The last entry in this series examined some of the most recent scientific evidence that has concluded that the world is rapidly running out of time to prevent dangerous climate change. The staggering magnitude of the challenge facing the international community to limit warming to 2 degrees C can be visualized by understanding the following chart that depicts three ghg emissions reductions pathways which would allow the world to stay within a specific remaining budget to achieve a specific atmospheric concentration of ghgs. As we explained in the last entry, the IPCC has in September of this year described a budget that would give the world a 66% confidence of preventing the 2 degree C warming limit which the international community has agreed upon.
Any atmospheric ghg concentration target can only state the warming that will be experienced at the concentration limited by a probability statement because there is scientific uncertainty about climate sensitivity, a term which is used to describe the warming that will be caused by different concentration of atmospheric ghgs. The level of certainty that we should seek to limit warming to a specific atmospheric concentration is itself an ethical question, not just a scientific question which often goes unexamined by the scientific community when discussing warming limits and emissions budgets to achieve warming limits.
One might ask why the budget prepared by IPCC was not based upon achieving the 2 degree C with much higher levels of certainty, a question which is not discussed in the IPCC report, yet one might speculate that IPCC’s failure to discuss a budget that would assure 100% certainty that the 2 degree C warming limit would not be exceeded was because it would leave no remaining budget for additional ghg emissions. The international community has already emitted so much CO2 that limiting warming to 2 degrees C with very high levels of certainty would mean that future emissions must be negative emissions, that is activities which remove ghg from the atmosphere while immediately ceasing ghg emissions activities.
As we have seen in the last entry, if the IPCC budget would have included all ghgs that have been emitted, it would have concluded that there remains only 269 billion tons of CO2e left to be emitted by the entire global community to stay within an emissions budget that will give a 66% confidence that the 20C warming limit would not be exceeded. Achieving the global reductions entailed by this budget is a civilization challenging problem of the highest magnitude.