The influence of climate forcings on global glacier evolution over the last millennium

Glacier evolution over the past century is, in part, caused by prior changes in the climate, resulting from both internal variability in the climate system and changes in external forcings. Therefore, the focus in this study is on the last millennium, to gain more insight into the build-up of the little ice age and the following glacier retreat. The role of the individual climate forcings (volcanic, greenhouse gasses (GHG), orbital, land cover and land use change (LULCC), solar and anthropogenically induced ozone and aerosols) is addressed through simulations with the Open Global Glacier Model (OGGM), using climate time series from the Community Earth System Model Last Millennium Ensemble (CESM-LME) as forcing. Our study is novel in both experimental set-up and in that it is the first global glacier attribution study on this time scale, simulating more than a small selection of glaciers.

How a glacier evolves is dependent on the state the glacier is in and on the climate. We take both these aspects into consideration in our global glacier last millennium attribution study. Instead of letting the glaciers freely evolve in the attribution experiments, we prescribe the glacier geometry based on a factual case, in order to avoid mass change being attributed based on the wrong glacier state (e.g. a glacier that has disappeared instead of an existing glacier). To create a factual case, the Last Millennium Re-analysis (LMR) is used as forcing in OGGM. This has the additional benefit that it gives the opportunity to address spin-up issues, as the LMR starts in 0 CE and the CESM-LME 850 years later.

Finally, the preliminary results show that changes in the volcanic forcing had a relatively minor role on the long term global glacier evolution over the last millennium. Instead LULCC and orbital forcing seem to have had a significant influence leading up to the little ice age maximum extent, and the GHG to the recent glacier retreat. Without anthropogenic forcing the glaciers would still be growing instead of retreating, as a result of GHG emissions.