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Medieval Warm Period (North America: Canada Plus) -- Summary
Climate alarmists claim that rising atmospheric CO2 concentrations due to the burning of fossil fuels, such as coal, gas and oil, have raised global air temperatures to their highest level in the past one to two millennia. And, therefore, investigating the possibility of a period of equal global warmth within the past one to two thousand years has become a high-priority enterprise; for if such a period could be shown to have existed, when the atmosphere's CO2 concentration was far less than it is today, there would be no compelling reason to attribute the warmth of our day to the CO2 released to the air by mankind since the beginning of the Industrial Revolution. Thus, in this review of the pertinent scientific literature, results of the search for such knowledge are presented for studies conducted within the borders of Canada and other regions north of the lower 48 states of the United States of America.

Arseneault and Payette (1997) analyzed tree-ring and growth-form sequences obtained from more than 300 spruce remains buried in a presently treeless peatland located near the tree line in northern Québec in order to produce a proxy record of climate for this region between AD 690 and 1591. This effort revealed that over the course of this 900-year time period, the trees of the region experienced several episodes of both suppressed and rapid growth, indicative of both colder and warmer conditions, respectively, than those of the present. Cooler (suppressed growth) conditions prevailed between AD 760-860 and 1025-1400, while warmer (rapid growth) conditions were prevalent between AD 700-750, 860-1000, 1400-1450 and 1500-1570.

Further analysis of the warm period between AD 860 and 1000 led the two researchers to conclude that the warmth experienced in northern Quebec during this time period coincided with the Medieval Warm Period that was experienced across the North Atlantic and Northern Europe, which "exceeded in duration and magnitude both the 16th and 20th century warm periods identified previously [by other scientists] using the same methods." Furthermore, on the basis of current annual temperatures at their study site and the northernmost 20th century location of the forest, which at that time was 130 km south of their site, they concluded that the "Medieval Warm Period was approximately 1°C warmer than the 20th century."

Three years later, Campbell and Campbell (2000) analyzed pollen and charcoal records obtained from sediment cores retrieved from three small ponds - South Pond (AD 1655-1993), Birch Island Pond (AD 1499-1993) and Pen 5 Pond (400 BC-AD 1993) - located within Canada's Elk Island National Park, which covers close to 200 km2 of the Beaver Hills region of east-central Alberta. And in so doing, and counter to the intuitive assumption that there would be an "increase in fire activity with warmer and drier climate," the Canadian researchers found that "declining groundwater levels during the Medieval Warm Period [MWP] allowed the replacement of substantial areas of shrub birch with the less fire-prone aspen, causing a decline in fire frequency and/or severity, while increasing carbon storage on the landscape." And they thus concluded that this scenario "is likely playing out again today," as all three of the sites they studied "show historic increases in Populus pollen and declines in charcoal."

In further discussing their results, the two researchers noted that the earth's present climate "is warmer and drier than that of either the Little Ice Age (which followed the MWP) or the early Neoglacial (preceding the MWP)," and they say we must therefore "consider the present pond levels to be more representative of the MWP than of the time before or after." But since their Pen 5 Pond data indicate that sediment charcoal concentrations have not yet dropped to the level characteristic of the MWP - even with what they describe as the help of "active fire suppression in the park combined with what may be thought of as unintentional fire suppression due to agricultural activity around the park" - it would appear that their study sites and their surroundings have not yet risen to the level of warmth and dryness that they experienced during the MWP, which they describe as having occurred over the period AD 800-1200.

Focusing on Alaska, Calkin et al. (2001) reviewed what they called "the most current and comprehensive research of Holocene glaciation" along the northernmost Gulf of Alaska between the Kenai Peninsula and Yakutat Bay; and in doing so, they noted several periods of glacial advance and retreat during the past 7000 years. Most recently, they described a general retreat during the Medieval Warm Period that lasted for "at least a few centuries prior to AD 1200." Then, following this Medieval Climatic Optimum, there were three major intervals of Little Ice Age glacial advance: the early 15th century, the middle 17th century, and the last half of the 19th century. And during these latter time periods, glacier equilibrium line altitudes were depressed from 150 to 200 m below present values as Alaskan glaciers "reached their Holocene maximum extensions."

Clearly, the existence of a Medieval Warm Period and Little Ice Age in Alaska is an obvious reality. But what is especially interesting to note is that glaciers there reached their maximum Holocene extensions during the Little Ice Age. Hence, it can logically be inferred that Alaskan temperatures reached their Holocene minimum during this time as well, leading a person to ask: should it come as any surprise if temperatures in Alaska rise significantly above the chill of the Little Ice Age in the region's natural recovery from the coldest period of the entire Holocene?

Also working in Alaska, and concurrently, were Hu et al. (2001), who "conducted multi-proxy geochemical analyses of a sediment core from Farewell Lake in the northwestern foothills of the Alaska Range," obtaining what they described as "the first high-resolution quantitative record of Alaskan climate variations that spans the last two millennia." And what did they find?

The team of five scientists said their results "suggest that at Farewell Lake SWT [surface water temperature] was as warm as the present at AD 0-300 [during the Roman Warm Period], after which it decreased steadily by ~3.5°C to reach a minimum at AD 600 [during the depths of the Dark Ages Cold Period]." And from that point in time, they stated that "SWT increased by ~3.0°C during the period AD 600-850 and then [during the Medieval Warm Period] exhibited fluctuations of 0.5-1.0°C until AD 1200." Completing their narrative, they indicated that "between AD 1200-1700, SWT decreased gradually by 1.25°C [as the world descended into the depths of the Little Ice Age], and from AD 1700 to the present, SWT increased by 1.75C," the latter portion of which warming initiated the Current Warm Period.

