A good explanation and discussion of the concept of carbon pie:
"Yet any realistic solution to the climate problem will have to resolve that conflict between the powerful drive to use fossil fuels and the real threat CO2 poses. One way to get an idea of both the scale of the problem and of what an equitable solution might look like is to think in terms of a "carbon pie". The pie represents the amount of CO2 we could still put into the atmosphere without disastrous effects. Its size is not easy to specify. We don't really know at what level the CO2 concentration will become truly dangerous - at what threshold the climate might shift so that rapid melting of the ice sheets becomes unavoidable, say, or the intensity of the drought in the American West is incompatible with the civilization we have built there. James Hansen, director of the NASA Goddard Institute for Space Studies in New York, puts the threshold at 450 parts per million. The Goddard climate model predicts a one-degree-Celsius warming from that concentration, and Hansen thinks a global average temperature one degree warmer than today is enough to threaten the long-term stability of the ice sheets.[...]
The drawback to setting that as a goal, however, is that it is probably not attainable.[...]
A more realistic goal would be 560 ppm - a doubling of pre-industrial CO2 - for which the middle-of-the-range climate-model forecast is a warming of three degrees Celsius. That would give us a carbon pie of 720 gigatons. How should the pie be sliced? The most equitable way would be for each country to get a slice proportional in size to its population. The industrialized countries as a group would then get around 20 percent of the pie, or 144 gigatons. At present they are emitting nearly 5 gigatons a year; at that rate, they will have eaten their pie in less than thirty years. Three decades to reduce their CO2 emissions to zero: that gives an idea of the challenge those countries face, if they want to take full responsibility for the consequences of their prosperity and do as much as possible - though much less than some researchers advocate - to protect the planet from dangerous climate change.
Clearly, the industrialized countries are not going to reduce their carbon emissions to zero in thirty years. Most of them are not even going to meet their much less challenging obligations under the Kyoto Protocol, which calls on them to reduce their emissions by 2012 to below the 1990 levels - on average 5 percent below. The United States, which signed but never ratified the 1997 protocol, has not even tried to reduce its CO2 emissions [...]. At the same time, one of the great shortcomings of the Kyoto Protocol, which conservative climate skeptics have stressed and which has become starkly evident in recent years, is that it placed no obligations on developing countries.
The carbon pie suggests a conceptual way out of this dilemma. It dramatizes the reality that any solution to the climate problem is going to require an overarching deal between industrialized and developing countries. In essence, the former will have to buy extra pieces of pie from the latter, to avoid the choice between protecting climate and torpedoing their economies. In return, the developing countries will get some kind of help with developing - ideally, in a way that helps alleviate rather than aggravate the CO2 problem. The bigger slices of pie that an equitable division would allot them would also allow them to use more fossil fuels for longer - which will in itself be an essential component of their development."
Wallace Broecker and Robert Kunzig, "CO2 - Fixing Climate", chapter "Green is not enough".
I think if such an agreement were reached at the Copenhagen Climate Conference in December, it would be a very good thing both for reducing the threats of global warming (although one can argue that if we choose a limit above the threshold at which climate change becomes dangerous for our civilization, then our efforts will be vain) and for helping the developing countries to get most of its people out of poverty, by transferring some of the riches the industrialized countries have gathered thanks to fossil fuels to those who have not yet enjoyed their benefits. But let's hope that these people don't follow our bad example and pollute as much as we did as they are developing. We therefore also need to develop cleaner energy production systems and to transfer these technologies to the developing countries as soon as possible!