The study compares two indicative climate mitigation policies with two ‘business-as-usual’ emissions pathways, and focuses on six catchments in the UK representing a diversity of catchment and climatic conditions. One of the indicative mitigation policies assumes that global emissions of greenhouse gases peak in 2016 and decline at 5% per year after, and the other assumes emisssions peak in 2030 and decline at 2% per year (Gohar and Lowe, 2009). The pathway with the 2016 peak produces a 50% probability that the increase in global mean surface temperature by 2100 would be below 2 °C (above pre-industrial; Gohar and Lowe, 2009), and is thus consistent with the aspirations of the Copenhagen Accord which underpins current global climate policy; the 2030 peak pathway produces a median temperature increase of 2.8 °C. The two ‘business-as-usual’ emissions pathways (SRES A1b and A1FI) result in median estimates of increases in global mean temperature by 2100 of 4 and 5.6 °C respectively; the difference between the two represents uncertainty in the effects of future economic development on emissions. Projected changes in hydrological regimes are known to be sensitive to the spatial and seasonal pattern of change in climate as represented by different climate models and, to a lesser extent, hydrological model uncertainty. It is also possible that the effects of climate mitigation policy on these impacts may vary with climate model pattern and representation of hydrological processes. This is assessed here by constructing climate scenarios from 21 global climate models, and by considering the effect of hydrological model parameter uncertainty on the impacts avoided by climate policy.