In 11 days, on Earth Day, world leaders will assemble at the United Nations in New York to sign the Paris climate agreement. That document pledges to hold the planet’s warming to “well below” 2 degrees Celsius above pre-industrial levels, and even to aspire to a 1.5 degree C temperature limit.
The urgency of signing the agreement has been underscored by recent climate news and events, including devastating coral bleaching around the world, newly shattered temperature records and disturbing news about the vulnerability of Arctic permafrost and the Antarctic ice sheet.

But there’s a problem: It is far from clear that, even if governments sign on to the Paris agreement and start implementing it rapidly, they actually know how to limit warming to 2 or 1.5 degrees Celsius. There are a number of problems with thinking that anyone does, argues Glen Peters, a researcher with the Center for International Climate and Environmental Research in Oslo, Norway, in the latest installment of Nature Climate Change.