Thirty percent of the world’s population is now exposed to potentially fatal heat waves for at least days each year, and as greenhouse gases accumulate in the atmosphere, more sections of the planet could experience deadly high temperatures.

This is the grim picture painted by a new study, which also discovered that up to three in four individuals will be exposed to death risks from heat by 2100 if there will be no dramatic reductions in emissions of CO2 and other greenhouse gases. Yet even with such decreases, one in two people by century’s end will still face 20 days or more of extreme heat.

Study Findings

Heat waves are already killing people around the globe today, in incidents including the 2010 Moscow event with its at least 10,000 casualties, as well as in the last few weeks in India and Pakistan where dozens died.

“Lethal heat waves are very common,” said lead author Camilo Mora of the University of Hawaii in a National Geographic report. “I don’t know why we as a society are not more concerned about the dangers.”

Mora cited a European heat wave in 2003 that killed about 70,000, or 20 times the victims of the 9/11 attacks.

For the analysis published in the journal Nature Climate Change, Mora’s team pored over more than 30,000 publications to find data on almost 2,000 case studies of places with heat and human death links. Cities with deadly heat waves range from New York City and Los Angeles to London and Toronto, and as far as Tokyo and Beijing.

The research showed that overall heat-related illness or death risk has increased steadily since 1980. About 30 percent of the global population too now lives in conditions packing a punch in high temperatures at least 20 days each year.

Most heat-related fatalities, however, do not take place during widely known disasters. In Phoenix, for instance, June saw an unusually hot spell that killed at least four.

Human Threshold To Deadly Temperatures, Prospects For Safety Amid Climate Change

The internal temperature of the human body likely lies between 98.6 to 100.4 degrees Fahrenheit. If this temperature moves close to 104 degrees Fahrenheit, one’s cellular network may begin to break down, and any higher will require urgent medical attention.

According to Mora, threshold to fatal conditions varies from one place to another, where some people could die in 23 degrees Celsius.

Left most vulnerable are the young and elderly, and climbing inequality could result in greater deaths from extreme heat, warned medical history professor Richard Keller of University of Wisconsin-Madison.

A crucial thing to determine: humidity paired with the heat.

“Your sweat doesn’t evaporate if it is very humid, so heat accumulates in your body instead,” Mora explained in a Guardian report, warning of heat toxicity or sunburn wreaking havoc on the body’s insides.

Deadly heat is expected to kill and harm more people and environments in the face of global warming, which has also brought about changes to human lives including sleep problems and altered mental health.

While India and other sections of the global south, for example, didn’t find heat a massive concern previously, extreme heat has now become more frequent and intense with climate change.

A University of California Irvine study published this month discovered that the likelihood of a heat wave killing more than 100 people in India has doubled because of a 0.5 degrees Celsius climb in temperature in the last half century. This means temperature increase is deemed as only mild compared to other countries and regions.

Small increases in temperature could translate to major effects, particularly among the poorest nations. Even among rich nations, the increased use of air-conditioning is hardly seen as a long-term solution.

Greater use of AC systems means failure of electrical grids, Mora said.

“We need to prevent heat waves rather than just trying to adapt to them,” he said.

2016 has been declared one of the hottest years on Earth. An official report has predicted that the drastic climatic change seen in that year is looking to make a comeback in 2017, even without a strong El Niño phenomenon.