The Holy Grail of climate change is a quantity known as the climate sensitivity—that is, how much the average global surface temperature will change from a doubling of the atmospheric carbon dioxide concentration. If we knew this number, we would have a much better idea of what, climatologically, was headed our way in the future and could make plans accordingly.
Thus far, however, this prize has been elusive. Back in 1990, in its very first Assessment Report, the Intergovernmental Panel on Climate Change (IPCC) suggested that the climate sensitivity was somewhere between 1.5°C to 4.5°C. In its latest Fourth Assessment Report published in 2007, the IPCC said the climate sensitivity was likely to be between 2.0°C and 4.5°C, and unlikely be to less than 1.5°C. Not a whole heck of a lot more certain than where things stood 20 years ago—and this despite a veritable scientific crusade to determine a more precise value.…