U.S. satellite data since 1979 has revealed that the troposphere--the weather-bearing layer of our atmosphere that extends more than seven miles up--warmed the most, by roughly 1.5 degrees Fahrenheit, in the middle latitudes. This band of warming crosses the southern U.S. as well as southern China and North Africa in the Northern Hemisphere and southern Australia, South Africa and lower South America in the Southern Hemisphere. This warmer air expands the reach of the tropics and pushes the jet streams toward the poles. "We estimate that the jet streams in both hemispheres have shifted poleward by roughly 1 degree latitude in both summer and winter seasons," the researchers, led by Qiang Fu of the University of Washington, write in today's Science. Each degree of latitude represents roughly 70 miles.
"The jet streams mark the edge of the tropics, so if they are moving poleward that means the tropics are getting wider," explains John Wallace, Fu¿s colleague at the University of Washington and coauthor of the report. "If they move another two to three degrees poleward in this century, very dry areas such as the Sahara Desert could nudge farther toward the pole, perhaps by a few hundred miles."
It remains unclear what specifically is driving the move, though climate change is a likely suspect, along with depletion of the ozone layer. "Regardless of the cause," the authors note, "the poleward shift of the jet streams and the associated subtropical dry zone, if it continues, could have important societal implications."