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[BusinessInsider] One of the fundamental climate constants that has helped define and divide the United States for centuries is shifting in response to global warming, according to new research.

The 100th meridian west— the invisible line of longitude that roughly bisects the continental US in half — was historically seen as a boundary separating the humid eastern states from their arid western counterparts.

But thanks to climate change, the world is not what it once was, and neither is this intangible barrier.

According to a new analysis of rainfall and temperature data by climate scientist Richard Seager from Columbia University, the climate boundary that once traced the 100th meridian has effectively been shifting eastwards ever since the 1980s, with the arid conditions of the western plains slowly expanding into the midwest.

While the meridian itself — which continues beyond US borders, northwards into Canada, and southwards into Mexico — is in the same place as always, the climate barrier that used to sit on top of it has moved approximately 225 kilometres (140 miles) to the east, Seager says, placing it closer to the 98th meridian west.

So far, this shift hasn't significantly affected farming and land use as the dryer conditions move across the US.

But the team's modeling suggests altered precipitation and rising temperatures as the century progresses will see the "effective 100th meridian" continue its shift eastward for decades to come, as arid conditions slowly colonize the east.