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This study explores the critical notion of how feasible it is to achieve long-term mitigation goals to limit global temperature change. It uses a model inter-comparison of three integrated assessment models (TIAM-Grantham, MESSAGE-GLOBIOM and WITCH) harmonized for socio-economic growth drivers using one of the new shared socio-economic pathways (SSP2), to analyse multiple mitigation scenarios aimed at different temperature changes in 2100, in order to assess the model outputs against a range of indicators developed so as to systematically compare the feasibility across scenarios. These indicators include mitigation costs and carbon prices, rates of emissions reductions and energy efficiency improvements, rates of deployment of key low-carbon technologies, reliance on negative emissions, and stranding of power generation assets. The results highlight how much more challenging the 2 • C goal is, when compared to the 2.5-4 • C goals, across virtually all measures of feasibility. Any delay in mitigation or limitation in technology options also renders the 2 • C goal much less feasible across the economic and technical dimensions explored. Finally, a sensitivity analysis indicates that aiming for less than 2 • C is even less plausible, with significantly higher mitigation costs and faster carbon price increases, significantly faster decarbonization and zero-carbon technology deployment rates, earlier occurrence of very significant carbon capture and earlier onset of global net negative emissions. Such a systematic analysis allows a more in-depth consideration of what realistic level of long-term temperature changes can be achieved and what adaptation strategies are therefore required.

Abstract:The scenarios generated by energy systems models provide a picture of the range of possible pathways to a low-carbon future. However, in order to be truly useful, these scenarios should not only be possible but also plausible. In this paper, we have used lessons from historical energy transitions to create a set of diagnostic tests to assess the feasibility of an example 2 • C scenario (generated using the least cost optimization model, TIAM-Grantham). The key assessment criteria included the rate of deployment of low carbon technologies and the rate of transition between primary energy resources. The rates of deployment of key low-carbon technologies were found to exceed the maximum historically observed rate of deployment of 20% per annum. When constraints were added to limit the scenario to within historically observed rates of change, the model no longer solved for 2 • C. Under these constraints, the lowest median 2100 temperature change for which a solution was found was about 2.1 • C and at more than double the cumulative cost of the unconstrained scenario. The analysis in this paper highlights the considerable challenge of meeting 2 • C, requiring rates of energy supply technology deployment and rates of declines in fossil fuels which are unprecedented.

Comparing emissions scenarios is an essential part of mitigation analysis, as climate targets can be met in various ways, with different economic, energy system and co-benefit implications. Typically, a central 'reference scenario' acts as a point of comparison, and often this has been a no-policy baseline, with no explicit mitigative action taken. The use of such baselines is under increasing scrutiny, raising a wider question around the appropriate use of reference scenarios in mitigation analysis. In this Perspective, we assess three critical issues relevant to the use of reference scenarios, demonstrating how different policy contexts merit the use of different scenarios. We provide recommendations to the modelling community on best practice in the creation, use and communication of reference scenarios.The Paris Agreement commits the global community to limiting warming to 'well below 2°C above preindustrial levels and pursuing efforts to limit the temperature increase to 1.5°C' 1 . To meet these ambitious goals, countries must embark on mitigation pathways towards a decarbonised future. Such pathways can be explored through the use of integrated assessment 2,3 and energy system 4 modelling. Integrated assessment models (IAMs) are a heterogeneous set of tools, varying substantially in model structure and behaviour. All IAMs however, attempt to couple different socio-economic, technical and biophysical systems together, allowing low-carbon futures to be explored in a systematic and selfconsistent manner. In this Perspective, we focus on the use of detailed-process IAMs to conduct mitigation analysis, as opposed to aggregate benefit-cost IAMs 2 . Our justification is that such IAMs (containing detailed representations of energy systems, as well as in many cases land and agricultural systems) are widely used in the scientific assessment of mitigation pathways, as reported in Intergovernmental Panel on Climate Change (IPCC) reports [5][6][7][8] . We also consider the use of standalone energy system models (i.e. those not integrated with biophysical systems) to produce low-carbon pathways at a national, regional and global scale.Many different mitigation scenarios could comply with the Paris Agreement. Scenarios may differ in their demographic, socio-economic and technological features, and hence there is a vast solution space of possible low-carbon futures meriting consideration. Making comparisons between scenarios is therefore an essential part of mitigation analysis.Modellers often rely on reference scenarios to enable different mitigation scenarios to be evaluated. We define a reference scenario as: 'a scenario which is referred to when evaluating mitigation scenarios, and hence is a central point of comparison in the analysis'. Such reference scenarios are often generated by one actor but intended for use by a wide range of other actors in mitigation analysis. Pertinent examples include the SSP-RCP framework 9-13 , scenarios generated by the International Energy Agency 14 , and the Annual Energy Outlook of t...

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This paper analyses the emissions and cost impacts of mitigation of non-CO 2 greenhouse gases (GHGs) at a global level, in scenarios aimed at meeting a range of long-term temperature goals (LTTGs). The study combines an integrated assessment model (TIAM-Grantham) representing CO 2 emissions (and their mitigation) from the fossil fuel combustion and industrial sectors, coupled with a model covering non-CO 2 emissions (GAINS), using the latest global warming potentials from the Intergovernmental Panel on Climate Change's Fifth Assessment Report. We illustrate that in general non-CO 2 mitigation measures are less costly than CO 2 mitigation measures, with the majority of their abatement potential achievable at US2005$100/tCO 2 e or less throughout the 21st century (compared to a marginal CO 2 mitigation cost which is already greater than this by 2030 in the most stringent mitigation scenario). As a result, the total cumulative discounted cost over the period 2010-2100 (at a 5% discount rate) of limiting global average temperature change to 2.5 • C by 2100 is $48 trillion (about 1.6% of cumulative discounted GDP over the period 2010-2100) if only CO 2 from the fossil fuel and industrial sectors is targeted, whereas the cost falls to $17 trillion (0.6% of GDP) by including non-CO 2 GHG mitigation in the portfolio of options-a cost reduction of about 65%. The criticality of non-CO 2 mitigation recommends further research, given its relatively less well-explored nature when compared to CO 2 mitigation.