The Copenhagen Accord reiterates the international community’s commitment to ‘hold the increase in global temperature below 2 degrees Celsius’. Yet its preferred focus on global emission peak dates and longer-term reduction targets, without recourse to cumulative emission budgets, belies seriously the scale and scope of mitigation necessary to meet such a commitment. Moreover, the pivotal importance of emissions from non-Annex 1 nations in shaping available space for Annex 1 emission pathways received, and continues to receive, little attention. Building on previous studies, this paper uses a cumulative emissions framing, broken down to Annex 1 and non-Annex 1 nations, to understand the implications of rapid emission growth in nations such as China and India, for mitigation rates elsewhere. The analysis suggests that despite high-level statements to the contrary, there is now little to no chance of maintaining the global mean surface temperature at or below 2 ° C. Moreover, the impacts associated with 2 ° C have been revised upwards, sufficiently so that 2 ° C now more appropriately represents the threshold between ‘dangerous’ and ‘extremely dangerous’ climate change. Ultimately, the science of climate change allied with the emission scenarios for Annex 1 and non-Annex 1 nations suggests a radically different framing of the mitigation and adaptation challenge from that accompanying many other analyses, particularly those directly informing policy.

The 2007 Bali conference heard repeated calls for reductions in global greenhouse gas emissions of 50 per cent by 2050 to avoid exceeding the 28C threshold. While such endpoint targets dominate the policy agenda, they do not, in isolation, have a scientific basis and are likely to lead to dangerously misguided policies. To be scientifically credible, policy must be informed by an understanding of cumulative emissions and associated emission pathways. This analysis considers the implications of the 28C threshold and a range of post-peak emission reduction rates for global emission pathways and cumulative emission budgets. The paper examines whether empirical estimates of greenhouse gas emissions between 2000 and 2008, a period typically modelled within scenario studies, combined with short-term extrapolations of current emissions trends, significantly constrains the 2000-2100 emission pathways. The paper concludes that it is increasingly unlikely any global agreement will deliver the radical reversal in emission trends required for stabilization at 450 ppmv carbon dioxide equivalent (CO 2 e). Similarly, the current framing of climate change cannot be reconciled with the rates of mitigation necessary to stabilize at 550 ppmv CO 2 e and even an optimistic interpretation suggests stabilization much below 650 ppmv CO 2 e is improbable.

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a b s t r a c tThe Tyndall decarbonisation scenarios project has outlined alternative pathways whereby a 60% reduction in CO 2 emissions from 1990 levels by 2050, a goal adopted by the UK Government, can be achieved. This paper, Part I of a two part paper, describes the methodology used to develop the scenarios and outlines the motivations for the project. The study utilised a backcasting approach, applied in three phases. In phase one, a set of credible and consistent end-points that described a substantially decarbonised energy system in 2050 were generated and reviewed by stakeholders. In phase two, pathways were developed to achieve the transition to the desired end-point. The impacts of the scenarios were assessed in phase three, by means of a deliberative multi-criteria assessment framework. The scenarios to emerge from this process are elaborated in Part II, and conclusions drawn in relation to the feasibility of achieving the 60% target.