Megadroughts

Our rain gauge stands at 6.21 inches for the water year (in California, precipitation is measured from October 1 through September 30, since it is a Mediterranean climate), which is about at 38% of what we’d expect to this date. And there is not much more of a rainy season left. Some say we’re on pace for the driest year since 1580 and discussion has begun on whether we’re entering a megadrought.

The paleoclimatic history of California is a fascinating story which I am just starting to read up on. (I’ve just started geographer Lynn Ingram’s recent book, The West without Water: What Past Floods, Droughts, and Other Climatic Clues Tell Us about Tomorrow) In the past several millenia, the Pacific Southwest has seen droughts lasting decades to upwards of a couple centuries. One of the more dramatic pieces of evidence comes from geomorpologist Scott Stine’s work at Mono Lake. In 1941 Los Angeles started diverting water from its tributary streams, causing the water level in the lake to fall some 50 feet, which exposed a lot of lakebed. When hiking across the lakebed at one point, Stine found dozens of tree stumps. Jeffrey pines do not grow in the middle of lakes, and the conclusion follows that the lake level was much lower in the past due to prolonged drought. Radiocarbon dating on the tree stumps showed that these trees came from two periods of drought, one lasting about 140 years ending about AD 1100, and the other lasting at least a century ending about AD 1350.

It is interesting how megadroughts illustrate an aspect of climate change that is independent of anthropogenic global warming (there was not much in the way of greenhouse gas emissions in the 13th century) that is nevertheless quite worrisome. Megadroughts have occurred recently enough so that they are clearly part of the general pattern of climatic variability in California at this point in geological history. Unlike a millenium ago, the past century-and-a-half has been fairly benevolent in terms of California’s climate. How would we cope now with a 140-year long drought?