Straight line divisions between climatic zones are very uncommon. However, one such boundary shapes the major climate patterns of the United States.
A clear longitudinal line along the 100th meridian west separates the dry climate of the West from the humid East. The 100th meridian thesis of a pioneering American geologist John Wesley Powell. Although it was known since the 19th century, its existence has been confirmed by modern climate science only recently. The boundary spans through Manitoba (Canada), the Dakotas, Nebraska, Kansas, Oklahoma, Texas and eastern Mexico. The only other straight line divide between arid and humid area is the one between the Sahara Desert and sub-Saharan Africa.
The Saharan Desert has been sprending for decades under the influence of climate change. Similarly, the North American 100th meridian climatic boundary is shifting, bringing dry climate patterns further east. Based on temperature and rainfall measurements from 1980s to this date, Richard Seager from Columbia University and his team found that the 100th meridian boundary has moved about 140 miles east, so now it is closer to the 98th meridian. By their projections, as the temperatures rise, evaporation increases, and precipitation patterns change, it will continue to move deeper east.
Changes related to 100th meridian are not a phenomenon that will affect only natural ecosystems. The divide has impacted the arrangements of human settlements for centuries, which consequently shaped historical events, and the entire society.
Today, Seager and his team have deepened the study of the 100th meridian boundary on human settlements. The research falls in the realm of psychogeography – the study of interactions between the natural environment and human lives and choices. In their two papers, the team is trying to paint a picture of how has the invisible border between the arid and the humid part of the continent influenced communities.
West of the 100th meridian the population density is smaller, development is more modest, and farms are generally larger and grow arid-resistant crops like wheat. On the contrary, when you look east of the boundary, there are more people and more infrastructure; farms are smaller and grow corn predominantly.
The team also gives predictions on further development of the region under the influence of climate change. With the spreading of the dry patterns further east, farms will have to adapt and follow the model of their western neighbours. To remain viable, they will have to become larger, turn to irrigation, and switch to more adaptable crops – for example, grow wheat in place of corn. Even with all the measures included, many croplands may fail altogether, becoming a part of the dry western pastureland.
Seager R et al. (2018). Whither the 100th Meridian? The Once and Future Physical and Human Geography of America’s Arid–Humid Divide. Part I: The Story So Far. American Meteorological Society https://journals.ametsoc.org/doi/abs/10.1175/EI-D-17-0011.1
Seager R et al. (2018). Whither the 100th Meridian? The Once and Future Physical and Human Geography of America’s Arid–Humid Divide. Part II: The Meridian Moves East. American Meteorological Society
A North American Climate Boundary Has Shifted 140 Miles East Due to Global Warming. (2018). Yale Environment 360:
Krajick, K. (2018.) The 100th Meridian, Where the Great Plains Begin, May Be Shifting Columbia University Blog: