Future annual aridity index (AI, precipitation divided by potential evapotranspiration [PET]) was calculated from monthly temperature and precipitation data extracted from 10 General Circulation Models from the Coupled-Model Intercomparison Project, Phase 5 (CMIP5) downscaled to 1/8° spatial resolution (Maurer et al. 2007, Bureau of Reclamation 2013). The models were selected for their independence (Knutti et al. 2013) and performance, assessed by comparing model hindcast results and observed climate values in the Pacific Northwest (Rupp et al. 2013) and southwest United States (David E. Rupp, personal communication). We selected projections based on the highest representative concentration pathway, or emissions scenario, available for CMIP5 projections, RCP8.5, which assumes “baseline” response to climate change without mitigation (8.5 W/m2; Riahi et al. 2011). We used the median prediction (average of the median two models out of ten) for each cell for the four variables (minimum, maximum, and mean temperature and precipitation) to account for model variability. This dataset represents the aridification trends, AI change per year, based on the slope of change from 2016 to 2075 (60-yr period). Additional details: Potential Evapotranspiration (PET) = 0.0023 x Solar Radiation x (Mean Temperature + 17.8) x Temperature Range^0.5 (Hargreaves and Samani 1985). Monthly solar radiation was calculated using mid-month daily extra-terrestrial radiation accounting for both latitude and season with solar angle and daylight hours (Table 2.6, Eq. 21, Allen et al. 1998). General Circulation Models: CESM1-CAM5, HadGEM2-ES, HadGEM2-CC, CanESM2, CMCC-CM, MIROC5, IPSL-CM5A-MR, NorESM1-M, CSIRO-Mk3-6-0, CNRM-CM5. Allen, R. G., L. S. Pereira, D. Raes, and M. Smith. 1998. Crop evapotranspiration: guidelines for computing crop requirements. 56, FAO, Rome, Italy. Bureau of Reclamation. 2013. Downscaled CMIP3 and CMIP5 Climate and Hydrology Projections: Release of Downscaled CMIP5 Climate Projections, Comparison with preceding Information, and Summary of User Needs. U.S. Department of the Interior, Bureau of Reclamation, Technical Services Center, Denver, Colorado. Hargreaves, G. H. and Z. A. Samani. 1985. Reference crop evapotranspiration from temperature. Applied Engineering in Agriculture 1:96-99. Knutti, R., D. Masson, and A. Gettelman. 2013. Climate model genealogy: Generation CMIP5 and how we got there. Geophysical Research Letters 40:1194-1199. Maurer, E. P., L. Brekke, T. Pruitt, and P. B. Duffy. 2007. Fine-resolution climate projections enhance regional climate change impact studies. EOS, Transactions, American Geophysical Union 88:504. Riahi, K., S. Rao, V. Krey, C. Cho, V. Chirkov, G. Fischer, G. Kindermann, N. Nakicenovic, and P. Rafaj. 2011. RCP 8.5-A scenario of comparatively high greenhouse gas emissions. Climatic Change 109:33-57. Rupp, D. E., J. T. Abatzoglou, K. C. Hegewisch, and P. W. Mote. 2013. Evaluation of CMIP5 20th century climate simulations for the Pacific Northwest USA. Journal of Geophysical Research-Atmospheres 118:10884-10906.