Rising temperatures and humidity will make the world’s tropics increasingly unliveable by pushing more people to the thresholds of their physical tolerance and beyond, a new international study finds.
As of 2000, about 30 per cent of the world’s population lived in regions where the climate exceeds deadly threshold levels – based on temperature and relatively humidity levels – for at least 20 days a year, researchers publishing in the Nature Climate Change journal estimate.
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Even with the most optimistic scenario for greenhouse gas emissions reductions, that share will rise to about 48 per cent by the end of the century. If so-called business as usual emissions continue, that share would climb to 74 per cent by then, the paper found.
“You are going to have all of those people in the tropics ‘cooking’ there because they are not going to have any possibility to cope with this [increase in heat and humidity],” said Camilo Mora, the paper’s lead author and an associate professor in the Department of Geography at the University of Hawaii, Manoa.
The research examined studies of 783 cases of excessive human deaths from heatwaves between 1980 and 2014. These included the 70,000 deaths from a huge European heatwave in 2003 and more than 10,000 deaths in Russia in 2010.
Professor Mora noted that identifying heatwave mortality rates was difficult because the causes – such as heart attacks and other organ failure – may only surface some time after the event.
“I think there is a huge underestimate of that even in developed countries,” such as in Europe, North America and Australia, he said. For developing nations with poorer record keeping, the deaths could be larger still.
Even though tropical areas, including northern Australia, were not projected to warm as much as regions further from the equator, conditions are already near tolerance levels with further warming forecast.
A family cools off in a stream during a heat wave last month in Islamabad, Pakistan. Photo: AP
“For many of these tropical places, the whole year becomes a heatwave,” he said.
Air-conditioning offered some relief but would make people “prisoners of their own homes” – so long as electricity supplies didn’t fail, Professor Mora said.
People cool off in a spray from a broken water pipe during a heat wave in Karachi, Pakistan last month. Photo: AP
Sophie Lewis, a heatwave specialist at the ARC Centre of Excellence for Climate System Science, said the study highlighted how poorer populations in particular were going to struggle.
“If we’re seeing an increase of 100 days a year that might be deadly in places that rely on subsistence agriculture, it’s going to be pretty hard for people to be outside working,” Dr Lewis said.
The paper provided an interactive graphic that estimated the populations most at risk under different emissions scenarios.
The chart [below] shows the risks under the most ambitious scenario of cuts out to 2100, based on the so-called 2.6 representative concentration pathway (RCP).
At the highest pollution scenario, the RCP8.5 pathway, most of the world’s tropics face high risks by the end of the century.
(See chart below.)
The heatwave report comes as much of south-western US faces a severe heatwave in coming days, particularly California and Arizona.
For populations at higher latitudes – such as Perth, Adelaide, Melbourne and Sydney – humidity levels may decline with future warming but those gains would be at least partly reversed by rising temperatures.
Soils, for instance, are becoming drier in those regions, reducing the moisture available for evaporative cooling, Professor Mora said. The process works much like a human loses heat through sweating.
“Unfortunately the mid-latitudes are drying out so much, the energy is retained there like a rechargeable battery,” he said.