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Medieval Warm Period (Europe) -- Summary
Was there really a Medieval Warm Period in Europe?  To cut to the end of the story, there certainly was.  But let's take a little time in getting there and review some of the evidence for that contention.

Based on analyses of subfossil wood samples from the Khibiny mountains on the Kola Peninsula of Russia, Hiller et al. (2001) were able to reconstruct a 1500-year history of alpine tree-line elevation.  This record indicates that between AD 1000 and 1300, the tree-line there was located at least 100 to 140 meters above its current location.  The researchers state that this fact implies a mean summer temperature that was "at least 0.8C higher than today."

Moving from land to water, in a study of a well-dated sediment core from the Bornholm Basin in the southwestern Baltic Sea, Andren et al. (2000) found evidence for a period of high primary production at approximately AD 1050.  Many of the diatoms of that period were warm water species that the scientists say "cannot be found in the present Baltic Sea."  This balmy period, they report, "corresponds to the time when the Vikings succeeded in colonizing Iceland and Greenland."  The warmth ended rather abruptly, however, at about AD 1200, when they note there was "a major decrease in warm water taxa in the diatom assemblage and an increase in cold water taxa," which latter diatoms are characteristic of what they call the Recent Baltic Sea Stage that prevails to this day.

In another marine study, Voronina et al. (2001) analyzed dinoflagellate cyst assemblages in two sediment cores retrieved from the southeastern Barents Sea, one spanning a period of 8300 years and one spanning a period of 4400 years.  The longer of the two cores indicated a warm interval from about 8000 to 3000 years before present, followed by cooling pulses coincident with lowered salinity and extended ice cover in the vicinity of 5000, 3500 and 2500 years ago.  The shorter core additionally revealed cooling pulses at tentative dates of 1400, 300 and 100 years before present.  For the bulk of the past 4400 years, however, ice cover lasted only two to three months per year, as opposed to the modern mean of 4.3 months per year.  In addition, August temperatures ranged between 6 and 8C, significantly warmer than the present mean of 4.6C.

Moving back towards land, Mikalsen et al. (2001) made detailed measurements of a number of properties of sedimentary material extracted from the bottom of a fjord on the west coast of Norway, deriving a relative temperature history of the region that spanned the last five millennia.  This record revealed the existence of a period stretching from A.D. 1330 to 1600 that, in their words, "had the highest bottom-water temperatures in Sulafjorden during the last 5000 years."

In eastern Norway, Nesje et al. (2001) analyzed a sediment core obtained from Lake Atnsjoen, deriving a 4500-year record of river flooding.  They observed "a period of little flood activity around the Medieval period (AD 1000-1400)," which was followed by "a period of the most extensive flood activity in the Atnsjoen catchment."  This flooding, in their words, resulted from the "post-Medieval climate deterioration characterized by lower air temperature, thicker and more long-lasting snow cover, and more frequent storms associated with the 'Little Ice Age'."

Working in both Norway and Scotland, Brooks and Birks (2001) studied midges, the larval-stage head capsules of which are well preserved in lake sediments and are, in their words, "widely recognized as powerful biological proxies for inferring past climate change."  Applying this technique to sediments derived from a lake in the Cairngorms region of the Scottish Highlands, they determined that temperatures there peaked at about 11C during what they refer to as the "Little Climatic Optimum" -- which we typically call the Medieval Warm Period -- "before cooling by about 1.5C which may coincide with the 'Little Ice Age'."

These results, according to Brooks and Birks, "are in good agreement with a chironomid stratigraphy from Finse, western Norway (Velle, 1998)," where summer temperatures were "about 0.4C warmer than the present day" during the Medieval Warm Period.  This latter observation also appears to hold for the Scottish site, since the upper sample of the lake sediment core from that region, which was collected in 1993, "reconstructs the modern temperature at about 10.5C," which is 0.5C less than the 11C value the authors found for the Medieval Warm Period.

