Dahl-Jensen et al. (1998) used data from two ice sheet boreholes to reconstruct the temperature history of Greenland over the past 50,000 years.  Their analysis indicated that temperatures on the Greenland Ice Sheet during the Last Glacial Maximum (about 25,000 years ago) were 23 ± 2 °C colder than at present.  After the termination of the glacial period, however, temperatures increased steadily to a value that was 2.5°C warmer than at present, during the Climatic Optimum of 4,000 to 7,000 years ago.  The Medieval Warm Period and Little Ice Age were also evident in the borehole data, with temperatures 1°C warmer and 0.5-0.7°C cooler than at present, respectively.  Then, after the Little Ice Age, the group of seven scientists reports that "temperatures reached a maximum around 1930 AD" and that "temperatures have decreased [our italics] during the last decades."

The results of this study stand in stark contrast to the predictions of general circulation models of the atmosphere, which consistently suggest there should have been a significant CO2-induced warming in high northern latitudes over the past several decades.  They also depict large temperature excursions over the last 10,000 years, when the air's CO2 content was relatively stable.  Each of these observations raises serious doubts about the models' ability to correctly forecast earth's climatic response to the ongoing rise in the air's CO2 content.

In another study of Greenland climate that included both glacial and interglacial periods, Bard (2002) reviews the concept of rapid climate change.  Of this phenomenon, he writes that "it is now recognized that the ocean-atmosphere system exhibits several stable regimes under equivalent external forcings," and that "the transition from one state to another occurs very rapidly when certain climatic parameters attain threshold values."  Specifically, he notes that in the models "a slight increase in the freshwater flux above the modern level F produces a decrease in the NADW [North Atlantic Deep Water] convection and a moderate cooling in the North Atlantic," but that "the system flips to another state once the flux reaches a threshold value F + deltaF," which state has no deep convection and "is characterized by surface temperatures up to 6°C lower in and around the North Atlantic."

With respect to what has been learned from observations, Bard concentrates on the region of the North Atlantic, describing glacial-period millennial-scale episodes of dramatic warming called Dansgaard-Oeschger events (with temperature increases "of more than 10°C"), which are evident in Greenland ice core records, as well as episodes of "drastic cooling" called Heinrich events (with temperature drops "of up to about 5°C"), which are evident in sea surface temperature records derived from the study of North Atlantic deep-sea sediment cores.

In the Greenland record, according to Bard, the progression of these events is such that "the temperature warms abruptly to reach a maximum and then slowly decreases for a few centuries before reaching a threshold, after which it drops back to the cold values that prevailed before the warm event."  He also reports that "models coupling the atmosphere, ocean, and ice sheets are still unable to correctly simulate that variability on all scales in both time and space," which suggests we do not fully understand the dynamics of these rapid climate changes.  Indeed, Bard forcefully states that "all the studies so far carried out fail to answer the crucial question: How close are we to the next bifurcation [which could cause a rapid change-of-state in earth's climate system]?"  In this regard, he notes that "an intense debate continues in the modeling community about the reality of such instabilities under warm conditions [our italics]," which is a particularly important point, seeing that all dramatic warming and cooling events that have been detected to date have occurred in either full glacials or transitional periods between glacials and interglacials.

This latter real-world fact clearly suggests we are unlikely to experience any dramatic warming or cooling surprises in the near future, as long as the earth does not begin drifting towards glacial conditions, which is but another reason to not be concerned about the ongoing rise in the air's CO2 content.  In fact, it suggests that allowing more CO2 to accumulate in the atmosphere actually provides an effective "insurance policy" against abrupt climate change; for interglacial warmth seems to inoculate the planet against climatic instabilities, allowing only the mild millennial-scale climatic oscillation that alternately brings the earth slightly warmer and cooler conditions typical of the Medieval Warm Period and Little Ice Age.  Hence, and in light of the fact that the four preceding interglacials basked in temperatures fully 2°C warmer than those of the current interglacial (Petit et al., 1999) without suffering any adverse climatic consequences, humanity would be wise to not surrender the true environmental insurance policy we worked so hard to put in place by depositing the excess CO2 we produced over the course of the Industrial Revolution into the atmosphere, where it clearly does both the climate and the biosphere great good.

