The 100th meridian affected all kinds of things, as this sign in South Dakota illustrates. It says, “Historically that meridian is significant. For two generations the Insurance Companies and other world-wide lending agencies would not, as a matter of agreed policy, lend a shiny dime west of this line. Their reason was that some geographer had labeled it the EAST EDGE of the Great American Desert. Neither the geographer nor the Insurance Companies had been east of 100. Today, more than a quarter of America’s new animal wealth alone, is produced from that misnamed desert. This unrealistic geographically limited loan policy forced South Dakota into the farm loan business. Our Rural Credit business cost us plenty and was a splendid illustration of why a State should not be in the loan business. But South Dakota has paid all its debts in full. The 100th Meridian is just another bad memory. Historically however the 100th Meridian was a most important one in Western economy.“

In 1879, John Wesley Powell established a line of longitude in the United States representing the boundary between the moist east and the arid west. The 100th meridian wasn’t selected just because it was a nice round number. It actually was one of the strongest natural boundaries in the world, marking the switch between the non-irrigated east and the irrigation-necessary west.

Thanks to global-scale wind patterns, to the west of the 100th meridian, rainfall drops sharply, and to the east of the line it picks up sharply. Powell described what he saw in 1890, writing, ““Passing from east to west across this belt a wonderful transformation is observed. On the east a luxuriant growth of grass is seen, and the gaudy flowers of the order Compositae make the prairie landscape beautiful. Passing westward, species after species of luxuriant grass and brilliant flowering plants disappear; the ground gradually becomes naked, with bunch grasses here and there; now and then a thorny cactus is seen, and the yucca plant thrusts out its sharp bayonets.”

The effect of this line is still easy to see, says Kevin Krajick, senior editor for science at the Earth Institute. “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.”

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). Thanks to State of the Planet for this map and caption.)

After determining that such a line really does exist, researchers from Penn State, Columbia Engineering, and the Lamont-Doherty Earth Observatory took a closer look at it. What they found is that the line has shifted from the 100th meridian to the 98th, about 140 miles to the east. Rainfall hasn’t changed much in the northern plains, but temperatures there are going up, increasing evaporation from the soil. Wind patterns have also changed, causing less rain to fall.

Land use hasn’t yet been affected because the changes are still very small and gradual. But Richard Seager of the Earth Observatory theorizes that as drying progresses, farms in the east may have to consolidate, irrigate, or change the crops they raise. He even sees the possibility that cropland will be converted back to pasture and western-style grazing.