James Hansen discussing environmental tipping points with Amy Goodman on Democracy Now, 12/22/09

"In the case of the ice sheets and sea level, we began in 2002 to get this spectacular data from the gravity satellite, which measures the gravitational field of the earth with such a high precision that you can get the mass of the Greenland ice sheet and the Antarctic ice sheets. And what we see is that from 2002 to 2005 we were losing mass from Greenland at the rate of about 150 cubic kilometers [of ice] per year. Well, now that's doubled to about 300 cubic kilometers per year. And likewise the mass loss from Antarctica has also doubled over that time period. So we can see where we're moving toward a tipping point where those ice sheets will begin to disintegrate more rapidly and sea level will go up."