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How can dangerous interference with the climate system be avoided? Science can help decision-makers answer this political question. Earlier publications have focused on the probability of keeping global mean temperature change below certain thresholds by stabilizing greenhouse gas concentrations at particular levels. We compare the results of such "stabilization profiles" with a set of "peaking profiles" that reduce emissions further after stabilization and thus result in a concentration peak. Given the inertia in the climate system, stabilization profiles lead to ongoing warming beyond 2100 until the temperature reaches equilibrium. This warming partly can be prevented for peaking profiles. In this way, these profiles can increase the likelihood of achieving temperature thresholds by 10–20% compared with the likelihood for the associated stabilization profiles. Because the additional mitigation efforts and thus costs for peaking profiles lie mainly beyond 2100, peaking profiles achieving temperature thresholds with the same likelihood as the original stabilization profile, but at considerably lower cost (up to 40%), can be identified. The magnitude of the cost reductions depends on the assumptions on discounting. Peaking profiles and overshoot profiles with a limited overshoot may, in particular, play an important role in making more ambitious climate targets feasible.

A post-2012 regime aimed at reducing greenhouse gas (GHG) emissions could develop towards a universal or fragmented regime. The fundamental difference between a universal and a fragmented regime is that the first involves a single comprehensive climate regime in which all countries participate, whereas the second involves either multiple treaties or a single treaty in which not all countries participate. This study assesses the literature on a wide range of different model studies concerning the environmental effectiveness and economic consequences of various universal and fragmented climate regimes. The most important conclusions (e.g. relative position of regions in terms of costs) are generally consistent across different studies, despite the differences in methodology. We conclude that stabilising GHG concentrations at low levels is more costly with a fragmented regime than with a universal regime, because reduction targets must be achieved by a smaller number of countries or because fragmented treaties may prevent reducing GHGs where it is cheapest to do so. However, establishing a universal regime will be challenging due to cost differences between regions if emissions are allocated based on specific allocation rules and incentives to free-ride on a universal regime. Even though alternative behaviours such as responsibility, the implementation of transfer schemes or exclusive membership can increase the likelihood of achieving a universal regime, a fragmented regime seems more feasible. Therefore, a transitional fragmented 'coalition of the willing' could be established first, which could provide the basis for a larger, universal regime in the long term. Adapted from the source document.

This article presents the regional emission targets corresponding to different climate regimes for differentiating commitments beyond 2012 on the basis of the Multi-Stage approach. This approach assumes a gradual increase in the number of Parties involved & their level of commitment according to participation & differentiation rules. The analysis focuses on two global greenhouse gas emission profiles resulting in CO2-equivalent concentrations stabilising at 550 & 650 ppmv in 2100 & 2150, respectively. Three Multi-Stage cases have been developed in order to assess different types of thresholds. These share three consecutive stages representing different commitments: stage 1 -- no quantitative commitments; stage 2 -- emission-limitation targets & stage 3 -- emission reduction targets. The analysis shows that by 2025 all three cases result in emission reduction objectives for all Annex I regions of at least 30-55% below their 1990 levels for 550 ppmv, whereas for 650 ppmv target they range from 0 to 20%. Furthermore, early participation is required of the major non-Annex I regions through emission limitation targets i.e. before 2025 & 2050 for the 550 & 650 ppmv targets, respectively. The first participation threshold for adopting emission-limitation targets on the basis of a capability-responsibility index (as introduced here) can provide for a balanced & timely participation of non-Annex I regions. Major strengths & weaknesses of the climate regimes are discussed & important obstacles & pre-conditions for their feasibility & acceptability are highlighted. Tables, Figures, Appendixes, References. Adapted from the source document.

The "South - North Dialogue" Proposal, developed by researchers from developing and industrialised countries, outlined equitable approaches to mitigation. These approaches were based on the criteria of responsibility, capability and potential to mitigate, and include deep cuts in industrialised (Annex I) countries and differentiated mitigation commitments for developing countries. This paper quantitatively analyses the implications of the proposal for countries' emissions and costs. The analysis focuses on a "political willingness" scenario and four stabilisation scenarios. The analysis shows that stringent stabilisation targets imply that many developing countries would have to take on quantitative mitigation obligations by 2030, even when the Annex I countries take on ambitious mitigation commitments far beyond the Kyoto obligations. The "political willingness scenario" will probably not suffice to limit a warming of the Earth's atmosphere to below 2 °C.

In order to evaluate the effectiveness of climate policy, it is important to understand emission trends and policies at the national level. The 2015 Paris Agreement includes (Intended) Nationally Determined Contributions, so-called (I)NDCs, outlining the contribution of different Parties to the overall target of the agreement to limit global mean temperature increase to well below 2 °C. Here, we assess emission trajectories and the energy system transition of 11 major economies (in the remainder: countries) projected by integrated assessment models (IAMs) for baseline and cost-optimal 450 ppm CO2 eq mitigation scenarios and compare the results with the (I)NDCs. Limiting global temperature increase to below 2 °C implies a substantial reduction of the estimated available carbon budget for each country. The national carbon budgets between 2010 and 2100 showed reductions between the baseline and the 2 °C consistent mitigation scenario ranging from 52% in South Korea to 95% in Brazil. While in the baseline scenario, the share of low-carbon primary energy sources is projected to remain around 15% (with Brazil being a notable exception, reaching 30%); in themitigation scenarios, the share of low-carbon energy is projected to increase to over 50% in 2050 in nearly all countries, with the EU, Japan and Canada reaching the largest shares. Comparison with the (I)NDCs shows that in Brazil, Canada, the EU, Mexico (conditional target), South Korea and the USA, the emission reduction targets of the NDCs are closer to the mitigation requirement of the 2 °C scenario; in other countries, however, there is still a large gap. The national detail of the indicators adds to the literature on low-carbon emission pathways, assists the assessment of the Paris Agreement and provides support to national policymakers to identify focus areas for climate policy in the coming years.

The RCP2.6 emission and concentration pathway is representative of the literature on mitigation scenarios aiming to limit the increase of global mean temperature to 2°C. These scenarios form the low end of the scenario literature in terms of emissions and radiative forcing. They often show negative emissions from energy use in the second half of the 21st century. The RCP2. 6 scenario is shown to be technically feasible in the IMAGE integrated assessment modeling framework from a medium emission baseline scenario, assuming full participation of all countries. Cumulative emissions of greenhouse gases from 2010 to 2100 need to be reduced by 70% compared to a baseline scenario, requiring substantial changes in energy use and emissions of non-CO2 gases. These measures (specifically the use of bio-energy and reforestation measures) also have clear consequences for global land use. Based on the RCP2. 6 scenario, recommendations for further research on low emission scenarios have been formulated. These include the response of the climate system to a radiative forcing peak, the ability of society to achieve the required emission reduction rates given political and social inertia and the possibilities to further reduce emissions of non-CO2 gases. © 2011 The Author(s).