Some have interpreted the “Hockey Stick” as definitive proof of the human influence on climate. However, others have suggested that the data and methodologies used to produce this type of figure are questionable (e.g. von Storch et al. 2004), because widespread, accurate temperature records are only available for the past 150 years. Much of the temperature record is recreated from a range of ‘proxy’ sources such as tree rings, historicalrecords, ice cores, lake sediments and corals.This simple fact - that carbon dioxide and methane reflect more infrared radiation than other gases - is incontrovertible, as solidly known as the force of gravity at the Earth's surface. There is no question that an atmosphere with more CO2 in it will reflect more infrared radiation than one with less. These are laws of physics, not models or conjecture.
Climate change arguments do not rest on “proving” that the warming trend is unprecedented over the past Millennium. Whether or not this debate is now settled, this is only one in a number of lines of evidence for human-induced climate change. The key conclusion, that the build-up of greenhouse gases in the atmosphere will lead to several degrees of warming,rests on the laws of physics and chemistry and a broad range of evidence beyond one particular graph.
Somehow, this always gets lost in the discussion. To claim that continuing emissions of CO2 won't lead to climate change is to claim that up is down, black is white, and the laws of physics no longer apply. Very, very good of Stern to state the facts of this matter.