodes are observed along the west


1 The confidence levels used in Table 3-1 are based on three scales: evidence and agreement; confidence; and likelihood (Mastrandrea et al., 2010). The summary terms used to describe the available evidence were limited, medium, or robust, and the degree of agreement was described as low, medium, or high. Levels of confidence were very low, low, medium, high, or very high. Likelihood (probability) was described as virtually certain (99–100%), very likely (90–100%), likely (66–100%), about as likely as not (33–66%), unlikely (0–33%), very unlikely (0–10%), and exceptionally unlikely (0–1%).

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