decade, the ability to anticipate specific climate events on that time-scale is still in its infancy. Improving our understanding of the ENSO cycle and its broader climatic implications currently offers the best route for extending our ability to make skillful forecasts of the risks of climate events from the current limit of a few weeks into the future to a few months or even one or two years. It will also be important to improve understanding of how the GHG-forced changes in climate might change the characteristics of climate variability, such as ENSO.

Despite their inability to predict specific events for the coming decade, climate models can still provide useful information about the range of plausible climate outcomes that could result from the combination of external forcing and variability of the climate system due to internal processes over the next decade. For example, several recent studies have evaluated the distribution of 10-year trends within model simulations that were forced with increasing concentrations of greenhouse gases (Easterling and Wehner, 2009; Santer et al., 2011). The purpose of these studies was to address claims that the lack of a significant positive global temperature trend over the 10-year period following 1998 refutes projections from climate models. And, indeed, the models did show that even under scenarios with relatively high greenhouse gas emissions, decades with a zero or negative temperature trend occur in more than 10 percent of the individual 10-year segments within models.

Figure 3-1 displays the probability distribution functions of 10-year trends from Easterling and Wehner (2009) for global mean temperatures, and it extends their method to look at the probability distributions of 10-year trends for a number of regions for the future (see Appendix C for an explanation of the method used for the regional projections). These functions were calculated using model simulations from the CMIP5 database under a scenario for the first half of the 21st century that assumes a “business as usual” rate of greenhouse gas increase of about 1 percent per year (known as the RCP8.5 scenario). The functions represent differences between projected average annual surface temperature, globally and for various regions, for the coming decade and the average temperature at the start of the decade, expressed in degrees Celsius. The estimates apply to any decadal start date up to 2040. Much as Easterling and Wehner (2009) found for global temperatures, each region has a reasonable chance of cooling over a period of a decade because of natural variability that can act to counteract the warming trend. At the same time, the upper tails of these distributions show that the increase in average global temperature could be as high as 1.8°F (1°C) in a single decade and the increase in average regional temperatures could be as high as 3.6°F (2°C) or more in a single decade— for example, about a 40 percent chance of an increase of at least 1.8°F (1°C) and about a 10 percent chance of an increase of at least 3.6°F (2°C)



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