Evidence that irreversible changes in Earth’s climate systems are underway means we are in a state of planetary emergency, leading climate scientists warn. A cascade of tipping points could amount to a global tipping point, where multiple earth systems march past the point of no return, they say.

That possibility is “an existential threat to civilization,” write Tim Lenton and colleagues in this week’s Nature.

Such a collapse of Earth’s systems could lead to “hothouse earth” conditions with a global temperature rise of 9 degrees F (5 degrees C), sea levels rising 20 to 30 feet, the complete loss of the world’s coral reefs and the Amazon forest, and with large parts of the planet uninhabitable.

A global emergency response is