In commenting on these findings, Hu et al. remarked that "the warmth before AD 300 at Farewell Lake coincides with a warm episode extensively documented in northern Europe … whereas the AD 600 cooling is coeval with the European 'Dark Ages'." They also reported that "the relatively warm climate AD 850-1200 at Farewell Lake corresponds to the Medieval Climatic Anomaly, a time of marked climatic departure over much of the planet." And they noted that "these concurrent changes suggest large-scale teleconnections in natural climatic variability during the last two millennia, likely driven by atmospheric controls."

Noting that "20th-century climate is a major societal concern in the context of greenhouse warming," Hu et al. concluded by reiterating that their record "reveals three time intervals of comparable warmth: AD 0-300, 850-1200, and post-1800," and they added that "these data agree with tree-ring evidence from Fennoscandia, indicating that the recent warmth is not atypical of the past 1000 years," in unmistakable contradiction of those who claim that it is.

The great importance of these observations resides in the fact that they testify to the reality of the non-CO2-induced millennial-scale oscillation of climate that brought the world, including Alaska, significant periods of warmth comparable to, or in some cases actually greater than, that of the present some 1000 years ago, during the Medieval Warm Period, and some 1000 years before that, during the Roman Warm Period. And, most importantly, these earlier periods of warmth were unquestionably not caused by elevated atmospheric CO2 concentrations (which were far less during those periods than they are today). Neither were they due to elevated concentrations of any other greenhouse gases. Thus, they were obviously caused by something else, which fact makes it very clear that the warmth of today could well be due to that same "something else" as well.

One year later, Kaplan et al. (2002) reported on paleolimnological inferences regarding Holocene climatic variability from a small lake in southern Greenland - Qipisarqo Lake (61°00'41"N, 47°45'13"W) - based on lake sediment physical-chemical properties, including magnetic susceptibility, density, water content, and biogenic silica and organic matter concentration. And in doing so, as they describe it, they found that "the interval from 6000 to 3000 cal yr B.P. was marked by warmth and stability." Thereafter, however, the climate cooled "until its culmination during the Little Ice Age." But from 1300-900 cal yr B.P., there was a partial amelioration during the Medieval Warm Period, which was associated with an approximate 1.5°C rise in temperature. Then, following another brief warming between A.D. 1500 and 1750, the second and more severe portion of the Little Ice Age occurred, which in turn was followed, as they describe it, by "naturally initiated post-Little Ice Age warming since A.D. 1850, which is recorded throughout the Arctic" and, most importantly, "has not yet reached peak Holocene warmth."

The three researchers also noted that "colonization around the northwestern North Atlantic occurred during peak Medieval Warm Period conditions that ended in southern Greenland by AD 1100." Norse movements around the region thereafter, however, occurred at what they described as "perhaps the worst time in the last 10,000 years, in terms of the overall stability of the environment for sustained plant and animal husbandry," with the result that the demise of the Norse colonies was clearly the result of "the most environmentally unstable period since deglaciation." And they concluded their paper with the further observation that "current warming, however rapid, has not yet reached peak Holocene warmth."

Returning to Canada, Campbell (2002) analyzed the grain sizes of sediment cores obtained from Alberta's Pine Lake (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. This effort revealed periods of both increasing and decreasing grain size (moisture availability) throughout the 4000-year record at decadal, centennial and millennial time scales, with the most predominant departures including four 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 100 BC to AD 700) and the Roman Warm Period (about 900-100 BC). In addition, a standardized median grain-size history indicated 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 which time grain sizes were about 2.5 standard deviations above the 4000-year mean. In contrast, the lowest rates of streamflow were observed around AD 1100, when median grain sizes were nearly 2 standard deviations below the 4000-year mean, while most recently, grain size over the past 150 years has generally remained above average.

The Pine Lake sediment record thus convincingly demonstrates the reality of the non-CO2-induced millennial-scale climatic oscillation that alternately brings several-century-long periods of alternating dryness and wetness to the southern Alberta region of North America, during concomitant periods of relative hemispheric warmth and coolness, respectively. And it also demonstrates that there is nothing unusual about the region's current moisture status, which suggests that the planet may still have a bit of warming to do before the Current Warm Period is fully upon us.

Working in both Canada and the United States were Laird et al. (2003), who studied diatom assemblages in sediment cores taken from three Canadian and three United States lakes situated within the northern prairies of North America. For five of the lakes, diatom-inferred salinity estimates were used to reconstruct relative changes in effective moisture (E/P), where E is evaporation and P is precipitation, with high salinity implying high E/P. For the sixth lake, diatom-inferred total phosphorus was used, while chronologies were based on 210Pb dating of recent sediments and radiocarbon dates for older sediments.

The seven scientists noted that their data showed 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. And in giving some context to these findings, they stated 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 noted 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."

Clearly, therefore, there is nothing unusual or unnatural about climate change. It happens on decadal scales, centennial scales and millennial scales. And over the past century or two, the earth has experienced a natural and not-unexpected millennial-scale climatic shift that may or may not have yet run its course. And the fact that the air's CO2 content increased in phase with this shift is simply due to the coincidental concurrent development of the Industrial Revolution and its subsequent transformative impact on humanity.

Turning our attention back to Greenland for a moment, Lassen et al. (2004) provided some historical background to their palaeoclimatic work by reporting that "the Norse, under Eric the Red, were able to colonize South Greenland at AD 985, according to the Icelandic Sagas, owing to the mild Medieval Warm Period climate with favorable open-ocean conditions." They also mentioned, in this regard, that the arrival of the gritty Norsemen was "close to the peak of Medieval warming recorded in the GISP2 ice core which was dated at AD 975 (Stuiver et al., 1995)," while Esper et al. (2002) independently identified the peak warmth of this period throughout North American extratropical latitudes as "occurring around 990." Hence, it would appear that the window of climatic opportunity provided by the peak warmth of the Medieval Warm Period was indeed a major factor enabling seafaring Scandinavians to establish stable settlements on the coast of Greenland.

