The different policies reduce damages by only a modest amount. Indeed, one of the surprises is how little the policies affect the damages from global warming. The reasons are that, because there is so much inertia in the climate system and because the Protocol reduced the global temperature increase by only a fraction of a degree over the next century.

Kyoto is likely to yield far less than the targeted emissions reduction. That failure will most likely be papered over with creative accounting, shifting definitions of carbon sinks, and so on. If this happens, the credibility of the international process for addressing climate change will be at risk.

If the relatively rich participating countries want to stabilize atmospheric concentrations of greenhouse gases, they will have to pay at least some poor countries to reduce their emissions. Achievement of substantial reduction in this way implies international transfers of wealth on a scale well beyond anything in recorded history. There is no effective political support for such a Herculean effort, particularly in the United States.

It will be nearly impossible to slow warming appreciably without condemning much of the world to poverty unless energy sources that emit little or no carbon dioxide become competitive with conventional fossil fuels.

With each passing year the difficulty of meeting any fixed quantitative target increases progressively. Moreover, plausible estimates of when the Protocol would go into effect leave such a small window of time before the first commitment period that achievement of the Kyoto targets will eventually pass out of reach.

The different policies reduce damages by only a modest amount. Indeed, one of the surprises is how little the policies affect the damages from global warming. The reasons are that, because there is so much inertia in the climate system and because the Protocol reduced the global temperature increase by only a fraction of a degree over the next century.

The impact of the Kyoto Protocol on global temperature is quite modest, especially for the first century. The reduction in global mean temperature in the Annex I case relative to the reference in 2100 is 0.13ºC; this compares with a difference of 0.17ºC from the Kyoto Protocol calculated by Wigley. The temperature reduction in the optimal run is essentially the same as the Kyoto runs by the 22nd century.