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Roman Warm Period -- Dark Ages Cold Period (Summary)
There has been much discussion of late about the magnitude and geographical extent of the Medieval Warm Period and Little Ice Age.  Soon et al. (2003) review a wealth of paleoclimatic data from around the world that suggest these relatively warmer and cooler epochs were of global extent and that the Medieval Warm Period was at least as warm as - and, in many places and times, even warmer than - the current Modern Warm Period.

On the other side of the debate, Mann et al. (2003) stridently claim these several-hundred-year climatic episodes were not nearly as dramatic nor as widespread as what Soon et al. (2003) suggest.  Why?  Because if they were, there would be nothing unusual about what climate alarmists, such as themselves, typically describe as the unprecedented global warming of the past century, which characterization makes the temperature increase of the past hundred-plus years appear so unusual they can further claim it must have been caused by something unnatural, such as the concomitant increase in anthropogenic CO2 emissions.

To bring another perspective to this debate, we here summarize the results of several scientific journal articles we have reviewed over the past few years that deal with the two prior incarnations of the Medieval Warm Period and Little Ice Age, i.e., the Roman Warm Period and Dark Ages Cold Period, which like their more current cousins are simply manifestations of a naturally-induced millennial-scale cycle of climate that is also likely responsible, in our view, for the establishment of the Modern Warm Period.

We begin with the paper of Andersson et al. (2003), wherein surface temperatures of the eastern Norwegian Sea were reconstructed from analyses of planktic stable isotopes and foraminiferal assemblages preserved in two seabed sediment cores.  The climate history derived from this study is remarkably similar to that derived by McDermott et al. (2001) from a high-resolution 18O record obtained from a stalagmite discovered in a cave in southwestern Ireland.  At the beginning of the 3000-year-long Norwegian Sea record, both regions were in the end-stage of the long cold period that preceded the Roman Warm Period.  Hence, both records depict warming from that point in time to the peak of the Roman Warm Period, which occurred about 2000 years before present (BP).  Then, both regions begin their descent into the Dark Ages Cold Period, which held sway until the increase in temperature that produced the Medieval Warm Period, which in both records prevailed from about 800 to 550 years BP.  Last of all, the Little Ice Age is evident, with cold periods centered at approximately 400 and 100 years BP, again in both records.  It is also interesting to note that neither record indicates the existence of a significant Modern Warm Period and that Andersson et al. say "surface ocean conditions warmer than present were common during the past 3000 years."

On the continent itself, Berglund (2003) 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."  Starting from the climatic warmth and elevated human productivity of the Roman Warm Period, there was, in Berglund's words, a "retreat of agriculture" centered on about AD 500 that was coincident with a period "of rapid cooling" - which ushered in the Dark Ages Cold Period - as "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."  The good times "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."

Also on the continent, Niggemann et al. (2003) studied the petrographical and geochemical properties of three stalagmites found in a cave in Sauerland, Germany, from which they developed a climatic history covering the last 17,600 years.  As they describe their findings, the records obtained from the three stalagmites "resemble records from an Irish stalagmite (McDermott et al., 1999)."  Specifically, they note that their data provide clear evidence for the existence of the Little Ice Age, Medieval Warm Period and Roman Warm Period, which also implies the existence of the Dark Ages Cold Period that separated the Medieval and Roman Warm Periods.

Other European evidence for the Dark Ages Cold Period is provided by Castagnoli et al. (2002), who developed a 1400-year record of 13C values from remains of the foraminifera Globigerinoides rubber, which were obtained from a sediment core extracted from the Mediterranean Sea.  Their data revealed an initial increase in 13C values that coincided with the climatic transition from the Dark Ages Cold Period to the Medieval Warm Period, over which period marine productivity in the Mediterranean rose dramatically.

Moving seaward to the west and north, Jiang et al. (2002) studied diatom assemblages from a high-resolution sediment core extracted from the seabed of the north Icelandic shelf.  The final 2700 years of their reconstructed temperature record witnessed a significant deterioration of the climate in the vicinity of the north Icelandic shelf, as the region moved ever further away from the benign weather of the Roman Warm Period.  After its descent into the Dark Ages Cold Period, however, the Icelandic record depicts a nearly complete recovery during the middle of the Medieval Warm Period; but this "Little Climatic Optimum" soon gave way to the rapid cooling that produced the Little Ice Age, which brought mean summer sea surface temperatures down by about 2.2C from what they were at the peak of the Medieval Warm Period.

