Europe's Medieval Warm Period
We begin our examination of Europe with the study of Axford et al. (2009), who note “the idea of a widespread and spatially coherent ‘Medieval Warm Period’ (MWP) has come under scrutiny in recent years,” while “it remains a viable hypothesis that a period of relative warmth in northwestern Europe and the northern North Atlantic region helped facilitate Norse expansion across the North Atlantic from the ninth to thirteenth centuries, including settlement of Iceland and Greenland” and “subsequent cooling contributed to the demise of the Norse settlements on Greenland.” To further explore the subject, they developed a regional climatic record from a sediment core retrieved from Lake Stora Vioarvatn in northeast Iceland (66°14.232’N, 15°50.083’W) in the summer of 2005, based on chironomid assemblage data—which were well correlated with nearby measured temperatures over the 170-year instrumental record—and total organic carbon, nitrogen, and biogenic silica content. This work revealed the occurrence of “warm temperatures in the tenth and eleventh centuries, with one data point suggesting temperatures slightly warmer than present.” They also discovered “temperatures were higher overall and more consistently high through much of the first millennium AD.”
In discussing their findings, the Icelandic, U.K., and U.S. scientists state, “the historical perception of a significant medieval climate anomaly in Iceland may be primarily a reflection of the human perspective,” in that “Iceland was settled ca. AD 870, during a period of relative warmth that was followed by many centuries of progressively colder and less hospitable climate,” that “had the Norse settled Iceland 1000 years earlier, the MWP might be viewed only as a brief period of climatic amelioration, a respite from a shift to colder temperatures that began in the eighth century,” near the end of several centuries of even greater warmth. In any event, and viewed from either perspective, it is clear there is nothing unusual or unnatural about the region’s present-day temperatures, which the researchers say “do not show much recent warming.”
In another significant study, Bonnet et al. (2010) developed a high-resolution record of ocean and climate variations during the late Holocene in the Fram Strait (the major gateway between the Arctic and North Atlantic Oceans, located north of the Greenland Sea)—based on detailed analyses of a sediment core recovered from a location (78°54.931’N, 6°46.005’E) on the slope of the western continental margin of Svalbard—that permitted the reconstruction of sea surface temperature (SST) conditions in both summer and winter. These histories were nearly identical and showed oscillations between -1°C and 5.5°C in winter and between 2.4°C and 10.0°C in summer; their graphical results indicate that between 2,500 and 250 years before present (BP), the mean SSTs of summers were warmer than those of the present about 80 percent of the time, while the mean SSTs of winters exceeded those of current winters approximately 75 percent of the time, with the long-term (2,250-year) means of both seasonal periods averaging about 2°C more than current means. The highest temperatures, however, were recorded during a warm interval that persisted from about AD 500 to 720, during the very earliest stages of the Medieval Warm Period, when the peak summer and winter temperatures of the MWP both exceeded the peak summer and winter temperatures of the first several years of the twenty-first century by about 3°C.
Moving to Finland, Haltia-Hovi et al. (2010) constructed detailed chronological histories of several magnetic properties of two sediment cores taken from Finland’s Lake Lehmilampi (63°37’N, 29°06’E), as well as a history of their total organic carbon content. Based on their analyses, they discovered a “conspicuous occurrence of fine magnetic particles and high organic concentration” evident around 4,700–4,300 Cal. yrs BP. This time interval, in their words, “is broadly coincident with glacier contraction and treelines higher than present in the Scandinavian mountains according to Denton and Karlen (1973) and Karlen and Kuylenstierna (1996).” They report from that time on toward the present, there was a “decreasing trend of magnetic concentration, except for the slight localized enhancement in the upper part of the sediment column at ~1,100–900 Cal. yrs BP,” where the year zero BP = AD 1950.
Changes of these types in prior studies have been attributed to magnetotactic bacteria (e.g. Magnetospirillum spp.), which Haltia-Hovi et al. describe as “aquatic organisms that produce internal, small magnetite or greigite grains” used “to navigate along the geomagnetic field lines in search of micro or anaerobic conditions in the lake bottom,” as described by Blakemore (1982) and Bazylinski and Williams (2007). They further state the studies of Snowball (1994), Kim et al. (2005), and Paasche et al. (2004) “showed magnetic concentration enhancement, pointing to greater metabolic activity of these aquatic organisms in the presence of abundant organic matter.” This is also what Haltia-Hovi et al. found in their study; they report the “concentration of organic matter in the sediment is highest, together with fine magnetic grain sizes, in the time period 1,100–900 Cal. years BP.” This time interval, they say, “is associated with warmer temperatures during the Medieval Climate Anomaly according to the varve parameters of Lake Lehmilampi,” citing the precise core-dating by varve-counting work of Haltia-Hovi et al. (2007). Taken together, these observations strongly suggest the peak warmth of the Medieval Warm Period (about AD 850–1050) was very likely somewhat greater than that of the Current Warm Period.
