The time between any kind of change and the response it elicits in a system. In geomorphology it is the interval between a disturbance or change in controls and observable morphological change. In hydrology, the bigger a catchment, the longer its reaction time. Harrison (2003) Am. Geophys. Union C22A-01 extends this term to glaciers, but Oerlemans (2007) J. Glaciol. 53, 182 thinks that reaction time is not a well-defined concept, ‘because the reaction time may depend on the glacier/climate history in a non-transparent way’.