Jökull - 01.12.1972, Blaðsíða 4

TABLE 1 - TAFLA 1
Classification of ice conditions in the period 1200—1968
Flokkun isára á tímabilinu 1200—1968
1201-1300 ...............................
1301-1400 ...............................
1401-1500 ...............................
1501-1600 ...............................
1601-1700 ...............................
1701-1800 ...............................
1801-1900 ...............................
1901-1968 ...............................
(1901-1968) • 100/68 ....................
land. Thoroddsen (1916—1917) has made an
extensive survey of most of the available
sources, but later Koch (1945) published a
graphical presentation of ice conditions near
Iceland until 1940, based mainly on Thorodd-
sen’s survey and other later ones. Thoroddsen
also made a graphical presentation of the ice
conditions from 1781 — 1915. Later extension has
been made by Sigtryggsson (1969), Sigtryggsson
et al. (1971) and in the present paper.
Koch constructs a numerical index for the
ice years, which increases with the amount of
ice. For this purpose he divides the sea around
Iceland into 10 areas, each around 135 kiío-
meters wide, except one, which extends frorn
Reykjanes to Látrabjarg. The index is a yearly
product of the number of weeks when ice was
observecl, and the number of areas where it
occurred.
Lauge Koch has maintained, on the basis of
his graphs, that the climate was much milder
when the country was settled, and in the fol-
lowing centuries, but became cooler later. I
consider it very likely that some changes took
place, but that the ice graphs are not complete
enough to ascertain this. The following discus-
sion may make this clearer.
It may be assumed that for one severe ice
year per century there should be several mode-
rate years and that the ratio between them
should be roughly constant so long as ice con-
ditions during a whole century would not be
exceptionally severe, or exceptionally mild.
2 JÖKULL 22. ÁR
Severe
ice years Ice years Slight ice No ice reports
4 (3)* 3 0 93 (94)
1 7 (6) 0 92 (93)
1 (0) 1 0 98 (99)
1 6 0 93
6 30 0 64
5 42 0 53
12 43 18 27
2 15 19 32
3 22 28 47
When the ice years are counted and classified,
the limits between the classes must be selected
rather arbitrarily. For the purpose of this
article, a severe ice year is one to which Koch
assigns an index of 126 or higher, a medium
year has an index of 5 to 125 inclusive but a
light ice year an index of 1 to 4. It might also
be pointed out, that most of the 20th century
has, until now, been rather ice free and the
ratio between years with ice and ice free years
during this period tends to be characteristic for
light ice periods.
The result of a count of the ice years accord-
ing to this classification is presented in the
table above. The last line shows the number of
years for the various classes for the whole of
the 20th century, assuming that the ratio be-
tween them for the century will be the same
as it has been from its beginning to 1968.
According to the table the ratio between
severe and average ice years is rather high in
the 13th and 15th centuries, compared to later
ones. In the 14th and 16th centuries, on the
other hand, this ratio is almost the same as in
the later centuries, when the ice reports have
become more reliable, but the total number of
years, when ice is reported is so small, compar-
ed to mild periods in the 20th century, that
*) According to Vilmundarson (1969) some of
the accounts frorn the 13th—15th centuries seem
to be unreliable. Revised figures appear in
parentheses.