Simulating the evolution of Hardangerjøkulen ice cap in southern Norway since the mid-Holocene and its sensitivity to climate change

Abstract

Abstract. Understanding of long-term dynamics of glaciers and ice caps is vital to assess their recent and future changes, yet few long-term reconstructions using ice flow models exist. Here we present simulations of the maritime Hardangerjøkulen ice cap in Norway from the mid-Holocene through the Little Ice Age (LIA) to the present day, using a numerical ice flow model combined with glacier and climate reconstructions.In our simulation, under a linear climate forcing, we find that Hardangerjøkulen grows from ice-free conditions in the mid-Holocene to its maximum extent during the LIA in a nonlinear, spatially asynchronous fashion. During its fastest stage of growth (2300-1300 BP), the ice cap triples its volume in less than 1000 years. The modeled ice cap extent and outlet glacier length changes from the LIA until today agree well with available observations. Volume and area for Hardangerjøkulen and several of its outlet glaciers vary out-of-phase for several centuries during the Holocene. This volume-area disequilibrium varies in time and from one outlet glacier to the next, illustrating that linear relations between ice extent, volume and glacier proxy records, as generally used in paleoclimatic reconstructions, have only limited validity.We also show that the present-day ice cap is highly sensitive to surface mass balance changes and that the effect of the ice cap hypsometry on the mass balancealtitude feedback is essential to this sensitivity. A mass balance shift by +0.5 m w.e. relative to the mass balance from the last decades almost doubles ice volume, while a decrease of 0.2 m w.e. or more induces a strong mass balance-altitude feedback and makes Hardangerjøkulen disappear entirely. Furthermore, once disappeared, an additional +0.1 m w.e. relative to the present mass balance is needed to regrow the ice cap to its present-day extent. We expect that other ice caps with comparable geometry in, for example, Norway, Iceland, Patagonia and peripheral Greenland may behave similarly, making them particularly vulnerable to climate change.

Topographic controls on plateau icefield recession: insights from the Younger Dryas Monadhliath Icefield, Scotland

“…This research complements recent modelling on non‐linear recession of plateau icefields (Åkesson et al ., ; Zekollari et al ., ), recognizing the potential for recession of the Younger Dryas Monadhliath Icefield to have been characterized by threshold behaviour, leading to topographically controlled rather than climatically controlled stagnation in the later stages. Further work modelling the progression of deglaciation in plateau icefield settings is required to fully integrate topography and climate through time and explore potential topographically controlled tipping points in detail.…”

“…Nesje et al, 2008;Giesen and Oerlemans, 2010). More recently, model simulations have investigated non-linear behaviour linked to hysteresis during icefield growth (Åkesson et al, 2017;Zekollari et al, 2017). However, there are currently few studies (but see Jiskoot et al, 2009) that investigate: (i) factors that influence differences in, and controls on, retreat patterns between outlet valleys, and (ii) how an icefield behaves during the final stages of recession once it is constrained to the plateau.…”

“…Improved understanding of these aspects of icefield recession is important for (i) improving modelled predictions of future icefield response to changes in climate as well as their contribution (rate and magnitude) to sea-level rise in coming decades; and (ii) unravelling the complex interplay between climatic, topographic, glaciological and other controls, including outlet glacier evolution during recession, which is crucial to accurately model the dynamics of ice-mass behaviour through time (cf. Oerlemans, 2008;Rowan, 2011;Åkesson et al, 2017;Zekollari et al, 2017).…”

Sensitivity, stability and future evolution of the world's northernmost ice cap, Hans Tausen Iskappe (Greenland)

“…In order to resolve the ice flow in the many outlet glaciers accurately, a higher-order (HO) approximation to the full force balance is used. This differs from other detailed ice cap and ice field modelling studies (Aðalgeirsdóttir et al, 2005(Aðalgeirsdóttir et al, , 2006Flowers et al, 2005Flowers et al, , 2007Flowers et al, , 2008Giesen and Oerlemans, 2010;Hannesdóttir et al, 2015;Ziemen et al, 2016;Åkesson et al, 2017) that are based on the SIA or similar approaches. We investigate the influence of the model complexity (SIA/HO) and resolution on the modelled geometries and run the ice cap into a steady state that is compared to field observations.…”

“…In this study an equilibrium ice cap with a similar size as observed could not be obtained, and the ice cap grew far beyond the present-day state or evolved to a very small ice cap. Since then several modelling studies have been performed on individual ice caps in Iceland (Aðalgeirs-dóttir et al, 2005(Aðalgeirs-dóttir et al, , 2006Flowers et al, 2005Flowers et al, , 2007Flowers et al, , 2008 and on Hardangerjøkulen (southern Norway) (Giesen and Oerlemans, 2010;Åkesson et al, 2017) with models based on the shallow-ice approximation (SIA). Schäfer et al (2015) used a full-Stokes ice flow model with an englacial temperature parameterization for a study on the Vestfonna ice cap (Svalbard) with a particular focus on the SMB-elevation feedback.…”