The glacier-wide mass balance that would have been observed if the glacier surface topography had not changed since a reference date. The time-invariant surface is called the 'reference surface', and is defined at some convenient time within a mass-balance programme, often at the start. The reference-surface balance is obtained when point measurements are extrapolated from their actual altitude to the altitude of the reference surface at the same horizontal position, and then extrapolated over the reference area. The reference surface is likely to differ from the actual surface in both area and area-altitude distribution. Differences in area and area-altitude distribution feed back on the magnitude of glacier response to climate. The reference-surface balance does not incorporate any of these feedback effects and is therefore more closely correlated to variations in climate than is the conventional balance.