Figure by Swid, Wikimedia Commons
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The 100th Meridian (west) is a very famous line of longitude. This line runs through the center of North America. As it crosses the United States it roughly bisects North Dakota, South Dakota and Nebraska, and then cuts through the western third of Kansas. It then crosses the Oklahoma panhandle and forms the border between Texas and Oklahoma as the eastern edge of the Texas panhandle. It then continues down through the middle of Texas into Mexico.
Climatologically, the 100th Meridian (west) marks the western edge of the typical movement of moist air coming off of the Gulf of Mexico and the eastern edge of the great rain-shadow of the Rocky Mountains. It is roughly the line where the average yearly rainfall equals 20 inches (51 cm). It is, therefore, the boundary between the moist ecosystems of the American East and the dry ecosystems of the American West. It was first defined as this border by John Wesley Powell in 1878.
Restored Tall Grass Prairie, Photo by Hyperik, Wikimedia Commons
The 100th Meridian is not a line of sudden change from the verdant green to arid brown. It is not a doorway like the one Dorothy steps through in the Wizard of Oz where the drab black and white of one world changes into the vibrant colors of another. It is part of a transition zone that may be as wide as a degree or two of longitude (roughly 60 to 120 miles at 38 degrees N latitude) (based on the graphs from Seager et al. 2018, mentioned below). This transition zone, though, may have remarkable annual rainfall variations. This
Yellowhouse Draw. Photo by Leaflet. Wikimedia Commons
Meridian line is, as we previously mentioned, the line of 20 inches (51 cm) of annual rain. The eastern edge of its transition zone, though, may receive 24 inches (61 cm) of rain and the western edge, 18 inches (or 46 cm) of rain. Adjacent counties along this line may have remarkably different weather patterns and remarkably different agricultural potentials!
In 2012, Richard Seager (Columbia University) and his Columbia colleague Mingfang Ting along with National Science Foundation funded undergraduate students from Columbia and Penn State began studying the climate and geography of the 100th Meridian. Their research was presented at the American Geophysical Union meeting in 2016 and at the American Meteorological Society meeting in 2017. It was subsequently published in two papers in the March 2018 edition of Earth Interactions (which were referred to above).
Seager developed an index that compared the loss of moisture from a site (the rate of evapotranspiration which is affected by temperature, wind speed, magnitude of incoming solar energy and relative humidity) to the addition of moisture to that site (i.e. the actual, measured precipitation). This index is an elaboration of the old ecological concept of “effective precipitation” (i.e the amount of moisture in rainfall that remains in an ecosystem and is available to sustain plant growth). Seager’s index was used to identify sites that could sustain farming without irrigation. It was, in fact, the old “20 inches of rain” line with a more precise interpretation.
National Atlas map modified by Cantner
Seager and his co-authors found that a line still exists between the humid East and the arid West, but it was no longer on the 100th Meridian. The line and its transition zone had moved east more than two degrees of longitude and was now primarily astride the 98th Meridian. Further, Seager asserts that this line will move another two or three degrees of longitude east by 2100 due to the predicted heating of the North American continent due to climate change. These warmer temperatures will accelerate the evaporation of water and may disrupt the delivery of moist air masses from the Gulf of Mexico. This shift is expected to be most dramatic in the central and southern Great Plains and will possibly stretch the climate transition line out into a slide curve with its northern portions extending far to the west and its southern portions extending far to the east. The roughly north/south line of the climate meridian, then, will become a long, slanted curve that affects more and more of the Great Plains.
Photo by H. Mulligan, Wikimedia Commons
As I have talked about before, there is very little of the Great Plains that is still in a natural state. Most of the vegetation has been destroyed or transformed into hodge-podges of invasive plants or agricultural crops. Further, most of the endemic animals of the Plains like the prairie dogs (Signs of Fall 4, October 3, 2019) and the American bison (Signs of Fall 5, October 22, 2020) have been almost exterminated and mostly replaced by domesticated grazing animals. It is very difficult, then, to talk about what these Climate Meridian changes will mean to the native flora and fauna and the natural ecosystems of the Plains.
Photo by T. Webster, Wikimedia Commons
The influence of these changes on agriculture, though, could be profound. Agriculture on either side of the Climate Meridian involves very different farm structures and very different crops. Farms on the humid, eastern side of the climate line are smaller and tend to be more closely packed together and more numerous. These eastern farms grow crops dominated by corn (70% of all crops grown in the east). Farms on the drier, western side of
Wheat field, Public Domain
the climate line are larger, less numerous and more widely spaced. The western crops are dominated by wheat (40% of all crops grown in the west). Places with abundant ground water and low irrigation costs may shift this crop distribution (for example, southern Nebraska grows a great deal of corn west of the 100th Meridian because of the ground water abundance of the Ogallala Aquifer). It is expected, though, that as the climate transition line moves to the east more and more eastern corn farmers will be forced to turn to crops like wheat that require much less water to grow.
Our world has changed and is changing more and more with each passing decade. The remarkable line of the 100th Meridian, wherever it is actually located, is a reminder of the consequences and magnitudes of many of these changes.