On Iceland itself, Olafsdottir et al. (2001) simulated the spatial relationship between temperature change and potential vegetation cover over the period of the Holocene, evaluating their results against real-world palynological and geomorphological data.  During the extended Holocene climatic optimum, they calculated that vegetation may have covered about 60% of the land.  Then, when the Roman Warm Period finally began to wane, a long-term vegetative decline commenced that lasted throughout the Dark Ages Cold Period.  With the onset of the Medieval Warm Period, the vegetation loss was actually reversed for about 400 years; but the subsequent appearance of the Little Ice Age resulted, in their words, in "an unprecedented low potential for vegetation for the Holocene that lasted c. 600 years, i.e., between AD c. 1300 and 1900."

Continuing on to North America, Willard et al. (2003) "examine[d] the late Holocene (2300 yr BP to present) record of Chesapeake Bay and the adjacent terrestrial ecosystem in its watershed through the study of fossil dinoflagellate cysts and pollen from sediment cores," determining 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.  The next such period (~AD 800-1200), in the words of Willard et al., "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."  They also note that these "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."

Willard et al. further indicate that the droughts "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)."  Their own work - and that of the others they cite - thus demonstrates the reality of the millennial-scale hydrologic cycle that accompanies the millennial-scale temperature cycle that has alternately produced the Roman Warm Period, Dark Ages Cold Period, Medieval Warm Period, Little Ice Age and now, potentially, the Modern Warm Period.

In Canada, Campbell (2002) analyzed grain sizes of sediment cores obtained from a lake in Southern Alberta 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 coarser and finer grain size (the former corresponding to moister climates with higher streamflow) were noted at decadal, centennial and millennial time scales.  The most predominant departures included 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).  In addition, a standardized median grain-size history revealed that the highest rates of stream discharge during the past 4000 years occurred during the Little Ice Age at approximately 300-350 years ago.  During this time, grain sizes were about 2.5 standard deviations above the 4000-year mean.  In contrast, the lowest streamflow rates were observed around AD 1100, when median grain sizes were nearly two standard deviations below the 4000-year mean.  These data, like those of the studies cited in the previous paragraph, also demonstrate the reality of the non-CO2-induced millennial-scale climatic oscillation that alternately brings widespread parts of the earth several-century periods of relative dryness and wetness during concomitant periods of relative warmth and coolness, respectively.

Crossing the Pacific, as we work our way around the Northern Hemisphere, Ma et al. (2003) assessed the climatic history of the past 3000 years at 100-year intervals on the basis of 18O data, the Mg/Sr ratio, and the solid-liquid distribution coefficient of Mg, which they derived by careful analysis of a stalagmite from Jingdong Cave about 90 km northeast of Beijing, China.  Between 200 and 500 years BP, the authors report "air temperature was about 1.2C lower than that of the present, corresponding to the Little Ice Age in Europe."  Earlier, between 1000 and 1300 years BP, there was an equally aberrant but warm period that peaked at about 1100 BP, which they say "corresponded to the Medieval Warm Period (~AD900-1300) in Europe."  This time of peak warmth in the Chinese record was further preceded by what we call the Dark Ages Cold Period that in turn had been preceded by the Roman Warm Period, which in the stalagmite record is best defined by the much colder period that preceded it.

Xu et al. (2002) studied plant cellulose 18O variations in cores retrieved from peat deposits at the northeastern edge of the Qinghai-Tibetan Plateau in China.  Following the demise of the Roman Warm Period, their data revealed the existence of three particularly cold intervals centered at approximately 500, 700 and 900 AD during the Dark Ages Cold Period.  Then, from 1100-1300 AD, they report that "the 18O of Hongyuan peat cellulose increased, consistent with that of Jinchuan peat cellulose and corresponding to the 'Medieval Warm Period'."  Finally, they note that "the periods 1370-1400 AD, 1550-1610 AD, [and] 1780-1880 AD recorded three cold events, corresponding to the 'Little Ice Age'."  Thus, they too detected the relative warmth and coolness of the Roman Warm Period, Dark Ages Cold Period, Medieval Warm Period and Little Ice Age.

