mid-2020s to be quite small. However, because of gaps in knowledge about the mechanisms and tipping points for the processes involved, it is difficult to assign a level of confidence to judgments about the preconditions for, or timing of, an instability affecting, for instance, a major section of a large ice sheet. Moreover, there may be other processes in the Earth system, not yet identified, that have tipping points that could lead to abrupt climate change. Because of such gaps in knowledge, the possibility of such events occurring in the next decade or so cannot be totally discounted.

Although the likelihood of such abrupt climate change scenarios happening in the next decade or so appears to be quite small, it cannot be estimated accurately. The consequences of some abrupt climate changes, if they occurred, could be quite severe. For example, droughts are a recurring event in the climate system, with a major drought occurring in some part of the world at least yearly. Some drought conditions, such as those that have continued off and on for the past decade in the western United States, could be harbingers of a mega-drought. If that proved to be the case in the western United States, the potential impacts on water supplies as well on as ecosystems, both natural and managed, in that region would be enormous. To the extent that the possible tipping elements leading to such major changes are known, it would be important to monitor those elements and the factors that affect them. It would also be important to monitor changes in the social, economic, and political factors that affect the size of the exposed populations, their susceptibility to harm, the ability of the populations to cope, and the ability of their governments to respond. Where potentially affected areas are important producers of key global commodities such as food grains, it would also be important to assess the effects of climate-induced supply reductions on global markets and vulnerable populations.

SINGLE EXTREME EVENTS

The fundamental science of climate change suggests that continued global warming will increase the frequency or intensity (or both) of a great variety of events that could disrupt societies, including heat waves, extreme precipitation events, floods, droughts, sea level rise, wildfires, and the spread of infectious disease. Underpinning many of these extreme events is an acceleration of the global hydrological cycle. For each 1.8°F (1°C) increase in the global mean surface temperature, there is a corresponding 7 percent increase in atmospheric water vapor. Because warm air holds more water vapor than cool air, this leads to more intense precipitation. Essentially, warm air increases evaporation from the ocean and dries out the land surface, providing more moisture to the atmosphere that will rain out downwind. Water vapor is also a powerful naturally occurring green-



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