Is a 2 degree C target realistic?

Al Gore one said that “the outer edge of the politically possible falls short of the inner edge of the necessary.” It is certainly an apt quote for the negotiations so far at Bali, where there seems to be a growing disconnect between the rhetoric of preventing dangerous anthropogenic climate change, usually defined as more than 2 degrees C relative to pre-industrial levels, and the discussions over what steps to actually take. With the Bali conference lowering expectations by the day and putting more substantive decisions off to 2009 and beyond, it raises the question of how viable the 2 degrees target really is, and if it is time to undergo a fundamental reassessment of what type of targets may be viable. This is not to suggest that we should not work as hard as we can to prevent more than 2 degrees warming; rather, that we should be prepared for more warming and redouble our efforts to prevent even greater climate impacts.

To reach a stabilization of 2 degrees C would require global emissions reductions between 60 and 85 percent by 2050 (this range is due to uncertainties in climate sensitivity). Since equity concerns require greater reductions on the part of developed countries, their emission reductions would likely be on the order of 80 percent or more. Global emissions would likely have to cap around 2015 to achieve a 450 ppm goal even with an overshoot. Furthermore, expected reductions in aerosol emissions—which have a net cooling effect—due to increasing air pollution concerns in countries like China and India would effectively require that a 2 degrees target limit us to current atmospheric concentrations. The IPCC AR4 WGIII estimates that global aerosol emissions will fall by around 50% relative to current levels. The current atmospheric budget of greenhouse gases include around 380 ppm CO2, 80 ppm CO2-equivalent (CO2e) gases (e.g. methane, N2O, PFCs, etc.), and –80 ppm sulphate aerosols. If aerosols were halved today, we would end up with around 420 ppm CO2e, generating roughly 1.8 degrees C warming at equilibrium. This effectively means that we have very little allowable growth in atmospheric concentrations—only 20 ppm CO2—allowed over the next century if we are to avoid more than 2 degrees warming.

However, no one at Bali is proposing measures nearly stringent enough to meet their stated goals. While it may be possible that developed country emissions would peak by 2015, no one believes that developing country emissions will follow accordingly. To meet emissions reductions greater than 80 percent in developed countries by 2050 would require an extremely high rate of emissions reductions. Sir Nick Stern calculates that such targets would require annual emissions reductions of over 7% per year. To put this in perspective, the economic collapse of the Soviet Union only resulted in emissions reductions of 5% per year.

The longer we delay in undertaking substantive commitments, the less realistic a 2 degrees C target appears. Delegates, policymakers, and the general public need to realize that the failure to begin the emission reduction process on a global scale in the near future will result in what James Hansen calls “a different planet”.