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Tracking the movement of the tropics 800 years into the past

For the first time, scientists have traced the north-south shifts of the northern-most edge of the tropics back 800 years, reports an international team of researchers. The movement of the tropical boundary affects the locations of Northern Hemisphere deserts including the Sonoran, Mohave and Saharan. Those deserts sit just north of the tropical belt, which includes the subtropics. Before now, scientists had information about the location of the tropical belt going back to around 1930, when reliable instrumental record-keeping began. On a standard map, the tropical belt spans roughly 30 degrees north latitude to 30 degrees south latitude. However, the new research reveals that from the year 1203 to the year 2003, the northern edge of the tropics fluctuated up to 4 degrees north and south of the northern 30th parallel. From 1568 to 1634, the tropics expanded to the north, the team found. That time period coincides with severe droughts and other disruptions of human societies, including the collapse of the Ottoman Empire in Turkey, the end of the Ming Dynasty in China and near abandonment of the Jamestown Colony in Virginia. To track the northern boundary of the Earth's tropical belt from 1203 to 2003, the team used the annual rings of trees from five different locations throughout the Northern Hemisphere. Researchers can figure out annual precipitation years into the past because each annual growth ring of a tree reflects the climate that year. Having an 800-year history also allowed the researchers to connect rare events such as huge volcanic eruptions with subsequent changes in climate.

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