Focusing on the more pertinent period of the current interglacial or Holocene, we next consider a number of papers that come to bear upon the reality of the Medieval Warm Period and Little Ice Age: two well-known multi-century periods of significant climatic aberration, whose existence the world's climate alarmists simply refuse to acknowledge.  And why do they refuse to acknowledge them?  Because these periods of modest climatic aberration, plus the analogous warm and cool periods that preceded them (the Roman Warm Period and Dark Ages Cold Period), provide strong evidence for the existence of a millennial-scale oscillation of climate that is unforced by changes in the air's CO2 content, which in turn suggests that the global warming of the Little Ice Age-to-Modern Warm Period transition was likely totally independent of the coincidental concomitant increase in the air's CO2 content that accompanied the Industrial Revolution.

We begin with the study of Keigwin and Boyle (2000), who briefly reviewed what is known about the millennial-scale oscillation of earth's climate that is evident in a wealth of proxy climate data from around the world.  Stating that "mounting evidence indicates that the Little Ice Age was a global event, and that its onset was synchronous within a few years in both Greenland and Antarctica," they remark that in Greenland it was characterized by a cooling of approximately 1.7°C. Likewise, in an article entitled "Was the Medieval Warm Period Global?", Broecker (2001) answers yes, citing borehole temperature data that reveal the magnitude of the temperature drop over Greenland from the peak warmth of the Medieval Warm Period (800 to 1200 A.D.) to the coldest part of the Little Ice Age (1350 to 1860 A.D.) to have been approximately 2°C, and noting that as many as six thousand borehole records from all continents of the world confirm that the earth was a significantly warmer place a thousand years ago than it is today.

McDermott et al. (2001) derived a δ¹⁸O record from a stalagmite discovered in Crag Cave in southwestern Ireland, after which they compared this record with the δ¹⁸O records from the GRIP and GISP2 ice cores from Greenland.  In doing so, they found evidence for "centennial-scale δ¹⁸O variations that correlate with subtle δ¹⁸O changes in the Greenland ice cores, indicating regionally coherent variability in the early Holocene."  They additionally report that the Crag Cave data "exhibit variations that are broadly consistent with a Medieval Warm Period at ~1000 ± 200 years ago and a two-stage Little Ice Age, as reconstructed by inverse modeling of temperature profiles in the Greenland Ice Sheet."  Also evident in the Crag Cave data were the δ¹⁸O signatures of the earlier Roman Warm Period and Dark Ages Cold Period that comprised the prior such cycle of climate in that region; and in concluding they reiterate the important fact that the coherent δ¹⁸O variations in the records from both sides of the North Atlantic "indicate that many of the subtle multicentury δ¹⁸O variations in the Greenland ice cores reflect regional North Atlantic margin climate signals rather than local effects."

Another study that looked at temperature variations on both sides of the North Atlantic was that of Seppa and Birks (2002), who used a recently developed pollen-climate reconstruction model and a new pollen stratigraphy from Toskaljavri, a tree-line lake in the continental sector of northern Fenoscandia (located just above 69°N latitude), to derive quantitative estimates of annual precipitation and July mean temperature.  The two scientists say their reconstructions "agree with the traditional concept of a 'Medieval Warm Period' (MWP) and 'Little Ice Age' in the North Atlantic region (Dansgaard et al., 1975)."  Specifically, they report there is "a clear correlation between our MWP reconstruction and several records from Greenland ice cores," and that "comparisons of a smoothed July temperature record from Toskaljavri with measured borehole temperatures of the GRIP and Dye 3 ice cores (Dahl-Jensen et al., 1998) and the δ¹⁸O record from the Crete ice core (Dansgaard et al., 1975) show the strong similarity in timing of the MWP between the records."  Last of all, they note that "July temperature values during the Medieval Warm Period (ca. 1400-1000 cal yr B.P.) were ca. 0.8°C higher than at present," where present means the last six decades of the 20th century.

Concentrating solely on Greenland and its immediate environs are several other papers, among which is the study of Wagner and Melles (2001), who retrieved a sediment core from a lake on an island situated just off Liverpool Land on the east coast of Greenland.  Analyzing it for a number of properties related to the past presence of seabirds there, they obtained a 10,000-year record that tells us much about the region's climatic history.

Key to the study were certain biogeochemical data that reflected variations in seabird breeding colonies in the catchment area of the lake.  These data revealed high levels of the various parameters measured by Wagner and Melles between about 1100 and 700 years before present (BP) that were indicative of the summer presence of significant numbers of seabirds during that "medieval warm period," as they describe it, which had been preceded by a several-hundred-year period of little to no inferred bird presence.  Then, after the Medieval Warm Period, the data suggested another absence of birds during what they refer to as "a subsequent Little Ice Age," which they note was "the coldest period since the early Holocene in East Greenland."  Their data also showed signs of a "resettlement of seabirds during the last 100 years, indicated by an increase of organic matter in the lake sediment and confirmed by bird observations."  However, values of the most recent data were not as great as those obtained from the earlier Medieval Warm Period; and temperatures derived from two Greenland ice cores led to the same conclusion: it was warmer at various times between 1100 to 700 years BP than it was over the 20th century.

