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The expanding tropics. Nature. Feb 2016.

One spring day in 2004, Qiang Fu was poring over atmospheric data collected from satellites when he noticed an unusual and seemingly inexplicable pattern. In two belts on either side of the equator, the lower atmosphere was warming more than anywhere else on Earth. Fu, an atmospheric scientist at the University of Washington in Seattle, was puzzled.

It wasn’t until a year later that he realized what he had discovered: evidence of a rapid expansion of the tropics, the region that encir- cles Earth’s waist like a green belt. The heart of the tropics is lush, but the northern and south- ern edges are dry. And these parched borders are growing — expanding into the subtropics and pushing them towards the poles.

Cities that currently sit just outside the tropics could soon be smack in the middle of the dry tropical edge. That’s bad news for places like San Diego, California. “A shift of just one degree of latitude in southern California — that’s enough to have a huge impact on those communities in terms of how much rain they will get,” explains climate modeller Thomas Reichler of the University of Utah in Salt Lake City.

Since Fu and his colleagues announced their discovery1 in 2006, many scientists have investigated the tropical bloating and tried to decipher its cause. Explanations range from global warming to ozone depletion or natural cycles that will reverse in the future. And there is little agreement on how quickly the border of the tropics is shifting: estimates run from less than half a degree of latitude per decade to several. At the more extreme end, the change in climate would be like moving London to the position of Rome over the course of a century. The problem is compounded by lack of consensus on how to define the tropics, which makes it hard for scientists to agree on the extent of the changes. Nevertheless, researchers investigating this phenomenon agree that it is real.

“There’s a big need to be concerned about this issue,” says climate scientist Chris Lucas at the Australian Bureau of Meteorology in Melbourne.

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Olive Heffernan

Olive Heffernan is a London-based freelance environment writer. Olive mostly writes about climate change and its impacts, but also writes more broadly on sustainable resource use. Here you can find an archive of her recent articles, link to her Twitter feed and her blog.

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