In 1878, John Wesley Powell assigned the 100th meridian line as the transition between the humid, moist east and the arid, dry west. A lot has been written about how the 100th is “moving” east due to climate change. The 100th marks an imaginary north-south line that follows the right-hand side of the Texas Panhandle down through the vicinities of Abilene, San Angelo, Uvalde, and Del Rio-Laredo. Some suggest that the 98th is the new 100th, with the 98th cutting near Mineral Wells, Stephenville, Lake Travis, Seguin, Karnes City, Kingsville, and Weslaco. This two-degree longitudinal difference suggests that the center of Dallas-Fort Worth and Waco now look more or less like Brownwood, Austin looks like Junction, San Antonio looks like Brackettville, and Houston looks like Bastrop.
I don’t know about that. First, Powell’s 100th is too simple a feature to definitively use as a demarcation between the moist east and the dry west. In his original writings, Powell noted that there was “a zone of semiarid land” defined by “about” 18 inches of rainfall on the west and “about” 24 inches on the east, with the 100th centered on this transition zone. In other words, he (1) recognized that there was a transition and (2) used rainfall to define his fuzzy edges.
Based purely on rainfall, the 100th is still the 100th. If anything, the 100th may have “moved” west rather than south, according to Powell’s definition, since the long-term rainfall trends in Texas are slightly up. With increased temperatures and perhaps a change in rainfall intensity, local water availability may have (and likely has) changed, probably downward. This would likely move the semiarid zone to the east, as defined by plant life and water availability.
Figure 1: Average annual maximum temperature (from the 2012 State Water Plan).
I suggest that instead of looking west for what we may become under a warming climate, we should look south. Why south? Two reasons. The first reason is because lines of average annual temperature (ye olde isotherms) tend to be oriented east and west, not north and south (Figure 1, although this shows average annual maximum temperatures). In other words, it gets warmer as we head south, not west. Unfortunately for this analysis, temperatures aren’t perfectly oriented east and west—there are topographic and coastal effects (why does no one cooperate?). But for this thought exercise, it’s close enough for a blog post.
Figure 2: Average annual precipitation (from the 2012 State Water Plan).
The second reason for looking south instead of west is because rainfall in Texas is not decreasing (as previously mentioned), and projections suggest that rainfall will remain about the same. Furthermore, lines of constant average annual precipitation (isohyets, pronounced eye-soh-high-et) tend to run north and south (Figure 2) with, again, some topographic and coastal effects.
I’ve combined these two figures into one, smoothed out the isohyets, and removed Far West Texas from consideration due to topographic effects on temperature and rainfall (Figure 3). I’ve also shown what some cities “look like” for the same rainfall amounts but following higher temperatures south. Note that my dots show temperature increases in Celsius. I did this because most people are aware of the 1.5 ºC to 2 ºC goals for the Paris Agreement on limiting global warming. One degree of change in Celsius equals 1.8 degrees of change in Fahrenheit. I am also assuming that those changes in average temperatures will apply to the figure, which represents average annual maximum temperatures (I’m starting to sound like an economist with all these assumptions!). The 1 ºC dot approximates where we are today (average annual temperature increases in Texas are about the same as the global increases; the world is actually a little warmer than 1 ºC these days, but that’s a discussion for another post/day). The numbers for 1.5 ºC and 2 ºC relate to the Paris Agreement, and 2.5 ºC is just over the path the world is on now with concrete actions to limit greenhouse gas emissions and assuming a well-behaved climate.
Figure 3: Average annual precipitation superposed on top of average annual maximum temperature, showing several cities with assumed levels of warming (figure modified from the 2012 State Water Plan).
Using this approach (with all its warts and assumptions) suggests that Dallas-Fort Worth looks more like Waco today than before 1 ºC of warming. A warming of 1.5 ºC pulls DFW down to the greater Austin area, while 2 ºC looks like the area halfway between San Antonio and Victoria. A warming of 2.5 ºC has Big D and Cowtown looking like Corpus Christi.
Today, Austin looks like the area halfway between San Antonio and Victoria. With 1.5 ºC of warming, Austin starts looking like the area halfway between San Antonio and Corpus Christi; at 2 ºC, it starts looking like Corpus Christi, and at 2.5 ºC, it’ll start looking like halfway between Corpus Christi and Brownsville. At present, San Antonio looks like the area halfway between it and Corpus Christi and looks like Corpus Christi at 1.5 ºC, then at 2 ºC, it looks like halfway between Corpus Christi and Brownsville. A warming of 2.5 ºC places San Antonio’s warmed climate south of the Rio Grande.
Today, San Angelo looks more like Del Rio and then heads toward Presidio with increased warming up to 2.5 ºC. Amarillo looks more like the Lubbock area today and through 2.5 ºC (not sure you’d be able to tell the difference…). Texarkana heads south to the Houston area as it warms up to 2.5 ºC. I’m unable to show Houston because warmer temperatures place it out in the Gulf of Mexico with this approach (although, to a certain degree, with sea level rise, there is some truth to this…). I can’t do El Paso unless I dig up precipitation and temperature maps for Mexico. I would need a round of margaritas to do that.
Although this analysis is flawed in many ways (you really need to use climate modeling to address local conditions in the most accurate way), I think it is useful and perhaps more appropriate than the “look-liking” done by peering west. Water-wise, Texarkana seems resilient—looking like Houston (hydrologically speaking) isn’t such a bad thing (except for that humidity). However, DFW, Austin, and San Antonio looking more like Corpus Christi, and the Lower Rio Grande Valley would not be so good. Amarillo’s and San Angelo’s movements to the south don’t seem too bad, and Wichita Falls – if it looks more like Austin – might actually be an improvement (Keep Wichita Weird!).
The central part of the state extending from DFW through San Antonio may simply be more sensitive to temperature changes than the eastern (already really wet) and western (already really dry) parts. Perhaps this sensitivity is a better way of designating the transition zone from the wetter east to the drier west. If that’s the case, then that transition area between 27 and 37 inches of rain always approximated the 98th (sorry, Mr. Powell). This also means that we should be more attentive to the vulnerabilities to water supplies in this region.