In another study, Larocque-Tobler et al. (2010) write that to better describe the amplitude of temperature change during the past millennium, “new records to increase the geographic coverage of paleoclimatic information are needed” and “only by obtaining numerous high-resolution temperature records will it be possible to determine if the 20th century climate change exceeded the natural pre-industrial variability of European climate.” To help achieve this important goal, they proceeded to obtain another such temperature record based on an analysis of fossil chironomids (non-biting midges) identified and quantified in four sediment cores extracted from the bed of Lake Silvaplana (46°26’56”N, 9°47’33”E) in the Upper Engadine (a high-elevation valley in the eastern Swiss Alps). This analysis produced a detailed history of that region’s mean July air temperature over the last millennium.
The results of this effort indicate, as the five researchers describe it, “at the beginning of the record, corresponding to the last part of the ‘Medieval Climate Anomaly’ (here the period between ca. AD 1032 and 1262), the chironomid-inferred mean July air temperatures were 1°C warmer than the climate reference period (1961–1990),” which would also make them warmer than most subsequent temperatures. And in looking at their graphs of 20- and 50-year running means, it can be seen that the peak warmth of the Medieval Warm Period exceeded that of the Current Warm Period by approximately 0.5°C in the case of 20-year averages and 1.2°C in the case of 50-year averages. Consequently, Larocque-Tobler et al. conclude, “there is no evidence that mean-July air temperature exceeded the natural variability recorded during the Medieval Climate Anomaly in the 20th century at Lake Silvaplana.” They note similar results “were also obtained in northern Sweden (Grudd, 2008), in Western Europe (Guiot et al., 2005), in a composite of Northern Hemisphere tree-ring reconstructions (Esper et al., 2002) and a composite of tree rings and other archives (Moberg et al., 2005).”
A few years earlier in Italy, Frisia et al. (2005) developed a 17,000-year record of speleothem calcite δ18OC data they obtained from a cave stalagmite located at the southeast margin of the European Alps (45°37’05” N, 13°53’10” E), which they calibrated against “a reconstruction of temperature anomalies in the Alps” developed by Luterbacher et al. (2004) for the last quarter of the past millennium. This work revealed—among several other things (due to the great length of time involved)—the occurrence of the Roman Warm Period and a Medieval Warm Period that was broken into two parts by an intervening central cold period. The five researchers say both portions of the Medieval Warm Period were “characterized by temperatures that were similar to the present.”
Also working in Italy, Giraudi (2009) examined “long-term relations among glacial activity, periglacial activity, soil development in northwestern Italy’s alpine River Orco headwaters, and downvalley floods on the River Po,” based on “studies carried out by means of geological and geomorphologic surveys on the glacial and periglacial features,” including a sampling of soils involved in periglacial processes that “provided a basis for development of a chronological framework of late Holocene environmental change” and an analysis of “a stratigraphic sequence exposed in a peat bog along the Rio del Nel” about 1 km from the front edge of the Eastern Nel Glacier. Among several interesting findings, these undertakings allowed Giraudi to determine that between about 200 BC and AD 100—i.e., during the Roman Warm Period—“soils developed in areas at present devoid of vegetation and with permafrost,” indicative of the likelihood that temperatures at that time “probably reached higher values than those of the present.” He also concluded “analogous conditions likely occurred during the period of 11th–12th centuries AD, when a soil developed on a slope presently characterized by periglacial debris,” while noting “in the 11th–12th centuries AD, frost weathering processes were not active and, due to the higher temperatures than at present or the longer duration of a period with high temperatures, vegetation succeeded in colonizing the slope.”
These several studies from Europe provide evidence for the millennial-scale oscillation of climate that has operated throughout glacial and interglacial periods alike, producing century-scale periods when temperatures were as warm as they are at present, or even warmer, even though the air’s CO2 content was much lower at those earlier times than it is today.
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