The twenty-first century may bring the United States more of the weather it’s already got, whether wet or dry. The U.S. National Climate Assessment, issued in May 2014, examined multiple model projections of seasonal precipitation over the rest of this century. In general, precipitation is projected to increase in the northernmost parts of the country, and decrease in the southwestern United States.
These maps show projected seasonal precipitation changes for the final decades of this century (2071-2099) compared to the end of the last century (1970-1999) depending on two possible scenarios for greenhouse gas emissions. One scenario assumes that greenhouse gas emissions peak sometime between 2010 and 2020 and rapidly decline afterwards. The other scenario assumes that greenhouse gas emissions continue increasing throughout the 21st century.
Precipitation increases appear in shades of blue-green, and decreases appear in shades of brown. The darker the color, the greater the departure from 1970-1999 precipitation levels. White areas mean that any porjected changes are not larger than the existing range of natural variability. Not surprisingly, continually increasing greenhouse gas emissions are projected to produce greater changes (positive and negative) in seasonal precipitation.
With either rapid emissions reductions or continued emissions increases, changes in projected precipitation vary by season and especially by location. Although the effects are milder under a rapid-reduction scenario, all the projections show more precipitation in the North. In most cases, precipitation in the southwestern United States decreases. A notable exception to this general pattern is the summer seasonal projection for increasing emissions; in this scenario, precipitation also decreases in the Pacific Northwest.
The contiguous United States spans the transition zone between the relatively dry sub-tropics and the wetter high latitudes. Although models agree on wetter conditions in the north and drier conditions in the southwest, pinpointing the exact boundary where more or less precipitation will dominate is tricky for some parts of the country. In general, though, the National Climate Assessment authors warn, “The contrast between wet and dry areas will increase both in the U.S. and globally—in other words, the wet areas will get wetter ,and the dry areas will get drier.”
Walsh, J., D. Wuebbles, K. Hayhoe, J. Kossin, K. Kunkel, G. Stephens, P. Thorne, R. Vose, M. Wehner, J. Willis, D. Anderson, S. Doney, R. Feely, P. Hennon, V. Kharin, T. Knutson, F. Landerer, T. Lenton, J. Kennedy, and R. Somerville, 2014: Ch. 2: Our Changing Climate. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 19-67. doi:10.7930/J0KW5CXT.