Arctic summer temperature variability since AD 800
Fredrik Charpentier Ljungqvist1 and Håkan Grudd2
1: Department of History, Stockholm University, SE-106 91 Stockholm.
2: Bert Bolin Centre for Climate Research, Department of Physical Geography and Quaternary Geology,
Stockholm University, SE-106 91 Stockholm.
Introduction
Interpretation in the light of historical data
The shrinking extent of Arctic summer sea-ice in recent years is often interpreted as the most clear and
unambiguous evidence of anthropogenic global warming. During the past century, the Arctic has
experienced a more rapid and pronounced temperature increase than most other parts of the world
(ACIA, 2005). The instrumental temperature record is too short to assess whether the 20th century
warming in the Arctic falls outside the range of natural variability on centennial to millennial timescales. In order to place the recent warming in a longer perspective, information must be drawn from
temperature-sensitive proxy data. However, the longest existing multi-proxy temperature reconstruction for the Arctic only reaches back to AD 1600 (Overpeck et al., 1997).
The large amplitude of pre-industrial temperature variability in our reconstruction is well in line with
qualitative information given in written and archaeological sources. Between c. AD 800–1200 Northern
European agriculture expanded, leading to a huge increase in population. The prevailing warm climate
was, of course, not the only reason for this expansion; major agricultural technical innovations and fairly
stable political conditions also prevailed. Parts of this agrarian expansion can nevertheless be plausibly
related to climate: farming expanded higher up in the Alps and in the Norwegian mountains and further
to the north than before. This circum-Arctic agrarian expansion is most evidently manifested in the
Norse expansion across the North Atlantic (Fagan, 2008). The warm temperatures c. AD 1000 was also
likely to have triggered the expansion of the Thule people, ancestors of the modern Inuit, from Alaska
into the Canadian Arctic and further on to Greenland (Dyke et al., 1996).
Here, we present a new 1200-year long reconstruction of Arctic summer temperatures based on three
temperature proxy records with annual resolution reaching back to AD 800. Such a reconstruction
enables us to better place the modern warming of the Arctic in a millennium perspective. Originally,
six annually resolvet proxy data series from north of 65°N were analyzed (see Table 1) but three were
excluded since they captured decadal variations in the instrumental record poorly. It was found that
a composite of the three records with the highest correlation to temperature could explain the temperature variance for the whole Arctic better than all six records together. The three series used in the
reconstruction were selected for their high sensitivity to both local and regional (e.g. Arctic) summer
temperatures. All the series have been normalized to the AD 1000–1900 mean and low-pass filtered
to remove variations on time-scales less than ten years. The arithmetic mean value of the three series
has then been calculated and calibrated by means of linear regression to fit the ten year low-pass filtered instrumental June to August temperature data from north of 65°N in the HadCruT3v record (Fig.
1 and Fig. 2). The inter-correlation between the records, as well as the correlation with the composite
reconstruction, is generally high and the reconstruction thus seems quite robust on decadal timescales (Table 2).
Severe famines caused by bad harvests were quite rare in northern Europe before c. AD 1280 but became
more frequent thereafter. This seems to coincide with the onset of colder summers in the Arctic. The altitude limit for agriculture dropped 100–220 metres in the Alps, cereal cultivation was mostly abandoned
in Norway north of c. 65°N, and wine cultivation was forsaken in England during this period (Lamb,
1977). The Greenland farmers suffered by far worse due to the colder climate: They died out. The people
on Iceland suffered tremendous hardships too and had to abandon cereal cultivation, resulting in a
decreasing and impoverished population (Fitzhugh and Ward, 2000). After a brief period of more favourable conditions in the 15th and 16th centuries, coinciding with warmer Arctic summer conditions in our
reconstruction, new hardships for far northern agriculture started in the 17th, 18th and 19th centuries
(Fagan, 2000).
Arctic (65–90°N) June to August temperature anomalys
2
Norsemen settle
Greenland
Results
Temperature deviation (°C)
Our new reconstruction of Arctic summer temperatures shows a clear Medieval Warm Period
(MWP), peaking c. AD 1000, fluctuating conditions c. AD 1100–1580 and a distinct Little Ice Age
(LIA, c. AD 1580–1920). Medieval summer temperatures show two notable peaks around c. AD
1000 and c. AD 1035. According to our reconstruction, in the Arctic, the MWP is over by c. AD
1100. The observed warming in the 1930s and 1940s is well captured as is the following period of
cooling. The MWP summer temperatures actually exceeded mean 20th century temperatures and part
of the MWP likely equalled or exceeded even those temperatures experienced during recent summers
in the Arctic. However, recent Arctic summer temperatures are the highest in at least 400 years. The
late 19th century seems to be the period with the lowest summer temperatures in the Arctic during the
last millennium. Not even the 17th century, often claimed to be the coldest period during the LIA,
shows as low Arctic summer temperatures as those c. AD 1900. Some of the subsequent 20th century
warming can thus be seen as a rebound to more “normal” conditions and not as a change to warmer
conditions.
Collapse of the northernmost Norse
settlement on Greenland
1
Small cod catches in
Norwegian waters
0
Norsemen settle
Iceland
Thule people expand
eastward and northward in
the Canadian Arctic
Collapse of the southernmost
Norse settlement on Greenland
-1
-2
800
†
† †
†††
1200
1300
†
900
1000
1100
†† † †
†
1400
1500
Proxy record
Torneträsk, tree-ring MXD
Polar Ural, tree-ring MXD
Taimyr, tree-ring width
Indigirka, tree-ring width
L. Murray Lake, varved sediment
Renland, δ18O ice-core
Arctic composite
R
0.78
0.75
0.34
0.45
0.49
0.75
0.86
R2
0.61
0.57
0.12
0.21
0.24
0.57
0.74
Latitude
68°13′N
66°83′N
73°00′N
70°53′N
81°21′N
71°3′N
Longitude
19°43′E
65°75′E
105°E
148°15′E
69°32′W
26°7′W
1700
†
† †
††
1800
1900
2000
Year AD
Agrarian expansion in northern Europe and in the
sub-Arctic North Atlantic
Agrarian stagnation in northern Europe and in the
sub-Arctic North Atlantic
Table 1. Correlations between the low-pass filtered instrumental June to August temperature data from
north of 65°N and the low-pass filtered proxy records.
1600
†
†
Minor or local famine
†
Major widespread famine
Proxy temperature reconstruction
Instrumental temperature
record
Agrarian retreat and depopulation in northern
Europe and in the sub-Arctic North Atlantic
Reference
Grudd 2008
Esper et al. 2002
Naurzbaev et al. 2002
Moberg et al. 2005
Cook et al. 2009
Vinther et al. 2008
Glaciers in the Alps larger than c. AD 1900
Glaciers in the Alps larger
than c. AD 1950
Glaciers in the Alps smaller
than c. AD 1950
Fig. 2. Our Arctic summer temperature reconstruction can explain many periods of expansion, stagnation and
retreat of agriculture, particularly in the marginal regions of northern Europe and the sub-Arctic North Atlantic.
This new natural–agricultural history fits well with historical records from periods of severe famine caused by
harvest failure.
Table 2. Correlation matrix of the analysed records after 10-year low-pass filtering.
Torneträsk
Polar Ural
Taimyr
Indigirka
L. Murray Lake
Renland
Arctic T_JJA
Torneträsk
1
0.73
0.66
0.48
0.55
0.79
0.75
Polar Ural
Taimyr
1
0.54
0.59
0.64
0.57
0.76
1
0.60
0.37
0.60
0.63
Indigirka
1
0.45
0.64
0.63
L. Murray Lake
Renland
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