Australian scientists warn of expanding tropics

Scientists report a rapid expansion of the world's tropical zones, a potentially disastrous change that would bring severe drought and higher temperatures.

Steve Turton, an Austrailian science professor, says he has proof that tropical rainforests will move further away from the equator.

SYDNEY // Australian scientists have reported a rapid expansion of the world's tropical zones, a potentially disastrous change that would bring severe drought and higher temperatures. The scientists warn that global warming is helping to drive a southwards drift of warm and humid conditions that could fundamentally alter the climate of the entire Australian continent, about half of which sits in the tropics.

"There is a consensus that the tropics have expanded in the order of five per cent in the last 25 years," said Steve Turton, a professor at the school of earth and environmental sciences at James Cook University in Cairns, who has documented the swift advance of the tropical region after reviewing more than 70 international scientific papers on the subject. "That expansion has been a few hundred metres vertically but more importantly there has been a displacement towards the poles anywhere between 300 and 500km in both hemispheres."

The tropics are home to half the world's population and the lion's share - about 80 per cent - of its biodiversity, much of it flourishing in the rainforests and coral reefs. The growth of the equatorial area that lies between the tropics of Cancer and Capricorn is likely to result in more rainfall in northern parts of Australia but drier conditions further south. "Some of the biggest effects of the expansion of the tropics are not actually going to be in the tropics," said Mr Turton.

"In particular the Mediterranean climates, which dominate parts of southern Australia, appear to be contracting and they are also very important for biodiversity." Mediterranean environments feature moderate temperatures and dry, hot summers due to subtropical high pressure systems, which are growing and pushing warmer conditions further south. "We are seeing these kinds of changes occurring already," said Mr Turton, who said Australia's south-western corner has experienced declining rainfall since the mid-1970s.

Researchers at James Cook University have suggested that significant drying will accelerate between 2030 and 2050 but believe that reductions in greenhouse gas emissions would prevent some of the worst effects. The scientists also believe that the impact of a shifting tropical belt would not only trigger more droughts but would also increase the incidence of mosquito-borne viruses. "The expansion of tropical diseases is, without doubt, one of the greatest threats to humanity," Mr Turton said.

A major concern for public health officials is the potential encroachment into the southern parts of Australia of dengue fever, which is endemic to the tropics. It thrives in warm and moist conditions, while bio-security consultants have warned that the disease could eventually penetrate as far south as New Zealand. "Dengue fever is the key risk," cautioned Phil Freeman from the Australian Conservation Foundation.

"It can incapacitate a healthy person for many weeks and for kids and the elderly it is particularly dangerous." "The southward spread could have some big economic costs on our health system. You've also got to think of the disruption to our key industries, including tourism. "It may be that fewer tourists want to come to a destination where tropical diseases are a risk and also for the broader workforce with losses in productivity if many workers are incapacitated," Mr Freeman added.

Unique flora and fauna in the tropics could also succumb to the effects of a shifting climate. In the hot, damp mountain ranges of Queensland's north-east coast, a Unesco World Heritage-listed strip of rainforest gives life to an abundance of species. Their finely balanced world is threatened by climatic upheaval, which could ultimately condemn many plants and animals to extinction. "We are concerned that we will get a shorter, wetter wet season possibly with an increase in the intensity of cyclones, which have a big impact on the structure of the rainforests but then we'll have a longer dry season and, in turn, that will challenge the ability of rainforests to persist year round," said Andrew McLean, executive director of the Wet Tropics Management Authority in Cairns.

"There is a species called the lemuroid ringtail possum that lives in the hills behind Cairns and it has in recent years become very rare and our concern is that it is an indication of the possible risk of climate change in our part of the world," he added. As government and campaigners prepare for talks to agree on a successor to the Kyoto Protocol in Copenhagen later this year, activists in Australia have demanded sweeping measures to protect the tropics and other vulnerable areas from man-made pollution.

"We know Australia is right in the firing line of climate change and as a nation we have probably more to lose than any other wealthy country," said Mr Freeman. "So, if we don't get a global deal it is clear that Australia has much to lose and could be in big, big trouble." [e-mail]