Linkages between short term and long term
For any chosen GHG-stabilization target, near-term decisions can be made regarding mitigation opportunities to help maintain a consistent emissions trajectory within a range of long-term stabilization targets. Economy-wide modelling of long-term global stabilization targets can help inform near-term mitigation choices. A compilation of results from short-and long-term models using scenarios with stabilization targets in the 3–5 W/m2 range (category II to III), reveals that in 2030, for carbon prices of less than 20 US$/tCO2-eq, emission reductions of in the range of 9-18 GtCO2-eq/yr across all GHGs can be expected. For carbon prices less than 50 US$/tCO2-eq this range is 14–23 GtCO2-eq/yr and for carbon prices less than US$100/tCO2-eq it is 17-26 GtCO2-eq/yr. (high agreement, much evidence).
Three important considerations need to be remembered with regard to the reported marginal costs. First, these mitigation scenarios assume complete ‘what’ and ‘where’ flexibility; that is, there is full substitution among GHGs, and reductions take place anywhere in the world as soon as the models begin their analyses. Second, the marginal costs of realizing these levels of mitigation increase in the time horizon beyond 2030. Third, at the economic-sector level, emission-reduction potential for all GHGs varies significantly across the different model scenarios (high agreement, much evidence) [3.6.2].
A risk management or ‘hedging’ approach can assist policy-makers to advance mitigation decisions in the absence of a long-term target and in the face of large uncertainties related to the cost of mitigation, the efficacy of adaptation and the negative impacts of climate change. The extent and the timing of the desirable hedging strategy will depend on the stakes, the odds and societies’ attitudes to risks, for example, with respect to risks of abrupt change in geophysical systems and other key vulnerabilities. A variety of integrated assessment approaches exist to assess mitigation benefits in the context of policy decisions related to such long-term climate goals. There will be ample opportunity for learning and mid-course corrections as new information becomes available. However, actions in the short term will largely determine long-term global mean temperatures and thus what corresponding climate change impacts can be avoided. Delayed emission reductions lead to investments that lock in more emission-intensive infrastructure and development pathways. This significantly constrains the opportunities to achieve lower stabilization levels and increases the risk of more severe climate change impacts. Hence, analysis of near-term decisions should not be decoupled from analysis that considers long-term climate change outcomes (high agreement, much evidence) [3.6; 3.5.2].