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“Damn! I think we just passed the last exit for the Holocene!” “I’m sorry, honey, I wasn’t looking.”

“We have to get off this highway.

What’s the next exit?” “It’s a long way ahead. Goes to somewhere called Perdition.”

(Ragged chorus from the back seat) “Are we there yet, Daddy?” The Holocene era is that blessed time of stable, warm climate (but not too hot) and unchanging sea levels in which human civilization was born and grew to its present size. In 10,000 years, our numbers have increased about a thousandfold -- but we may be about to leave the Holocene, and that would be too bad. No other climatic state would let us maintain our current numbers, and massive diebacks are no fun at all.

James Hansen, the director of NASA’s Goddard Space Flight Center in New York, is one of the most respected scientists working in the field of climate studies. It was his famous speech to the US Congress 20 years ago that put climate change on the US political agenda, and led indirectly to the Earth Summit and the Framework Convention on Climate Change of 1992 and the Kyoto Protocol of 1997.

Now he has something else to say.

For most of the past decade, Hansen adhered to the emerging consensus among climate scientists that the maximum permissible concentration of carbon dioxide in the atmosphere is 450 parts per million (ppm). That was believed to give us a 50 per cent chance of getting away with an average global temperature only two degrees C (3.6 degrees F) hotter than it was at the beginning of the 1990s.

Now Hansen doesn’t believe in 450 ppm any more.

Four hundred and fifty ppm was chosen partly because it seemed impossible to stop the rise in carbon dioxide before that -- we’re already at 387 ppm, and going up almost three ppm per year -- and partly because it seemed relatively safe.

Two degrees C hotter would turn a lot of sub-tropical land into desert, cause bigger hurricanes, and turn most of Asia’s big rivers into seasonal watercourses that are empty in summer, but it would not melt the icecaps.

At least that’s what they thought, although everybody knew that the numbers were soft.

You can do a lot with climate models, but the Earth hasn’t actually seen a carbon dioxide (CO2) concentration as high as 450 ppm since about 35 million years ago. So Hansen and some colleagues went to work on exactly that period, and came back with some bad news. If you leave the world at even 425 ppm for very long, all the ice will probably melt: Greenland, Antarctica, the lot. And the sea level will go up 70-80 metres (240-270 ft.) How do they know? Because the world was very hot and completely ice-free for a long time before 35 million years ago, but the amount of carbon dioxide in the atmosphere was falling gradually. When it reached 425 ppm, Antarctica began to freeze over.

So if that’s where the first permanent ice appeared while CO2 was on the way down, it’s probably where the last permanent ice will disappear when CO2 is on its way back up.

Now, there’s a big margin of error when you are dealing with 35 million years ago: plus or minus 75 ppm, in this case. That means that the fatal number when all the ice disappears could be as high as 500 ppm -- or it could be as low as 350 ppm. If that is the range within which ALL the world’s ice will eventually melt, and you like living in the Holocene, then you probably should not put all your money on a 450 ppm ceiling for CO2.

So James Hansen is spearheading a campaign to get 350 ppm recognized as the real long-term target we should be aiming at. Tricky, since we are already at 387 ppm and rising fast, but last week, when I spoke to him at the Tallberg Forum’s annual conference in Sweden, he explained: “To figure out the optimum is going to take a while, but the fundamental thing about the 350 [ppm target], and the reason that it completely changes the ball-game, is precisely the fact that it’s less than we have now.”

“Even if the optimum turns out to be 325 or 300 or something else, we’ve got to go through 350 to get there. So we know the direction now that we’ve got to go, and it’s fundamentally different.

It means that we really have to start to act almost immediately. Even if we cut off coal emissions entirely, CO2 would still get up to at least 400, maybe 425, and then we’re going to have to draw it down, and we’re almost certainly going to have to do it within decades.”

But there is time. The oceans and the ice-sheets react so slowly to changes in the air temperature that you can overshoot the limit for a while, so long as you get the temperature back down before irreversible changes set in. Stop at 450 ppm in 25 years’ time, then get back below 400 in another 25, and down to 350 by, say, 2075. It could work: there is still one last exit for the Holocene.

Gwynne Dyer is a London-based, Newfoundland-born independent journalist whose articles are published in 45 countries.