As time progressed, however, the glowing promise of the apex of Medieval warmth gave way to the debilitating reality of the depth of Little Ice Age cold. Jensen et al. (2004), for example, reported that the diatom record of Igaliku Fjord "yields evidence of a relatively moist and warm climate at the beginning of settlement, which was crucial for Norse land use," but that "a regime of more extreme climatic fluctuations began soon after AD 1000, and after AD c. 1350 cooling became more severe." Lassen et al. additionally noted that "historical documents on Iceland report the presence of the Norse in South Greenland for the last time in AD 1408," during what they described as a period of "unprecedented influx of (ice-loaded) East Greenland Current water masses into the innermost parts of Igaliku Fjord." They also reported that "studies of a Canadian high-Arctic ice core and nearby geothermal data (Koerner and Fisher, 1990) correspondingly showed a significant temperature lowering at AD 1350-1400," when, in their words, "the Norse society in Greenland was declining and reaching its final stage probably before the end of the fifteenth century." Consequently, what the relative warmth of the Medieval Warm Period provided the Norse settlers, the relative cold of the Little Ice Age took from them: the ability to survive on Greenland.

Many more details of this incredible saga of five centuries of Nordic survival at the foot of the Greenland Ice Cap are provided by the trio of papers addressing the palaeo-history of Igaliku Fjord. Based on a high-resolution record of the fjord's subsurface water-mass properties derived from analyses of benthic foraminifera, Lassen et al. concluded that stratification of the water column, with Atlantic water masses in its lower reaches, appears to have prevailed throughout the last 3200 years, except for the Medieval Warm Period. During this time interval, which they describe as occurring between AD 885 and 1235, the outer part of Igaliku Fjord experienced enhanced vertical mixing (which they attributed to increased wind stress) that would have been expected to increase nutrient availability there. A similar conclusion was reached by Roncaglia and Kuijpers (2004), who found evidence of increased bottom-water ventilation between AD 960 and 1285. Consequently, based on these findings, plus evidence of the presence of Melonis barleeanus during the Medieval Warm Period (the distribution of which is mainly controlled by the presence of partly decomposed organic matter), Lassen et al. concluded that surface productivity in the fjord during this interval of unusual relative warmth was "high and thus could have provided a good supply of marine food for the Norse people."

Shortly thereafter, however, the cooling that led to the Little Ice Age was accompanied by a gradual re-stratification of the water column, which curtailed nutrient upwelling and reduced the high level of marine productivity that had prevailed throughout the Medieval Warm Period. These linked events, according to Lassen et al., "contributed to the loss of the Norse settlement in Greenland." Indeed, with deteriorating growing conditions on land and simultaneous reductions in oceanic productivity, the odds were truly stacked against the Nordic colonies; and it was only a matter of time before their fate was sealed. As Lassen et al. described it, "around AD 1450, the climate further deteriorated with further increasing stratification of the water-column associated with stronger advection of (ice-loaded) East Greenland Current water masses." This development, in their words, led to an even greater "increase of the ice season and a decrease of primary production and marine food supply," which they noted "could also have had a dramatic influence on the local seal population and thus the feeding basis for the Norse population."

The end result of these several conjoined phenomena, in the words of Lassen et al., was that "climatic and hydrographic changes in the area of the Eastern Settlement were significant in the crucial period when the Norse disappeared." Also, Jensen et al. report that "geomorphological studies in Northeast Greenland have shown evidence of increased winter wind speed, particularly in the period between AD 1420 and 1580 (Christiansen, 1998)," noting that "this climatic deterioration coincides with reports of increased sea-ice conditions that caused difficulties in using the old sailing routes from Iceland westbound and further southward along the east coast of Greenland, forcing sailing on more southerly routes when going to Greenland (Seaver, 1996)."

In conclusion, therefore, Jensen et al. wrote that "life conditions certainly became harsher during the 500 years of Norse colonization," and that this severe cooling-induced environmental deterioration "may very likely have hastened the disappearance of the culture." At the same time, it is also clear that the more favorable living conditions associated with the peak warmth of the Medieval Warm Period - which occurred between approximately AD 975 (Stuiver et al., 1995) and AD 990 (Esper et al., 2002) - were what originally enabled the Norse to successfully colonize the region. Furthermore, in the thousand-plus subsequent years, there has never been a sustained period of comparable warmth, nor of comparable terrestrial or marine productivity, either locally or hemispherically (and likely globally, as well). And, therefore, since the peak warmth of the Medieval Warm Period was caused by something quite apart from elevated levels of atmospheric CO2, or any other greenhouse gas for that matter, there is no reason to not believe that a return engagement of that same factor or group of factors is responsible for the even lesser "peak" warmth of today.

Around this same timeframe, D'Arrigo et al. (2004) sampled trees of white spruce (Picea glauca (Moench) Voss) from fourteen sites near the elevational treeline on the eastern Seward Peninsula of Alaska, obtaining 46 cores from 38 trees, which they used to develop a maximum latewood density (MXD) chronology for the period AD 1389 to 2001. Calibrating a portion of the latter part of this record (1909-1950) against May-August monthly temperatures obtained from the Nome meteorological station, they then converted the entire MXD chronology to warm-season temperatures. This work revealed, in their words, that "the middle-20th century warming is the warmest 20-year interval since 1640." In viewing their plot of reconstructed temperatures, however, it can readily be seen there is a nearly equivalent warm period near the end of the 1600s, as well as a two-decade period of close-to-similar warmth in the mid-1500s. What is more, in the latter part of the 1400s, there is a decade of warmth that is actually warmer than that of the mid-20th century. Thus, the new temperature reconstruction, which the five researchers described as "one of the longest density-based records for northern latitudes," provides yet another indication that 20th-century warmth was by no means unprecedented when compared to the past millennium or two, contrary to the claims of Mann et al. (1998, 1999) and Mann and Jones (2003). Quite to the contrary, in fact, it rather supports the findings of Esper et al. (2002, 2003), McIntyre and McKitrick (2003), and Loehle (2004), which indicate there were several periods over the past millennium or more when it was equally as warm as, or even warmer than, it was during the 20th century.

