The 100th meridian, where the Great Plains begin, shifting eastward

“Powell talked eloquently about the 100th meridian, and this concept of a boundary line has stayed with us down to the current day,” said Richard Seager, a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory and lead author of the two papers.  “We wanted to ask whether there really is such a divide, and whether it’s influenced human settlement.” He calls the studies an example of “psychogeography”—the examination of how environment affects human decisions. The papers appear in the current edition of the journal Earth Interactions.

While the climate divide is not a literal line, it is about the closest thing around, easily seen on maps. Due to global-scale wind patterns, to the west of this longitude, rainfall drops off sharply. East of the line, it picks up sharply. Powell noted correctly that the western plains are dry in part because they lie in the rain shadow of the Rocky Mountains, which rake off almost all the moisture blowing in from the Pacific Ocean. Seager’s team identifies two other factors. In winter, Atlantic storms bring plenty of moisture into the eastern plains and Southeast, but don’t make it far enough to moisten the western plains. In summer, moisture from the Gulf of Mexico moves northward, but that also curves eastward, again providing the East with plenty of precipitation, while the West gets cheated. Seager says there is only one other such major straight-line climate divide on the global map: the one separating the Sahara Desert from the rest of Africa, also due to cutoffs of prevailing oceanic winds.

In the United States, the effects show up in obvious ways. To the west, population density drops sharply. There are fewer homes, commercial facilities and roads. Farms are fewer, but bigger, reflecting the economics of less water and thus lower productivity. To the east, 70 percent of the crop is moisture-loving corn; to the west, aridity-resistant wheat is dominant.

Now, the researchers say, warming climate appears to be pushing the divide east. In the northern plains, rainfall has not changed much, but temperatures are going up, increasing evaporation from the soil. Further south, concurrent shifts in wind patterns are in fact causing less rain to fall. Either way, this tends to push western aridity eastward. Data collected since about 1980 suggests that the statistical divide between humid and arid has now shifted closer to the 98th meridian, some 140 miles east. (In Texas, this would move it roughly from Abilene to Fort Worth.) Seager says year-to-year weather variations may blur the data, and in any case the changes are still too small and gradual to yet affect land use over wide areas. But he is confident that aridity will perceptibly move eastward during the 21st century, and eventually effect large-scale changes.

Seager predicts that as drying progresses, farms further and further east will have to consolidate and become larger in order to remain viable. Unless farmers turn to irrigation or otherwise adapt, they will have to turn from corn to wheat or some other more suitable crop. Large expanses of cropland may fail altogether, and have to be converted to western-style grazing range. Water supplies could become a problem for urban areas.

Some historians say it could be argued that white settlement beyond the meridian influenced everything from the end of slavery (plantations could not expand beyond the line, weakening the South) to the development of modern firearms (settlers with single-shot muskets couldn’t compete with native peoples’ rapid-fire arrow attacks, until they became the first, best customers for new Colt repeating revolvers and rifles). The meridian itself is still registered in the popular imagination by historical roadside signs, books such Wallace Stegner’s Beyond the Hundredth Meridian, and the Canadian rock hit “At the Hundredth Meridian.” “It’s a reminder that climate really matters, then as it does today,” said Seager.

— Read more in Richard Seager et al., “Whither the 100th Meridian? The Once and Future Physical and Human Geography of America’s Arid–Humid Divide. Part I: The Story So Far,” Earth Interactions 22, no. 1 (21 March 2018; 22): (DOI: 10.1175/EI-D-17-0011.1); and Richard Seager et al., “Whither the 100th Meridian? The Once and Future Physical and Human Geography of America’s Arid–Humid Divide. Part II: The Meridian Moves East,” Earth Interactions 22, no. 5 (21 March 2018 (DOI: 10.1175/EI-D-17-0012.1)