Climate Change Mitigation Scenarios - Target Levels of CO₂

Target Levels of CO₂

Contributions to climate change, whether they cool or warm the Earth, are often described in terms of the radiative forcing or imbalance they introduce to the planet's energy budget. Now and in the future, anthropogenic carbon dioxide is believed to be the major component of this forcing, and the contribution of other components is often quantified in terms of "parts-per-million CO₂-equivalent" (ppm CO₂e), or the increment/decrement in carbon dioxide concentrations which would create a radiative forcing of the same magnitude.

At present, non-CO₂ contributions to climate change, positive and negative, are believed to roughly cancel out, so that the net radiative forcing being experienced at present, expressed in ppm CO₂-e, is more or less the same as the actual current level of carbon dioxide (393 ppm CO₂, as of April 2011). To some extent this legitimates the statement of targets just in terms of ppm CO₂, as is usually the case. However, the positive and negative non-CO₂ will not necessarily balance in future, and so a target stated in terms of CO₂e is less ambiguous.