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Megadroughts

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Posted: Thursday, August 4, 2011 5:00 am | Updated: 9:32 am, Thu Aug 4, 2011.

Everything is bigger in Texas, including droughts

 

By BOB VARMETTE

Pioneer staff writer

 

An Internet search for "Texas drought" will return numerous pages of news stories, on subjects ranging from forecast agricultural losses to the challenges encountered by river tubers.

Crop and livestock losses could double the record of $4.1 billion set in 2006, according to the Texas AgriLife Extension Service. Meanwhile, several Central Texas businesses dependent on river tubers are facing uncertain futures as the ongoing drought exposes obstacles and river bottoms that render tubing unsafe.

There are stories of wildfires that continue to plague the state, cattle being sold and how the drought is adversely affecting wildlife and insects.

It has been uniformly dry across Texas, with 248 counties having enacted burn bans. Fort Stockton-Pecos County Airport has reported just .57 inches of precipitation in 2011 -- and since Oct. 1, 2010 -- with .3 inches coming in one day: July 22.

Fort Stockton has actually fared better than other areas of the Permian Basin. Midland International Airport is currently in its driest 10-month period in the history of local weather observations with just .18 inches of precipitation since Oct. 1, 2010.

Worse, there apparently is not any substantial relief forecast in the near future.

The National Weather Service Climate Prediction Center in its latest discussion issued July 21 expects much of Texas, including the Permian Basin, to see drought conditions that "persist or intensify" through October.

The CPC's outlook forecasts the possibility there may be "some improvement" in the drought situation in parts of Far West Texas.

In its latest assessment of the 2010 drought, CPC said the Summer 2010-Spring 2011 La Nina event is the primary cause for the abnormally dry conditions across the southern U.S., "promoting widespread drought development and intensification."

For the last several weeks, it adds, a persistent ridge of high pressure centered over the southern Great Plains has consistently blocked or weakened cold fronts that might have brought relief.

That ridge was also responsible for the rapid demise of Tropical Storm Don, which was hoped to deliver some drought relief to South Texas and parts of West Texas last weekend, but only produced scattered showers and thunderstorms.

The failure of the monsoon to bring little more than higher dew points to West Texas and New Mexico has also been instrumental in the area's nearly nonexistent rainfall, CPC said.

Because West Texas and New Mexico have seen so little precipitation during the spring and summer, CPC forecasts for the eastern monsoon region, which includes Texas, are not enthusiastic for the next three months.

 

PAST IS KEY TO PRESENT

 

Weather is variable from day to day or week to week. Climate also changes, but the variability is measured in years, decades, centuries or millennia.

While the U.S. Southwest has been in a warming and drying trend since the end of the Last Glacial Maximum, there have been periods where the climate was cooler and wetter.

Barry Saltzman in his textbook "Dynamical Paleoclimatology" notes that climate variability has actually been the norm during the 4.6 billion years of Earth's existence.

"(T)here has always been so much variability of climate that it seems doubtful that we can ever speak of a single climatic norm for the Earth," Saltzman writes. "Judging from the past, the climate we are experiencing today is almost certain to be transient, giving way to something different in the future."

Just as this year -- a drought year -- follows last year -- a year of slightly more than average precipitation in Fort Stockton -- dry climatic periods follow wet ones, which were preceded by dry climate regimes.

It is likely of no comfort to agricultural producers or river tubing entrepreneurs, but the current drought, or the worst droughts of the 20th century, nowhere approach the severity of droughts that have occurred in Texas earlier in the current Holocene.

In Texas, increasing aridity is the product of enhanced anti-cyclonic circulation aloft over the Gulf of Mexico and reduced lower-level moisture in the southern Great Plains, according to Noah Diffenbaugh and others in "Summer aridity in the United States: Response to mid-Holocene changes in insolation and sea surface temperature."

Diffenbaugh et al add insolation (incoming solar radiation) is likely the most important cause of summer aridity in the U.S., and prior to the Industrial Revolution external climate forcing was largely controlled by "seasonality and latitudinal distribution of insolation."

Combined, they helped produce what are called megadroughts, and these most severe of droughts have been numerous in the last 10,000 years in the North American Great Plains.

"(T)he intensity, duration, and frequency of 20th-century drought are dwarfed by those inferred from paleoclimatic records," according to Jian Tian, David Nelson and Fen Sheng Hu in "Possible linkages of late-Holocene drought in the North American midcontinent to the Pacific Decadal Oscillation and solar activity."

The variability in drought occurrence, as shown by oxygen isotope studies in Minnesota, was abnormally low during the 20th century, they argue, and the low variability is not typical when compared to the last 3,100 years.

