Rajendra Pachauri, chairman of the policy-neutral Intergovernmental Panel on Climate Change, has never been shy of speaking his mind on climate policy.

“I feel I have responsibility far beyond being a spokesman for the IPCC. If I feel there are certain actions that can help us meet this challenge, I feel I should articulate them,” he told Nature two years ago (Nature, 450, 1150-1155; 2007; subscription required).

He’s just articulated them again, calling for atmospheric carbon dioxide concentrations to be kept below 350 parts per million. (Current levels are around 387 ppm, and in its 2007 report, the IPCC took 450 ppm as a key target):

“As chairman of the Intergovernmental Panel on Climate Change, I cannot take a position because we do not make recommendations. But as a human being I am fully supportive of that goal. What is happening, and what is likely to happen, convinces me that the world must be really ambitious and very determined at moving toward a 350 target.” (AFP)

The statement was music to the ears of environmental writer Bill McKibben, the founder of 350.org, whose url explains its mission. In a Guardian blog, he called it “amazing news”.

Nature features editor Rich Monastersky wrote in an April 2009 article (Nature 458, 1091-1094; 2009, subscription required): “The difference between 350 and 450 is not just one of degree. It’s one of direction. A CO₂ concentration of 450 p.p.m. awaits the world at some point in the future that might conceivably, though with difficulty, be averted. But 350 p.p.m. can be seen only in the rear-view mirror.”

Here’s McKibben making his point again on the Colbert Report a fortnight ago: