Simulating the climatic response of Hardangerjøkulen in southern Norway since the Little Ice Age

Glacier and ice cap volume changes currently amount to half of the total contribution from the cryosphere to sea-level rise. Ice caps and their outlet glaciers are dynamically different from the Greenland and Antarctic ice sheets, and respond considerably faster to climate change. We use a shallow-ice version of the Ice Sheet System Model (ISSM) to model the dynamics and evolution of the maritime-continental Hardangerjøkulen ice cap We force the ice flow model with a dynamically calibrated mass balance history based on moraine evidence from the Little Ice Age maximum in 1750, as well as later outlet glacier front positions from moraines and direct measurements. Glaciological mass balance measurements force the model from the 1960s onwards, and we validate the model using a surface digital elevation model from 1995 and aerial photographs. The model successfully reproduces most of the LIA extent of the ice cap. Outlet glaciers retreat too far in the model for the early 1900s, while observed ice extent after 1960 is accurately represented. This coincides with the period where direct mass balance data is used as forcing, indicating its key role. A comparison with a digital elevation model from 1995 reveals a very good agreement of surface topography, except for a too thick eastern ice cap. We find a non-linear relationship between mass balance perturbations and ice volume response, where Hardangerjøkulen is more sensitive to negative than positive mass balance changes. We discuss these findings in light of reconstructed past changes and future predictions