Pushing the Great American Desert Eastward – Reality or Hype?

Pushing the Great American Desert Eastward – Reality or Hype?

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Guest essay by Rick Yarnell

In 1877 an explorer and scientist named John Wesley Powell conceived a dividing line in the North American continent that marked the separation of the green and humid East from the dry, desert West. That dividing line was the 100th Meridian. East of this line, conventional agriculture, which he asserted depends on at least 20 inches of rain, would succeed. West of the line, with its desert conditions, could not support conventional, non-irrigated agriculture.

Mr. Powell and the story of the 100th Meridian was recently explored in a beautifully written essay in the Wall Street Journal, by John F. Ross, (The Prophet of the Dust Bowl, § Review, June 16-17, 2018, pC4).

The essay is adapted from Mr. Ross’s book “The Promise of the Grand Canyon: John Wesley Powell’s Perilous Journey and his Vision for the American West”, Viking, 2018.

Toward the end of the essay, one conclusion Mr. Ross puts forward is that Mr. Powell, in his warnings that the arid west was not farmable, “did not consider one crucial factor, because he did not know about it: the effects of a warming atmosphere, caused by greenhouse gas emissions from the burning of fossil fuels.”

Mr. Ross tells us, “Columbia University climatologist Richard Seager and his team of researchers…looking at data collected since 1980…found that warming conditions have pushed Powell’s line 140 miles to the east.”

The 100th meridian west (solid line) has long been considered the divide between the relatively moist eastern United States, and the more arid West. Climate change may already have started shifting the divide eastward (dotted line).
CREDIT Modified from Seager et al. Earth Interactions, 2018

Now, Mr. Powell’s 100th Meridian line stretched from the Dakotas to Texas, a region with vastly different temperature extremes, so, of course, the Dry Line was not marked by primarily by temperature, but rather by rainfall. Locations west of the 100th Meridian usually received less than 20 inches of rainfall in a year, while those locations east of the 100th Meridian would get greater than 20 inches of rain.

In my home state of Kansas anyone with a passing familiarity of the state’s agriculture knows that the western third of the state is dry. Early maps of the region in fact noted the high plains through which passed the Oregon, California and Santa Fe Trails as the Great American Desert. I happen to own a small farm in the Flint Hills about an hour east of Wichita, Kansas, and I keep pretty good records of rainfall on my farm. It struck me that, if the Dry Line had moved eastward, my own records would provide a clue. But my farm wasn’t experiencing a significant decrease in rainfall. In fact, last year, 2017, was perfect growing weather that yielded a bumper crop. I wondered, to what data was Mr. Seager looking?

So, I did my own research.

An article in Earth Magazine gave more information. “Dividing Line, the Past, Present and Future of the 100th Meridian”, by Harvey Leifert, (January 22, 2018). Mr. Leifert states, “The researchers developed a standard measure called the aridity index to assess aridity across a geographic area, using observational data from the North American Land Data Assimilation System. They estimated potential evapotranspiration — the amount of water the atmosphere could potentially extract from the surface — from the land based on temperature, wind speed, solar radiation and relative humidity. The ratio of actual precipitation to potential evapotranspiration provides the aridity index.”, (emphasis mine).

Notice that this is not the same measurement used by Mr. Powell in his “Report on the Lands of the Arid Region of the United States” (Second Edition, Washington, Government Printing Office, 1879). Mr. Powell wrote in that report, “records are such as to indicate approximately the boundary between the Arid Region, where irrigation is necessary to agriculture, and the Humid Region, where the lands receive enough moisture from the clouds for the maturing of crops. Experience teaches that it is not wise to depend upon rainfall where the amount is less than 20 inches annually, if this amount is somewhat evenly distributed throughout the year.” The “mean annual rainfall line of 20 inches, as indicated on the rain chart accompanying [Mr. Powell’s] report, begins on the southern boundary of the United States, about 60 miles west of Brownsville, on the Rio Grande del Norte, [note: just south of Palmview, TX at about 98.5 degrees W] and intersects the northern boundary about 50 miles east of Pembina [note: at Maida, ND at about 98.4 degrees W]. Between these two points the line is very irregular, but in middle latitudes makes a general curve to the westward.” Mr. Powell also noted three other modifiying conditions to successful non-irrigated agriculture along this line: altitude, latitude and temperature.

Mr. Seager’s research team, according to Mr. Leifert, presented their findings “at the American Geophysical Union’s fall meeting in December 2016” and “at the American Meteorological Society’s meeting in January 2017. The work has been accepted for publication in the journal Earth Interactions.”

