Fróðskaparrit - 01.01.1998, Síða 177

SEINHOLOSENA VEÐURLAGIÐ ELVDITIL JARÐFORMANDI VIRKSEMI í FØROYUM
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was obtained. This simple transformation
brings the point agglomeration close to the
empirical position for modern ELA’s, sug-
gesting that climatic conditions in the high-
lands during cold intervals of the Little Ice
Age were rather close to those of glacia-
tion. However, also precipitation varies
with altitude due to orographic effects, and
usually increases by 10-20% per 100 m al-
titude (Søgaard, 1996). If a conservative
precipitation adjustment of only 10% per
100 m is introduced in the above analysis,
most of the fully transformed annual data
(filled circles) plot very close to the empir-
ical ELA-zone or even within the zone of
glaciation. By this, a number of sites with-
in the highest mountains in the Faroe Is-
lands with little doubt experienced a posi-
tive snow mass balance in certain years
during cold intervals of the Little Ice Age,
and from a glaciological point of view were
on the brink of glaciation. One obvious
candidate for incipient glaciation is
Givrabotnur on the eastern side of Slæt-
taratindur (Fig. 1), where snow is known to
have survived several summers during the
latter part of the 20th century. There is,
however, no geomorphic indication of any
Holocene reglaciation in the Faroes (Hum-
lum et al, 1996), and the number of con-
secutive years with positive mass balance
during the Little Ice Age was probably too
small. Presumably, an uninterrupted row of
at least 15-25 years with positive mass bal-
ance is required to accumulate an ice body
thick enough to generate significant inter-
nal deformation.
Permafrost in the highlands ?
Permafrost most likely was absent in the
Faroe Islands since the Younger Dryas
(since 11500 cal. years BP; Humlum,
1998), although the highest mountains
presently only are 300-500 m below the
theoretical level for discontinuous per-
mafrost (Humlum and Christiansen, in
press). Considering empirical knowledge,
sporadic permafrost may be met with from
a MAAT of about 0°C, discontinuous per-
mafrost from about -2°C, while continuous
permafrost usually requires a MAAT below
-6.5°C (see, e.g., French, 1996).
The meteorological observations from
Tórshavn have been used to estimate the
temperature conditions during the instru-
mental period at the summit of Slættaratin-
dur (882 m a.s.l.), the highest mountain in
the Faroe Islands (Fig. 7). The estimated
average MAAT for Slættaratindur AD
1867-1995 is only slightly above freezing,
about 0.8°C, assuming a standard vertical
lapse rate of 0.0065°Cm-l, corresponding
rather well to actual measurements (Fig. 4)
at the summit of neighbouring Gráfelli
(856 m a.s.l.). However, significant varia-
tions are seen to have occurred during the
instrumental period. Especially during the
final cold interval of the Little Ice Age the
summit MAAT apparently was below 0°C
for several years (AD 1888, 1892, 1917,
1919), and apparently this was also the case
for a single year in the latter half of the 20th
century (AD 1979). Several other years plot
close to 0°C. During years like these, a
number of high summits in the Faroes
probably were close to the establishment of
sporadic permafrost, especially at wind ex-