The range of estimates is rather wide, starting at $20 per ton (Nordhaus 2007), through $50, which is an upper bound in Tol’s (2005) meta-analysis of 103 estimates from 28 studies, up to $311 in Stern’s (2007) influential book (all estimates in 2005 USD). However, the main source of variation in these estimates does not come from differences in methodologies; rather studies differ in their assumptions (Pindyck 2013a; Stern 2013). Notably the choice of the discount rate on future consumption has (an exponential) impact on the estimates (Nordhaus 2007; Pindyck 2013a; Stern 2008; Tol 2005).