The United Nations Framework Convention on Climate Change (FCCC) calls for a “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.” That level is nowhere defined, however, nor is the definition addressed in any of the reports of the UN Intergovernmental Panel on Climate Change [IPCC, 1996]. A reasonable interpretation might be that “dangerous” refers to the stability of the climate system rather than to a simple warming. For example, could higher GH gas (mainly CO2) concentrations produce a sudden, major change in the state of the climate?