Spatial and temporal distribution of mass loss from the Greenland Ice Sheet since AD 1900

Abstract

The response of the Greenland Ice Sheet (GIS) to changes in temperature during the twentieth century remains contentious, largely owing to difficulties in estimating the spatial and temporal distribution of ice mass changes before 1992, when Greenland-wide observations first became available. The only previous estimates of change during the twentieth century are based on empirical modelling and energy balance modelling. Consequently, no observation-based estimates of the contribution from the GIS to the global-mean sea level budget before 1990 are included in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Here we calculate spatial ice mass loss around the entire GIS from 1900 to the present using aerial imagery from the 1980s. This allows accurate high-resolution mapping of geomorphic features related to the maximum extent of the GIS during the Little Ice Age at the end of the nineteenth century. We estimate the total ice mass loss and its spatial distribution for three periods: 1900-1983 (75.1 ± 29.4 gigatonnes per year), 1983-2003 (73.8 ± 40.5 gigatonnes per year), and 2003-2010 (186.4 ± 18.9 gigatonnes per year). Furthermore, using two surface mass balance models we partition the mass balance into a term for surface mass balance (that is, total precipitation minus total sublimation minus runoff) and a dynamic term. We find that many areas currently undergoing change are identical to those that experienced considerable thinning throughout the twentieth century. We also reveal that the surface mass balance term shows a considerable decrease since 2003, whereas the dynamic term is constant over the past 110 years. Overall, our observation-based findings show that during the twentieth century the GIS contributed at least 25.0 ± 9.4 millimetres of global-mean sea level rise. Our result will help to close the twentieth-century sea level budget, which remains crucial for evaluating the reliability of models used to predict global sea level rise.

Evaluating Model Simulations of Twentieth-Century Sea-Level Rise. Part II: Regional Sea-Level Changes

“…Observation-based estimates and reanalysis-based estimates of Greenland SMB indicate a contribution of Greenland SMB to global sea level change of up to 25 6 9 mm and 10 6 4 mm, respectively (Kjeldsen et al 2015; Part I), which is significantly larger than the estimate based on extended historical simulations from climate models. The reason for the difference with observations is likely internal variability leading to more warming around Greenland in 1920-50 that is not present in the CMIP5 model historical simulations (Church et al 2013a; Part I).…”

Reassessment of 20th century global mean sea level rise

“…The Greenland ice sheet contribution to sea level is estimated using the recent mass balance estimate from ref. 19. The Antarctic ice sheet is modeled with the RACMO2.3 model (40), as in ref.…”

“…To produce homogeneous synthetic fields of historical sea level fields, we combine historical fields of sea surface height from CMIP5 models and the SODA reanalysis over the period from 1871 to 2005 with independent estimates of glacier melting (18). Also added are the observation-based estimates of the ice sheets (19,20) and TWS containing both groundwater depletion (21) and water impoundment behind dams (22). For the simulation of ocean dynamics, sea surface height fields (variable "zos" in CMIP5 terminology) are obtained from the historical simulations of 11 CMIP5 models (see Fig.…”

“…S2A). After accounting for VLM, each tide gauge is further corrected for geoid changes from ongoing GIA (17), glacier/ice-sheet melting (18)(19)(20) (Fig. S2 C and D), and TWS (21,22) (Fig.…”

“…1B) before ∼1970 are the result of methodological limitations with respect to the heterogeneous spatial and temporal tide gauge distribution, we test our approach in a set of 12 synthetic sea level fields from the Simple Ocean Data Assimilation (SODA) reanalysis and historical simulations of CMIP5 models during their common period from 1871 to 2005; these are combined with the corresponding components of the glacier contribution (in the case of SODA, the glacier reconstruction from ref. 18) and historical fingerprints from TWS (21,22) and the Antarctic and Greenland ice sheets (19,20) (Materials and Methods). Because in the synthetic sea level fields the true model GMSL, as well as the individual ice-melt and TWS fingerprints, are a priori known, they are an excellent testbed for our reconstruction approach.…”