Paulsen et al. (2003) used high-resolution records of 13C and 18O derived from a stalagmite taken from Buddha Cave "to infer changes in climate in central China for the last 1270 years in terms of warmer, colder, wetter and drier conditions."  Among the climatic episodes evident in their data, they specifically identified "those corresponding to the Medieval Warm Period, Little Ice Age and 20th-century warming, lending support to the global extent of these events."  In addition, their record begins in the depths of the Dark Ages Cold Period, which ends about AD 965 with the commencement of the Medieval Warm Period, which continues to approximately AD 1475, whereupon the Little Ice Age sets in and holds sway until about AD 1825, after which the warming responsible for the Modern Warm Period begins.

Yang et al. (2002) used nine separate proxy climate records, derived from peat, lake sediment, ice core, tree ring and other proxy sources, to compile a single weighted temperature history for China spanning the past two millennia.  The composite record revealed five distinct climate epochs: a warm stage from AD 0 to 240 (the tail-end of the Roman Warm Period), a cold interval between AD 240 and 800 (the Dark Ages Cold Period), a return to warm conditions from AD 800-1400 (which included the Medieval Warm Period between AD 800 and 1100), a cool interval between 1400 and 1820 (the Little Ice Age), and the current warm regime (the Modern Warm Period) that followed the increase in temperature that began in the early 1800s.  Another important finding of the study was the fact that the warmest temperatures of the past two millennia were observed during the second and third centuries AD near the end of the Roman Warm Period.

Working with an ice core retrieved from Dasuopu glacier in the central Himalayas, Tibet, Yao et al. (2002) derived a 2000-year proxy temperature (18O) history, which revealed that temperature in the first century A.D. "was low and [was] followed by a significant increase until 730 A.D.," whereupon it "reached its maximum during 730-950 A.D., then it lowered again, which persisted until 1850 A.D.," after which "temperature has increased gradually to its present levels."  These intervals correspond, respectively, to the Dark Ages Cold Period, the Medieval Warm Period, the Little Ice Age, and - near the very end of the record - the Modern Warm Period.

The Dasuopu temperature record once again demonstrates the importance of considering more than just the past thousand years when attempting to gain an appreciation for the degree of natural climate variability one must consider when attempting to assign a cause to the temperature increase of the past century and a half.  As Yao et al. put it, "if we just analyse temperature changes in [the most] recent 1 ka, we may draw a wrong conclusion that [the] temperature recorded in [the] Dasuopu ice core[near its end] goes beyond the natural variability range."

Over in India at the Gangotri Glacier, which is considered to be the source of the Holy Ganga, Kar et al. (2002) explored the nature of climate change over the past 2000 years via pollen analyses of a 1.25-meter sediment profile in an outwash plain located about 2.5-3 km from the glacier's snout.  Their data revealed the existence of a cooler climate "than the one prevailing at present" between 2000 and 1700 years BP.  Comparing this result with the findings of McDermott et al. (2001) in Ireland, this period of time is seen to be part of the Dark Ages Cold Period.  Between 1700 and 850 years BP, there then occurs what Kar et al. call an "amelioration of climate," which represents the transition from the depth of the Dark Ages Cold Period to the midst of the Medieval Warm Period.

Subsequent to 850 BP, the climate "became much cooler," indicative of its transition to Little Ice Age conditions.  Between 300 and 200 years ago, in fact, Kar et al. note that the long-term "retreat of the Gangotri Glacier ceased, possibly with some minor advancement."  During the last 200 years, however - when the study of Esper et al. (2002) indicates there has been a rather steady warming of the Northern Hemisphere - the glacier's snout has retreated by about 2 km.

The results of this study clearly demonstrate the exquisite harmony of climate change in the region of the North Atlantic Ocean and the distant Himalayas, providing ever-increasing evidence for the reality of the oscillatory climatic phenomenon that has brought the world the Medieval Warm Period, Little Ice Age and Modern Warm Period, as well - in this case - as the even earlier Dark Ages Cold Period.

