Tropical Boundaries Expand

The tropics have expanded more than 200 miles toward the poles in the past 50 years, raising fears that dry, sub-tropical climates will also migrate poleward and cause prolonged droughts in the Mediterranean and the southwestern United States, according to a new study. The tropics are currently defined as the region between the Tropics of Cancer and Capricorn. But a study by scientists at the University of Utah and the U.S. National Center for Atmospheric Research shows that tropical boundaries have been expanding at a rate of 43 miles per decade for the past half-century. The study, published in the journal Geophysical Research Letters, studied an atmospheric region called the tropopause, located at the boundary of the troposphere, where weather systems form, and the stratosphere. In tropical regions, the tropopause tends to be located roughly 2 miles higher than in temperate regions, and the researchers tracked the expansion of the tropics by following the poleward migration of the tropopause. This could lead to the expansion of sub-tropical regions and bring persistent drought to more temperate zones, the lead author said.