By 2100, if nations continue to burn fossil fuels at the current rates, three out of four people will be at risk from lethal heatwaves.

And even if the governments of the world act on promises they made in 2015 and drastically reduce greenhouse gas emissions, almost one in two could face the risk of sickness and death by intolerable heat.

That is because, as the temperatures rise, heat and humidity begin to challenge human physiology. Humans are adapted to body temperatures of around 37C. If humidity the levels of water vapour in the air go up with the thermometer, then people caught in a zone of extreme heat cannot adjust body temperatures by perspiration.

And with every 1C rise in temperatures, the capacity of the air to hold moisture goes up by 7%. People with no access to air conditioning or a cool breeze become at high risk.

It happened in Europe in 2003, when an estimated 70,000 died. A heatwave in Moscow in 2010 killed around 10,000. And researchers warned years ago that under global warming predictions, more such extremes of heat would become inevitable by 2020.

Heatwaves pose a considerable risk to human life because hot weather, aggravated with high humidity, can raise body temperature, leading to life-threatening conditions.

We are running out of choices for the future, said Camilo Mora, a geographer at the University of Hawaii at Manoa, who led the study.

For heatwaves, our options are now between bad or terrible. Many people around the world are already paying the ultimate price of heatwaves, and while models suggest that this is likely to continue, it could be much worse if emissions are not considerably reduced.

The human body can only function within a narrow range of core body temperatures around 37C. Heatwaves pose a considerable risk to human life because hot weather, aggravated with high humidity, can raise body temperature, leading to life-threatening conditions.

Dr Mora and colleagues warned years ago that by 2016 climate would change inexorably in at least some regions of the globe. More recently he and colleagues calculated that the relentless increase in carbon dioxide levels in the atmosphere could limit the growing season and pose a threat to world food security. History has yet to deliver a verdict on either prediction.

But the warning about heatwaves starts from facts already available. One scientific group has calculated the humidity and temperature hazards and predicted that at least one climate zone the Gulf between Iran and the Arabian Peninsula could become murderously hot by the centurys end.

A second study of heatwaves over recent decades in India has established a link between extremes of heat, climate change and mass death.

Dr Mora and colleagues in the US and Britain report in Nature Climate Change that they found evidence on a global scale. They began with 30,000 relevant publications and identified 911 scientific papers with data on 1,949 case studies of cities or regions where deaths were associated with high temperatures.

From this mass of information they found 783 lethal heatwaves in 164 cities across 36 countries, with most cases recorded in developed countries at mid-latitudes since 1980: in cities such as New York, Washington, Los Angeles, Chicago, Toronto, London, Beijing, Tokyo, Sydney and So Paulo.

From this data, they found a common threshold at which temperatures and humidities became lethal: that is, as relative humidity climbed, even lower temperatures could kill. And then they devised a world map of those cities and regions most at risk.

Right now, one human in three lives in a climate zone in which death by extreme heat is or could be possible. The area in which such conditions are liable to happen on at least 20 days a year is predicted to grow.

By 2100 New York could have around 50 days in which conditions could be potentially lethal. In Sydney, Australia the number of such deadly days could be 20, for Los Angeles 30.

For Orlando, Florida and for Houston, Texas the entire summer could exceed the thresholds at which people have been known to die.

People are talking about the future when it comes down to climate change, but what we found from this paper is that this is already happening. In fact since 1980 we found close to 2,000 cases of these places and cities when people died from this, and this is obviously going to get a lot worse, Dr Mora said.

Notoriously, President Trump has announced that he will withdraw the US from the Paris Agreement of 2015 to contain average global warming to well below 2C by the centurys end.

The implication of the Hawaiian research is that if nations act in a concerted way to reduce fossil fuel emissions, an estimated 48% of the human population could be at risk of summer extremes. And if they do not, this hazard rises to 74%.

Climate change has put humanity on a path that will become increasingly dangerous and difficult to reverse if greenhouse gas emissions are not taken much more seriously, Dr Mora said.

Action like the withdrawal from the Paris Agreement is a step in the wrong direction that will inevitably delay fixing a problem for which there is simply no time to waste.

This story was originally published by Climate News Network.

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Heatwaves are testing the limits of human physiology more and more - Fusion