Nearly all the world’s 2 billion children will be exposed to frequent heat waves by 2050, according to a new report from UNICEF.

Already today, around 559 million — or 24 percent — of the world’s children are exposed to high heat wave frequency, or an average of 4.5 or more heat waves annually, the report found.

It added that some 624 million children encounter one of three other high heat measures: high heat wave duration, high heat wave severity or extreme high temperatures.

“The mercury is rising and so are the impacts on children,” UNICEF Executive Director Catherine Russell said in a statement.

“Already, one in three children live in countries that face extreme high temperatures and almost one in four children are exposed to high heat wave frequency, and it is only going to get worse,” Russel warned.

The authors define a heat wave as any period of three days or more in which the maximum temperature each day falls in the top 10 percent of the local 15-day average.

High heat wave duration, when the average heat wave lasts 4.7 days or longer, currently impacts 538 million — or 23 percent — of children worldwide, according to the report.

This figure is poised to rise to 1.6 billion children in 2050 if the planet warms by 1.7 degrees Celsius, or 3.06 degrees Fahrenheit, above preindustrial averages. And in a scenario of 2.4 degrees Celsius — 4.32 degrees Fahrenheit — of warming, 1.9 billion children could be facing high heat wave duration, the authors found.

Millions more children will also be exposed to high heat wave severity, in which the average heat wave event is 2 degrees Celsius or more above the local 15-day average, according to UNICEF. 

Also poised to rise are extreme high temperatures, when more than 83.54 days a year exceed 35 degrees Celsius — 95 degrees Fahrenheit — per the report.

Children who live in northern parts of the world, particularly in Europe, will experience the most dramatic surges in high severity heat waves, the authors noted.

By 2050, almost half of all children in Africa and Asia will face sustained exposure to extreme high temperatures, per the report.

While today just 23 countries fall in the highest category for childhood exposure to extreme high temperatures, that figure is expected to rise to 33 countries by 2050 under a low emissions scenario and 36 countries under a very high emissions scenario.

The nations most likely to fall in the highest category under both scenarios include Burkina Faso, Chad, Mali, Niger, Sudan, Iraq, Saudi Arabia, India and Pakistan, according to UNICEF.

“More children will be impacted by longer, hotter and more frequent heat waves over the next 30 years, threatening their health and wellbeing,” Russel said.

“How devastating these changes will be depends on the actions we take now,” she added.

Russel urged governments to double their funding for climate adaptation measures by 2025 and ensure that global heating does not rise 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, above preindustrial averages — a goal of the 2015 Paris climate agreement.

“This is the only way to save children’s lives and futures — and the future of the planet,” Russel said.