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The mystery of the expanding tropics

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In their study, Lu and his colleagues found that climate models generally forecast that the outer edge of the Hadley cell will shift because of global warming. But the models predict a much slower rate of tropical expansion than has been seen so far — which has led researchers to suspect that something else is going on.

The mystery of the expanding tropics

As Earth’s dry zones shift rapidly polewards, researchers are scrambling to figure out the cause — and consequences.

Olive Heffernan 02 February 2016 PDF Nature 530, 20–22 (04 February 2016) doi:10.1038/530020a

….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 century2, 3, 4, 5. 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. That’s because of the possible impacts: some of the world’s most fertile fishing grounds could disappear, global grain production could shrink and biodiversity could suffer….

Some of the changes in the tropics could be a result of global warming. Reichler investigated that possibility in a study6 led by Jian Lu, an Earth systems scientist now at the Pacific Northwest National Laboratory in Richland, Washington. Working with Gabriel Vecchi, a climate scientist with NOAA in Princeton, New Jersey, the researchers looked at climate forecasts to see how warming might affect an atmospheric circulation pattern called the Hadley cell, which transports heat from the warmer parts of Earth towards the cooler regions (see ‘Bulging waistline’).

 

As part of the Hadley cell, warm, moist air soars skywards above the Equator and cool, dry air tumbles towards Earth at about 30 ° latitude in the Northern and Southern Hemispheres. That downward limb of the Hadley cell helps to create some of the driest deserts on the planet, such as the Kalahari in southern Africa and the Sahara in northern Africa, and it is one of the most common measures of the boundary between the tropics and the drier subtropics. In their study, Lu and his colleagues found that climate models generally forecast that the outer edge of the Hadley cell will shift because of global warming. But the models predict a much slower rate of tropical expansion than has been seen so far — which has led researchers to suspect that something else is going on. ….

 

On land, biodiversity is also potentially at risk. This is especially true for the climate zones just below the subtropics in South Africa and Australia, on the southern rim of both continents. In southwestern Australia, renowned as one of the world’s biodiversity hotspots, flowers bloom during September, when tourists come to marvel at some of the region’s 4,000 endemic plant species. But since the late 1970s, rainfall there has dropped by one-quarter. The same is true at South Africa’s Cape Floristic Province, another frontier known for its floral beauty. “This is the most concrete evidence we have of tropical expansion,” says Steve Turton, an environmental geographer at James Cook University in Cairns, Australia.

 

Turton worries that the rate of change will be too rapid for these ecosystems to adapt. “We’re talking about rapid expansion that’s within half or a third of a human lifetime,” he says. In the worst-case scenario, the subtropics will overtake these ecologically rich outposts and the hotter, drier conditions will take a major toll.…..

 

But that long wait for an answer will be no comfort for the residents of cities such as Santiago, San Diego and Melbourne, and for the billions of others who live near the boundary between the tropics and subtropics. “We need to understand this issue,” says Lucas, “to have a sustainable civilization there.”

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