Absent new, more ambitious climate policies, the world is headed for a magnitude of climate change that would put about 2 billion people at risk of extreme heat by the end of the century, a new study finds.

Why it matters: Limiting global warming to the Paris Agreement's target of 1.5°C (2.7°F) above preindustrial levels would yield a five-fold reduction in the population exposed to unprecedented heat by the end of this century.

• The nearly 1.2°C (2.16°F) increase in global average surface temperatures to date has already knocked more than 600 million people out of the "human climate niche" in which society has historically thrived.
• The researchers of the study, published Monday in Nature Sustainability, define that niche by looking at how human population density varies with temperature and precipitation.

The intrigue: They find two peaks in population density, one associated with a mean annual temperature of about 13°C (55.4°F), and the other tied to more tropical climates, at 27°C (80.6°F).

• Outside of these temperatures, conditions tend to be either too wet or dry, or too hot or cold, for high concentrations of people to thrive, the study states.

What they're saying: "The human climate niche shows how human population density varies with average temperature and average precipitation," said Tim Lenton, study lead author and director of the University of Exeter's Global Systems Institute.

• "It thus shows the temperature and rainfall levels we flourish most at, and how population density drops moving away from those peaks," he told Axios in an email.

Between the lines: The researchers sought to shed light on the current and projected human toll of climate change.

• Unprecedented heat exposure is defined as having a mean annual air temperature of 84.2°F (29°C), which the scientists found correlated with more frequent spikes to greater than 40°C (104°F), and potentially lethal wet bulb temperatures of greater than 28°C (82.4°F).
• Wet bulb temperatures incorporate humidity; once they climb to about 35°C (95°F), they can cause potentially fatal heat-related illness by impeding the human body's ability to cool itself through sweat.

Of note: The study found that if countries only meet existing emissions reductions based on current policies, and warming were to reach 2.7°C by 2100 (4.8°F), the top 5 countries most vulnerable to unprecedented heat (based on the number of people exposed) would be India, Nigeria, Indonesia, the Philippines and Pakistan.

• In this scenario, up to a third of humanity would be exposed to such extreme heat and be well outside the human climate niche, the study found.

Yes, but: The heat would not hold off until the end of the century, but ratchet up noticeably over time.

• The study concludes that for every 0.1°C (0.18°F) increase in global average surface temperatures, another 140 million people will be exposed to dangerous heat.

The big picture: Extreme heat this week illustrates the challenges these countries are already facing face, with temperatures in Delhi, India reaching 46.5°C (115.7°F) on Monday.

• Heat waves already have toppled monthly and all-time records across swaths of Asia during April into May, which scientists have tied in part to human-caused climate change.
• Other studies have pointed to the dangers of temperatures hitting intolerable limits, which challenge humans' physiological abilities to withstand such conditions, with increasing frequency.

The bottom line: Unless nations take more stringent actions to rein in global greenhouse gas emissions, extreme heat will only get more dangerous and prevalent.