PARIS: Many millions of people in the tropics could be exposed to dangerous heat for half the year by 2100 even if humanity manages to meet climate goals, researchers warned Thursday.

In the most likely scenario, the world would miss those targets — potentially subjecting people across the tropics to harmful temperatures most days of each typical year by the end of the century, the study found. If emissions go unchecked, large numbers of people in these regions could face potentially “nightmarish” periods of extreme heat.

“There’s a possibility that if we don’t get our act together, billions of people are going to be really, really overexposed to these extremely dangerous temperatures in a way that we just fundamentally haven’t seen,” said lead author Lucas Vargas Zeppetello of Harvard University.

Severe heatwaves — made hotter and more frequent by climate change — are already being felt across the world, threatening human health, wildlife and crop yields. Most climate projections predict temperature increases under different policy scenarios, but do not say which of those pathways is more likely.

In this study, published in the journal Communications Earth and Environment, researchers estimated potential exposure to dangerous heat and humidity.

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They used statistical projections to predict levels of carbon dioxide emissions from human activity and the resultant levels of global warming.

They found that many people in tropical regions could face dangerous heat levels for half the year by the end of the century, even if the world limits temperature rises to the Paris climate deal goal of less than two degrees Celsius (35.6 degrees Fahrenheit) above preindustrial levels.

Outside the tropics, they said deadly heatwaves will likely become annual occurrences.

The researchers used a heat index that puts “dangerous” levels at 39.4C, while temperatures above 51C are considered “extremely dangerous” and totally unsafe for humans.