Southwest U.S. Experiencing Its Worse Drought in 1200 Years

A recent paper on the historic 22-year drought in the Southwest U.S., published in Nature Climate Change by Williams and colleagues, received a great deal of attention in the media. Even though it is not focused on the Sierra Nevada, it used data from the Sierra Nevada and also has important ramifications for the range; therefore, I thought it was appropriate to discuss it here. In addition, this blog provides the opportunity to present interesting and important details that may have been left out in the press accounts.

The yellow box on this map outlines the study area. This map also shows the vapor pressure deficit (VPD) anomaly, a measure of how 'thirsty' the air has been during the past twenty years relative to previous years. This anomalously high vapor pressure deficit is thought to be responsible for the outbreak of large fires in the region. 

One of the many strengths of this paper is that it focuses on soil moisture, rather than temperature or precipitation. Soil moisture is an interesting variable to look at because it integrates both temperature and precipitation and, also, because it is what the plants care about the most. It's easy enough to measure the moisture of the soil in today's forests, but how do you measure the moisture of soils from 1000 years ago? For this, the authors measured the widths of tree rings. Simply put, when soil moisture is high, trees grow quickly and lay down wide tree rings and when soil moisture is low, trees grow slowly and lay down narrow rings.

Tree ring width changes with the climate. Tree rings from a 16th century megadrought are narrower than adjacent rings. Photo credit: Daniel Griffin.

To calibrate tree ring width to soil moisture, the authors compared tree ring widths from modern trees to modern soil moisture measurements. Fortunately, there is a relatively long record of soil moisture measurements so there were sufficient data to develop a robust mathematical relationship between tree ring width and soil moisture.

The plot below compares the soil moisture predicted from the tree rings (black line) with the actual soil moisture (red line). The prediction lines up quite nicely with the actual measurements. Note that these are 'anomalies,' meaning that they are deviations from the average.

Once a mathematical relationship was established between ring width and soil moisture for the modern data, that relationship was used to calculate soil moisture back in time using much older trees. In their analysis, the authors found that soil moisture in the Southwest U.S. is presently at its lowest point in the past 1200 years.

This plot shows how soil moisture anomalies (ie., deviation from the average) have changed over the past 1200 years. Soil moisture in the Southwest U.S. is presently lower than during the major dips in the 12th and 16th centuries. The blue line at the right end of the data shows what the soil moisture would be without anthropogenic global warming.

An important question is whether this present, historic drought is a consequence of just natural variability or whether its unusual severity is due to anthropogenic climate change. The authors used the result from climate models to show that, without the effects of climate change, the period from 2000 - 2021 wouldn't be considered unusual with respect to past dry periods.

The most worrying part of this study is that there is a strong likelihood that the drought will continue into the near future. Using the durations of past droughts identified in the tree ring record, the authors conclude that the drought will likely continue into a 23rd year and that it could even last another 8 years. If this current drought lasts a total of 30 years - and the likelihood of that is not insignificant - the effects on the regional ecosystems and our infrastructure could be catastrophic. We've already seen how 22 years of drought have damaged forests in the Sierra due to fires, bark beetle infestations, and low soil moisture. Water supplies for food production and home use would also continue to get tighter, potentially leading to social and political unrest. A recent example of this comes from Syria; there is evidence that a drought in that region contributed to the ongoing civil war.

This is a plot of flow down the Colorado River - the dashed horizontal line represents the long-term average. Since about 1980, there has been a decline in the river's discharge, with many years well below the average. About 12% of the U.S. population relies on water from the Colorado River.