Moving to Switzerland, Filippi et al. (1999) analyzed a sediment core extracted from Lake Neuchatel in the western Swiss Lowlands.  During this same transition from the Medieval Warm Period (MWP) to the Little Ice Age (LIA), they detected a drop of approximately 1.5C in mean annual air temperature.  To give some context to this finding, they say that "the warming during the 20th century does not seem to have fully compensated the cooling at the MWP-LIA transition."  And to make the message even more clear, they add that during the Medieval Warm Period, the mean annual air temperature was "on average higher than at present."

Over in Ireland, in a cave in the southwestern part of the country, McDermott et al. (2001) derived a 18O record from a stalagmite that provided evidence for climatic variations that are "broadly consistent with a Medieval Warm Period at ~1000 200 years ago and a two-stage Little Ice Age."  Also evident in the data were the 18O signatures of the earlier Roman Warm Period and Dark Ages Cold Period that comprised the preceding millennial-scale cycle of climate in that region.

In another study of three stalagmites found in a cave in northwest Germany, Niggemann et al. (2003) discovered that the climate records they contained "resemble records from an Irish stalagmite (McDermott et al., 1999)," specifically noting that their own records provide evidence for the existence of the Little Ice Age, the Medieval Warm Period and the Roman Warm Period, which evidence also implies the existence of what McDermott et al. (2001) call the Dark Ages Cold Period that separated the Medieval and Roman Warm Periods, as well as the existence of the unnamed cold period that preceded the Roman Warm Period.

Continuing our mini-review of the Medieval Warm Period in Europe, Bodri and Cermak (1999) derived individual ground surface temperature histories from the temperature-depth logs of 98 separate boreholes drilled in the Czech Republic.  From these data they detected "the existence of a medieval warm epoch lasting from A.D. 1100-1300," which they describe as "one of the warmest postglacial times.  Noting that this spectacular warm period was followed by the Little Ice Age, they went on to suggest that "the observed recent warming may thus be easily a natural return of climate from the previous colder conditions back to a 'normal'."

Filippi et al. (1999) share similar views, as is demonstrated by their citing of Keigwin (1996) to the effect that "sea surface temperature (SST) reconstructions show that SST was ca. 1C cooler than today about 400 years ago and ca. 1C warmer than today during the MWP."  Citing Bond et al. (1997), they further note that the MWP and LIA are merely the most recent manifestations of "a pervasive millennial-scale coupled atmosphere-ocean climate oscillation," which, we might add, has absolutely nothing to do with variations in the air's CO2 content.

Last of all, we report the findings of Berglund (2003), who identified several periods of expansion and decline of human cultures in northwest Europe and compared them with a history of reconstructed climate "based on insolation, glacier activity, lake and sea levels, bog growth, tree line, and tree growth."  In doing so, he determined there was a positive correlation between human impact/land-use and climate change.  Specifically, in the latter part of the record, where both cultural and climate changes were best defined, there was, in his words, a great "retreat of agriculture" centered on about AD 500, which led to "reforestation in large areas of central Europe and Scandinavia."  He additionally notes that "this period was one of rapid cooling indicated from tree-ring data (Eronen et al., 1999) as well as sea surface temperatures based on diatom stratigraphy in [the] Norwegian Sea (Jansen and Koc, 2000), which can be correlated with Bond's event 1 in the North Atlantic sediments (Bond et al., 1997)."

Next came what Berglund calls a "boom period" that covered "several centuries from AD 700 to 1100."  This interval of time proved to be "a favourable period for agriculture in marginal areas of Northwest Europe, leading into the so-called Medieval Warm Epoch," when "the climate was warm and dry, with high treelines, glacier retreat, and reduced lake catchment erosion."  This period "lasted until around AD 1200, when there was a gradual change to cool/moist climate, the beginning of the Little Ice Age ... with severe consequences for the agrarian society."