One year later, Luckman and Wilson (2005) used new tree-ring data from the Columbia Icefield area of the Canadian Rockies to present a significant update to a millennial temperature reconstruction published for this region in 1997. The new update employed different standardization techniques, such as the regional curve standardization method, in an effort to capture a greater degree of low frequency variability (centennial to millennial scale) than reported in the initial study. In addition, the new data set added over one hundred years to the chronology that now covers the period 950-1994.

The new tree-ring record was found to explain 53% of May-August maximum temperature variation observed in the 1895-1994 historical data and was thus viewed as a proxy indicator of such temperatures over the past millennium. And based on this relationship, the record showed considerable decadal- and centennial-scale variability, where generally warmer conditions prevailed during the 11th and 12th centuries, between about 1350-1450 and from about 1875 through the end of the record. The warmest reconstructed summer occurred in 1434 and was 0.23°C warmer than the next warmest summer that occurred in 1967, while persistent cold conditions prevailed between 1200-1350, 1450-1550 and 1650-1850, with the 1690s being exceptionally cold (more than 0.4°C colder than other intervals).

Among other things, the revised Columbia Icefield temperature reconstruction provides further evidence for natural climate fluctuations on centennial-to-millennial time scales and indicates, once again, that temperatures during the Current Warm Period are no different from those observed during the Medieval Warm Period (11-12th centuries). And since atmospheric CO2 concentrations had nothing to do with the warm temperatures of those earlier periods, one cannot rule out the possibility that they also have had nothing to do with the warm temperatures of the modern era.

But if not CO2, then what? According to Luckman and Wilson, the Columbia Icefield reconstruction "appears to indicate a reasonable response of local trees to large-scale forcing of climates, with reconstructed cool conditions comparing well with periods of known low solar activity," which is a polite way of suggesting that some solar-related phenomenon may well be the main driver of the low frequency temperature trends.

About this same time, D'Arrigo et al. (2005) used a new tree-ring width data set that was derived from 14 white spruce chronologies obtained from the Seward Peninsula, Alaska, covering the years 1358-2001, to combine with additional tree-ring width chronologies from northwest Alaska to produce two versions of a much longer data series that extended all the way back to AD 978. The first chronology was created using traditional methods of standardization (STD), which do not perform well in capturing multi-decadal or longer climate cycles, while the second chronology utilized the regional curve standardization (RCS) method, which better preserves low-frequency variations at multi-decadal time scales and longer.

The new-and-improved (and extended) final temperature history of this study provided further evidence for natural climate fluctuations on centennial-to-millennial time scales, capturing the temperature oscillations that produced the Medieval Warm Period (11-13th centuries) and Little Ice Age (1500-1700). What the records failed to do, however, was provide evidence for what climate alarmists call the unprecedented warmth of the last decade of the 20th century. Quite to the contrary, in fact, the northwest Alaska temperatures of the last four decades actually hovered around the long-term average.

Moving forward in time another year, Hallett and Hills (2006) reconstructed the Holocene environmental history of Kootenay Valley in the southern Canadian Rockies, based on relevant data obtained from the sediments of Dog Lake, British Columbia (50°46'N, 116°06'W). In doing so, they found that in the centuries leading up to AD 800, that area had developed "a more open landscape," and that "fire frequencies and summer drought appear to increase," concluding that this increased fire activity was "supported by higher dry-open/wet-closed [forest] pollen ratios and indicates a return to dry-open forest conditions around Dog Lake," which lasted about 400 years. Thereafter, they also determined that "wet-closed forest cover reaches its maximum extent from 700-150 cal years BP [AD 1250-1800]" in what "appears to be a response to Little Ice Age cooling." Last of all, they say that "current global warming trends ... may again create the conditions necessary for dry-open ... forest to expand in the Kootenay Valley."

In contemplating these observations, Hallett and Hills opined that current global warming may recreate climatic conditions similar to those that prevailed in the Kootenay Valley prior to the global chill of the Little Ice Age, which suggests that it has not been as warm there yet, nor for as long a time, as it was between AD 800 and 1200, when the Medieval Warm Period held sway in that part of the world. For this region, therefore, it would appear that the Current Warm Period cannot yet hold a candle to both the level and duration of medieval warmth, which their work suggests was unprecedented over the past millennium. And the fact that there was around 115 ppm less CO2 in the air of that earlier record warm period than there is today suggests that something far more potent than the atmosphere's CO2 concentration is in control of earth's climate and is the cause of 20th-century global warming.

Also with a paper appearing in the same year were Loso et al. (2006), who presented "a varve thickness chronology from glacier-dammed Iceberg Lake [60°46'N, 142°57'W] in the southern Alaska icefields," where "radiogenic evidence confirms that laminations are annual and record continuous sediment deposition from AD 442 to AD 1998," and where "varve thickness increases in warm summers because of higher melt, runoff, and sediment transport." And they report that the temperatures implied by the varve chronology "were lowest around AD 600, warm between AD 1000 and AD 1300 [which they called "a clear manifestation of the Medieval Warm Period"], cooler between AD 1500 and AD 1850, and have increased dramatically since then."

In light of these findings, the four scientists said their varve record "suggests that 20th century warming is more intense ... than the Medieval Warm Period or any other time in the last 1500 years." However, the intense warming of the 20th century peaked somewhere in the vicinity of 1965 to 1970 (as best as can be determined from their graphical representation of varve thickness), after which it was followed by equally intense cooling, such that by 1998 (the supposedly warmest year of the past two millennia, according to the world's climate alarmists), temperatures are implied to have been less than they were during the Medieval Warm Period.

