An alternative approach to establishing trade-offs among greenhouse gases

The Kyoto Protocol permits countries to meet part of their emission reducti on obligations by cutting back on gases other than CO2 (ref. 1). This appro ach requires a definition of trade-offs among the radiatively active gases. The Intergovernmental Panel on Climate Change has suggested global warming potentials for this purpose(2), which use the accumulated radiative forcin g of each gas by a set time horizon to establish emission equivalence. But it has been suggested that this approach has serious shortcomings: damages or abatement costs are not considered(3-10) and the choice of time horizon for calculating cumulative radiative force is critical, but arbitrary(5). H ere we describe an alternative framework for determining emission equivalen ce between radiatively active gases that addresses these weaknesses. We foc us on limiting temperature change and rate of temperature change, but our f ramework is also applicable to other objectives. For a proposed ceiling, we calculate how much one should be willing to pay for emitting an additional unit of each gas. The relative prices then determine the trade-off between gases at each point in time, taking into account economical as well as phy sical considerations. Our analysis shows that the relative prices are sensi tive to the lifetime of the gases, the choice of target and the proximity o f the target, making short-lived gases more expensive to emit as we approac h the prescribed ceiling.