Global glacier evolution over the last millennium and the influence of climate forcings on the mass balance

Mass loss of glaciers and ice caps has been one of the major contributors to sea-level rise over the past century. Glaciers respond slowly to a changing climate. Therefore, glacier evolution over the past century is partly a result of prior changes in the climate, resulting both from internal variability in the climate system and changes in external forcings. Here we present a simulation of global glacier evolution over the period 850-2000 CE and assess the influence that different climate forcings have on the glacier mass balance. The glacier evolution simulation thus serves as a base for the mass balance attribution experiment.

The Open Global Glacier Model (OGGM) was used to simulate glacier geometry and mass balance evolution of land-terminating glaciers. The dynamic simulations were forced with the full length of the Last Millennium Reanalysis (LMR), a climate timeseries covering the period 0-2000 CE, using the first part for spin-up only. The initialization of the glacier states in 850 CE was done with a calibration procedure, making use of glaciers with a relatively short memory for initializing those with a longer one.

To assess the influence of different climate forcings (volcanic, greenhouse gases (GHG), orbital, land cover and land use, solar and anthropogenic ozone and aerosols) on glacier mass balance, simulations of the Community Earth System Model Last Millennium Ensemble (CESM-LME) are being used. The CESM-LME fully forced, single forced and 850 CE control simulations are used to force OGGM in climatic mass balance simulations. In those simulations the glacier geometries are prescribed with those from the LMR forced dynamic simulation, in order to avoid biases in the attribution caused by deviating glacier evolutions under the different forcings.

Results show that the changes in the GHG forcing have little influence on the SMB from 850 to ~1850 CE. After that the influence becomes increasingly more negative. All other forcings that have been assessed here have positive contribution to glacier mass balance over the last millennium. Although the influence of land use and land cover change has not received a lot of attention before in this context, it has a substantial influence on global glacier mass in our simulations. However, the influence of the forcings differs strongly between regions.