Arseneault and Payette (1997) analyzed tree-ring and growth-form sequences obtained from more than 300 spruce remains buried in a presently treeless peatland in northern Quebec to produce a proxy record of climate for this region of the continent between 690 and 1591 AD.  Perhaps the most outstanding feature of this history was the warm period it revealed between 860 and 1000 AD.  Based on the fact that the northernmost 20th century location of the forest tree-line is presently 130 km south of their study site, the scientists concluded that the "Medieval Warm Period was approximately 1°C warmer than the 20th century."

Shifting to the other side of the continent, Calkin et al. (2001) carefully reviewed what they termed "the most current and comprehensive research of Holocene glaciation" along the northernmost Gulf of Alaska between the Kenai Peninsula and Yakutat Bay, where they too detected a Medieval Warm Period that lasted for "at least a few centuries prior to A.D. 1200."  Also identifying the Medieval Warm Period, as well as other major warm and cold periods of the millennial-scale climatic oscillation that is responsible for them, was Campbell (2002), who analyzed the grain sizes of sediment cores obtained from Pine Lake, Alberta, Canada (52°N, 113.5°W) to provide a non-vegetation-based high-resolution record of climate variability for this part of North America over the past 4000 years.  Periods of both increasing and decreasing grain size (related to moisture availability) were noted throughout the 4000-year record at decadal, centennial and millennial time scales.  The most predominant departures were several-centuries-long epochs that corresponded to the Little Ice Age (about AD 1500-1900), the Medieval Warm Period (about AD 700-1300), the Dark Ages Cold Period (about BC 100 to AD 700) and the Roman Warm Period (about BC 900-100).

Working our way southward, Laird et al. (2003) studied diatom assemblages in sediment cores taken from three Canadian and three United States lakes situated within the northern prairies of North America, finding that "shifts in drought conditions on decadal through multicentennial scales have prevailed in this region for at least the last two millennia."  In Canada, major shifts occurred near the beginning of the Medieval Warm Period, while in the United States they occurred near its end.  In giving some context to these findings, the authors state that "distinct patterns of abrupt change in the Northern Hemisphere are common at or near the termination of the Medieval Warm Period (ca. A.D. 800-1300) and the onset of the Little Ice Age (ca. A.D. 1300-1850)."  They also note that "millennial-scale shifts over at least the past 5,500 years, between sustained periods of wetter and drier conditions, occurring approximately every 1,220 years, have been reported from western Canada (Cumming et al., 2002)," and that "the striking correspondence of these shifts to large changes in fire frequencies, inferred from two sites several hundreds of kilometers to the southwest in the mountain hemlock zone of southern British Columbia (Hallett et al., 2003), suggests that these millennial-scale dynamics are linked and operate over wide spatial scales."

In an effort to determine whether these climate-driven millennial-scale cycles are present in the terrestrial pollen record of North America, Viau et al. (2002) analyzed a set of 3,076 ¹⁴C dates from the North American Pollen Database used to date sequences in more than 700 pollen diagrams across North America.  Results of their statistical analyses indicated there were nine millennial-scale oscillations during the past 14,000 years in which continent-wide synchronous vegetation changes with a periodicity of roughly 1650 years were recorded in the pollen records.  The most recent of the vegetation transitions was centered at approximately 600 years BP (before present).  This event, in the words of the authors, "culminat[ed] in the Little Ice Age, with maximum cooling 300 years ago."  Prior to that event, a major transition that began approximately 1600 years BP represents the climatic amelioration that "culminat[ed] in the maximum warming of the Medieval Warm Period 1000 years ago."

And so it goes, on back through the Holocene and into the preceding late glacial period, with the times of all major pollen transitions being "consistent," in the words of the authors of the study, "with ice and marine records."  Viau et al. additionally note that "the large-scale nature of these transitions and the fact that they are found in different proxies confirms the hypothesis that Holocene and late glacial climate variations of millennial-scale were abrupt transitions between climatic regimes as the atmosphere-ocean system reorganized in response to some forcing."  They go on to say that "although several mechanisms for such natural [our italics] forcing have been advanced, recent evidence points to a potential solar forcing (Bond et al., 2001) associated with ocean-atmosphere feedbacks acting as global teleconnections agents."  Furthermore, they note that "these transitions are identifiable across North America and presumably the world."

Additional evidence for the solar forcing of these millennial-scale climate changes is provided by Shindell et al. (2001), who used a version of the Goddard Institute for Space Studies GCM to estimate climatic differences between the period of the Maunder Minimum in solar irradiance (mid-1600s to early 1700s) and a century later, when solar output was relatively high for several decades.  Their results compared so well with historical and proxy climate data that they concluded, in their words, that "colder winter temperatures over the Northern Hemispheric continents during portions of the 15th through the 17th centuries (sometimes called the Little Ice Age) and warmer temperatures during the 12th through 14th centuries (the putative Medieval Warm Period) may have been influenced by long-term solar variations."

Rounding out our mini-review of the Medieval Warm Period in North America are two papers dealing with the climatic history of the Chesapeake Bay region of the United States.  The first, by Brush (2001), consists of an analysis of sediment cores obtained from the Bay's tributaries, marshes and main stem that covers the past millennium, in which it is reported that "the Medieval Climatic Anomaly and the Little Ice Age are recorded in Chesapeake sediments by terrestrial indicators of dry conditions for 200 years, beginning about 1000 years ago, followed by increases in wet indicators from about 800 to 400 years ago."

Willard et al. (2003) studied the same region for the period 2300 years BP to the present, via an investigation of fossil dinoflagellate cysts and pollen from sediment cores.  Their efforts revealed that "several dry periods ranging from decades to centuries in duration are evident in Chesapeake Bay records."  The first of these periods of lower-than-average precipitation, which spanned the period 200 BC-AD 300, occurred during the latter part of the Roman Warm Period, as delineated by McDermott et al. (2001) on the basis of a high-resolution speleothem ð¹⁸O record from southwest Ireland.  The next such dry period (~AD 800-1200), in the words of the authors, "corresponds to the 'Medieval Warm Period', which has been documented as drier than average by tree-ring (Stahle and Cleaveland, 1994) and pollen (Willard et al., 2001) records from the southeastern USA."

Willard et al. go on to say that "mid-Atlantic dry periods generally correspond to central and southwestern USA 'megadroughts', described by Woodhouse and Overpeck (1998) as major droughts of decadal or more duration that probably exceeded twentieth-century droughts in severity."  They further indicate that "droughts in the late sixteenth century that lasted several decades, and those in the 'Medieval Warm Period' and between ~AD 50 and AD 350 spanning a century or more have been indicated by Great Plains tree-ring (Stahle et al., 1985; Stahle and Cleaveland, 1994), lacustrine diatom and ostracode (Fritz et al., 2000; Laird et al., 1996a, 1996b) and detrital clastic records (Dean, 1997)."

In summing up these several findings, it is evident that the Medieval Warm Period has left its mark throughout North America in the form of either warm temperature anomalies or periods of relative dryness.

References
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