Temperatures reached nearly 50 ºC in Delhi this week, according to the New York Times.

Also this week, Pope Francis declared a global "climate emergency" while hosting oil executives at the Vatican.

"Our children and grandchildren should not have to pay the cost of our generation's irresponsibility," the pontiff said.

And now, the group 350.org has called for global all-ages climate strikes as part of a week of actions starting on September 20. 

"Millions of school strikers have shown us they're serious about climate action," the group declares. "Adults, will you join our youth?"

Recent measurements at the Mauna Loa Observatory in Hawaii show that the concentration of carbon dioxide equivalents has exceeded 415 parts per million in the atmosphere.

That's the highest in 800,000 years, leading to fears of a climate breakdown.

The climate strike movement was launched by Swedish teenager Greta Thunberg when she walked out of classes on Fridays, beginning last year, to protest inaction by adults.

Thunberg, who refuses to fly, has inspired millions of other secondary students to hold their own climate strikes in cities around the world.