Here Come the Tropics!

Earth's tropical belt seems to have expanded a couple hundred miles over the past quarter century, which could mean more arid weather for some already dry subtropical regions, new climate research shows.

Geographically the tropical region is a wide swath around Earth's middle stretching from the Tropic of Cancer, just south of Miami, to the Tropic of Capricorn, which cuts Australia almost in half.

It's about one-quarter of the globe and is generally considered hot, steamy, and damp, but it also has areas of brutal desert.

To meteorologists, however, the tropics region is defined by long-term climate and what's happening in the atmosphere. Recent studies show changes that indicate an expansion of the tropical atmosphere.

The newest study, published Sunday in the new scientific journal Nature Geoscience, shows that by using the weather definition, the tropics are expanding toward Earth's Poles more than predicted. And that means more dry weather is moving to the edges of the tropics in places like the U.S. Southwest.

Independent teams using four different meteorological measurements found that the tropical atmospheric belt has grown by anywhere from 2 to 4.8 degrees latitude since 1979. That translates to a total north and south expansion of 140 to 330 miles (225 to 530 kilometers).

One key determination of the tropical belt is called the Hadley circulation—prevailing rivers of wind that move vertically as well as horizontally, carrying moisture to rainy areas while drying out arid regions on the edges of the tropics. That wind is circulating over a larger area than it was a couple decades ago.

But that's not the only type of change meteorologists have found that shows an expansion of the tropics. They've seen more tropical conditions by measuring the amount of ozone in the atmosphere, the depth of the lower atmosphere, and the level of dryness in the atmosphere at the edges of the tropics.

Climate scientists have long predicted a growing tropical belt toward the end of the 21st century because of global warming.

But what has happened in the past quarter century is larger and more puzzling than initially predicted, said Dian Seidel, a research meteorologist with the National Oceanic and Atmospheric Administration lab in Silver Spring, Maryland. She is the author of the newest study.

"They are big changes," she said. "It's a little puzzling."

She said this expansion may only be temporary, but there's no way of knowing yet.

There's an obvious potential explanation. It will be interesting to see if that is really the one for the expansion of the Tropics.