Here is the abstract of the research of Mr. Seager’s team: “The potential evapotranspiration (PET) is first computed using a suite of three NLDAS-2 land surface models and the Penman-Monteith Equation, and the aridity index (AI), defined as precipitation divided by PET, is used as the aridity metric. There is a sharp gradient in aridity along and just east of the 100th Meridian, verifying Powell’s observations. We further determined that this arid-humid boundary is primarily caused by strong spatial gradients in precipitation and humidity, which in turn are caused by the seasonal cycle in wind direction and moisture transport. Using CMIP5 climate model data, the future was projected in 20-year increments from the present through 2100. Models project that the arid-humid boundary will shift eastward by approximately 2 to 3 degrees by the end of the 21st Century, the gradient will weaken, and that the entire continental US will experience at least some degree of aridification.” (Whither the 100th Meridian: The once and future physical geography of America’s arid-humid divide; http://adsabs.harvard.edu/abs/2016agufmgc11b1136t), again, emphases mine.

Wow, America’s Breadbasket will dry up and we will all starve to death. Oh, woe.

So, really, have warming conditions pushed Mr. Powell’s line 140 miles to the east, as asserted by Mr. Ross in his Wall Street Journal essay? That’s easy enough to find out: let’s look at rainfall records for towns along the 100th Meridian, and of towns that are approximately 140 miles east of that line.

First, I looked at a map of the high plains to find village or town sites that had weather observational records. The high plains are sparsely settled, so there are not a lot of sites that sit very close to the 100th Meridian. I chose four sites in North Dakota, four in South Dakota, three in Nebraska, four in Kansas and five in Oklahoma/Texas, then looked up the average annual rainfall reported on various weather and climate websites. The annual rainfall records indicate that reporting stations near the 100th Meridian in the northern high plains are much drier than those in the southern high plains. The average annual rainfall for the four chosen sites in North Dakota was 18.22 inches. Averages proceeding southward: South Dakota: 20.65”, Nebraska: 22.67”, Kansas: 23.30”, OK/TX: 24.08”. Reported annual average rainfall for sites near the 100th Meridian ranged from a low of 16.12” in Strasburg, ND to 27.16” in Eden, TX. The seven northernmost reporting stations all had annual average rainfall of less than twenty-one inches, as did the southernmost station in Carrizo Springs, Texas. I then looked up a corresponding weather station that was approximately 140 miles east of these twenty sites, to determine average annual rainfall.

For the twenty stations that were approximately 140 miles east of the 100th Meridian, approximately between the 97th and 98th Meridians, we find as we did further west, that the northern plains are drier than the southern plains. The stations in ND/Minn. recorded average rainfall of 22.42” annually. Averages proceeding southward: South Dakota: 24.27”, Nebraska: 27.89”, Kansas: 33.20”, OK/TX: 36.35”. Reported annual average rainfall for sites about 140 miles east of the 100th Meridian ranged from a low of 21.66” in Argyle, Minnesota to 37.54” in Fort Worth, Texas.

The average annual rainfall for all twenty stations that are about 140 miles east of the 100th Meridian is greater than, or much greater than, 20”. From what I see in the average annual rainfall records, it seems that the assertion by Mr. Ross “that warming conditions have pushed Powell’s line 140 miles to the east” is untrue. But, remember, that is not what Prof. Seager’s team reported. The team wrote, “Models project that the arid-humid boundary will shift eastward by approximately 2 to 3 degrees (or the 98th and 97th Meridians West) by the end of the 21st Century.”

I questioned, next, whether the average rainfall amounts were skewed by larger numbers back further in time, and that, maybe, the most recent years are drier than earlier years. So I chose a set of eleven stations to examine the actual reported rainfall for each month in the five most recent years: 2013-2017. For this data I used www.wunderground.com/history. This website has monthly history data for only larger regional weather stations, thus, the limit to eleven stations.

Here is the average annual rainfall for the years 2013-2017 for weather stations near the 100th Meridian, and the corresponding overall average annual rainfall from www.usclimatedata.com:

2013-17 overall

North Dakota*

Winner, SD 17.39 23.93

Ainsworth, NE 20.56 23.29

Dodge City, KS 22.39 21.49

Canadian, TX 18.88 21.72

Abilene, TX 26.62 24.83

* Note: none of the towns along the 100th Meridian in North Dakota have reliable history data on wunderground.com (Rugby, Harvey, Steele, Strasburg).

Here is the average annual rainfall for the years 2013-2017 for weather stations about 140 miles east of the 100th Meridian, and the corresponding overall average annual rainfall from www.usclimatedata.com:

2013-17 overall

Grand Forks, ND 21.59 20.73

Sioux Falls, SD 28.98 26.35

Wayne, NE 21.28 27.39

Wichita, KS 38.47 34.33

Oklahoma City, OK 39.04 36.46

Fort Worth, TX 33.44 37.54

There is no noticeable pattern of decreased rainfall in any of the above records. Of the five locations near the 100th Meridian, two have more average annual rainfall in the last five years than their usual average, and three have less. Of the six locations east of the 100th Meridian, four have more average annual rainfall in the last five years than their usual average, and two have less.

Conclusion

John Wesley Powell made his conception of the Dry Line using his observations of plant and animal life and especially rainfall. When we use that same rainfall methodology today, we confirm that the Dry Line is, today, still near the 100th Meridian. No modeling needed.


Supporting data:

ICT DDG rainfall 13-18 (Excel)

Superforest,Climate Change

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