The tropics are coming

The tropical regions of the planet are rapidly expanding towards the poles, and we’re set to see conditions that humans have never experienced before.

In 2016, when James Cook University ecologist Dr Mark Ziembicki trekked to the relatively cool and mossy summit of Mt Lewis in tropical Far North Queensland, he had one thing on his mind – finding an animal that sits at the brink of extinction.

After several nights of searching in vain, Dr Ziembicki and the team finally spotted one – a white lemuroid ringtail possum, perched high in the forest canopy, its eyes flashing bright in the beam of a torch.

The elusive marsupial is particularly susceptible to the effects of climate change, as it’s found only in the rapidly shrinking mountaintop cloud forests of northern Australia. It can’t survive for more than five hours in temperatures above 30°C, making it extremely vulnerable to the heat-waves now striking its habitat with ever-increasing frequency.

This variant of the lemuroid ringtail possum is just one of many plants and animals found in dwindling island-like patches of habitat atop mountains in the world’s equatorial regions. And they’re all highly imperilled by rising global temperatures.

Lemuroid ringtail possum. Credit Dr Mark Ziembicki

Credit: Dr Mark Ziembicki

You might think of polar bears and melting ice caps when it comes to climate change, but tropical species are far more susceptible to the small changes in temperature that we’re currently experiencing.

Roughly 23 per cent of tropical mammals are currently threatened with extinction, compared to 13 per cent of mammals in other parts of the world.

“In terms of biodiversity impacts, the Tropics are set to lose far more, because they’ve not only got the highest number of threatened species, but also the highest proportion,” says Dr Ziembicki, a senior researcher with JCU’s State of the Tropics project – a global alliance of institutions focused on the social, economic, and environmental issues affecting tropical regions.

Can’t sweat it out

As global temperatures rise, and the heat and humidity we associate with the Tropics spread across a wider belt of our planet, we can expect to lose biodiversity; suffer more intense extreme weather events; endure the spread of tropical diseases; face more deaths from heat-waves and lethal temperatures, and experience reduced agricultural productivity.

And those are just a handful of the issues we’ll have to grapple with in an increasingly tropical world.

The tropics have historically been defined as the region between the Tropics of Capricorn and Cancer – the part of Earth’s surface where the Sun is directly overhead at least once a year.

But tropical climes are more loosely said to cover the warm region between a latitude 30° north and 30° south of the equator, which has little seasonal change and a consistent mean monthly temperature above 18°C.

A variety of measurements are now showing that the subtropical regions found to the north and south are expanding. What’s recently been dubbed “Earth’s bulging waistline” is said by some researchers to be heading poleward in both directions at a rate of between 56 and 111 km per decade.

Worst-case scenario modelling of this suggests that London could be experiencing a climate more like current-day Rome in less than a century.

“Those climate conditions that we associate with the Tropics are likely to become more prevalent in the subtropics,” says Ann Penny, State of the Tropics project manager.

“In Australia, the most recent projections show that, towards the end of the century, Sydney will become more like Brisbane, and Brisbane will be more like Bundaberg.”

Map indicating the world's tropical and sub-tropical regions. Credit: KVDP/Wikimedia

Map indicating the world’s tropical and sub-tropical zones. Credit: KVDP/Wikimedia

If you live somewhere that’s currently cool, like London or Moscow, this added warmth might sound rather nice, but the consequences for people living in the tropical regions of the planet are dire.

A combination of heat and humidity could see conditions reaching the point where people are unable to sweat efficiently enough to keep their body temperature below 37.5°C, leading to heat illness, heat stroke, and potentially death, researchers have predicted.

“Singapore will become the hottest city on Earth, with year-round rainfall – conditions that human communities have never experienced,” Ms Penny notes.

Kinshasa in the Democratic Republic of Congo and Lagos in Nigeria are expected to suffer similarly unbearable conditions.

The economic consequences

Well before this point, the economic productivity of some tropical and subtropical regions will begin to fall, as the warmer conditions affect people’s ability to work, particularly in developing regions where they cannot afford air-conditioning, and creaking electricity grids fail to keep up with demand.

Darwin currently has just nine or 10 days a year above 35°C, but according to some models, this is projected to rise to more than 300 by the year 2100, Ms Penny says. “Combined with humidity, this will make it impossible to work outside for much of the year.”

According to a study published in late 2017, while 30 per cent of the world’s population is currently exposed to potentially deadly heat 20 days per year, this could rise to nearly three-quarters of the planet by 2100.

Many of the worst affected people will be in tropical and subtropical regions of the world, such as the 1.5 billion people living on the Indian subcontinent, where few have air-conditioning, and many work outside.

“Singapore is relatively rich, and people have air-conditioning. But what will the implications be like for the most vulnerable communities around the equator?” says Dr Ziembicki. “Conditions are going to be hotter and wetter than at any other time in human history.”

Vector-borne diseases that are now largely confined to the Tropics – malaria, dengue fever, and schistosomiasis – will extend their ranges north and south with the climate. This will impact a much larger chunk of the world’s population, 42 per cent of which is currently found within tropical regions.

Extreme weather events also typically associated with tropical and subtropical regions, such as flooding rains and intense tropical storms, are also predicted to spread over a much greater area of the planet.

Already in Taiwan, the frequency of typhoons has doubled since 2000 as ‘Typhoon Alley’ shifts north, while typhoons striking an area from Vietnam and the Philippines to Korea have increased in intensity by 12 to 15 per cent since 1970.

In Australia, we may see tropical cyclones travelling hundreds of kilometres further south by 2050, Ms Penny says, with the remnants of these maelstroms even hitting Sydney.

But it might not all be bad news.

There are potential benefits of the poleward shift of warmer climes. Some nations, such as Russia and Canada, will see an upswing in their agricultural productivity, and currently subtropical and Mediterranean regions may one day be able to grow rice, sugarcane, and other tropical crops, while wine-growing regions expand north and south.

But sadly for the white lemuroid ringtail possum of Australia; the mountain gorilla of Africa’s Virunga Mountains; and the Huon tree kangaroo of Papua New Guinea, the displacement of their habitats by encroaching hotter tropical conditions could very well seal their fate.

The best chance they’ve got is for researchers to better understand what they’re up against, so they can inform the world’s conservation efforts in response to this looming tropical threat.

By John Pickrell

Find out more about the research being carried out by the State of the Tropics project at JCU.

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