When Hurricane Harvey blasted ashore in August, drowning south Texas in a year’s worth of rain in just a few days, it left behind an estimated $150 billion in damage to sodden homes and inundated factories, and claimed about 60 lives.

Two weeks later, Hurricane Irma churned into Florida, killing at least 33 people there and causing billions more in damages - as well as brutal loss of life in the Caribbean.

But these storms may not be 2017’s deadliest U.S. disaster. Instead, that title may go to a largely unseen killer: rising temperatures.

Over the last 30 years, increasingly broiling summer heat has claimed more American lives than flooding, tornadoes or hurricanes, according to the U.S. National Weather Service.

And the problem has not been limited to the United States. More than 35,000 people died during a European heatwave in 2003, and tens of thousands perished in Russia during extreme heat in 2010.

The threat is particularly severe in already sweltering places, from South Asia to the Gulf, and has been linked to a rise in migration out of hot and poor parts of rural Pakistan.

But experts say heat remains underestimated as a threat by governments, aid agencies and individuals. That’s both because it’s an invisible, hard-to-document disaster that claims lives largely behind closed doors - and because hot weather just doesn’t strike many people as a serious threat.

“If you have a natural disaster like a cyclone or an earthquake or a flood, the impacts are immediate. Things get washed away, people drown. But heat is a silent killer,” said Sarah Perkins-Kirkpatrick, a climate change researcher at Australia’s University of New South Wales.

“In Australia, heatwaves kill more people than any other natural disaster - but no one realises the destruction they can cause. The attitude is, ‘It’s hot, suck it up, get on with it’.”

Around the world, heat is a neglected and poorly understood disaster, in part because few of the deaths it produces are directly attributed to heatwaves.

Victims - many elderly, very young, poor or already unhealthy - often die at home, and not just of heat stroke but of existing health problems aggravated by heat and dehydration.

In India, for instance, a major risk factor for women - who die of heat far more often than men, researchers say - is the lack of an indoor toilet.

To avoid embarrassment or harassment, many women refrain from drinking water during the day to limit their trips to the toilet - a potentially deadly strategy during heatwaves.

“These deaths are recorded as normal deaths. But they wouldn’t have happened if it wasn’t so hot,” said Gulrez Shah Azhar, an Indian heat researcher who works for the RAND Corporation, a global think tank.

HOTTER CITIES

To find the true rate of deaths during heatwaves, health officials look at “excess” deaths - how many more people died than would otherwise be expected during that period.

In places used to dealing with hot conditions, there is a “diagnostics failure” in recognising the risks of extreme heat, noted Eric Klinenberg, an American sociologist and expert on a deadly 1995 Chicago heatwave.

In steamy cities like Miami, “there’s a sense we know how to deal with heat here, while everybody else is complaining”, he said. “There’s a will not to see the risk.”

City dwellers, from Bangkok to Cairo, face particular - and growing - risks. In many rural areas, trees and open land planted with crops help daytime heat subside at night, providing some respite.

But in cities, acres of concrete and asphalt absorb warmth during the day and radiate it back at night, creating heat islands that can be nearly as hot at night as during the day.

During Chicago’s three-day heatwave in 1995, more than 730 people died, many of them older people living alone and already facing health problems. With city services overwhelmed, hospitals turned away emergency cases, and the city’s morgue had to rent refrigerated trucks to store the dead.

With more than half the world’s population now living in cities - and two-thirds of people expected to live in them by 2050 - finding ways to reduce urban heat will be crucial to saving lives as climate change ramps up heat extremes.

COOLING DOWN

Many parts of the world, especially those that have already seen substantial heat deaths, are experimenting with ways to lower the risks.

In New York, and a host of other cities, authorities are painting roofs white to reflect more heat and cool inhabitants. They are planting more trees and preparing hospitals to deal with more heat stroke and other health problems when temperatures soar.

A pioneering heat action plan in Ahmedabad, a city in western India, for instance, automatically triggers deliveries of water to slum areas when temperatures hit dangerous levels, as well as opening neighbourhood cooling centres.

Simple measures like this are a clear, economical way to save lives, their backers say.

“The cost of setting up a heat preparedness plan is orders of magnitude cheaper than the cost of lives. It’s background noise,” said Azhar, who helped develop the Ahmedabad plan after a 2010 heatwave that killed more than 1,300 people in the city.

In other countries, architectural styles designed to deal with heat - often abandoned when air conditioning became available - are being revived.

Construction workers, who can’t avoid being outside on hot days, are being kitted out with gel-filled cooling vests and collars.