We now shift gears and travel to the Southern Hemisphere, beginning with the study of Jenny et al. (2002), who analyzed geochemical, sedimentological and diatom-assemblage data derived from sediment cores extracted from one of the largest natural lakes in Central Chile to obtain information about the hydrologic climate of that region over the past two millennia.  From 200 BC, when the record began, until AD 200, conditions were primarily dry, concurrent with the Roman Warm Period of Europe.  Subsequently, from AD 200-700, with a slight respite in the central hundred years of that period, there was a high frequency of flood events.  This period of time coincided with the Dark Ages Cold Period of the North Atlantic region.  Then came a several-hundred-year period of less flooding that was coeval with the Northern Hemisphere's Medieval Warm Period.  This more benign period was followed by another interval of frequent flooding from 1300-1700, which resumed briefly about 1850 and was coeval with the Little Ice Age.  The striking temporal correspondences of these several climatic oscillations in Central Chile and those of many parts of the Northern Hemisphere bear strong testimony to the global nature and coherency of the planet's climate variability over two full cycles of naturally-induced oscillatory behavior.

Pretty much the same thing was found in neighboring Peru by Chepstow-Lusty et al. (1998), who analyzed pollen in sediment cores obtained from a recently in-filled lake at Marcacocha, Peru.  From AD 100-1050 there was an overall decline in pollen content, which they indicate is reflective of increasingly colder conditions (the Dark Ages Cold Period) relative to the period before that time (the Roman Warm Period).  Indeed, they state that the "low proportion of Chenopodiaceae and Ambrosia is a clear indication that temperatures were still suppressed" toward the end of this 1000-year interval.  However, a "more optimum climate," characterized by greater warmth and dryness, prevailed for several centuries after about AD 900, as indicated by the establishment and rapid growth of the tree Alnus acuminata (Aliso) during this period; but between AD 1700 and 1800, during what the authors call the "most intense episode of the Little Ice Age," a major decline of Alnus occurred, indicating once again a return to colder conditions.

Another study that suggests the existence of the same set of climatic epochs, but from a more roundabout way, is that of Moy et al. (2002), who retrieved two 8-m cores and two 0.5-m cores from the center of a lake in the southern Ecuadorian Andes and derived from them a continuous history of El Nio/Southern Oscillation (ENSO) events over the past 12,000 years.  In coming out of the Dark Ages Cold Period at about 1200 years BP, the number of ENSO events indicated by their record drops by a full order of magnitude, from a high of approximately 33 events per 100 years to a low of about 3 events per 100 years, centered approximately on the year AD 1000, which is right in the middle of the Medieval Warm Period, as delineated by the work of Esper et al. (2002).  Then, at approximately AD 1250, the frequency of ENSO events exhibits a new peak of approximately 27 events per 100 years in the midst of the longest sustained cold period of the Little Ice Age, again as delineated by the work of Esper et al.  Finally, ENSO event frequency declines in zigzag fashion to a low on the order of 4 to 5 events per 100 yr at the start of the Modern Warm Period, which according to the temperature history of Esper et al. begins at about 1940.

Going back in time to 2000 years BP, the Roman Warm Period as delineated by McDermott et al. is near its peak warmth and ENSO event frequency is again at a very low level.  By 3000 years BP, however, ENSO event frequency is yet again significantly elevated in response to another very distinctive millennial-scale cold period that appears in the climatic reconstruction of McDermott et al.  Hence, it can be appreciated that ENSO event frequency as recorded in lake sediments in the southern Ecuadorian Andes is essentially a mirror-image of millennial-scale climatic change as recorded throughout the world and particularly in the North Atlantic Ocean, a point that is readily recognized by Moy et al., who note that "two processes known to operate at this timescale are the deposition of ice-rafted detritus in the North Atlantic (Bond events)" and "changes in the carbon cycle represented by the residual 14C record."

Dropping to latitudes below South America, Noon et al. (2003) analyzed oxygen isotopes preserved in authigenic carbonate retrieved from the freshwater sediments of Sombre Lake on Signy Island in the Southern Ocean, which they used to construct a 7000-year history of that region's climate.  Over the past seven millennia, the general trend of temperature at the study site was downward, as it was most everywhere.  Of primary interest to us, however, is that approximately 2000 years ago, after a thousand-year gap in the data, Signy Island is seen to have experienced the relative warmth of the last vestiges of the Roman Warm Period, as delineated by McDermott et al. (2001).  Then comes the Dark Ages Cold period, which is also contemporaneous with what McDermott et al. observed in the Northern Hemisphere, after which the Medieval Warm Period appears at the same point in time and persists for the same length of time that it does in the vicinity of Ireland, whereupon the Little Ice Age sets in just as it does in the Northern Hemisphere.  Finally, there is an indication of late 20th century warming, but with still a long way to go before conditions comparable to those of the Medieval Warm Period are achieved.

