Cook's team ran the models a thousand times and determined the statistically best results, the so-called Monte Carlo technique. The results are consistent: more heat and greater evaporation mean less water in the soil. For the second half of this century, the results point to a state of unrelenting severe drought.

Their next step was to compare their 21st-century forecasts to the climate record of the past. For that they took the North American Drought Atlas (NADA), a well-regarded record of summer droughts based on tree rings that goes back 2000 years and translates directly to PDSI numbers.

Here are the results for the Southwest region since about 1750—approximately the historical record. The tree-ring record shows that the upcoming droughts would greatly exceed anything in the historical record. Note that California is part of this region.

The comparison for the Central Plains region is similar. Here it's interesting to note that in the mid-1800s, just before settlement began, this part of the West was known as the "Great American Desert." The study's projections show the area is likely to return to that state and then get worse.

Cook's team looked farther back in time, and compared their 21st-century forecasts against the last thousand years of the tree-ring drought record. That record is our best gauge of natural variation, how much the climate swings back and forth on its own. The graph, at the top of this post, shows a future state of persistent drought exceeding everything in the last millennium.

Severe "megadroughts" have happened in the past that lasted 10 years or longer. The authors fix the odds of one at about 12% during historical time. For the second half of this century, though, they put the chances of a megadrought at 80% or greater.

"Our results," they conclude, "point to a markedly drier future that falls far outside the contemporary experience of natural and human systems in Western North America." Because we already pump more water out of the ground than the rains put back, the outlook is grim under business as usual and "nearly identical," they say, under the more moderate scenario. It doesn't take much thought to realize the changes that may be in store for the residents and ecosystems of these areas.

Look for this paper, which is publicly readable, to be discussed wherever climate change is debated. The beauty of open-access science like this is that it helps keep arguments focused and gives everyone the chance to consult the evidence for themselves.