Jökull

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Jökull - 01.01.2012, Qupperneq 97

Jökull - 01.01.2012, Qupperneq 97
Mass and volume changes of Langjökull ice cap, Iceland, ∼1890 to 2009 intervals of distinctly different climate conditions, and are thus valuable for further studies of glacier re- sponse to climate variations. During the first two decades of the 20th century, the mass balance of Langjökull was close to zero, then highly negative ∼1925–1950 (-1.6 mwe yr−1), and close to zero again in the 1970s and 1980s. From 1997–2009, Langjökull has been losing mass at a high rate of -1.3 mwe yr−1, which is in accord with the ∼1.4◦C warmer average summer temperature than in the close to zero mass balance period in the 1980s. The mass balance sen- sitivity of Langjökull is high; -2 mwe yr−1K−1 es- timated from a coastal meteorological station and -1 mwe yr−1K−1 derived from an inland station. This is lower than obtained in previous studies of the mass balance sensitivity of the more maritime (higher pre- cipitation) south coast glacier, Eyjafjallajökull (-2.8 mwe yr−1K−1). Acknowledgements This work was financially supported by Rannís (The Icelandic Research Fund), University of Iceland Re- search Fund, the National Power Company of Ice- land, the Jules Verne French-Icelandic program and the Nordic Projects Climate and Energy (CE) and Cli- mate and Energy Systems (CES) supported by the Nordic Energy Research. This publication is contri- bution number 15 of the Nordic Centre of Excellence program SVALI (Stability and Variations of Arctic Land Ice), funded by the Nordic Top-level Initiative (TRI). E.B. acknowledges support by the TOSCA (CNES) and PNTS programs. SPOT5 HRG images were made available by the two OASIS (Optimising Access to Spot Infrastructure for Science) projects number 36 and 94. The Icelandic Meteorological Office provided the meteorological data from both Hveravellir and Stykkishólmur and the National Land Survey of Iceland the published maps. We express thanks to all participants in the field surveys, espe- cially Þorsteinn Jónsson and Sveinbjörn Steinþórsson, Institute of Earth Sciences, University of Iceland, and Hlynur Skagfjörð Pálsson, the HSSR Rescue Team, Reykjavík, Iceland. We thank Simon Carr and an anonymous reviewer for constructive comments that improved the paper. ÁGRIP Afkoma Langjökuls hefur verið mæld á 22 til 23 mælistöðvum á jöklinum frá jökulárinu 1996–1997 og eftir mælingunum unnin stafræn kort af afkomu jökulsins. Nákvæm hæðarkort voru unnin eftir þétt- um GPS-sniðmælingum frá apríl 1997 og þrívíddar- myndum teknum með SPOT5 gervitunglinu í ágúst 2004. Þessi kort voru notið til viðmiðunar við leið- réttingu á bjögunum og villum í eldri yfirborðskortum (1937, 1945–1946, 1986) og nákvæmni þeirra þannig bætt. Breyting ísrúmmáls á árabilunum milli kortanna var metin með því að draga eitt kort frá öðru. Lega ysta jaðars Langjökuls (í lok litlu ísaldar um 1890) var dregin upp eftir gervitunglamyndum, ljósmynd- um og ýmsum rituðum heimildum; þannig fékkst mat á flatarmál jökulsins við lok litlu ísaldar. Þetta flatar- mál og samhengið milli flatarmáls og ísrúmmáls ár- in sem hæðarkort eru til var notað til að áætla ísrúm- mál við lok litlu ísaldar. Langtímabreyting afkomu öll þessi tímabil er metin út frá rúmmálsbreytingum. Rýrnum metin eftir yfirborðshæðarbreytingum áranna 1997–2004 reynist sú sama og samanlögð afkoma ein- stakra ára sem metin eftir afkomumælingunum á jökl- inum. Það bendir til að vel hafi tekist við staðsetningu afkomumælistaða, mælingar og túlkun þeirra. Gögn sem ná frá um 1890 til fyrsta áratugar 21. aldar sýna að jökullinn hafi rýrnað mjög hratt á hlýju tímabili 1936 til 1946, þegar meðalársafkoman var neikvæð um 1,6 m vatns að jafnaði, og aftur á árunum eft- ir 1997, með neikvæða meðalársafkomu um 1,3 m vatns. Á kaldari tímabilunum frá 1946 til 1986 og 1986 til 1997 var afkoma jökulsins mun nær því að vera í jafnvægi eða - 0,3 til - 0,2 m vatns að jafnaði. Ísrúmmál Langjökuls minnkaði frá um 250 km3 við lok litlu ísaldar í 188 km3 árið 2004, sem er um 24% heildarrýrnun, en 40% þeirrar rýrnunar varð á hlýju árunum 1936 til 1946 og eftir 1997. REFERENCES Aðalgeirsdóttir, G., T. Jóhannesson, H. Björnsson, F. Pálsson and O. Sigurðsson 2006. Response of Hofsjökull and southern Vatnajökull, Iceland, to climate change. J. Geophys. Res. 111, F03001, doi:10.1029/2005JF000388. Aðalgeirsdóttir, G., S. Guðmundsson, H. Björnsson, F. Páls- son, T. Jóhannesson, H. Hannesdóttir, S. Þ. Sigurðsson and E. Berthier 2011. Modelling the 20th and 21st century JÖKULL No. 62, 2012 95
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