The last study we have reviewed that touches on the occurrence of the Roman Warm Period and Dark Ages Cold Period in the Southern Hemisphere is that of Haug et al. (2003), which draws heavily on the study of Haug et al. (2001).  Based on analyses of titanium and iron concentrations in an ocean sediment core extracted from the Cariaco Basin on the Northern Shelf of Venezuela, the earlier of the two studies developed a hydrologic history of the entire Holocene for Mesoamerica and northern tropical South America.  Then, based on a more detailed analysis of the titanium content of a smaller part of the record, the more recent of the two papers developed a hydrologic history of pertinent portions of the extended record that yielded roughly bi-monthly resolution of the annual signal.

How is this detailed hydrologic history related to the fortunes of the Maya of Mesoamerica and northern tropical South America?  Haug et al. (2003) say the Pre-Classic period of Maya civilization flourished "before about 150 A.D.," which, according to the climate history of McDermott et al. (2001), corresponds to the latter portion of the Roman Warm Period.  Subsequently, however, during the transition to the Dark Ages Cold Period, which was accompanied by a slow but persistent decline in precipitation, Haug et al. report that "the first documented historical crisis hit the lowlands, which led to the 'Pre-Classic abandonment' (Webster, 2002) of major cities."

This crisis occurred during the first intense multi-year drought of the transition period, which was centered on about the year 250 A.D.  Although the drought was devastating to the Maya, Haug et al. report that when it was over, "populations recovered, cities were reoccupied, and Maya culture blossomed in the following centuries during the so-called Classic period."

Ultimately, however, there came a time of total reckoning, between about 750 and 950 A.D., during what Haug et al. determined was the driest interval of the entire Dark Ages Cold Period, when they say "the Maya experienced a demographic disaster as profound as any other in human history," in response to a number of other intense multi-year droughts.  During this Terminal Classic Collapse, Haug et al. note that "many of the densely populated urban centers were abandoned permanently, and Classic Maya civilization came to an end."

As they assess the significance of these several observations near the end of their paper, Haug et al. conclude that, "given the perspective of our long time series, it would appear that the droughts we have highlighted were the most severe to affect this region in the first millennium A.D."  Although some of these spectacular droughts were "brief," lasting only between three and nine years, Haug et al. note "they occurred during an extended period of reduced overall precipitation that may have already pushed the Maya system to the verge of collapse," which suggests to us that these droughts within dry periods were likely the proverbial straws that broke the camel's back.

After the Maya civilization had faded away, the authors' data depict the subsequent development of the Medieval Warm Period, when the Vikings established a foothold in Greenland.  Then comes the Little Ice Age, which just as quickly led to the Vikings demise in that part of the world.  This distinctive cold node of the planet's millennial-scale climatic oscillation must have also led to hard times for the inhabitants of Mesoamerica and northern tropical South America; for according to the data of Haug et al., the Little Ice Age produced by far the lowest precipitation regime (of several hundred years duration) of the last two millennia in that part of the world.

There are a number of conclusions that may be drawn from these several observations. One is that both climatic and human history tend to repeat themselves.  Another is that the millennial-scale climatic oscillation, which manifests itself throughout glacial and interglacial periods alike, does so totally independently of what the atmosphere's CO2 concentration is doing.  Yet another logical conclusion is that the two nodes of this climate cycle - of which the Medieval Warm Period (and its antecedent Roman Warm Period) and Little Ice Age (and its antecedent Dark Ages Cold Period) are typical - are truly global phenomena, manifesting themselves in some parts of the world primarily in terms of thermal extremes and in other parts of the world primarily in terms of moisture extremes.  Most important of all, however, is that all of these observations clearly demonstrate there was nothing abnormal or unusual about the global warming of the 20th century.  It was simply the natural transition from cool-node to warm-node global climate that was only to be expected with the naturally-induced demise of the Little Ice Age.

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