Why estimates of deglacial ice loss should be biased low

Abstract

Sea level rose by about 130 m over the last deglaciation, but the Last Glacial Maximum (LGM, 24,000 to 18,000 yr before present) is estimated to have had only enough grounded ice to raise sea level by about 115 m. These estimates of the mass of grounded ice are typically expressed as an equivalent sea-level rise by normalizing with the oceanic surface area and seawater density. Here we diagnose the deglacial freshwater budget by comparing the freshwater content of the modern ocean to four reconstructions of the Last Glacial Maximum constrained by data from porewaters, benthic foraminifera, and faunal assemblages. A deglacial ice loss of 127.4 to 128.0 m of sea-level equivalent is necessary to balance the freshwater budget, suggesting a bias of 2.0 to 2.6 m less than the assumed true sea-level rise. Oceanic thermal expansion explains 0.8 to 1.4 m of the bias. Independent of deglacial warming, an additional 1.2 m of bias is intrinsic to the formulation of the equivalent sea-level metric. Thus, estimates of deglacial ice loss should underestimate the true sea-level rise, leading to a reduction in the amount of glacial land ice that needs to be found.