Uncertainties in global warming science and near-term emission policies

It is argued here that stringent, early emission reductions are necessary in order to minimize 'dangerous anthropogenic interference in the climate system'(DAI), the stated Objective of Article 2 of the UNFCCC (United Nations Framework Convention on Climate Change). Given probability distribution functions (pdfs) for climate sensitivity and the temperature threshold for harm consistent with currently available evidence, and accepting a 10% risk of unacceptable damage as the threshold for 'danger', it is not possible to avoid DAL Having adopted a precautionary approach in setting emission trajectories, the possibility arises that future resolution of uncertainties concerning climate sensitivity and the harm threshold may show the climate sensitivity to be low (1-2 K) and the harm threshold high (2 K rather than 1 K). Using a simple coupled climate-carbon cycle model, it is shown that if the climate sensitivity were to be definitively determined to be 2 K in 2020, then the emission reductions achieved by that time and planned for the next two decades are still fully needed. Only if climate sensitivity is very low (1 K) and the harm threshold is high (2 K) would the emissions achieved by 2020 not have been fully necessary. However, this would still lead to changes in ocean chemistry that are likely to be highly detrimental to marine life. Thus, when the full spectrum of impacts is considered, there is no plausible set of assumptions under which stringent near-term emission reductions are rendered unnecessary.