In light of this varied array of empirical evidence, the story from Europe seems quite clear.  There was a several-hundred-year period in the first part of the last millennium that was significantly warmer than it is currently, contrary to the claims of climate alarmists who say the warmth of that time was nothing special.  In addition, there is reason to believe that the planet may be on a natural climate trajectory that is taking it back to a state reminiscent of the Medieval Warm Period, which we could call the Modern Warm Period.  And there is nothing we can do about it except, as is implied by the study of Berglund (2003), reap the benefits!

References
Andren, E., Andren, T. and Sohlenius, G.  2000.  The Holocene history of the southwestern Baltic Sea as reflected in a sediment core from the Bornholm Basin.  Boreas 29: 233-250.

Berglund, B.E.  2003.  Human impact and climate changes - synchronous events and a causal link?  Quaternary International 105: 7-12.

Bodri, L. and Cermak, V.  1999.  Climate change of the last millennium inferred from borehole temperatures: Regional patterns of climatic changes in the Czech Republic - Part III.  Global and Planetary Change 21: 225-235.

Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., deMenocal, P., Priori, P., Cullen, H., Hajdes, I. and Bonani, G.  1997.  A pervasive millennial-scale climate cycle in the North Atlantic: The Holocene and late glacial record.  Science 278: 1257-1266.

Brooks, S.J. and Birks, H.J.B.  2001.  Chironomid-inferred air temperatures from Lateglacial and Holocene sites in north-west Europe: progress and problems.  Quaternary Science Reviews 20: 1723-1741.

Eronen, M., Hyvarinen, H. and Zetterberg, P.  1999.  Holocene humidity changes in northern Finnish Lapland inferred from lake sediments and submerged Scots pines dated by tree-rings.  The Holocene 9: 569-580.

Filippi, M.L., Lambert, P., Hunziker, J., Kubler, B. and Bernasconi, S.  1999.  Climatic and anthropogenic influence on the stable isotope record from bulk carbonates and ostracodes in Lake Neuchatel, Switzerland, during the last two millennia.  Journal of Paleolimnology 21: 19-34.

Hiller, A., Boettger, T. and Kremenetski, C.  2001.  Medieval climatic warming recorded by radiocarbon dated alpine tree-line shift on the Kola Peninsula, Russia.  The Holocene 11: 491-497.

Jansen, E. and Koc, N.  2000.  Century to decadal scale records of Norwegian sea surface temperature variations of the past 2 millennia.  PAGES Newsletter 8(1): 13-14.

Keigwin, L.D.  1996.  The Little Ice Age and Medieval Warm Period in the Sargasso Sea.  Science 174: 1504-1508.

McDermott, F., Frisia, S., Huang, Y., Longinelli, A., Spiro, S., Heaton, T.H.E., Hawkesworth, C., Borsato, A., Keppens, E., Fairchild, I., van Borgh, C., Verheyden, S. and Selmo, E.  1999.  Holocene climate variability in Europe: evidence from delta18O, textural and extension-rate variations in speleothems.  Quaternary Science Reviews 18: 1021-1038.

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.

Mikalsen, G., Sejrup, H.P. and Aarseth, I.  2001.  Late-Holocene changes in ocean circulation and climate: foraminiferal and isotopic evidence from Sulafjord, western Norway.  The Holocene 11: 437-446.

Nesje, A., Dahl, S.O., Matthews, J.A. and Berrisford, M.S.  2001.  A ~ 4500-yr record of river floods obtained from a sediment core in Lake Atnsjoen, eastern Norway.  Journal of Paleolimnology 25: 329-342.

Niggemann, S., Mangini, A., Richter, D.K. and Wurth, G.  2003.  A paleoclimate record of the last 17,600 years in stalagmites from the B7 cave, Sauerland, Germany.  Quaternary Science Reviews 22: 555-567.

Velle, G.  1998.  A paleoecological study of chironomids (Insecta: Diptera) with special reference to climate.  M.Sc. Thesis, University of Bergen.

Voronina, E., Polyak, L., De Vernal, A. and Peyron, O.  2001.  Holocene variations of sea-surface conditions in the southeastern Barents Sea, reconstructed from dinoflagellate cyst assemblages.  Journal of Quaternary Science 16: 717-726.