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The Medieval and Roman Warm Periods in Scandinavia
Reference
Hormes, A., Karlen, W. and Possnert, G. 2004. Radiocarbon dating of palaeosol components in moraines in Lapland, northern Sweden. Quaternary Science Reviews 23: 2031-2043.

What was done
The authors identified and dated periods of soil formation in moraines in the Kebnekaise mountain region of Swedish Lapland in the foreground of the Nipalsglaciaren (6758'N, 1833'E), after which they compared the climatic implications of their results with those of other proxy climate records derived throughout other areas of northern and central Scandinavia.

What was learned
Two periods of soil formation were identified (2750-2000 and 1170-740 cal yr BP), which spans of time coincide nearly perfectly with the Roman and Medieval Warm Periods delineated by McDermott et al. (2001) in the high-resolution δ18O record they developed from a stalagmite in southwestern Ireland's Crag Cave.

What it means
Hormes et al. say that the periods during which the soil formation processes they discovered took place "represent periods where the Nipalsglacier did not reach the position of the moraine," and that "the glacier was most likely in a position similar to today, and climate conditions were also similar to today." Comparing these findings with those of other investigators, they note the following, the first set of which observations apply to the Medieval Warm Period.

(1) Pollen profiles derived from sediments of Lake Tibetanus in Lapland (Hammarlund et al., 2002) "infer increased mean July temperatures ... peaking around 1000 cal yr BP."

(2) Oxygen isotope studies in nearby Lake 850 "record changes around 1000 cal yr BP towards favourable climate conditions (Shemesh et al., 2001)."

(3) At Lake Laihalampi in southern Finland, "pollen-based reconstructions of mean temperatures indicate 0.5C higher values between 1200 and 1100 cal yr BP (Heikkila and Seppa, 2003)."

(4) Radiocarbon ages of mosses in front of Arjep Ruotesjekna in the Sarek Mountains of Swedish Lapland "support the conclusion that between 1170 and 920 cal yr BP the glaciers had not reached the 1970s limit (Karlen and Denton, 1975)."

(5) Reconstructed temperatures of a pine dendrochronology from northern Fennoscandia "show temperatures between 1100 and 750 cal yr BP to have been around 0.8C higher than today (Grudd et al., 2002)."

(6, 7) At Haugabreen glacier (Matthews, 1980) and Storbreen glacier (Griffey and Matthews, 1978) in southern maritime Norway, "soil formation on moraines were dated between 1060 and 790 cal yr BP."

(8) Alder trees were melted out from Engabreen glacier (Worsley and Alexander, 1976), "suggesting a smaller extension of this Norwegian glacier between 1180 and 790 cal yr BP supporting warm/dry conditions during that time in central Norway."

(9) Jostedalsbreen glacier "receded between 1000 and 900 cal yr BP (Nesje et al., 2001)."

With respect to their identification of the Roman Warm Period, Hormes et al. report prior findings of soil formation at (1) Svartisen glacier between 2350 and 1990 cal yr BP by Karlen (1979), (2) Austre Okstindbreen glacier between 2350 and 1800 cal yr BP by Griffey and Worsley (1978), and (3) Austre Okstindbreen glacier between 2750 and 2150 by Karlen (1979). In addition, they note the following:

(4) The pine tree-based temperature history of northern Fennoscandia developed by Grudd et al. (2002) "discloses a spike +2C higher than today's around 2300 cal yr BP."

(5, 6, 7, 8, 9) "The lacustrine records in Lapland and Finland are also consistent with supposition of a warmer climate than at present before 2000 cal yr BP and cooler temperatures before 2450 cal yr BP (Rosen et al., 2001; Seppa and Birks, 2001; Shemesh et al., 2001; Hammarlund et al., 2002; Heikkila and Seppa, 2003)."

In view of these many research findings, it is clear that both the Medieval and Roman Warm Periods were very real features of Scandinavian climatic history, and that they were likely even warmer than the Modern Warm Period has been to date, all without any help from elevated atmospheric CO2 concentrations, which were a full 100 ppm less than today's concentration during those earlier high-temperature periods. Hence, there is no compelling reason to believe that the modest warmth of today has anything to do with the air's CO2 content either.

References
Griffey, N.J. and Matthews, J.A. 1978. Major neoglacial glacier expansion episodes in southern Norway: Evidences from moraine ridge stratigraphy with 14C dates on buried palaeosols and moss layers. Geografiska Annaler 60A: 73-90.

Griffey, N.J. and Worsley, P. 1978. The pattern of neoglacial glacier variations in the Okstindan region of northern Norway during the last three millennia. Boreas 7: 1-17.

Grudd, H., Briffa, K.R., Karlen, W., Bartholin, T.S., Jones, P.D. and Kromer, B. 2002. A 7400-year tree-ring chronology in northern Swedish Lapland: natural climatic variability expressed on annual to millennial timescales. The Holocene 12: 657-665.

Hammarlund, D., Barnekow, L., Birks, H.J.B., Buchardt, B. and Edwards, T.W.D. 2002. Hoolocene changes in atmospheric circulation recorded in the oxygen-isotope stratigraphy of lacustrine carbonates from northern Sweden. The Holocene 12: 339- 351.

Heikkila, M. and Seppa, H. 2003. A 11,000-yr palaeotemperature reconstruction from the southern boreal zone in Finland. Quaternary Science Reviews 22: 541-554.

Karlen, W. 1979. Glacier variations in the Svartisen area, northern Norway. Geografiska Annaler 61A: 11-28.

Karlen, W. and Denton, G.H. 1975. Holocene glacial variations in Sarek National Park, northern Sweden. Boreas 5: 25-56.

Matthews, J.A. 1980. Some problems and implications of 14C dates from a podzol buried beneath an end moraine at Haugabreen, southern Norway. Geografiska Annaler 62A: 85-208.

McDermott, F., Mattey, D.P. and Hawkesworth, C. 2001. Centennial-scale Holocene climate variability revealed by a high-resolution speleothem 18O record from SW Ireland. Science 294: 1328-1331.

Nesje, A., Matthews, J.A., Dahl, S.O., Berrisford, M.S. and Andersson, C. 2001. Holocene glacier fluctuations of Flatebreen and winter-precipitation changes in the Jostedalsbreen region, western Norway, based on glaciolacustrine sediment records. The Holocene 11: 267-280.

Rosen, P., Segerstrom, U., Eriksson, L., Renberg, I. and Birks, H.J.B. 2001. Holocene climatic change reconstructed from diatoms, chironomids, pollen and near-infrared spectroscopy at an alpine lake (Sjuodjijaure) in northern Sweden. The Holocene 11: 551-562.

Seppa, H. and Birks, H.J.B. 2001. July mean temperature and annual precipitation trends during the Holocene in the Fennoscandian tree-line area: pollen-based climate reconstruction. The Holocene 11: 527-539.

Shemesh, A., Rosqvist, G., Rietti-Shati, M., Rubensdotter, L., Bigler, C., Yam, R. and Karlen, W. 2001. Holocene climatic changes in Swedish Lapland inferred from an oxygen isotope record of lacustrine biogenic silica. The Holocene 11: 447-454.

Worsley, P. and Alexander, M.J. 1976. Glacier and environmental changes - neoglacial data from the outermost moraine ridges at Engabreen, Northern Norway. Geografiska Annaler 58: 55-69.

Reviewed 30 March 2005