Global climate change has caused the tropics to widen by between two and 4.8 degrees of latitude since 1979, new research carried out by US scientists suggests.

Geographers define the tropics as the area between 23.5 degrees north and 23.5 degrees south; however, for atmospheric scientists, the zone is more fluid, its boundaries defined by high-altitude features such as the jet stream and circulation patterns known as Hadley cells.

The US team, led by Dian Seidel of the National Oceanic and Atmospheric Administration at Silver Spring, Maryland, examined stratosphere changes based on five sets of data gathered by satellites and weather balloons. The different data sets suggested various amounts of widening of the tropical zone representing north- and southward movement of between 200 and 480 kilometres.

Changes in the locations of the edges of the tropical belt are likely to cause changes in precipitation and ‘could lead to fundamental shifts in ecosystems and in human settlements,’ the researchers write in the journal Nature Geoscience. The specific mechanisms behind the expansion are unclear, but may include ocean surface warming and an increase in the tropopause, a layer between the troposphere and the stratosphere.


February 2008