The Climate Science Special Report: Science Informing Mitigation Issues (Invited Presentation)

This chapter of CSSR provides scientific context for the long-term mitigation of climate change. Policy analysis and recommendations are beyond scope. Limiting and stabilizing warming to any level implies that there is an upper limit to the cumulative amount of CO2 that can be added to the atmosphere. Eventually stabilizing the global temperature requires CO2 emissions to approach zero. For a 3.6°F (2°C) or any desired global mean temperature target, an estimated range of allowable cumulative CO2 emissions from the current period onward can be calculated. Accounting for the temperature effects of non-CO2 species, cumulative CO2 emissions are required to stay below about 800 GtC in order to provide a two-thirds likelihood of preventing 3.6°F (2°C) of warming, meaning approximately 230 GtC more could be emitted globally. Assuming global emissions follow the range between the RCP8.5 and RCP4.5 scenarios, emissions could continue for approximately two decades before this cumulative carbon threshold is exceeded. Meeting a 2.7°F (1.5°C) target implies much tighter constraints; only about 30 GtC more could be emitted, which is projected to occur in the next few years. Mitigation of non-CO2 species (e.g., methane and black carbon) contributes substantially to near-term cooling benefits but cannot be relied upon for ultimate stabilization goals. There is increased interest by some scientists and policy makers in exploring additional measures designed to reduce net radiative forcing through other, as yet untested actions, which are often referred to as geoengineering or climate intervention (CI) actions. The evaluation of the suitability and advisability of potential CI actions requires a decision framework that includes important dimensions beyond scientific and technical considerations. Among these dimensions to be considered are the potential development of global and national governance and oversight procedures, geopolitical relations, legal considerations, environmental, economic and societal impacts, ethical considerations, and the relationships to global climate policy and current GHG mitigation and adaptation actions.