Acceptable Concentration Levels
Both the basic concept of Contraction and Convergence and the specific formulas that the Global Commons Institute advocate can be adapted to both very moderate restrictions on carbon emissions through to much more drastic measures depending on what final level of greenhouse gases concentration is deemed to carry an acceptable level of risk. However, supporters of Contraction and Convergence believe that a safe level at which concentrations of greenhouse gases should stabilize is much lower than many suggest. Specifically they believe that suggestions that atmospheric concentrations of carbon dioxide of 550 parts per million by volume (ppmv) would be safe are wrong. This they consider runs the risk of entering a phase of runaway “climate feedback”, where one change sparks off another with unpredictable results. They advocate erring on the side of caution, with atmospheric concentrations of carbon dioxide being stabilized at 350-450 ppmv. GCI's numerical analysis of the unrealistically optimistic 'sink-assumptions' behind the UK Climate Act is here: - http://www.gci.org.uk/animations/Sources_and_Sinks_UK_Climate_Act.swf
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