Further, they add, the drought occurrence pattern of the 20th century should not be the standard for future drought events and their timing or duration.

The droughts that plagued Minnesota also affected the southern Great Plains into Texas. Moreover, the Great Plains in the mid-Holocene saw the development of a north-to-south gradient of increasing aridity and attendant decreases in surface-water availability and resource abundance, reports David Meltzer in "Human Reponses to Middle Holocene (Altithermal) Climates in the North American Great Plains."

Based upon pollen and plant remains records, Meltzer believes there was considerable change in vegetation, both in density and type. The southern Great Plains, into most of Texas, saw a transformation to a "desert-plains grassland."

 

ALTITHERMAL

 

The Altithermal, also known as the Holocene Climatic Optimum, the Hypsithermal, the Holocene Thermal Maximum and Holocene Megathermal, among others, was a warm climatic period from the early to mid-Holocene.

Michelle Rich, in "Temporal and Spatial Analysis of Paleoindian and Archaic Site Distribution in the Chihuahuan Desert Region of Texas" pegs the period of the Altithermal in the Trans Pecos as from about 7,400 to 4,500 years BP.

Some areas of northern North America saw average temperature increases of nearly eight degrees Fahrenheit. However, there was likely a declining gradient of average annual temperature increases from north to south across the continent, according to Michael Gagan and others in "Temperature and Surface-Ocean Water Balance of the Mid-Holocene Tropical Western Pacific," where they report little change in mean temperatures at low and middle latitudes based upon tropical reef proxy records that reveal temperature increases of less than two degrees Fahrenheit.

While the Texas climate may not have been much warmer than present, it was certainly drier, with substantial archeological and biological evidence having been uncovered.

"The climate of the Southern High Plains during the middle Holocene is indicated by data from a variety of sources," Vance Holliday reports in "Middle Holocene Drought on the Southern High Plains," noting significant stratigraphic, geomorphic and soil research in the area. "Climate models and very limited isotopic data from Bison bone suggest that summers in the middle Holocene were warmer than present, with reduced effective precipitation.

"All lines of evidence indicate that the Southern High Plains was subjected to prolonged drought in the middle Holocene with a maximum between 6500 and 4500 yr B.P., conforming climatically and chronologically to the Altithermal."

Holliday adds that widespread aeolian erosion may have begun as early as 9,000 years BP.

Robert Mallouf in "West Texas Traveler Magazine" hypothesizes the Altithermal may have begun and ended later, with the change to a wetter climate being completed about 2,000 years BP.

Mallouf does concur with Rich and Holliday on effects, though. The Altithermal saw a significant reduction of surface water in the southern High Plains and the Trans Pecos, evidenced primarily by the complete evaporation of numerous playas and basin lakes.

Vegetation cover was also affected, Mallouf reports, with a significant retreat of pine trees and other conifers from the lower elevations.

"Plateau grasslands, foothills and basins now had admixtures of desert succulents such as sotol, lechugilla, and yucca that had slowly expanded their ranges with increasing aridity," Mallouf writes.

 

THE COMMON ERA

 

Moving forward in time to the late Holocene, specifically the last two millennia, data indicate climate oscillations continued, and with them the recurrence of megadroughts.

Connie Woodhouse and Jonathan Overpeck in "2000 Years of Drought Variability in the Central United States" report there were at least four periods of drought between 1 and 1200 CE, based upon a large volume of proxy records recovered from the Great Plains and the Southwest.

The droughts that occurred in the western U.S. prior to 1600 were often much more severe than the droughts of the 1930s and 1950s, they report, and affected much larger areas for periods that were often multi-decadal.

As during the Altithermal, megadroughts resulted in decreased availability of surface water, declines in accessible groundwater resources, changes in vegetation cover and the retreat of floral and faunal species to more restricted habitats, if not their disappearance.

The megadroughts and the concomitant changes to the environment had profound, lasting and sometimes fatal effects upon aboriginal populations in North America.

David Stahle, a geosciences professor at the University of Arkansas who has conducted extensive research in dendrochronology, or tree-ring dating, has reported in several papers there were at least four megadroughts during the last 12 centuries.

Stahle concludes two of these megadroughts, which dwarfed the droughts of the 1930s and the 1950s in severity, duration and areal coverage, may have been aggravating factors in the fall of two Pre-Columbian civilizations.

Stahle writes that a megadrought from 1149 to 1167 likely played a significant role in finishing off a declining Toltec empire. Earlier, a megadrought that reached from southern North America to the highlands of Central America from 897-922 is thought to have been an important factor in the disappearance of the Mayan civilization.

