Náttúrufræðingurinn

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Náttúrufræðingurinn - 2004, Síða 62

Náttúrufræðingurinn - 2004, Síða 62
N áttúrufræðingurinn son's43 and Thorarinsson's9 conclusions about glacier maxima in the northwest- ern peninsula (Vestfirðir) during the middle of the 18th century and the mid- dle of the 19th century must be viewed with caution. Therefore, Thorarinsson's "known facts" are not beyond doubt. A caim built for geodetic purposes on top of the highest peak (Sjónfríð) in the Gláma massif in the year 1806 is still standing (Fig. 3). This proofs that there was no glacier during the last century of the "Little Ice Age'' on top of the pro- tmding peaks in the area. Therefore, the location of any glacier must be sought in the lower passes between the peaks or as isolated ice masses in cirque basins. The 19th century in Iceland was a period of expansion of glaciers, most of which reached their maximum areal extent about 1890. There is no reason to expect that glaciers in the northwestem penin- sula had a trend which was appreciably different. The glacier Unadalsjökull, northem Iceland, has a similar topographic set- ting as the postulated Glámujökull; it is situated in a saddle between two valleys. The time of its maximum area is not documented but might very well be toward the end of the 19th century, as with most other glaciers in Iceland. Its previous maximum extent is still clearly visible by the dark colour of the scoured proglacial area (see Fig. 5). During the warm 20th century, Unadalsjökull shrank from about 3 km2 to about 2 km2. Many glaciers that in historical time had a maximum area of less than 1 km2 still exist as such despite the fact that the 20th century was probably the mildest centu- ry of the last millennium. In general, masses of snow and ice need to be about 40-50 m thick and show some signs of flow to meet the definition of a glacier. According to mea- surements of Icelandic glaciers in the last decades, annual net mass balance has rarely exceeded ±1 m water equivalent. Therefore, a new glacier is not likely to form in less than about a century during a period with a distinct trend of positive mass balance. Likewise, it will take a glacier, even a small one, at least a centu- ry to disappear during a distinct and extended period of warming. Ac- cordingly, there is no reason to believe that a substantial glacier, present in the Gláma highlands as late as about 1800, would have disappeared entirely by the visits of Stefán Stefánsson in 1893 or even Herrmann in 1914, taking the cool climate of the 19th century into consider- ation. The remnants or traces of such a glacier would certainly still have been easily recognized in the field. About 10-15 km north of Gláma there are a few cirque glaciers (Fig. 7) present on the north side of a mountain (Lambadalsfjall) about 40 m higher in elevation than Sjónfríð. They have not during historical time been much bigger than they are now. Small glaciers are problematic to map. They are usually covered by snow and, therefore, not easy to distinguish from fim fields, either in the field or on aerial photographs and other types of remote sensing. Their areal extent can only be reliably traced in late summer in an extended period of negative mass bal- ance. Most maps of Iceland are of very little value when estimating the areal extent of small glaciers because of the inability of cartographers to differentiate glacier ice from fim fields. A legal question has been brought before the High Court of Iceland (Icelandic: Hæstiréttur) about the possi- bility of the existence of a glacier (Icelandic: jökull) on the Fimmvörðuháls mountain pass between the Eyjafjalla- jökull and Mýrdalsjökull ice caps in southem Iceland. Artifacts such as the series of caims situated along the fre- quently crossed mountain track and the lack of geological evidence for glacier action indicate that the two ice caps were not contiguous (continuous glacier ice) during historical time. On the other hand, it may not be necessary to prove the existence of a glacier that strictly meets the glaciological definition, because the Icelandic term jökull has always denoted extensive fim fields and large masses of snow as well as glaciers. The following conclusions may be drawn from the discussion above: • There is a big difference between the processes associated with a glacier and a long-lived, persist- ent fim field; they must be clearly distinguished from each other in any discussion. Of course, there may be borderline cases not easy to define, but they are expected to be few and insignificant glacio- logically. • Glaciers tend to be quite stable in time and space. Observations of glaciers in Iceland during the cli- mate variations from the cold 19th century to the very mild 20th cen- tury indicate that it would take a glacier, even a small one, at least a century to disappear. Extensive fim fields, on the other hand, may melt away in a few years. • The preponderance of evidence indicates that Glámujökull did not exist in historical time, with particular reference to the fact that there was no glacier on Gláma in the summer of 1893. This was at the end of the coldest 30 years on instmmental record in Iceland, when most if not all other glaciers in the country expanded and reached their historical maximum areal extent. • The ice caps of Eyjafjallajökull and Mýrdalsjökull, southern Iceland, were not connected by glacier ice during historical time. 60
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