One additional thought. In several of the sessions I attended at the Nature Conservancy meeting, reference was made to the need to stabilize global temperature rise at no more than 2°C, and correspondingly to stabilize CO2 levels at no greater than 450 ppm (strictly speaking, this should be 450 ppm CO2-radiative-equivalent; there is a big difference, and it is often neglected). In each case, reference was made to the IPCC reports as the source of these numbers. Yet these numbers really don't derive from the IPCC, which (rightly) shied away from being policy prescriptive. Rather, they have their origin in a small number of documents, notably the 1995 report of the German Advisory Council on Global Change, papers related to the Exeter conference on "avoiding dangerous climate change" and in European Union Council decisions (see the 1996 and 2006 Presidency Conclusions here). Clearly, the 2°C/450 ppm numbers have completely permeated the policy-advocacy realm. Yet while they are arguably derivable from the IPCC reports, it is actually not clear to what extent the larger scientific community really believes these are the right numbers. There simply has not been a process to evaluate this that compares in depth and breadth with the IPCC. A new and much more comprehensive analysis, by a much greater group of scientists, would be valuable at this juncture. Scientists are fond of saying that they cannot summarize their projections with a small handful of simple numbers, but simple numbers are what are being discussed in policy circles. If the "right" numbers are really so low as Jim Hansen believes (see our post on this, here), then the Nature Conservancy has an even more difficult task ahead.
