The ESME (Energy Systems Modelling Environment) model is national integrated energy systems model for the UK, used to simulate a range of different low carbon pathways. Built in the AIMMS environment, the model uses linear programming to assess cost-optimal technology portfolios across all sectors of the energy system, to deliver CO2 emission reductions (Heaton 2014). It is spatially resolved, providing insights on the energy system change across different regions of the UK (Li, Pye, and Strachan 2016). ESME also features a module for simulating large numbers of runs to explore parametric uncertainty of model inputs, through Monte Carlo sampling (Pye, Sabio, and Strachan 2015; Pye, Usher, and Strachan 2014), and it is this feature that is used in the REEEM project. In addition to its use in research, ESME has been used to inform energy policy and strategy in the UK, both for the Department for Energy and Climate Change (DECC 2011),  and the UK Committee on Climate Change (CCC) (CCC 2013, 2011).