气候学家谈论哥本哈根大会

TONY JONES: Okay let's go quickly through a couple of the other key arguments put forward by the sceptics. Why worry about carbon dioxide when water vapour is a stronger greenhouse gas and actually occurs naturally?

JAMES HANSEN: Yeah, that's the screwiest argument which keeps being made again and again and again. The amount of water vapour in the atmosphere is determined by the atmosphere's temperature, everyone should know that. Look at the difference between winter and summer.

As you go to a warmer climate the atmosphere holds more water vapour because at the places where the humidity reaches 100 per cent the water vapour falls out as water or snow. And therefore, as the planet becomes warmer, the atmosphere holds more water vapour.

That's why we get heavier rain falls as the planet gets warmer. So this water vapour is an amplifying feedback. It makes the greenhouse effect much stronger. But it's not something that just changes on its own accord; it changes in response to the temperature changes.

TONY JONES: Okay, if I understand it correctly your argument is that climate change is not only about droughts, but that effect you're talking about will cause much more frequent and much more severe storms; is that correct?

JAMES HANSEN: Yeah, the, both extremes of the hydrologic cycle must increase, become more intense as the planet becomes warmer. At the times and places where it's dry, the increased heating of the surface makes it hotter and drier.

On the other hand, the oceans, the places where you have water, the increased heating evaporates more water, so the atmosphere holds more water vapour and at the times when you get rainfall you will get heavier rainfall and greater floods, so the extremes of the climate increase, the extremes of the hydrologic cycle.

Now as far as storms are concerned, the storms that are driven by latent heat - that means thunderstorms, tornados, tropical storms - the strongest ones will get stronger because there's more fuel. The water vapour provides the fuel for those types of storms.

Not all of them will be stronger, but the strongest ones will be stronger than the strongest ones now. But in addition to that, and one thing I talk about in my book, Storms of my Grandchildren, I'm talking about the mid-latitude storms, the fact that as the icesheets on Greenland and Antarctica begin to melt more rapidly than they are now, they will discharge ice fast enough that it will cool the surface of the ocean, nearby ocean, in the North Atlantic and in the circum Antarctic Ocean.

That will cause the temperature gradient between low latitudes and high latitudes to increase, so the storms that are driven by horizontal temperature gradients will become stronger, and these can be very damaging storms, this is like the storms that hit the Netherlands and England in the 1950.

They can do enormous damage. So, yes, it's true that all the storms that we can think of will become stronger as the climate becomes warmer.