Governments are experimenting with cloud-seeding to bring cooling rain. And some scientists are already discussing “geoengineering” the planet using sun shields in space or sulfur particles in the atmosphere to reflect the sun’s heat.

China, one of the countries grappling with worsening heatwaves, is trying to create “sponge cities” with abundant trees and green areas that can absorb heavy rainfall, then gradually evaporate the cooling moisture back into the air.

POWER TO THE PEOPLE

In most hot places, people are advised to stay inside on sweltering days, drink more water, wear cool clothing and avoid strenuous activity. But those things can be hard to manage in practice when there’s work to be done and deadlines to be met.

“I was working at home on a 45-degree (113-degree Fahrenheit) day, and the guy across the street was still building a home in that heat,” recalled Perkins-Kirkpatrick, who lives in Sydney. “There needs to be more education that it’s not okay to be outside in those conditions.”

In rural areas, as well as many urban ones, lack of access to electricity can be one of the biggest risks during heatwaves.

India alone has 300 million people without a power connection, which means they cannot turn on a fan or air conditioning when temperatures soar.

In New Delhi, some of the poorest of the poor, living on the streets, sleep near the curb of busy roads at night, hoping to catch a breeze from passing cars.

When Azhar was growing up in the Indian city of Lucknow, in a home without electricity, “there was no escape” from debilitating summer heatwaves. “You’re trapped and there is literally nothing you can do,” he said.

Experts say one clear way to reduce growing health risks from heatwaves is to provide more of the world’s population with access to power, particularly in the hottest areas.

“The best way to mitigate (heat deaths) is to get electricity” to run fans or air conditioning, said Steven J. Davis, a University of California, Irvine earth system science professor and one of the authors of a 2017 report that predicted a growing risk of widespread deaths during Indian heatwaves.

Global efforts, including as part of the Sustainable Development Goals, to bring power to those without it could play a significant role in reducing heat deaths, experts say.

But if action to curb climate change is not robust enough, heatwaves could more often overwhelm or break down power grids, leaving rich and poor without help to cool down, they warn.

GAINS AND LOSSES

In many poor communities, deadly heat is driving innovation. In the slums of Bhubaneswar, a humid city in eastern India where temperatures last summer hit 46.8 degrees Celsius (116 degrees Fahrenheit), families have learned to soak jute sacks in water and place them on their tin roofs, to cool the inside.

And rickshaw drivers have added wet cooling mats to their vehicle roofs, drawing more customers and boosting their income.

“There was never a day all this summer that he didn’t bring home 500 rupees ($8)- a third more than others did,” said Kumari Behera, a Bhubaneswar resident whose son drives one of the “air-conditioned” rickshaws.

Intensifying heat presents many risks beyond growing loss of life. It is a contributor to longer and more intense droughts and water shortages that are destroying harvests and creating more severe forest fires, with choking smoke.

Higher temperatures can increase smog production in cities, as heat “cooks” pollution from vehicles, and extend the length of allergy seasons, prolonging periods of misery for millions.

And while rising temperatures may boost harvests in some parts of the world, extreme heat threatens to slash farm production in many areas, particularly of staple crops such as wheat, maize, rice and soybeans, scientists say.

In the shorter term, rising temperatures present opportunities for some cooler parts of the world, however.

In northern Canada, for instance, warmer weather is opening once frost-prone land to farming for the first time, raising the prospect of new harvests in new places.

“There’s a lot more interest in taking a look at underdeveloped land in northern Ontario and Quebec because of changes in climate,” said Rod Bonnett, president of the Canadian Federation of Agriculture.

Fishermen around the world, meanwhile, are finding different species in their nets, many driven into new waters by ocean warming. Iceland’s fishermen alone have spotted more than 30 new species in the last two decades, as some old ones disappear.

“The gains and losses seem to be balanced for now. But in the long term, I think (the change) will be slightly positive,” said Hreidar Thor Valtysson, an Icelandic natural resources specialist, whose children now catch mackerel near the Arctic Circle.

DISEASE, MIGRATION

Rising temperatures also look set to change the world’s disease threats. Mosquitoes that spread potentially deadly viruses from Zika and dengue to chikungunya thrive better in warmer climates than their malaria-carrying cousins, researchers at Stanford University found.

That means malaria rates could fall in some parts of sub-Saharan Africa, for instance, as temperatures rise and malaria-carrying mosquitoes struggle - or move to cooler areas of the continent.

But communities that see malaria diminish likely will face new threats, including from diseases that are less researched and have lower-funded eradication efforts.

“We have this intriguing prospect of the threat of malaria declining in Africa, while Zika, dengue and chikungunya become more of a danger,” said Erin Mordecai, a Stanford University professor.

