The past is never dead. The past is not even past.
When William Faulkner wrote that, he was thinking of human history, but it's true on here on planet earth as well. Cycles repeat. For that reason, and because they were troubled by the drought they saw in the deep time record, paleoclimatologist B. Lynn Ingram and a fellow researcher at UC Berkeley set out to present the full record of climate extremes in the Southwest to the public in a new book, The West Without Water.
How extreme is this year in California's climate history? To answer this, we need to look back further than the 119 years we have on record, to the geologic past. Based on the growth rings of trees cored throughout the Western United States, AD 1580 stands out as the driest year in the last half a millennium, drier than 1976-77. It was so devastatingly dry in 1580 that the giant sequoias in the Sierra Nevada essentially failed to grow at all; the cores show either extremely thin or absent tree rings. If the current drought continues in California through Oct. 1, this water year will be the driest not only in our modern records but in half a millennium.
But that's actually a little bit reassuring, because it implies that this sort of drought, when eventually it is over, at least will not return soon. For perhaps as long as 500 years.
But what if it doesn't quit?
In an interview, Ingram gets even scarier:
If you look at the archaeological record, you see that the Native American population in the West expanded in the wet years that preceded those long droughts in the Medieval period. Then during the droughts, they were pretty much wiped out. There was the so-called Anasazi collapse in the Southwest about 800 years ago. In some ways, I see that as an analogy to us today. We’ve had this wetter 150 years and we’ve expanded. Now we’re using up all the available water, yet our population is still growing.
We’re vulnerable just like they were, but on an even larger scale.
[Image of groundwater levels in Central Valley from GRACE, NASA's gravity-measurement satellite. Red line shows groundwater levels from l962 based on USGS measurements — green line shows satellite measurements since 2003]