The 2011 Texas Drought in a Historical Context
Texas climatologists have recently stated that the ongoing dry spell is the worst one-year drought since Texas rainfall data started being recorded in 1895. The majority of the state has earned the highest rating of “exceptional” drought and the remaining areas are not far behind with “extreme” or “severe” ratings by the U.S. Drought Monitor. So far, Texas has only received 6.5 inches of the 16 inches that has normally accumulated by this time of year.
|Figure 1: SOURCE: Brent McRoberts and John Nielsen-Gammon, Texas A&M University|
Streams throughout Texas are running well below normal and reservoirs are running at 50 percent of capacity. Only one boat ramp remains open between Lake Travis and Lake Buchanan and water levels are falling by a foot per week. For farmers and ranchers who depend on Mother Nature to provide water for their livestock and crops, this lack of water has been crippling. Agricultural losses have already mounted to a record 5.2 billion, and the drought has not yet broken.
Texas has suffered through mega-droughts in the past, so how does the current one compare?
Figure 1 shows the year of the worst 6-12 month drought for various areas in Texas. For 55.8 percent of the state, the current drought is the worst on record. No other drought was as bad in so many places. The previous standard for a one year drought, 1925, can now only be considered the worst ever in 14.6 percent of the state.
For July, the statewide Palmer Drought Severity Index (PDSI), which is a measure of dryness that takes both temperature and moisture into account, recorded its lowest ever reading. This surpassed the worst July readings for 1918, 1925 and 1956, the droughts of record in Texas.
Figure 2 shows whether the twelve-month precipitation amounts for the period 1896-2011 were above or below normal, and by how much. With less than six inches of rain since January and a 13 inch rainfall deficit since last August, this period of dryness is unprecedented in recorded Texas history, significantly below the previous minimum readings in the 115-year record.
Figure 2: Rainfall anomaly for the 12 months preceding July for the 1895-2011 period. DATA SOURCE: http://lwf.ncdc.noaa.gov/temp-and-precip/time-series/
However, the drought is not unprecedented in every way, and much longer droughts have occurred in the past. The worst extended drought remains the massive 1950’s event when Texas suffered under drought conditions for 10 years from the late 1940’s until the late 1950’s. In the 1918 case, severe dryness began in 1917 and peaked in 1918 before rebounding to wetter than normal conditions. As the current drought has only been ongoing for the past 6-12 months, it can only be described as the most acute in Texas history; it is nowhere near the longest—yet. With the possibility of another La Nina developing in the Pacific—an event historically correlated with Texas drought—there is no sign that the current drought will break anytime soon, but it is impossible to predict whether its duration will ultimately match its intensity.
Even though we can’t predict what will happen with individual droughts, Figure 2 does communicate useful information about drought risk in Texas going forward. The record shows that 10-year droughts are possible. Going back even further in time, climate data from tree rings shows that in the past, Texas has suffered through droughts that are measured in multiple decades.
As for the future, there is 80 percent agreement among climate models that Texas soils will get drier over this century if greenhouse gas emissions continue to grow (Figure 2).
Figure 3: Multi-model mean changes in soil moisture content. Stippled areas indicate at least 80 percent agreement among models agree on the direction of change. Changes are annual means for the SRES A1B scenario for the period 2080 to 2099 relative to 1980 to 1999. Changes are shown at land points with valid data from at least 10 models. SOURCE: http://ipcc.ch/publications_and_data/ar4/wg1/en/figure-10-12.html.
What we know from the past climate record, how the trend fits with our physical understanding of climate change, and what climate models project for the future, is strong evidence of an increase in drought risk that must be managed to avoid increasing costs to citizens, communities, and businesses of the Lone Star state. Residents, water managers, and community leaders in Texas would do well to both prepare for the possibility that the current drought will last longer than anticipated and that the future climate in Texas will be at risk of more severe and longer droughts (regardless of how long the current drought continues). The current drought represents an opportunity for Texans to identify drought adaptations that will allow them to better respond to the increased risks of a drier future.
Check the C2ES's work on climate change and the risks of extreme weather.
Dan Huber is a Science & Policy Fellow