Seen Lake Mead lately? The bathtub ring - what locals call the white walls of the canyon covered with mineral deposits - keeps growing taller. If UCLA researchers are right, a confluence of natural circumstances will continue to make the bathtub ring grow - and that spells trouble for states that depend on Colorado River water.
UCLA's Glen MacDonald and a former graduate student of his, Abbie Tingstad, went to the Uinta Mountains region in northeastern Utah to examine tree rings in that region. Studying the rings of Pinyon Pines, they reconstructed about a millenium of snowpack and river flow information for the region - whose climate is representative of the upper Colorado River.
They compared their data to three already-available climate signifiers - variations in sea surface temperature. One was records for La Nina - a climate phenomenon where sea surface temperatures at the equator in the Pacific have cooled over time as much as 18 degrees or more (before cycling back upward). The second was records for the Pacific Decadal Oscillation, a monthly indicator of variable sea surface temperature in the North Pacific. The third was records for the Atlantic Multidecadal Oscillation - another sea surface temperature measurement that's variable, this one over a 20-40 year period.
For the PDO and the AMO, each of those indicators has a "positive phase" - when temperatures are up - and a "negative phase," when temperatures are down. Scientists speak of the strength of the La Nina effect. When the three of them line up just so, water gets scarce - and drought can get serious.
In the past, a warm phase in that Atlantic indicator, a cold phase in the Pacific indicator, and a strong phase for the La Nina effect have added up to - well, less water in the river. That's what co-authors Tingstad (now at RAND) and MacDonald (still in charge of the Institute of the Environment at UCLA) are worried could happen in the next year. Based on their numbers, they see that these three variable conditions could converge again - and when that did happen in the last century, diminished water flows were often the result.
Persistent drought for the last decade or so has left the level of water in Lake Mead at a smidge under 1100 feet above sea level. If Lake Mead drops more than 9 feet below that, the drop would trigger some belt-tightening for states with secondary claims to Colorado River water - Arizona and Nevada. California's got a primary claim on that river, so it doesn't risk the same limits. But southern California relies heavily on the water in this system, and predicting the length or duration of conditions contributing to water shortage is tricky business.
So, it's something to keep an eye on. Not a hole in our bucket per se, but worth checking nonetheless.