By 2070, heatwave could force up to 400 million Chinese to find refuge in the south

400 million refugees heading south. They're desperate to escape what can kill them in just six hours. The year is 2070. Photo / Getty Images

Now we know why Beijing is so interested in the South China Sea: Killer heatwaves are set to sweep across northern China within just 50 years. If they don't leave, 400 million citizens could face a day where they're left with only hours to live.

And that day will be a pressure cooker.

A report published this week in the science journal Nature Communications details the effect of climate change on China's great North Plain, which contains the megacities Beijing and Tianjin. The area's once fertile open fields have become among the most densely inhabited places on Earth.

People trying to cool off at a water park in Suining, southwest China's Sichuan province, as a heatwave hit several provinces in China. Photo / Getty Images

But things are warming up. Fast.

"This spot is going to be the hottest spot for deadly heatwaves in the future, especially under climate change," warned lead author MIT professor Elfatih Eltahir.

And the kinds of heatwaves the data predicts will be among the worst on Earth.

Even in the shade, the ambient heat and humidity could end up killing humans within six hours.

Perfect heatwave

Ask any Territorian: it's not just the heat that's the problem. It's the amount of moisture in the air that determines whether or not you get a chance to cool down.

It's called 'wet bulb' temperature.

The human body's ability to withstand heatwaves depends on its ability to sweat — and for that sweat to cool the skin through evaporation.

Extreme humidity means there's no room in the air for that sweat to go. So it just clings to you.

And your body heats up.

Even a healthy adult cannot survive outdoors in a 'wet bulb' of 35C for more than six hours.

"If the wet bulb temperature exceeds the human body's skin temperature of 35C, perspiration no longer works as a cooling mechanism," Seaver College of Science and Engineering Professor Jeremy Pal said. "The body will quickly overheat, resulting in death."

A 'wet bulb' of 35C can be produced with 44.4C heat in 55 per cent humidity.

At 85 per cent humidity, 'wet bulb' conditions are created at just 37.8C.

"When it is both very hot and humid outside, heat in the body cannot be expelled," University of Hawaii researcher Camilo Mora said. He developed the model used to calculate deadly heat days under different climate change scenarios.

"This creates a condition called 'heat cytotoxicity' that is damaging to many organs," he told AFP.

"It's like a sunburn, but inside the body."

The Nature Communications report says weather conditions capable of generating a killer 'wet bulb' could be in place by 2070.

Children cool off at a fountain in a park. A heatwave has hit many regions in China. Photo / Getty Images

Blowtorch on China

The historical weather archives for northern China point in a terrifying direction.

Records show that, since 1970, heatwaves there have become both more intense and more frequent. Since 1990, their frequency has exploded.

According to the paper, the average temperatures in the North China Plain are already consistently an average of 1.35C above those recorded during the 1950s.

Extreme heatwaves have begun persisting for periods of up to 50 days, the study shows.

As just one example, Shanghai, East China's largest city, broke a 141-year temperature record in 2013. Dozens died.

Essentially, the region is experiencing climate change at double the rate of the rest of the world.

New #climate paper shows risk of deadly #heatwaves increased by intensive irrigation in the North #China plain https://t.co/pYNmpXOlk6

— Nature Communications (@NatureComms) August 1, 2018

And that means Beijing has a problem of a scale unlike anything seen in history.

And it's being made worse by its solution to another problem: hunger.

Much of the North China Plain has been irrigated to maximise its agricultural output.

It's China's 'bread basket'. Tens of millions of farmers toil the fields by day to feed enormous cities such as Beijing.

Computer models show that same irrigation network is adding a further 0.5C to the region's temperatures, as well as the humidity.

"Irrigation exacerbates the impact of climate change," Professor Eltahir says.

A Chinese H-3K strategic bomber takes off into a red dawn. China has been dramatically increasing its military presence along its southern borders. Photo / supplied

Burning issue

"The North China Plain is likely to experience deadly heatwaves with wet bulb temperatures exceeding the threshold defining what Chinese farmers may tolerate," Professor Eltahir says.

So, in the 2070s, the day is coming where those farmworkers will die from heat stroke within just six hours — whether they're resting in the shade or not. Conditions within the cities will be terrible — but survivable through airconditioning.

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But food supplies will reach crisis point. And living conditions would be untenable.

Some 400 million people could be forced to flee for cooler climates.

The report concludes: "China is currently the largest contributor to the emissions of greenhouse gases, with potentially serious implications to its own population: Continuation of the current pattern of global emissions may limit habitability of the most populous region of the most populous country on Earth."