There are several indicators we have that if plotted on a graph over time do not give a linear slope. Dr. Peltier and Dr. Weaver mentioned the acceleration of the Greenland and Arctic ice shields. I talked about the decline of the Arctic sea ice. If you look even at something as simple as temperatures, the rate of increase of temperature is actually faster than it has been. That is not linear. If you look at sea level rise, that again is not linear. In many places you look, it seems as though climate change is actually accelerating and that the impacts we're seeing are happening at a faster rate than we had anticipated in the IPCC's third assessment report.
On February 19th, 2007. See this statement in context.