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Climate Change Assessment for Water Resources Region 03 South Atlantic -Gulf Annual precipitation totals have become more variable in recent years compared to earlier in the 201h century. Evidence has also been presented, but with limited consensus, of mildly increasing trends in the magnitude of annual and seasonal precipitation for parts of the study area. These results are seemingly contradicted by a number of studies that have shown decreasing trends in streamflow throughout the area, particularly since the 1970s. This paradox is discussed by Small et al. (2006), who attribute it largely to seasonal differences in the timing of the changes in precipitation vs. streamflow. The study authors evaluated watersheds that experienced minimal water withdrawals and /or transfers. Results presented here also suggest that increasing temperatures may also play a role in decreasing streamflows, despite the lack of corresponding precipitation decline. 3. Projected Climate Trends While historical data is essential to understanding current and future climate, non- stationarity in the data (i.e., a changing climate) dictates the use of supplemental information in long -term planning studies. In other words, the past may no longer be a good predictor of the future (Milly et al., 2008). Consequently, the scientific and engineering communities are actively using computer models of the Earth's atmosphere and associated thermodynamics to project future climate trends for use in water resources planning efforts. Although significant uncertainties are inherent in these model projections, the models, termed global climate models (GCMs), are widely accepted as representing the best available science on the subject, and have proven highly useful in planning as a supplement to historical data. A wealth of literature now exists on the use of GCMs across the globe. This section summarizes projected climate trends, as projected by GCMs, within the South Atlantic -Gulf Region identified in a review of recent peer- reviewed literature. The information presented should be considered an overview and, similar to Section 2 on observed climate trends, does not focus on attribution or causation of the projected climate trends or the causal relationships between climate variables. These relationships are complex and influenced by multiple natural and unnatural (i.e., anthropogenic greenhouse gas emissions) forcings that influence the Earth's climate system. Typical of projected climate studies, often specific (and sometimes multiple) greenhouse gas emission scenarios (or representative concentration pathways) are modeled by a single GCM (or ensemble of GCMs). The spectrum of scenarios offer a wide range of "climate futures" so each study's assumed emission scenario(s) are noted. When additional detail is needed, the reader is referred to the specific references cited, including the third NCA, which includes not only regional assessments, but also foundational resources related to climate science literacy, GCMs, and emission scenarios. Results of this review indicate a strong consensus in the scientific literature that air temperatures will trend sharply upward over the next century in the South Atlantic -Gulf Region. There is much less consensus on the future trending, or lack thereof, in precipitation and streamflow in the region. 3.1. Temperature GCMs have been used extensively to project future climate conditions across the country. At a national scale, model projections generally show a significant warming trend throughout the 21" USACE Institute for Water Resources 20 January 9, 2015