Edward Cook and others in "Megadroughts in North America: placing IPCC projections of hydroclimatic change in a long-term palaeoclimate context" report:

"There is abundant evidence now that some droughts in North America prior to the 20th century were remarkably more severe compared to anything we have experienced since that time. Perhaps the most famous example is the 'Great Drouth' (sic) of AD 1276-1299 described by A. E. Douglass (1929, 1935) as the

likely cause of the abandonment of Anasazi cliff dwellings across the Colorado Plateau. This 24-year drought easily exceeds the duration of the worst 20th-century droughts ... (.)"

Evidence of additional severe and often multi-decadal droughts -- including a 38-year drought in Nebraska centered on the turn of the 14th century -- appears in the Great Plains record through the beginning of modern meteorological observations.

 

SO, NOW WHAT?

 

As noted previously, despite climate oscillations that have occurred since the Last Glacial Maximum, the overall trend for Texas is toward a warmer and drier regime.

Climatologists and others continue to debate whether climate change has an anthropogenic forcing component, but long-term model solutions reveal an increase in average annual temperature in Texas through the end of the century.

In its 2007 State Water Plan, the Texas Water Development Board references a study by the Finnish Environment Institute. The 2003 study, "Future climate in world regions: an intercomparison of model-based projections for the new IPCC emissions scenarios" by Kimmo Ruosteenoja and others, predicts the state's annual average precipitation should remain stable overall, but forecasts temperatures to increase three to 10 degrees Fahrenheit by 2099, enhancing evaporation rates.

More troubling, the Finnish study also predicts the number and severity of droughts to increase.

The environmental and economic toll from the next megadrought to visit Texas could be catastrophic, particularly for residents of arid counties. Pecos County, for example, is heavily dependent upon groundwater for its existence, primarily the Edwards-Trinity (Plateau) Aquifer for irrigation, and for municipal and industrial uses.

The correspondence between the Edwards-Trinity and megadroughts is not clear due to a lack of available research on the subject. It is generally accepted among workers who have studied the aquifer that the Edwards-Trinity is a drought resistant aquifer, but that acceptance is based upon aquifer response to 20th-century droughts; how the aquifer responded to past multi-decadal or multi-centennial megadroughts is still a subject in need of study.

One could infer, though, that a multi-decadal or multi-centennial drought -- broken up by only a few short-lived wet periods -- would have a considerable negative impact upon Edwards-Trinity storage, thus placing in jeopardy any withdrawals from the aquifer.

As the solution channel that feeds Comanche Springs is highly sensitive to aquifer levels, it is highly probable the springs ceased to flow during past megadroughts, and a cessation would likely result from any future megadroughts, with or without aquifer withdrawals for human use.

The total interruption of flow from Comanche Springs would likely continue until the climate cycle swings back to cooler and wetter. With the next glacial maximum the discharge from Comanche Springs will be much larger, as it was during past subpluvial and pluvial periods, based upon satellite-map interpretation that finds paleochannels from Comanche, Diamond Y and Santa Rosa springs.

During the late Pleistocene, it appears the discharge from Comanche Springs was large enough to create a widening channel north of Fort Stockton. Further northeast the discharge appears to also have flowed laterally, at least intermittently, onto a wide flood plain and was joined by outflow from Diamond Y Spring before discharging into a much more robust Pecos River.

 

CONCLUSION

 

Due to the numerous variables that can control the onset of droughts or megadroughts, it is difficult to propose a definitive proximate cause, although there is some limited agreement on poleward migration of the subtropical dry zone.

Various locations saw varying degrees of drought that may have been initiated by varying factors, including short- and long-term atmospheric dynamics that affect low-level moisture transport, local and regional insolation anomalies and changes in eastern Pacific Ocean sea surface temperatures. Variances in local and regional albedo values, and the Earth's precessional cycle may also impart influences.

Regardless of the direct or indirect causes, there have been several megadroughts in the Southwest and southern Great Plains during the Holocene, each worse than the Texas drought of record of the 1950s.

While there are arguments for an anthropogenic forcing component to the megadroughts that will inevitably occur in the future, the validity of this assumption still provokes debate, and evidence is not conclusive either for or against this proposition.

If one wishes to peer even further into the future, anthropogenic contributions to global climate change will likely be inconsequential.

Texas climate is likely to be alternately warmer and drier and cooler and wetter -- although an overall trend of increasing aridity and temperature is forecast to persist -- until a combination of orbital, atmospheric and oceanic factors end the current interglacial and initiate the next glacial maximum in about 30,000 years AP.

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