in the physical parameterization schemes of the models (Collins, 2000). The models are not consistent in their projections of ENSO amplitude or frequency in the 21st century, and it is not yet possible to tell which models’ results are most credible. The IPCC study concluded that “it is not possible at this time to confidently predict whether ENSO activity will be enhanced or damped due to anthropogenic climate change” (Intergovernmental Panel on Climate Change, 2012:157). Climate scientists continue to explore links between increased greenhouse gas concentrations and ENSO because the fundamental principles of atmospheric science indicate that greenhouse warming will drive changes in ENSO.

It seems reasonable to infer from this research that ENSO is likely to change in coming decades, even though we cannot at this point determine which parameters will change or to what extent. In our judgment it would be worthwhile for the intelligence community to consider the security implications of a few of the scientifically plausible scenarios for how ENSO might change because changes in the amplitude and nature of ENSO constitute one of the few components of the coupled climate system capable of having a global synchronized impact. However ENSO changes, there will be simultaneous consequences in many places. For example, if the future will bring stronger El Niño events, as some models project, the results will likely include drought in Australia and excess precipitation across parts of the southern tier of the United States occurring in the same year. Such a confluence of events could affect global grain production in ways that could be disruptive, depending on other conditions affecting food supply and demand.

Conclusion

Over the next decade the most likely scenario for single extreme events is a continuation of the trends of recent decades—probably a slow rate of change for now, but with the possibility of a faster one later. Many of these trends involve the continuation of trends that are already being observed, such as those summarized in Table 3-1, such as a warming of days and nights and a trend toward heavier precipitation, with wet areas tending to get wetter and dry areas drier, and with more of the precipitation concentrated in heavy events, particularly in certain regions. The effects of the ENSO cycle, superimposed on these longer-term climate trends, will exacerbate some extremes and dampen others in complex ways. In addition to these events, there remains the possibility of unprecedented extreme events that might occur as a result of abrupt climate change or other climatic phenomena discussed later in the chapter.



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