The same story is also told by tree ring-width anomalies from the adjacent Wrangell Mountains of Alaska, which Loso et al. portrayed as updated from Davi et al. (2003). Hence, it can be concluded from two different data bases that the region's current temperature is, in fact, lower than it was during the warmest part of the Medieval Warm Period, adding more weight to the growing mountain of evidence that indicates there is nothing unusual about the planet's current level of warmth.

One year later, Hay et al. (2007) analyzed the vertical distributions of diatoms, silicoflagellates and biogenic silica found in two sediment cores recovered from the inner and outer basins (49°04'N, 125°09'W and 49°02'N, 125°09'W, respectively) of Effingham Inlet, British Columbia, Canada, after which they described the climatic implications of what they had found. And what they found was evidence that "a period of warmer and drier climate conditions and possibly increased coastal upwelling offshore occurred ca. 1450-1050 calendar years before present," i.e., from about AD 500-900. Also, and noting that "the patterns observed in the diatom record of Effingham Inlet are consistent with regional marine and terrestrial paleoenvironmental records," they went on to report that "coast range glaciers ... showed a hiatus from 1500 to 1100 calendar years before present," and that this "period of more productive conditions ... was correlative with increased regional primary and marine fish production." In addition, their data indicated that concentrations of Skeletonema costatum, which they say "is limited by low temperatures," were much greater over the AD 550-950 period (which appears to represent the Medieval Warm Period in this part of the world) than in any portion of the following (most recent) millennium. And so it was that Hay et al.'s work presented yet another example of the widespread occurrence of the Medieval Warm Period and its superiority to the Current Warm Period in terms of maximum temperatures.

Also hard at work in the same timeframe were Zabenskie and Gajewski (2007), who extracted sediment cores from Lake JR01 (69°54'N, 95°4.2'W) on the Boothia Peninsula, Nunavut, Canada, using a 5-cm diameter Livinstone corer, and who were careful to note that "the uppermost part of the sediment was sampled in a plastic tube with piston to ensure that the sediment-water interface was collected," while further stating that "the upper 20 cm of sediment were sub-sampled into plastic bags at 0.5-cm intervals." Then, from the fossil pollen assemblages thereby derived, July temperatures were estimated "using the modern analog technique," as per Sawada (2006). And what did they thereby learn?

The two researchers reported that "maximum estimated July temperatures were reached between 5800 and 3000 cal yr BP, at which time they exceeded present-day values." Thereafter, however, temperatures decreased, but with a subsequent "short warming," which they said "could be interpreted as the Medieval Warm Period," or MWP, which they identified as occurring "between 900 and 750 cal yr BP." Then, following this latter period of warmth, they found that "temperatures cooled during the Little Ice Age," as pollen percentages "returned to their values before the [MWP] warming." And last of all, during the final 150 years of the record, they noted that a "diverse and productive diatom flora" was observed. However, the two researchers state in their paper that "July temperatures reconstructed using the modern analog technique remained stable during this time," which suggests that the Lake JR01 region of the Boothia Peninsula is currently not as warm as it was during the MWP.

Close to simultaneously, Podritske and Gajewski (2007) evaluated the relationship that exists between diatoms and temperature by comparing a diatom stratigraphy based on high-resolution sampling with independent paleoclimatic records, after which they used a high-resolution diatom sequence of the past 9900 years that they developed from sediment-core data acquired from a small lake (unofficially named KR02) on Canada's Victoria Island (located at 71.34°N, 113.78°W) to place recent climatic changes there "in an historical context." And in doing so, the two researchers found, as they described it, that "there is evidence of diatom community response to centennial-scale variations such as the 'Medieval Warm Period' (~1000-700 cal yr BP), 'Little Ice Age' (~800-150 cal yr BP) and recent warming." In addition, and most importantly, they reported that the recent warming-induced changes "are not exceptional when placed in the context of diatom community changes over the entire Holocene," stating that "although recent changes in diatom community composition, productivity, and species richness are apparent, they were surpassed at other periods throughout the Holocene." And they explicitly add that the most recent rate-of-change "was exceeded during the Medieval Warm Period."

Moving ahead one year, Wiles et al. (2008) used "comparisons of temperature sensitive climate proxy records with tree-ring, lichen and radiocarbon dated histories from land-terminating, non-surging glaciers for the last two millennia from southern Alaska" to help them "identify summer temperature as a primary driver of glacial expansions," based on "field and laboratory work over the past decade" that yielded "five new or updated glacier histories," one each for Bear Glacier (Kenai Mountains), Marathon Mountain Cirque (Kenai Mountains), Amherst Glacier (Chugach Mountains), Crescent Glacier (Chugach Mountains) and Yakutat Glacier (St. Elias Mountains), all located just above the Gulf of Alaska (about 60°N) between approximately 140 to 150°W.

The four researchers' findings suggested the presence of the Roman Warm Period near the beginning of their 2000-year record, because of detected "general glacier expansions during the First Millennium AD" that experienced their "strongest advance" at AD 600, which latter cold interval - with ice extent "as extensive as [the] subsequent Little Ice Age" - is typically known as the Dark Ages Cold Period. This latter cold interval was then followed by the Medieval Warm Period (MWP), the evidence for which consisted of "soil formation and forest growth on many forefields in areas that today are only just emerging from beneath retreating termini," which suggests that the MWP was likely both warmer and longer-lived than what we have so far experienced of the Current Warm Period. And they also report, in this regard, that at the Sheridan, Tebenkof and Princeton glaciers, "tree-ring chronologies show that forest growth on these forefields was continuous between the 900s and 1200s."

