The IPCC’s fifth assessment reports (AR5) have been widely heralded as delivering unequivocal and stark messages to policy makers. Of particular relevance to this paper is the inclusion, for the first time in the IPCC’s history, of explicit carbon budgets for differing probabilities of meeting a range of twenty-first century temperatures rises, from 1.5°C to 4°C. these carbon budgets provide a clear and quantifiable framework against which to assess technical and socio-economic policies for delivering the requisite rates and timeframes of mitigation.With specific focus on CO2-only emissions from the energy system, this paper will revisit the framing of the mitigation challenge in accordance with the ar5 carbon budget range for a “likely” (66%), “likely as not” (50%) and “not likely” (33%) probability of maintaining the rise in global mean surface temperature below the 2°C characterisation of dangerous climate change. new estimates of the process- carbon emissions from the cement industry, combined with a revised carbon budget estimate for deforestation will be used to determine what CO2-only budget remains for the energy sector.the paper will demonstrate that even assuming an unparalleled agreement at the paris negotiations in December 2015 (COp 21), alongside highly ambitious policies for reducing emissions from cement and deforestation, the energy-only budget of CO2 post-2020 will be radically more challenging than implied in ar5’s post-2011 budgets. For a “likely” chance of 2°C, and assuming global emissions peak in 2020, mitigation of energy-only CO2 would need to rise rapidly to well over 10% p.a. by 2025 and be maintained at that rate until the virtual elimination of CO2 by 2050. the story for a 50:50 chance of 2°C though slightly less dramatic, is nonetheless beyond anything yet countenanced by policy makers and only seldom referred to in the literature.Whilst the implications of such 2°C pathways are interpreted by some as highly regressive, this paper seeks to outline a positive solutions-oriented agenda. the scale of the challenge in the twenty-first century, with globalisation and an increasing population, demands moving beyond the reductionist disciplinary tools of the twentieth century. a systems-oriented and interdisciplinary approach designs infrastructure and develops institutions to offer resilience and facilitate iteration; a radical departure from abstract and theorised optimisation. set within this context of unprecedented rates of mitigation and rapidly shrinking timeframes, it is this intellectually more exciting agenda, that informs the papers quantitative, technical and social- science content.