“It’s almost too late. We need to get cracking with this,” said Manfred Lenzen, a sustainability researcher at the University of Sydney and co-author of the new paper. “A lot of people think 1.5 is dead already, that we have to realistically aim for staying below 2C,” he said, adding that green growth—decoupling emissions from economic expansion—might have worked if the world had taken it seriously in 2000.

Starting then, it would have taken only a 2 percent annual reduction of greenhouse gas emissions to reach net zero by 2050; starting now means cutting global emissions by 7 percent a year, but the green growth approach is not cutting emissions by anywhere near the required rate, he said. And, particularly as outlined in the policy guiding reports from International Panel on Climate Change, it relies on deployment of unproven technologies.