Abstract
This study compares emission pathways aimed at limiting temperature increase to 2°C under varying constraints. In a first set of pathways, the timing of emission reductions is such that over the 2010–2100 period, assuming full participation from 2013 onwards, mitigation costs are minimized. In a second set of pathways, we set emissions in 2020 at a level based on the pledges of the Copenhagen Accord. In the ‘Copenhagen Potential’ scenario, climate talks result in satisfying conditions linked by countries to their ‘most ambitious’ proposals. Contrasting, in the ‘Copenhagen Current’ scenario, climate talks fall short of satisfying the conditions to move beyond current unilateral pledges. We include scenarios with and without the availability of bio-energy in combination with carbon capture and storage. We find that for a ‘Copenhagen Potential’ scenario, emissions by 2020 are higher (47 GtCO2eq/yr) than for a least-cost pathway for 2°C (43 GtCO2eq/yr with a 40–46 GtCO2eq/yr literature range). In the ‘Copenhagen Potential’ scenario the 2°C target can still be met with a likely chance, although discounted mitigation costs over 2010–2100 could be 10 to 15 % higher, and up to 60 % in the 2040–2050s, than for least-cost pathways. For the ‘Current Copenhagen’ scenario, maintaining an equally low probability of exceeding 2°C becomes infeasible in our model, implying higher costs due to higher climate risks. We conclude that there is some flexibility in terms of 2020 emissions compared to the optimal pathways but this is limited. The 2020 emission level represents a trade-off between short-term emission reductions and long-term dependence on rapid reductions through specific technologies (like negative emission reductions). Higher 2020 emissions lead to higher overall costs and reduced long-term flexibility, both leading to a higher risk of failing to hold warming below 2°C.
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Notes
Framework to Assess International Regimes for the differentiation of commitments—Simple Model for Climate Policy Assessment
Emissions in this paper refer to the sum of the six Kyoto greenhouse gas emissions (CO2 equivalent emissions weighted by global warming potentials as presented in the IPCC Fourth Assesment Report (Solomon et al. 2007)) as listed in Annex A of the Kyoto Protocol and include land-use related CO2 emissions
An annual reduction rate of 3.5–4.5 % of 2000 emission levels, applied to Kyoto greenhouse gases excluding land use CO2, for respectively with and without bio-energy with CCS (BECCS) technology. The potential reduction rate with BECCS is higher because this technology results in net negative emissions, as it replaces fossil fuels with biomass and captures and stores the CO2 from combustion.
The Joint Research Centre of the European Commission in Ispra, Italy, has calculated the estimated credits accounting for land use and forestry for all Annex I countries, as described in den Elzen et al. (2011), which would result in additional emission allowances of 2.5 % of 1990 Annex I emissions (about 0.5 GtCO2eq) by 2020.
A scenario is considered feasible if there is sufficient mitigation potential available to meet the climate target at all times during the evaluation period, at carbon prices below 1000$/tC.
Note that our reported total emissions comprise all GHG emissions of gases controlled under the Kyoto Protocol, and Ozon Depleting Substances (ODS) emissions are dealt with separately in our climate model. Total emission levels reported in Montzka et al. (2011) include ODS controlled under the Montreal Protocol, and higher CO2 emissions from land use. This explains about 3 Gt CO2eq of the difference.
Note that our estimates—in contrast with the UNEP figures—are based on non-harmonized historical data. With harmonization, the optimal emission level would be about 1 GtCO2eq higher by 2020 (also see SI)
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van Vliet, J., van den Berg, M., Schaeffer, M. et al. Copenhagen Accord Pledges imply higher costs for staying below 2°C warming. Climatic Change 113, 551–561 (2012). https://doi.org/10.1007/s10584-012-0458-9
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DOI: https://doi.org/10.1007/s10584-012-0458-9