By the end of the century, extremely humid heatwaves that could kill even healthy people within hours could begin to appear, according to a new report released this week by the journal Science Advances.

The lethal heatwaves are most likely to occur in certain areas of the Indian subcontinent. Beyond these hot spots, the majority of the subcontinent’s 1.7 billion population will face “extremely dangerous” levels of humid heat.

Take Action: Stand with climate change refugees

Humid heat can be measured by a metric called wet-bulb temperature (WTB), which takes into account heat stress in direct sunlight. Withstanding a wet-bulb temperature of 35 degrees celsius (or 95 degrees fahrenheit) for over six hours can send humans into hyperthermia and eventually lead to death, because the body won’t be able to cool itself by sweating.

The study predicts that wet-bulb temperatures in Southeast India are likely to approach or even exceed this dangerous level by the end of the century.

Southeast Asia faces annual monsoons which carry humid air to the land, and evaporation from widespread irrigation systems heighten humidity as well. Since much of India’s jobs are in outdoor agriculture, huge amounts of people will be  vulnerable to the crippling heat.

Read More: 5 Consequences of Trump Dumping the Paris Climate Agreement

“Climate change, without mitigation, presents a serious and unique risk in South Asia, a region inhabited by about one-fifth of the global human population,” the study warns.

The scientists examined computer climate models that simulate past climate trends of South Asia and then conducted a high resolution analysis of the region. They discovered that if carbon emissions continue at their current rates, by 2071-2100, at least 4% of the region's population would suffer lethal six-hour heat waves.

The affected areas encompass the cities of Lucknow in Uttar Pradesh and Patna in Bihar, which are currently home to over two million people each.

Elfaith Eltahir, one of this study’s authors, previously looked at heatwaves in the middle east, an area that has faced nearly deathly, 35C wet-bulb temperatures in 2015. But Elthair’s findings regarding southeast Asia are even more concerning to him, given the region’s uniquely dense population and high poverty levels.

“That combination is what makes, what shapes this acute vulnerability,” Eltahir told Climate Central.

There is hope to curb this impending danger, though. If countries reduce their carbon emissions along the lines of the Paris Climate Agreement, the 35C WBT heatwaves could be completely prevented.

While President Trump pulled the United States out of the Paris Climate Agreement in June, other nations like France and India  – and even states within the US – are maintaining their commitment to the environment and to preventing disasters like extreme heatwaves from endangering millions of people.

Read More: Climate Change Has Been Linked to 59,300 Farmer Suicides in India, Study Says

Some cities are already implementing plans to inform and prepare citizens for these impending heatwaves, creating comprehensive guides for people to stay safe during the extreme temperatures. But climate scientists hope that nations will take more proactive measures, like adopting cleaner energy sources to help prevent these heat waves from happening in the first place.

“Our hope is that this [work] will inform the policy debate,” Elthair said.