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 balance–altitude 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.
Highlights
The 211 000 glaciers and ice caps (GICs) (Pfeffer et al, 2014; Arendt et al, 2015) in the world are relatively small compared to the Greenland and Antarctic ice sheets, but they constitute about half of the current cryospheric contribution to sea level rise (Shepherd et al, 2012; Vaughan et al, 2013), a distribution projected to remain similar throughout the 21st century (Church et al, 2013; Huss and Hock, 2015)
We find that Hardangerjøkulen is exceptionally sensitive to surface mass balance changes and that the surface mass balance–altitude feedback and ice cap hypsometry are crucial to this sensitivity
We show that commonly used scaling relations overestimate ice volume and suggest that glacier and climate reconstructions could benefit from quantifying the impact on proxy records of bed topography, glacier hypsometry and the surface mass balance–altitude feedback (Sect. 5.5)
Summary
The 211 000 glaciers and ice caps (GICs) (Pfeffer et al, 2014; Arendt et al, 2015) in the world are relatively small compared to the Greenland and Antarctic ice sheets, but they constitute about half of the current cryospheric contribution to sea level rise (Shepherd et al, 2012; Vaughan et al, 2013), a distribution projected to remain similar throughout the 21st century (Church et al, 2013; Huss and Hock, 2015). Since areas of GICs are more readily available than their volume, scaling methods are commonly employed to estimate total ice volumes and their sea level equivalents (e.g., Bahr et al, 1997, 2015; Grinsted, 2013). Many of these GICs are ice caps, though little is known about their response to longterm climate change, how a particular ice cap geometry contributes to this sensitivity or how scaling methods perform for ice caps. Åkesson et al.: Simulating the evolution of Hardangerjøkulen ice cap Parameter
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