Millions of people around the world are experiencing a scorching summer, as records are broken and thermostats climb this week in parts of Europe. Temperatures in Paris and Brussels exceeded 90 degrees Fahrenheit at a time of year when 70-degree weather is the norm, according to Accuweather.com.

In Bandar-e Mahshahr, Iran, temperatures climbed to 115 °F last week. The temperature, together with high humidity, felt like 163 °F to hapless people directly exposed to the weather, according to Accuweather.

That is the second-highest known “heat index” value ever recorded, said Maximiliano Herrera, a climatologist and weather aficionado who maintains one of the world’s most comprehensive datasets of extreme temperatures. The highest heat-index value ever recorded was 174 °F in 2003 in Dhahran, Saudi Arabia, he said. The highest air temperature in an inhabited area was recorded in Gotvand and Dehloran, Iran, and Turbat and Sibi, Pakistan, in the 1990s, when the thermostat climbed to 127.4 °F (53 degrees Celsius), Herrera said.

In June, Pakistan experienced a heat wave so severe that more than 1,229 people died. A month earlier, temperatures in parts of India climbed up to 113 °F, killing at least 2,500 people.

Including June, four months out of the first six in 2015 have broken global temperature records. July appears to be tracking the trend, even as a strong El Niño has formed, which will exacerbate global temperatures.

“I’d not be surprised if 2015 ends up the warmest year on record,” said Derek Arndt, chief of the monitoring branch at National Oceanic and Atmospheric Administration’s National Centers for Environmental Information, during a media call in June.

While scientists are still deciphering if particular heat waves, such as the ones in Pakistan and India, could be tied to climate change, it is accepted science that heat waves, broadly speaking, will become more frequent, intense and prolonged with global warming.

They are already the deadliest weather phenomenon in the world. Nine out of 10 heat waves with the most fatalities have occurred since 2000, according to data in EM-DAT, an international disaster database. They have caused 128,885 deaths around the world, according to the database.

Some areas may become uninhabitable

So what is the highest temperature a person can tolerate?

It depends on the amount of heat stress a person undergoes. This, in turn, is dictated not simply by the air temperature, but also by the humidity, wind speed, and the amount of long- and shortwave radiation a person is exposed to.

In a 2010 study in the Proceedings of the National Academy of Sciences, scientists explored the level of temperature and humidity—called the “wet-bulb temperature”—beyond which a human body can no longer dissipate heat by sweating, and the body temperature rises to life-threatening levels.

The scientists found that if humans were to be exposed to wet bulb-temperatures higher than 95 °F (35 ºC) for more than six hours, they would not survive.

Wet-bulb temperatures average 78 °F (26 ºC) in most places in the world today. That’s true even in the hottest deserts, where air temperatures can soar but the humidity tends to be low, resulting in tolerable wet-bulb temperatures.

But low wet-bulb temperatures may not hold in a warming world, the study found.

As humans double the levels of CO2 in the atmosphere above preindustrial levels, global temperatures are expected to rise by between 1.9 and 4.5 ºC, according to the Intergovernmental Panel on Climate Change.

If much of the fossil fuel reserves on the planet are burned, CO2 levels would more than double. As a result, global temperatures may rise by 12.6 °F (7 ºC), and many parts of the world would become uninhabitable, the study finds.

And if all the fossil fuel reserves are burned, temperatures would rise by 21.6 °F (12 ºC), and all of Earth will be intolerable to humans, the study finds.

Adaptation harder for the poor

Wet-bulb temperature does not correlate directly with the number of fatalities from a given heat wave. Although Bandar-e Mahshahr residents experienced wet-bulb temperatures exceeding comfortable limits last week, there were few fatalities.

That’s because the nation has infrastructure to minimize residents’ exposure to intolerable heat. Middle Eastern nations, which are sweltering hot throughout the year, are highly air conditioned.

People in Bandar-e Mahshahr do not stay outside in the summer for more than 15 minutes without cooling off, one resident told The Washington Post.

In contrast, poorer people in Pakistan and India do not have the economic means to cope with deadly heat. They may also have a poorer nutritional status and chronic diseases related to poverty that puts them at higher risk of heat-related mortality, said Patrick Kinney, a professor of environmental health sciences at Columbia University.

“People that are less economically capable and living in conditions where they can’t protect themselves will be more vulnerable,” he said.

Some Indian states, like Odisha and Gujarat, have launched awareness campaigns to inform people of heat waves. But much of the nation has not adapted to the threat. So 16 years after a heat wave killed 2,500 people across the nation in 1999, India experienced similar levels of mortality this May.