气候学家谈论哥本哈根大会

TONY JONES: Well if I understand you correctly your biggest fear now is that these built in temperature rises will trigger what you call feedback mechanisms. Can you explaining how they work, and what are the implications of them?

JAMES HANSEN: Yeah, well that's what makes climate a really dangerous situation, because of the inertia of the system. It takes the ocean a long time to warm up, it's four kilometres deep, and it takes icesheets a long time to get started to move, they're very thick and have a lot of inertia.

The problem is that as these changes begin to occur, and they are beginning to occur - Greenland is losing ice faster and faster and Antarctica is beginning to lose ice at a rate of about 150 cubic kilometres per year - as you get to a certain point, you can get to a point where the dynamics of the system begins to take over.

If the icesheets begin to collapse, by that time it's too late. You've passed the tipping point and the icesheet is going to end up in the ocean. So, that's one of the tipping points. Another one is methane hydrates. We're beginning to see methane bubble out of the tundra as it's melting.

There's a lot more methane hydrates on continental shelves. As the ocean warms that methane hydrate can also begin to release methane, which is a very strong greenhouse gas and can cause amplifying feedback which makes the global warming much larger.

And this is not idle speculation, because we can look at the history of the earth. And in past global warming events we have seen those kind of amplifying feedbacks which then make the change extremely large.

TONY JONES: Okay, well you're talking about what you find from the examination of ice core data. Is there a comparable period in history, the history of the planet that is, where warming accelerates due to these feedback mechanisms, and do you get much more rapid sea level rises during that period?

JAMES HANSEN: Yeah, well, in the relatively recent paleoclimate, coming from the last ice age to the present interglacial period that we've been living in for 10,000 years, when that icesheet, the big icesheet on North America began to disintegrate, sea level went up five metres per century. That's one meter every 20 years for several centuries. So once an icesheet begins to melt and begins to disintegrate, things can move very rapidly.