Noting that the alternating warm-cold-warm-cold-warm sequence of the past 2000 years "is consistent with millennial-scale records of ice-rafted debris flux in the North Atlantic and Northern Hemisphere temperature reconstructions," and that "variable Holocene solar irradiance has been proposed as a potential forcing mechanism for millennial-scale climate change," they concluded that "this is supported by the Southern Alaskan glacial record," which implies that the past century's lead-in to the Current Warm Period may well have been similarly orchestrated and have had essentially nothing to do with the concomitant increase in the air's CO2 content.

Close to the same time, Besonen et al. (2008) derived thousand-year histories of varve thickness and sedimentation accumulation rate for Canada's Lower Murray Lake (81°20'N, 69°30'W), which is typically covered for about eleven months of each year by ice that reaches a thickness of 1.5 to 2 meters at the end of each winter. With respect to these parameters, they stated that "field-work on other High Arctic lakes clearly indicates that sediment transport and varve thickness are related to temperatures during the short summer season that prevails in this region, and we have no reason to think that this is not the case for Lower Murray Lake." And, therefore, in the words of the six scientists, the story told by both the varve thickness and sediment accumulation rate histories of Lower Murray Lake was that "the twelfth and thirteenth centuries were relatively warm," and in this regard their data indicate that Lower Murray Lake and its environs were often much warmer during this time period (AD 1080-1320) than they were at any point in the 20th century, which has also been shown to be the case for Donard Lake (66.25°N, 62°W) by Moore et al. (2001).

Also in this same timeframe, and based on several years of field investigations designed to exhaustively map and accurately date the occurrences of all fires per each 100-year interval over the last 2000 years within a 40-km2 area of northern boreal forest-tundra within the Riviere Boniface watershed in northeastern Canada (57°45'N, 76°W), Payette et al. (2008) developed a long-term, spatially-explicit fire history of the northernmost boreal forest in that region. And as a result of their efforts, they found there was a "70% reduction of forest cover since 1800 yr BP and nearly complete cessation of forest regeneration since 900 yr BP," with the result that "the northern part of the forest tundra in Eastern Canada has been heavily deforested over the last millennium," while further noting that "the climate at the tree line was drier and warmer before 900 cal. yr BP."

As for what this all means, the chief direct cause of the post-900 yr BP deforestation, in the words of the three Canadian researchers, was "climate deterioration coinciding with the phasing-out of the Medieval Warmth and incidence of the Little Ice Age." In addition, they concluded that since "the latitudinal position of successful post-fire regeneration of lichen-spruce woodlands is situated approximately 1.5° south of the Boniface area, as a rule of thumb it is probable that a drop of at least 1°C in mean annual temperature occurred after 900 cal. yr BP." And as a result of that added fact, they went on to state that "recovery of the boreal forest after a long period of deforestation will require sustained warming," which they add has only been occurring "since the mid-1990s in Eastern subarctic Canada." Hence, it would appear that this particular part of North America has not yet experienced a sustained warming of magnitude great enough to return it to Medieval Warm Period conditions.

About this same time, Edwards et al. (2008) developed a cellulose δ13C dendrochronology "from cross-dated 10-year increments of 16 sub-fossil snags and living-tree ring sequences of Picea engelmannii (Englemann spruce) from upper alpine treeline sites near Athabasca Glacier and subfossil material from the forefield of Robson Glacier plus living and snag material of Pinus albicaulis (whitebark pine) adjacent to Bennington Glacier, spanning AD 951-1990," as well as from an oxygen isotope (δ18O) dendrochronology pertaining to the same time period, from which data they were able to calculate past changes in relative humidity and temperature over Canada's Columbia Icefield in the general vicinity of 53°N, 118°W. And in doing so, they reported several "intriguing new discoveries," one of which was "evidence of previously unrecognized winter warmth during the Medieval Climate Anomaly (~AD 1100-1250)," as can be seen in the following figure.


Columbia Icefield mean winter temperature z-scores relative to that of the period AD 1941-1990.

In viewing the four researchers' results, it can be seen that the peak winter temperature of the Medieval Climate Anomaly throughout Canada's Columbia Icefield was warmer than the peak temperature of the Current Warm Period (which appears to have occurred ~1915), while it was even warmer than the mean temperature of the 1941-1990 base period, as well as the mean temperature of the last ten years of that period (1980-1990). Hence, it is becoming ever more evident that recent temperatures around the world have not been "unprecedented" over the past one to two millennia, as climate alarmists typically claim they have.

One year closer to the present, and noting that "tree-ring crossdates of glacially killed logs have provided a precisely dated and detailed picture of Little Ice Age (LIA) glacier fluctuations in southern Alaska," Barclay et al. (2009) extended this history back into the First Millennium AD (FMA) by integrating similar data obtained from additional log collections made in 1999 with the prior data to produce a new history of advances and retreats of the Tebenkof Glacier spanning the past two millennia.

In the figure that follows, it can be seen that between the FMA and LIA extensions of the Tebenkof Glacier terminus, there was a period between about AD 950 and 1230 when the terminus dropped further than two kilometers back from the maximum LIA extension that occurred near the end of the 19th century. It can also be seen that this warmer/drier period of glacier terminus retreat had to have been much more extreme than what was experienced at any time during the 20th century, because at the century's end the glacier's terminus still had not retreated more than two kilometers back from the line of its maximum LIA extension. Also, this 280-year period of likely greater warmth/dryness falls right in the middle of the broad peak of maximum warmth during the global Medieval Warm Period, as defined by the abundance of data plotted on co2science.org's Interactive Map and Time Domain feature of their MWP Project.


The temporal history of the distance by which the terminus of the Tebenkof Glacier fell short of its maximum LIA extension over the past two millennia. Adapted from Barclay et al. (2009).

