The UK is committed to ambitious medium- and long-term climate change targets, including a commitment to an 80% reduction in emissions from 1990 levels by 2050. Whilst emissions have fallen significantly since 1990, further reductions will be increasingly difficult to achieve. The government has agreed carbon budgets to the late 2020s that are consistent with the long-term 80% target. However, increasing energy prices since the mid-2000s and the 2008 financial crisis have led to cracks in the political consensus in support of these budgets and targets. This paper carries out an assessment of the feasibility of the UK's agreed low carbon pathway over the medium term, with a particular focus on the fourth carbon budget (2023-27). It analyses the uncertainties associated with the specific changes that may be necessary to comply with this carbon budget - including measures to decarbonise electricity, heat and transport. This analysis focuses on 'instrumental' uncertainties associated with specific areas of the energy system (e.g. the decarbonisation of heat in households) and 'systemic' uncertainties that tend to have more pervasive implications for the energy system as a whole (e.g. uncertainties associated with public attitudes). A framework is developed that sets out and analyses the key uncertainties under those two broad categories, in terms of their complexity and their potential impact on the fourth carbon budget. Through the application of this framework the paper also considers strategies to mitigate or manage these uncertainties, and which actors could help develop and implement these strategies. (C) 2015 Elsevier Ltd. All rights reserved.