this year, the United nations Conference of the parties in paris is tasked with delivering a land-mark agreement on avoiding the 2°C warming associated with ‘dangerous interference with the climate system’. if such an agreement is reached, it will re-invigorate analyses of how global and national energy systems can deliver the rates of mitigation accompanying the 2°C threshold. Commonly such studies rely on detailed integrated assessment models combining physical and economic relationships to postulate future climate and energy systems. typically these models optimise on the basis of minimum costs in developing ‘feasible’ scenarios in terms of technology, infrastructure and efficiency change, as well as providing longitudinal outputs related to parameters such as capital costs, carbon tax rates, etc.This paper reflects upon the reliance of decision makers on the outcomes of these models, and their suitability for producing plausible outcomes. One criticism is aimed at their limited ability to explore future societies under the pressures of climate change mitigation and adaptation, given that their economic parameterisations are underpinned by historical relationships fit for a world unperturbed by climate change. a second questions whether their theoretical basis is appropriate for articulating the outcome of non-marginal change, when the very futures they are set up to explore involve non-marginal rates of change; for example radical cuts in CO2, or severe climate change impacts. Quantifying societal responses within such models is a particular challenge. Finally, these models downplay risks through disregarding low-probability, high-impact events and their consequences, including wars and migration. it is argued here that as currently formulated these tools are unsuitable for modelling the revolutionary transformations necessary to stay within 2°C carbon budgets, or similarly, futures with higher levels of warming and subsequent impacts.To address these deficiencies, this paper takes an alternative approach to contextually explore the ‘possibility space’ appropriate for avoiding 2°C. in contrast to exercises that build future scenarios using ‘immutable’ relationships within and between the energy and climate systems, a more transparent and dynamic framing based on highly constrained cumulative carbon budgets is proposed. Building on previous assessments that use a similar approach by authors anderson and Bows, this analysis looks beyond the contested ‘annex 1’ and ‘non- annex 1’ division to backcast what the remaining CO2 budget implies for the world’s top emitting nations. the analysis takes the top 25 nations, responsible for 85% of global CO2, and groups these nations on the basis of similarities within their energy systems. Using a range of explicit variables such as short-term CO2 growth, a suite of emission scenarios for these groups are developed, constrained within a range of 2°C carbon budgets. Byvarying the levels of near-term emissions from each group’s energy system, under a highly constrained CO2 budget, important sensitivities are revealed. results demonstrate the significance of the rates of CO2 growth in the highest emitting groups, the importance of short- term change and the relevance of bunker-fuel emissions in shaping our collective futures. they also illustrate that only non-marginal change resulting in radical transitions across all energy systems can now be reconciled with the 2°C policy objective.