Based on the data depicted in the figure above, it would appear that the central portion of the Medieval Warm Period in southern Alaska was likely significantly warmer/drier than it was at any time during the 20th century. And as a result, it can be further concluded that there is nothing unprecedented or unusual about that region's current degree of warmth/dryness, which means there is no need to invoke anthropogenic CO2 emissions as the cause of the region's current heat and moisture levels. All that is needed to have created its current warmth/dryness is just a little less of whatever it was that caused the greater warmth/dryness of the Medieval Warm Period, which was clearly not carbon dioxide.

In another paper from the same year, Rolland et al. (2009) described how - while working in Nunavut, Canada - they reconstructed the late-Holocene evolution of a Southampton Island lake known as Tasiq Qikitalik (65°05'70'N, 83°47'49'W) by studying fossil chironomid distributions along with sedimentological data (X-ray fluorescence, grain size and C/N ratios) that they obtained from a sediment core retrieved from the lake's deepest reachable point, deriving in the process a 1200-year history of inferred August temperatures. This accomplishment led to their discovery that (1) "higher temperatures were recorded from cal yr AD 1160 to AD 1360, which may correspond to the Medieval Warm Period," and that (2) "between cal yr AD 1360 and AD 1700, lower temperatures were probably related to a Little Ice Age event," the latter of which periods exhibited a minimum August temperature that was "ca. 2°C colder than the maximum observed during the Medieval Warm Period." Also of note, the most recent August temperature (which occurs at the end of the record at about 2008) is approximately 0.9°C less than the maximum August temperature of the Medieval Warm Period. And so we have another example of the reality of the Medieval Warm Period and its thermal superiority (greater warmth) compared to that of the Current Warm Period, which even at its 1980 peak was still about 0.2°C cooler than the Medieval Warm Period was during its peak warmth, when the air's CO2 concentration was far less than it is currently.

Meanwhile, Laird and Cumming (2009) developed a history of changes in the level of Lake 259 (Rawson Lake, 49°40'N, 93°44'W) within the Experimental Lakes Area of northwestern Ontario, Canada, based on a suite of near-shore gravity cores they analyzed for diatom species identity and concentration, as well as organic matter content. This effort led them to discover there was "a distinct decline in lake level of ~2.5 to 3.0 m from ~800 to 1130 AD." And this interval, in their words, "corresponds to an epic drought recorded in many regions of North America from ~800 to 1400 AD," which they said was "often referred to as the Medieval Climatic Anomaly or the Medieval Warm Period," and which also "encompasses 'The Great Drought' of the thirteenth century (Woodhouse and Overpeck, 1998; Woodhouse, 2004; Herweijer et al. 2007)." In addition, they noted that the Canadian prairies were at that time "experiencing reductions in surface-water availability due to climate warming and human withdrawals (Schindler and Donahue, 2006)," and that many regions in the western U.S. had experienced water supply deficits in reservoir storage with the multi-year drought described by Cook et al. (2007). However, they went on to say that "these severe multi-year drought conditions pale in comparison to the many widespread megadroughts that persisted for decades and sometimes centuries in many parts of North America over the last millennium (Woodhouse, 2004)." And combining these observations with what was known about the close association between the severity and duration of drought and warmth throughout the affected region of North America suggests that the degree of warmth experienced during the Medieval Warm Period in the Experimental Lakes Area of Canada was likely much greater than the degree of warmth so far experienced there during the Current Warm Period.

Moving one year closer to the present, Clegg et al. (2010) conducted a high-resolution analysis of midge assemblages found in the sediments of Moose Lake (61°22.45'N, 143°35.93'W) in the Wrangell-St. Elias National Park and Preserve of south-central Alaska (USA), based on data obtained from cores removed from the lake bottom in the summer of AD 2000 and a midge-to-temperature transfer function that yielded mean July temperatures (TJuly) for the past six thousand years.

Some of the results of this study are portrayed in the figure below, where it can be seen that from about 2600 cal BP to the present, there is a clear multi-centennial oscillation about the declining trend, with peaks and valleys defining the temporal locations of the Roman Warm Period, the Dark Ages Cold Period, the Medieval Warm Period, the Little Ice Age - during which the coldest temperatures of the entire interglacial or Holocene were reached - and, finally, the start of the Current Warm Period, which is still not expressed to any significant degree compared to the Medieval and Roman Warm Periods.


Mean July near-surface temperature (°C) vs. years before present (cal BP) for south-central Alaska (USA). Adapted from Clegg et al. (2010).

In discussing their results, the seven scientists wrote that "comparisons of the TJuly record from Moose Lake with other Alaskan temperature records suggest that the regional coherency observed in instrumental temperature records (e.g., Wiles et al., 1998; Gedalof and Smith, 2001; Wilson et al., 2007) extends broadly to at least 2000 cal BP," while noting that (1) climatic events such as the LIA and the MWP occurred "largely synchronously" between their TJuly record from Moose Lake and a δ18O-based temperature record from Farewell Lake on the northwestern foothills of the Alaska Range, and that (2) "local temperature minima likely associated with First Millennium AD Cooling (centered at 1400 cal BP; Wiles et al., 2008) are evident at both Farewell and Hallet lakes (McKay et al., 2008)."

It is instructive to note here that even with the help of the unprecedented anthropogenic-induced increase in the air's CO2 concentration that occurred over the course of the 20th century, the Current Warm Period has not achieved anywhere near the warmth of the MWP or RWP, which suggests that the climatic impact of the 20th-century increase in the air's CO2 content has been negligible, for the warming that defined the earth's recovery from the global chill of the LIA - which should have been helped by the concurrent increase in the air's CO2 content, if it were a well-mannered greenhouse gas - appears no different from the non-CO2-induced warming that brought the planet out of the Dark Ages Cold Period and into the Medieval Warm Period.

