"We now realize that we have passed or are on the verge of passing several tipping points that pose grave risks for humanity and especially for a large fraction of our fellow species on the planet," he wrote in a draft letter (PDF) last week to Gordon Brown, Prime Minister of the United Kingdom, urging him not to approve the building of a new coal-burning plant.

Based on his previously-published calculations of the energy balance of the earth, Hansen calculates we as a species passed these tipping points when we as a species exceeded 300-350 parts per million of carbon dioxide in the atmosphere, a point passed decades ago.

"We have taken it as a God-given fact that we will burn all the fossil fuels," he said at the press conference. "But we simply cannot do that, if we want to keep the planet we now have."

The question Hansen now asks is: Have we passed the point of no return?

On this question, he is somewhat more hopeful. We have "probably not" passed the point of no return on the melting of the ice sheets, he said, and stressed that if we can avoid building carbon dioxide-emitting coal plants, burning tropical forests, and releasing greenhouse gases captured in soils, we have a reasonable chance of avoiding disaster.

When I asked him if this shift was based on recent research, he said no -- it was a rhetorical change designed to clarify an issue that has confused the public, which is the difference between the climate change that is already in the pipeline, and the climate change that will give us "a different planet."