Roads have twisted in the heat. Hospitals are overwhelmed by thousands of dehydrated people, the poor, the elderly and children among the worst hit. Urgent instructions to wear wide-brimmed hats and light-coloured cotton clothes, use umbrellas and drink lots of fluid have been issued by the government.

India is struggling to cope with one of the deadliest heatwaves to hit the subcontinent. And its attempt to do so is raising a question for the whole planet – how can humans cope with the kinds of temperatures that scientists fear may become ever more common?

In only 10 days, the death toll is reported to have reached around 1,800, a 20-year high. The brunt of the burden has fallen on the southern state of Andhra Pradesh, where 1,300 people have died, the highest loss of life due to heat the state has known, according to officials. By comparison, 447 people in the state died from the heat last year.

The blistering temperatures may, mercifully, be about to break, as there are signs that the longed-for monsoon may be arriving in the form of showers and thunderstorms in the south. But as temperatures neared 50C in recent weeks, leave for doctors has been cancelled and the government has issued warnings for people to stay indoors and stay hydrated. For many, though, that isn’t an option.

The greatest loss of life has been among the elderly and the poor, in particular day labourers who have little alternative but to risk their lives in the heat for fear of losing their livelihoods.

“People are dying and suffering, and I think it shows us the vulnerability that exists even in an area that is used to very hot weather,” said Peter Stott at the Met Office in Exeter. “It can still have a major impact.”

It is hard to say for sure whether any single extreme weather event is a result of manmade climate change, but this is a scenario we should get used to seeing more of, scientists say. As the effects of climate change take hold and global temperatures creep up, extreme heat events will become more common.

“Recent research shows that heatwaves are currently five times more frequent than they would be in the absence of human-caused warming, and the chance of any particular heatwave being caused by climate change is 80%,” a spokesperson for Greenpeace said, adding that unless efforts are made to cut carbon emissions, heatwaves are forecast to be 12 times more likely by 2040.

These trends will push the capabilities of the human body beyond the limits of the cooling systems it has evolved and raise serious questions about the future of food production and farming. Globally, more than half of all working hours outside the home are spent outdoors, mainly in agriculture.

As the heatwave in India shows, those who have to work outside in hot conditions are at the greatest risk of heat exhaustion and heatstroke. There are reports that construction workers in the Indian city of Gurgaon had no choice but to continue working all last week in temperatures higher than 43C, in order to earn their wage of 200 rupees a day (about £2).

“If everybody had the same lifestyle as Europeans and Americans, it wouldn’t be so much of an issue, but most of the food that is generated and grown in south-east Asia, Latin America and parts of Africa comes from small landholders,” said Mark Maslin, professor of climatology at University College London (UCL).

“In the south of the US people in air-conditioned tractors are farming vast areas in an industrial way, but most of the world works on smallholders producing enough food to feed themselves and enough surplus to provide food for the local towns and cities.”

This problem is exacerbated by the fact that humidity in the areas most affected by climate change poses an insurmountable challenge to the way the human body deals with heat.

“Humans can survive for short periods in environments hot enough to cook a steak, as long as the air is dry,” said Matthew Huber of the Department of Earth Sciences at the University of New Hampshire, who has studied the extent to which humans might adapt to heat stress as a result of global warming. Those same temperatures in more humid climates would kill us. “Humans have a vast and nearly unique capacity to cool themselves by sweating – but this only works when the air is relatively dry,” he added.

In very dry conditions, people can work outside in temperatures of up to 40C. But the safety cutoff drops below 30C when you have very high humidity. To calculate the limits in which it is safe for people to work in extreme heat, scientists rely on a measure of temperature that takes into account both the heat and the humidity.

This is known as the wet-bulb globe temperature. At wet-bulb temperatures higher than 35C, human skin can no longer itself cool down through evaporation. The US military suspends training and physical exercise when this temperature exceeds 32C. Peak wet-bulb temperatures measured in the heatwave in India are around 30-31C.

“While this value is less than the 35C threshold, that threshold is the value for a healthy, fully hydrated person in the shade who is not engaged in labour,” said Huber, explaining why the heatwave in India has been so lethal. “On the other hand, for poor workers in India, not working can also carry serious consequences.”

Facebook Twitter Pinterest A man takes bath under the tap of a water tanker on a hot day in Ahmadabad. Photograph: Ajit Solanki/AP

In recent years, several groups have used this measure to make predictions about what rising temperatures will mean for workers worldwide, and to paint a picture of what global productivity will look like as average temperatures creep up. A recent study published in Nature Climate Change estimates that heat stress has already reduced global labour to 90% of capacity during the hottest months of the year. Under the most dire climate change projections, this could fall as low as 40% by 2200. The regions predicted to be worst affected include India, northern Australia and the south-east of the US.

But countries that cannot afford technological adaptations such as air conditioning will be affected the most. “As climate change starts to bite and these heatwaves come, people in poor countries are very vulnerable because they don’t have another source of income,” said Maslin. “So if their crops fail and they are not able to work outside, they don’t have the money to buy food so they starve.”

The effects of crop failure are also likely to have global repercussions in terms of food security. In 2009, Maslin was involved in a report published by UCL and medical journal The Lancet on how to manage the health effects of climate change. It said that the biggest threat from climate change was to food and water security.

“You have a double whammy: your populations are increasing and the heatwaves are increasing. So the amount of food you can produce is going to decrease and there you have a major crisis,” Maslin said. He pointed to a Russian heatwave in 2010 that affected global wheat prices.

Although numbers are hard to pin down, it is forecast that many people will be forced to leave their homes, making climate migration an international issue. About 17 million people were displaced by natural hazards in 2009, and 42 million in 2010, according to a British government report. People who work outside, in extreme cases of climate change, may become seasonal migrant workers, Huber said.

In the short term, policies that could help include instituting international work standards for heat conditions, or even switching to night-time working to reduce exposure. “I predict that access to air conditioning – or an equivalent technology – will become a fundamental right enshrined in international law,” Huber said.

Nevertheless, such technological solutions come with their own problems. Relying on air conditioning for survival means that power cuts – which are common during heatwaves – would become life threatening. Much better, scientists say, would be to reduce global warming. The next Lancet commission, a follow-up to the 2009 report, is due next month. “It focuses on cutting emissions and why this is so important for global health,” Maslin said.“The reason is that, as far as economists can see, there is no way of ensuring those people have enough money to be protected from the problem of not being able to grow food.”

Stott agreed. Bleak as the picture is, he said: “The key thing is where emissions go. When we think about the future, it’s a very different world depending on what happens to carbon emissions – and there’s still plenty of opportunity to avoid those worst effects. The next few years are crucial. It’s all to play for.”

HOW THE BODY REACTS TO HEAT

Cooling The body works hard to keep its core temperature at around 37C. As it heats up, blood vessels expand, sending blood to the surface of the skin to cool it down. Sweating increases in an attempt to cool the surface of the skin through evaporation, though high humidity can make this impossible.

Heat exhaustion After several days of heat exposure, the body’s attempts to cool itself down begin to take their toll. Lost fluids and salts cause muscles to stop working, leading to cramps and weakness. Reduced blood flow to the brain causes headaches and dizziness. The elderly and people with high blood pressure are most at risk.

Heatstroke The body’s cooling system becomes overwhelmed and shuts down, and body temperature can rocket to 41C or more in 10 minutes. Organs can no longer function, and if heatstroke isn’t treated fast, the brain overheats, sometimes leading to death.

Sunstroke This is the most common type of heatstroke, caused by the sun shining directly on the head and neck for long periods.

• The article was amended on 6 June 2015 to correct the affiliation of academic Matthew Huber.

