Similarities and differences in the response to climate warming of two ice caps in Iceland S. Guðmundsson, H. Bjo ¨ rnsson, T. Jo ´ hannesson, G. Aðalgeirsdo ´ ttir, F. Pa ´ lsson and O. Sigurðsson ABSTRACT S. Guðmundsson (corresponding author) H. Bjo ¨ rnsson F. Pa ´ lsson Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavı´k, Iceland E-mail: sg@hi.is T. Jo ´ hannesson O. Sigurðsson Icelandic Meteorological Office, Bu ´staðavegur 9, 150 Reykjavı´k, Iceland G. Aðalgeirsdo ´ ttir Danish Climate Centre, Danish Meteorological Institute, Lyngbyvej 100, 2100 Copenhagen, Denmark The transient response to projected climate change of two ice caps in the central Icelandic highland was simulated with a vertically integrated ice-flow model coupled to a degree-day mass- balance model. The ice caps, Langjo ¨ kull and Hofsjo ¨ kull, are of similar size (area ,900 km 2 and volume ,200 km 3 ) and located only ,30 km apart. The climate change simulations were started in 1990 from steady states corresponding to the average climate of 1981–2000 and driven with observed weather parameters until 2005. Thereafter, the forcing was according to a Nordic climate change scenario based on the IPCC B2 emission scenario. The simulations during the period 1990–2005 compare reasonably well with observations of mass-balance and glacier extent. Both ice caps are projected to essentially disappear during the next 100 to 200 years. Langjo ¨ kull, which disappears within the next 150 years, shows larger mass-balance sensitivity to warming than the higher elevated Hofsjo ¨ kull, where ice on the highest peaks may last over 200 years. A large proportion of the simulated runoff increase with respect to a 1981–2000 average has already taken place within the period 1990–2005. The runoff will increase further during the next 40–60 years and remain considerably higher than at present until the end of the 21st century. Key words | climate change, glacial runoff change, glacier flow model, glacier mass balance, Iceland INTRODUCTION A comparison of the sensitivity of the two neighbouring Langjo ¨ kull and Hofsjo ¨ kull ice caps in Iceland (64.58 –658N, 19.58 – 21.58W, Figure 1) to climate warming is of special interest as their area and volume are similar but the elevation distributions differ. Hofsjo ¨ kull covers a circular volcanic caldera, at an elevation of more than 300 m higher, and has 100 – 200 m larger maximum ice thickness than Langjo ¨ kull, which covers mountain ridges (Table 1, Figure 2). A consequence of this difference in hypsometry is a considerable difference in the response to warming. Both ice caps have extensive monitoring programmes, which makes them suitable for glacier modelling. Two major rivers drain Langjo ¨ kull, but a significant part of the glacial runoff is directed into groundwater aquifers. Most of the runoff from Hofsjo ¨ kull is delivered directly to rivers. Glacier river discharge provides a significant contribution of inflow to hydroelectric power plants in Iceland, and variations in discharge affect the design and operation of power plants. Potential future changes in glacier runoff are therefore highly important for the Icelandic hydropower industry. This study uses ice flow and degree-day mass-balance models that were previously applied to Hofsjo ¨ kull and southern Vatnajo ¨ kull (Aðalgeirsdo ´ ttir et al. 2006) and a revised climate change scenario for the Icelandic highlands, constructed within the Nordic project Climate and Energy (CE) (Fenger 2007). The study forms a part of the CE project. doi: 10.2166/nh.2009.210 495 Q IWA Publishing 2009 Hydrology Research | 40.5 | 2009 Downloaded from http://iwaponline.com/hr/article-pdf/40/5/495/364433/495.pdf by guest on 08 June 2022