1. "Equal rights for all"
The formula: each person on earth would have an equal right to emit carbon, so countries would be allocated emission targets strictly according to population. To exceed these tartets, the would have to buy spare entitlements from poor nations and those that had invested in low carbon technologies
Advantages: fairness, and no need for complex negotiations. Fixed emissions targets can be tied to scientific criteria for protecting the climate
Disadvantages: possible incentives for countries to increase their populations
2. "To those that have..."
The formula: targets based on the ratio of national carbon emissions to GDP. Countries might be asked to reduce the carbon intensity of their economies (that is, the amount of wealth produced per unit of greenhouse gas pollution) by some percentage. In 2002, the George W Bush administration set a target for the US of improving its carbon intensity by 18 per cent in the coming decade.
Advantages: a clear incentive for moving to clean technologies while not penalising countries that are efficient at using carbon emissions to create wealth.
Disadvantages: total carbon emissions may continue to rise.
Get our weekly email