Drought in the Southwest Can Last for Decades -- or Centuries

One of the interesting reports coming out of the American Geophysical Union conference this year was on "abrupt" climate change. For a long time the Dust Bowl droughts of the l930's, which were indeed severe, were considered the worst the Southwest could expect. But now, based on tree-ring and pollen "proxy" studies, scientists can with confidence state that the past two millenia suffered droughts considerably worse than we saw in the Dust Bowl. Even if we choose to ignore the 18 out of 19 general circulation models that forecast a poleward shift of the winds that bring the Southwest rains, we have reason to believe that droughts in our region can last for decades -- or centuries.

Here's a brief discussion from the paper (released by the US Climate Change Research Program) with a helpful graphic to follow [pp172]:

...a period of elevated aridity was found in the A.D. 900-1300 period that included four particularly widespread and prolonged multi-decadal megadroughts (Fig. 3.8). This epoch of large-scale elevated aridity was corroborated by a number of independent, widely scattered, proxy records of past drought in the West (Cook et al., 2004). In >addition, the four identified megadroughts agreed almost perfectly in timing with those identified by Woodhouse and Overpeck (1998), which were based on far fewer data.

These findings were rather sobering for the West because they (1) verified the occurrence of several past multidecadal megadroughts prior to 1600, (2) revealed an elevated background state of aridity that lasted approximately four centuries, and (3) demonstrated that there are no modern analogs to the A.D. 900-1300 period of elevated aridity and its accompanying megadroughts. This is clearly a cause for concern because the data demonstrate that the West has the capacity to enter into a prolonged state of dryness without the need for greenhouse gas forcing.