The duration of the morphometric adjustments following change in net mass balance. This is a vague concept, because the reaction time may depend on the glacier history in an untransparent way. A distinction should be made between response times for glacier volume and glacier length. When the specific balance changes, the volume will respond immediately. For glacier length it may take a bit more time, in particular when the changes are more apparent at higher parts. ‘Numerical experimentation has indeed shown that response times for volume are typically 30% shorter than response times for length’ (J. Oerlemans 2001).