A few months later, based on their study of an 11.6-m sediment core that they extracted in June of 2001 from the deepest point of Felker Lake (51°57.0'N, 121°59.9'W), which sits in the rainshadow generated by Canada's Coast, Cascade and Columbia Mountains, Galloway et al. (2011) had a paper published that described how they had analyzed diatom assemblages, together with pollen and spore types and quantities, in a program designed to produce an 11,670-year record of hydrological change throughout the Holocene, based on a calibration data set of 219 lakes from British Columbia, including Felker Lake, and select lakes from the Northern Great Plains (Wilson et al., 1996). This work provided evidence for what they called a "millennial-scale pacing of climate" throughout the Holocene, as well as the fact that "the most extreme episode of hydrological change occurred from ca. 1030 cal. year BP to ca. 690 cal. year BP," a period that they noted was "broadly coeval with the Medieval Warm Period." And buttressing their conclusion, they remarked that "a coeval warm and dry interval is recognized in numerous paleoclimate studies in western North America," citing as some examples the work of Hallett et al. (2003), Laird et al. (2003) and Bracht et al. (2007).

As for the significance of Galloway et al.'s findings, the fact that the warm and dry interval they discovered at Felker Lake during the heart of the Medieval Warm Period was the most extreme such period of the entire Holocene to be recorded there indicates just how unique the Medieval Warm Period was in this regard. And this finding testifies to the non-uniqueness of the warmth and dryness experienced in that part of the world during the establishment of the planet's Current Warm Period, which further suggests that the historical increase in the air's CO2 concentration likely had next to nothing to do with the development of the much milder warmth and dryness of that region's current climatic state, with potentially similar implications for the rest of the world.

About this same time, Kobashi et al. (2011) wrote that "Greenland recently incurred record high temperatures and ice loss by melting, adding to concerns that anthropogenic warming is impacting the Greenland ice sheet and in turn accelerating global sea-level rise." However, they went on to state that "it remains imprecisely known for Greenland how much warming is caused by increasing atmospheric greenhouse gases versus natural variability." And in rigorously exploring this question of the source of recent warmth attribution, Kobashi et al. reconstructed Greenland surface snow temperature variability over the past 4000 years at the GISP2 site (near the Summit of the Greenland ice sheet; hereafter referred to as Greenland temperature) with a new method that utilizes argon and nitrogen isotopic ratios from occluded air bubbles, as described in detail by Kobashi et al. (2008a,b).

In describing their findings, the eight researchers reported that "the temperature record starts with a colder period in 'the Bronze Age Cold Epoch'," which they say was followed by "a warm period in 'the Bronze Age Optimum'," which was followed by a 1000-year cooling that began "during 'the Iron/Roman Age Optimum'," which was followed by "the Dark Ages," which was followed by "the Medieval Warm Period," which was followed by "the Little Ice Age" - which they describe as "the coldest period of the past 4000 years" - which was followed, last of all, by "the recent warming." For comparative purposes, they also noted that "the current decadal average surface temperature at the summit is as warm as in the 1930s-1940s, and there was another similarly warm period in the 1140s (Medieval Warm Period)," indicating that "the present decade is not outside the envelope of variability of the last 1000 years." In fact, they say that "excluding the last millennium," there were fully "72 decades warmer than the present one, in which mean temperatures were 1.0 to 1.5°C warmer," and that during two centennial intervals, average temperatures "were nearly 1.0°C warmer than the present decade" (see figure below).


Reconstructed Greenland snow surface temperatures for the past 4000 years as adapted from Kobashi et al. (2011). The blue line and blue band represent the reconstructed Greenland temperature and 1? error, respectively. The green line represents a 100-year moving average of the blue line. The black and red lines indicate the Summit and AWS decadal average temperatures, respectively, as calculated by others.

Since the Greenland summit's decadal warmth of the first ten years of the 21st century was exceeded fully six dozen times over the prior four millennia, it is clear that it was in no way unusual, unnatural or unprecedented; and, therefore, it is clear that none of Greenland's recent warming need have been caused by increasing concentrations of greenhouse gases. Indeed, it is far more likely that its recent warmth is nothing more than the next expected phase of the natural oscillation of climate that has produced numerous several-hundred-year periods of alternating warmth and cold over the past four thousand years.

In one of the more recent pertinent studies to be included in this review, Bunbury and Gajewski (2012) wrote that "although the nature of the Little Ice Age is quite well known and it is recognized that the climate variations during this time occurred globally, knowledge of medieval warming is less established and there is still debate about its geographic extent." Thus, to help shed more light on the subject, Bunbury and Gajewski obtained sediment cores from two lakes in the interior southwest of Canada's Yukon Territory - Jenny Lake (61.04°N, 138.36°W) and Upper Fly Lake (61.04°N, 138.09°W) - which, in their words, "yielded chironomid records that were used to provide quantitative estimates of mean July air temperature." And as a result of these efforts, the two researchers were able to state that their chironomid-inferred temperature estimates from the two lakes "compare well with one another and also with other paleoclimate evidence from the region," noting that their data suggest "relatively warm conditions during medieval times, centered on AD 1200, followed by a cool Little Ice Age, and warming temperatures over the past 100 years." More specifically, it can be estimated from the graphical representations of their data that the Medieval Warm Period at both lake sites extended from about AD 1100 to 1350. And it can similarly be estimated that the most recent (AD 1990) of their temperature determinations were about 0.8°C cooler than the peak warmth of the Medieval Warm Period at Jenny Lake and approximately 0.5°C cooler at Upper Fly Lake. And these results now join the many other similar results, from all around the world, which have been archived in the databases of co2science.org's Medieval Warm Period Project, where it can be seen that the Medieval Warm Period was not only a global phenomenon, but that its peak warmth was very likely significantly greater than that of the Current Warm Period.

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Last updated 16 April 2014