Global temperature rise is a bad starting point for negotiating climate change targets
The EU, the G8, and many other countries consider that limiting global temperature rise to two degrees Celsius is an attractive goal for climate change mitigation policy to avoid 'dangerous climate change'. If taken as the international basis for setting climate change mitigation targets the use of a global temperature rise limit is likely to pose difficulties for climate change negotiators.
Two degrees Celsius ('2C') is a fine headline target for politicians to sign up to, and a useful slogan for the public, however its translation into practical targets for greenhouse gas (GHG) emissions reduction and into desirable levels of CO2 in the atmosphere is more problematic, and there are also those who consider '2C' too high a value.
A global climate change target needs to provide a practical framework for national GHG mitigation targets and actions. Reducing CO2 emissions is certainly the key step to take in combating the effects of climate change, notwithstanding the fact that other GHG emissions - methane, nitrous oxide etc also need to be reduced.
In Copenhagen the international agreement that is most urgently needed is one that sets clear and unambiguous targets for drastically reducing CO2 emissions, since they comprise the largest proportion of GHG.
The task faced by world leaders and politicians - that of translating the scientific information about climate change into politically acceptable and practical national targets - is already difficult and there is a desperate need to put policy in place speedily.
So why use temperature rise as an overall global target as the EU and G8 have done? Framing negotiations in terms of '2C' surely serves only to increase the complexity of the task.
Politically expressed as 'limiting average global temperature rise at sea level to two degrees Celsius relative to pre-industrial levels' a limit to global temperature rise seems a straightforward goal. However, temperature rise as a key indicator focuses on the end result - global warming, not on the GHG emissions that cause it. Global temperature limit is an indicator that is distanced from the practical issues of quantified GHG emissions limitation.
Using a global '2C' target as the starting point for international climate change negotiations could leave open a chain of opportunities for varied interpretations of meaning and inhibit progress towards practical targets.
Targets, if framed in the context of 'global average temperature rise' would need to be mapped to the causes of global temperature rise because, in international negotiations, all parties would need to agree a common interpretation of what a global '2C' target means and how it would relate in practical terms to potential mitigation actions.
Do we really want negotiators to dissipate good will in Copenhagen arguing about possible interpretations of the meaning of '2C global average temperature change' when what is needed is to negotiate quantifiable mitigation targets that can be evolved into practical policy?
Temperature rise relates to atmospheric GHG change by means of best estimates and likely ranges not with certainty*. Such uncertainties would need to be incorporated in any translation from 'global temperature rise' into national targets for practical actions to mitigate human induced warming. Undertaking such a translation process raises the spectre of endless political wrangling over nuances related to temperature rise, rather than negotiation of the tonnage emissions targets that are the heart of the matter.
It is however possible to quantify directly, by country, the millions of tonnes of fossil fuels burnt each year and hence each country's contribution to global warming in terms of CO2 emissions.
It is more practical to set targets in terms of CO2 emissions, which are the cause global warming, and which connect directly to national fossil fuel energy consumption, rather than work through to reach national targets from '2C' temperature rise.
Targets closely related to the causes of warming can be defined in terms of CO2 (or CO2e) emissions, CO2 concentrations or indeed tonnes of carbon in fossil fuels burnt. Use of these quantities provides a more direct starting point for negotiating climate change mitigation targets than global temperature limits. If required emissions based targets could then be related back, through models, to temperature rise to check that they are consistent with the desired temperature limit.
While global temperature rise appears in the headlines, and the need to mitigate the unprecedented rise in global temperatures is urgent, framing negotiations around '2C' temperature rise is likely to impede progression to a new international agreement and using emissions based targets would provide a clearer route for negotiations.
*Anthropogenic GHG emissions are the cause of the recent rapid global temperature rise but their relationship with temperature rise is not a simple one. The underlying science linking global temperature rise to GHG concentrations is couched in terms of probability not certainty. The climate system is complex and highly variable due to both natural processes and to human induced changes, and is not now in thermal equilibrium. In addition to the known causes of the global warming (anthropogenic GHG), the earth's energy balance is also influenced by uncertainties in, for example, the thermal storage of oceans, changes in albedo as ice sheets disintegrate, the release of methane as permafrost melts, and these effects may be subject to time lags.