The tropical zone, commonly called the tropics, is the geographic region of the Earth that includes the equator and extending northward to about 23.5 degrees North latitude, at the Tropic of Cancer), and extending southward to about 23.5 degrees South latitude, at the Tropic of Capricorn.

For example, southern Florida and the U.S. Southwest, are on the northern boundary of the Tropic of Cancer, while portions of the Mediterranean and southern Australia are on the southern boundary of the Tropic of Capricorn.

The results of the study (“Widening of the tropical belt in a changing climate”) were published on December 3, 2007, in the publication Nature Geoscience.

Lead research meteorologist in the study is Dian J. Seidel, who is with NOAA’s Air Resources Laboratory, in Silver Spring, Maryland, U.S.A. Seidel stated in a NOAA website (“Study: Expanding Tropical Belt Could Affect Climate”): “We looked at how certain aspects of the structure and circulation of the atmosphere have been altered over the past few decades and how models predict they may change as the climate changes in the future. We are seeing indications that a warming climate is associated with expansion of the tropical region toward the poles, and the rate of expansion that has occurred in recent decades is greater than projected by climate models to occur in the 21st century.”

The NOAA researchers used various measurement techniques to make their conclusions, including satellites that measure ozone concentrations and atmospheric temperatures; and weather balloons in the tropopause (the boundary region between the stratosphere and the troposphere, and is the atmospheric layer where weather occurs).

When the data was inserted into computer simulations, it was discovered that the jet streams and other horizontal and vertical wind and precipitation patterns (such as the Hadley circulation) were tending to move more northward and southward from the equator as the result of the warming of the planet.