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Climate Change Assessment for Water Resources Region 03 South Atlantic -Gulf 1.1 A Note on the Water Resources Region Scale USACE and other resource management agencies require reliable, science -based methods for incorporating climate change information into the assessments that support water resources decisions and actions. Such planning assessments must quantify projections of future climate and hydrology. One common practice is to begin by developing relationships between the currently observed climate and the projected future possible climate over the assessment region. However, the numerical models producing these multiple projections of future possible climate were not designed to support these assessments for local -to- regional scale operations. This means that intervening steps have to be taken to correct obvious biases in the models' outputs and to make the outputs relevant at the scales where hydrologic resource assessments can take place. The commonly used name for these post - processing steps is "downscaling" because one step is using one or another method to spatially (and temporally) disaggregate or interpolate (or other) the results produced at the numerical climate models' native scale to the scale of the water resources assessment. The current generation of climate models, which includes the models used to generate some of the inputs described in this work, have a native scale on the order of one to two hundred kilometers on each side of the grids used to simulate climate for Earth, substantially too coarse for the watershed assessments needed to inform resource assessment questions and decisions. On the other hand, these questions and decisions should not be addressed with model inputs at scales so fine that they impart false precision to the assessment. False precision would appear by suggesting that the driving climate model information can usefully be downscaled, by any method, to individual river reaches and particular project locations, for example. The approach at USACE is to consider the questions in need of climate change information at the geospatial scale where the driving climate models retain the climate change signal. At present, USACE judges that the regional, sub - continental climate signals projected by the driving climate models are coherent and useful at the scale of the 2 -digit USGS HUC (Water Resources Region), and that confidence in the driving climate model outputs declines below the level of a reasonable trade -off between precision and accuracy for areas smaller than the scale of the 4 -digit HUC (Water Resources Subregion). Hence, these summaries group information at the Water Resources Region scale both to be guides into the climate change literature and to support the informational analyses USACE is conducting at the Water Resources Subregion scale. For Water Resources Region 03, both the 2 -digit and 4 -digit HUC boundaries are shown in Figure 1.2. 2. Observed Climate Trends Observed climate trends within the Water Resources Region 03 are presented in this section to generally characterize current, or past, climate in the study region. While the primary cause for global warming is attributed by the scientific community to human - induced increases in atmosphere levels of heat - trapping gases (Walsh et al., 2014) this section is not focused on attribution or cause (either natural or unnatural). Rather, it is specifically focused on the identification and detection of climate trends in the recent historical record. The interrelationships of Earth's climate system are complex and influenced by multiple natural and USACE Institute for Water Resources 6 January 9, 2015