The 2007 Bali conference heard repeated calls for reductions in global greenhouse gas emissions of 50 per cent by 2050 to avoid exceeding the 2°C 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 2°C 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 (CO2e). Similarly, the current framing of climate change cannot be reconciled with the rates of mitigation necessary to stabilize at 550 ppmv CO2e and even an optimistic interpretation suggests stabilization much below 650 ppmv CO2e is improbable.
One contribution of 12 to a Theme Issue ‘Geoscale engineering to avert dangerous climate change’.
↵CO2 data from the Carbon Dioxide Information Analysis Centre (CDIAC) including recent data from G. Marland (2006, personal communication); non-CO2 greenhouse gas data from the USA Environmental Protection Agency (EPA 2006) including the projection for 2005, and assuming deforestation emissions in 2005 to be 5.5 GtCO2 (1.5 GtC), with a 0.4 per cent growth in the preceding 5 years in line with data within the Global Forest Resources Assessment (FAO 2005).
↵FAO (2005) contains rates of tropical deforestation for the 1990s revised downward from those in the 2000 Global Forest Resources Assessment (FAO 2000; R. A. Houghton 2006, personal communication). An earlier estimate based on high-resolution satellite data over areas identified as ‘hot spots’ of deforestation, estimated the figure at nearer 3.7 GtCO2 (1 GtC) for 2000 (Achard et al. 2004). It is Houghton's more recent estimate that is used in this paper.
↵There remains considerable uncertainty as to the actual level of radiative forcing associated with aerosols, exacerbated by their relatively short residence times in the atmosphere and uncertainty as to future aerosol emission pathways (Cranmer et al. 2001; Andreae et al. 2005). Similarly, there remain significant uncertainties as to the radiative forcing impact of non-CO2 emissions from aviation, particularly contrails and linear cirrus (e.g. Stordal et al. 2004; Mannstein & Schumann 2005).
↵For example, and in particular, the reduced uptake of CO2 in the Southern Ocean (Raupach et al. 2007) and the potential impact of low level ozone on the uptake of CO2 in vegetation (Cranmer et al. 2001).
↵While the scenarios are at least as optimistic as those underpinning, for example, the 2005 Forest Resource Assessment (FAO 2005) and the 2006 Stern report, it could be argued they are broadly in keeping with the high profile deforestation gained during the 2007 United Nations Climate Change Conference in Bali.
↵DL per cent change value is the mean for the period between 2030 and 2050, and DH is the mean value for 2040–2060.
↵EPA values for global warming potential of the basket of six gases are slightly different from those used in IPCC. The difference, though noted here, does not significantly alter the analysis or results.
↵Comparing values outlined in Stern (2006, p. 233) with those in AB1 and AB2 for 2015. In addition, Stern envisages a global CO2e emissions increase of approximately 5 GtCO2e between 2000 and 2015 compared with provisional estimates for China alone of between 4.2 and 5.5 GtCO2e, extending up to 12.2 GtCO2e (T. Wang & J. Watson of the Sussex Energy Group (SEG) 2008, personal communication). If the lower SEG estimate for China is correct, Stern's analysis implicitly assumes that global emissions (excluding China) remain virtually unchanged between 2000 and 2015.
↵The 450 ppmv figure is from AR4 (IPCC 2007a), while the 550 and 650 ppmv figures are from Jones et al. (2006) and include carbon-cycle feedbacks (used in Stern's analysis). Although the Jones et al. figures are above the mid-estimates of the impact of feedbacks, there is growing evidence that some carbon-cycle feedbacks are occurring earlier than was thought would be the case, e.g. the reduced uptake of CO2 by the Southern Ocean (Raupach et al. 2007).
↵Meinshausen (2006) estimates the mid-range probability of exceeding 4°C at approximately 34 per cent for 600 ppmv and 40 per cent for 650 ppmv. Given this analysis has not factored in a range of other issues with likely net positive impacts, adapting for estimated impacts of at least 4°C appears wise.
↵At 650 ppmv the range of global decarbonization rate is 3–4 per cent per year (table 7, columns 1 and 4). As OECD nations represent approximately 50 per cent of global emissions, and assuming continued CO2 emission growth from non-OECD nations for the forthcoming two decades, the OECD nations will need to compensate with considerably higher rates of emission reductions.
↵This is not assumed desirable or otherwise, but is a conclusion of (i) the quantitative analysis developed within the paper, (ii) the premise that stabilization in excess of 600–650 ppmv CO2e should be avoided and (iii) Stern's assertion that annual reductions of greater than 1 per cent have ‘been associated only with economic recession or upheaval’ (Stern 2006, p. 231).
- © 2008 The Royal Society