Fiercer heatwaves also are raising questions about the limits of humanity’s ability to adapt.

Christian Clot, a French-Swiss explorer, has been testing humans’ limits in extreme conditions, including the nearly 60-degree Celsius (140-degree Fahrenheit) heat of Iran’s Dasht-e Lut desert.

What he and doctors have found is that the ability to do just about anything - including sweat, work and think - diminishes as exposure to extreme heat grows.

That is a worry with three in four people in the world expected to face deadly heat by the turn of the century, according to a study published in the journal Nature.

“We think we’re stronger than nature - but we’re not,” Clot warned.

Researchers believe heat-related migration - already underway - will ramp up.

A 2014 study in Pakistan found that increasing heat - rather than worsening flooding, as thought - was a strong driver of migration out of agricultural villages over a 20-year period.

And in India’s southern state of Tamil Nadu, Vinod Kumar’s family is already on the move after heat and drought repeatedly wrecked their crops.

“At this time of year, these fields should be green with paddy shoots, but no one seems to be farming,” he said, driving past arid land overgrown with scrub and thorns.

With too much heat and too little water, “it has become impossible to make a living from farming”, Kumar said. He now drives a taxi in Chennai to get by.

HEAT DAMAGE

Higher temperatures are also a driver of worsening wildfires that have scorched woods from Los Angeles to Italy and Canada this year, and killed more than 60 people in Portugal alone.

In northeast Spain, farmers spooked by soaring fire risk have moved herds of goats and sheep into nearby forests to clear underbrush, in an effort to reduce the chance of runaway blazes.

“We’re very worried,” said Pau Figueras Mundo, a 36-year-old herder, as his animals nibbled at the fringes of a forest.

Paradoxically, however, it’s the relative lack of obvious damage from most heatwaves that results in them being underestimated as a threat, experts say.

“Heat is by no means taken as seriously by governments as hurricanes or earthquakes or floods. I believe that’s because a lot of our system for providing relief and assistance is based on property damage,” U.S. sociologist Klinenberg said.

Extreme heat can lead to the buckling of roads and railway lines, and reduce the capacity of power grids. But most of the damage remains “less photogenic” than other disasters, he said.

Preparing for worsening heat shocks will require planning and broader resilience-building efforts, experts say.

In Ethiopia, aid agency World Vision has helped rural communities re-grow felled forests to provide shade, hold more moisture in fields, and give farmers alternative ways to earn money when drought destroys crops.

“The more prepared we are for heat stress, the more that communities can conserve water and change their agricultural practices, (and) the more they can absorb acute stress,” said Maggie Ibrahim, a resilience manager with World Vision.

Similar preparation needs to happen in many parts of the world, said Australian heat expert Perkins-Kirkpatrick.

“In Australia we have heat plans but they’re not very detailed. There needs to be a national plan and money put into infrastructure - not just advice like drink water, stay inside and check on your elderly neighbours,” she said.

That advice is good and necessary, but “it’s not enough”, she said. Soon “everyone” will be affected, including people out on a run or mothers walking their kids to school, she added.

INVISIBLE DISASTER

One key problem in getting heat action plans in place - already a challenge with a largely invisible disaster threat - is the politics surrounding climate change.

Heatwaves are the hazard most clearly tied to global warming. But U.S. President Donald Trump’s Republican-led administration has denied climate change is a significant risk.

That has hampered determined efforts by many U.S. cities, states and companies to prepare for what they see as increasingly evident climate threats.

Despite an international agreement to curb climate change reached in Paris in 2015, cuts in the use of fossil fuels around the world are not yet ambitious enough to meet the accord’s goal of keeping warming to 1.5 to 2 degrees Celsius above pre-industrial times.

Instead the world is on a path towards at least 3 to 4 degrees Celsius of warming by the turn of the century, scientists say.

“How is the global community going to respond to that?” Ibrahim asked. “Are we just going to accept millions of deaths? We do now around drought, but will we do that around heat exhaustion? And how are we going to manage the migration flows?”

“We are not at all prepared globally for the big numbers that will be affected,” she said.

Texas and Florida, still recovering from Hurricanes Harvey and Irma, may have particular reason to worry in the years ahead, scientists say.

An analysis by Climate Central, a U.S. non-profit science and media organisation, found that Houston by 2030 is likely to face “heat danger days” - when combined heat and humidity make temperatures feel like 105 degrees Fahrenheit (40 degrees Celsius) - 110 days a year.

Miami will face 126 such days each year by 2030, it noted.

“A one-off every now and then we can recover from,” said Perkins-Kirkpatrick. “But we’ll be seeing this almost every summer in the next 40 or 50 years. We need to do something about it.”