The paleoclimate record suggests multidecadal epochs of aridity (megadroughts) have occurred in the past; climate models suggest that they may be more likely. Understanding just how likely in vulnerable regions throughout the world comprises a key aspect of this research. Doing so, however, requires us to use a "wide lens" to try and characterize the underlying properties of the climate system over the last few millennia to better understand how frequently these events have occurred in the past, and how probable it is that they occur again.

Below is what we estimate for the risk of a decadal drought (10year, 1930's dust bowl severity):

This map shows the risk of a multidecadal megadrought (35year):

Because the map above map smooths out some of the local variability in megadrought risk, and because it only looks at the lower end of risk estimates (raw CMIP5), we went ahead and made an enlarged version of the southwest only (cf., Figure 11, panel (i) in Ault et al, 2014):

State by state data
At the link after this list of caveats are the raw data used to produce the first map shown above. But first, a few things to note:
(1) These are state-by-state averages from our results.

(2) They are for "decadal drought" (10 years): our study focused on this timescale, which sometimes gets called megadrought in the scientific literature, as well as multi-decadal megadrought (35 year).

(3) They are for the most severe warming scenario (RCP8.5) and apply mid-to-late century.

(4) They only apply to precipitation, making them "conservative" in the sense that evaporative demands from warmer temperatures could actually make the true levels of risk even higher.

(5) They do not factor in paleoclimate data because we have not turned the crank on this part of analysis for the coterminous US, only the southwest where there are high-quality tree-ring (and other) paleoclimate data that we could use. Factoring in paleoclimate data makes the levels of risk even higher in the Southwest.

(6) Overall, the risks are linked to the drying, which in turn is linked to the warming. In some ways, this shouldn't be surprising because drier average conditions load the dice to make megadroughts more likely.

(7) Each one is a range (+/- 1 standard deviation of the average). Averages below 5% just say <5%. Data are here:

Ault, T.R., S. St. George, J.E. Smerdon, S. Coats, J.S. Mankin, C.M. Carrillo, B.I. Cook, and S. Stevenson, 2018: A Robust Null Hypothesis for the Potential Causes of Megadrought in Western North America. J. Climate, 31, 3–24, (

Ault, T. R., Mankin, J. S., Cook, B. I., and Smerdon, J. E. (2016): Relative impacts of mitigation, temperature, and precipitation on 21st-century megadrought risk in the American Southwest. Science Advances, 2(10):e1600873 (

Cook, B.I., T.R. Ault, and J.E. Smerdon, 2015: Unprecedented 21st-century drought risk in the American Southwest and Central Plains. Sci. Adv., 1, no. 1, e1400082, doi:10.1126/sciadv.1400082.

Ault, T. R., J.E. Cole, J.T. Overpeck, G.T. Pederson and D. Meko, (2014): Assessing the risk of persistent drought using climate model simulations and paleoclimate data, Journal of Climate, doi: 10.1175/JCLI-D-12-00282.1 (

St. George, S., and Ault, T. R., (2014): The imprint of climate within Northern Hemisphere trees, Quaternary Science Reviews, 89, doi: 10.1016/j.quascirev.2014.01.007 (

St. George, S., Ault, T.R., and Torbenson, M (2013): The rarity of absent growth rings in Northern Hemisphere forests outside the American Southwest. Geophysical Research Letters, 40 (14), doi: 10.1002/grl.50743 (

Ault, T.R., J.E. Cole, G.T. Pederson, J.T, Overpeck, S. St. George, B. Otto-Bliesner, C. Deser, and C. Woodhouse (2013): The continuum of hydroclimate variability in western North America during the last millennium. Journal of Climate, 26 (16), doi: 10.1175/JCLI-D-11-00732.1 (

Ault, T.R., J.E. Cole, and S. St. George, (2012): The amplitude of decadal to multidecadal variability in precipitation simulated by state-of-the-art climate models. Geophysical Research Letters, 39 (21), L21 705, doi:10.1029/2012GL053424 (

St. George, S. and T.R. Ault (2011): Is energetic decadal variability a stable feature of the central Pacific Coast’s winter climate? Journal of Geophysical Research, 116, D12102, doi:10.1029/2010JD015325 (

Pederson, G.T., S.T. Gray, T.R. Ault, W. Marsh, D.B. Fagre, A.G. Bunn, C. Woodhouse, and L.J. Graumlich (2011): Climatic controls on the snow melt hydrology of the northern Rocky Mountains, USA. Journal of Climate, 24(6): 1666-1687, doi: 10.1175/2010JCLI3729.1 (

Truebe, S., T.R. Ault and J. E. Cole (2010): A forward model of cave drip-water δ18O and application to speleothem records. IOP Conference Series: Earth and Environmental Science, Vol. 9, 012022, doi: 10.1088/1755-1315/9/1/012022 (

Ault, T.R. and S. St George (2010): The magnitude of decadal and multi-decadal variability in North American precipitation. Journal of Climate, 23 (4): 842-850, doi: 10.1175/2009JCLI3013.1 (

Ault, T.R., J. E. Cole, M. N. Evans, H. Barnett, N.J. Abram, A.W. Tudhope, B.K. Linsley (2009): Intensified decadal variability in tropical climate during the late 19th century. Geophysical Research Letters, 36 (8), L08602, doi: 10.1029/2008GL036924.