Kaplan et al. (2002) also worked with data obtained from a small lake, this one in southern Greenland, analyzing sediment physical-chemical properties, including magnetic susceptibility, density, water content, and biogenic silica and organic matter concentrations.  They discovered that "the interval from 6000 to 3000 cal yr BP was marked by warmth and stability."  Thereafter, however, the climate cooled "until its culmination during the Little Ice Age," but from 1300-900 years BP, there was a partial amelioration of climate (the Medieval Warm Period) that was associated with an approximate 1.5°C rise in temperature.

Following another brief warming between AD 1500 and 1750, the second and more severe portion of the Little Ice Age occurred, which was in turn followed by "naturally initiated post-Little Ice Age warming since AD 1850, which is recorded throughout the Arctic."  Last of all, they report that Viking "colonization around the northwestern North Atlantic occurred during peak Medieval Warm Period conditions that ended in southern Greenland by AD 1100," noting that Norse movements around the region thereafter "occurred at 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."

We further explore these aspects of Greenland's climatic history in our Editorial of 2 Jun 2004, which draws heavily upon a set of three important papers that reconstructed environmental conditions in the vicinity of Igaliku Fjord, South Greenland, before, during and after the period of Norse habitation of this and other parts of the ice-covered island's coast, beginning with the study of Lassen et al. (2004), who provide 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 mention, 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 we additionally note that 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 long-enduring 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, report 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 note that "historical documents on Iceland report the presence of the Norse in South Greenland for the last time in AD 1408," during what they describe as a period of "unprecedented influx of (ice-loaded) East Greenland Current water masses into the innermost parts of Igaliku Fjord."  They also report that "studies of a Canadian high-Arctic ice core and nearby geothermal data (Koerner and Fisher, 1990) correspondingly show 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 palaeohistory 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. conclude 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 period, which they describe as occurring between AD 885 and 1235, the outer part of Igaliku Fjord experienced enhanced vertical mixing (which they attribute 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.  Hence, 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. conclude 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, 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. describe 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 "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 light of these observations, Jensen et al. state 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), the strident protestations of Mann et al. (2003) notwithstanding.  Hence, 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 warmth of today.

Concentrating finally on the 20th century, Hanna and Cappelen (2003) determined the air temperature history of coastal southern Greenland from 1958-2001, based on data from eight Danish Meteorological Institute stations in coastal and near-coastal southern Greenland, as well as the concomitant sea surface temperature (SST) history of the Labrador Sea off southwest Greenland, based on three previously published and subsequently extended SST data sets (Parker et al., 1995; Rayner et al., 1996; Kalnay et al., 1996).  The coastal temperature data showed a cooling of 1.29°C over the period of study, while two of the three SST databases also depicted cooling: by 0.44°C in one case and by 0.80°C in the other.  Both the land-based air temperature and SST series followed similar patterns and were strongly correlated, but with no obvious lead/lag either way.  In addition, it was determined that the cooling was "significantly inversely correlated with an increased phase of the North Atlantic Oscillation (NAO) over the past few decades."  The two researchers say that this "NAO-temperature link doesn't explain what caused the observed cooling in coastal southern Greenland but it does lend it credibility."

In referring to what they call "this important regional exception to recent 'global warming'," Hanna and Cappelen note that the "recent cooling may have significantly added to the mass balance of at least the southern half of the [Greenland] Ice Sheet."  Consequently, since this part of the ice sheet is the portion that would likely be the first to experience melting in a warming world, it would appear that whatever caused the cooling has not only protected the Greenland Ice Sheet against warming-induced disintegration but actually fortified it against that possibility.

Several other studies have also reported late-20th-century cooling on Greenland. Based on mean monthly temperatures of 37 Arctic and 7 sub-Arctic stations, as well as temperature anomalies of 30 grid-boxes from the updated data set of Jones, for example, Przybylak (2001) found that "the level of temperature in Greenland in the last 10-20 years is similar to that observed in the 19th century."  Likewise, in a study that utilized satellite imagery of the Odden ice tongue (a winter ice cover that occurs in the Greenland Sea with a length of about 1300 km and an aerial coverage of as much as 330,000 square kilometers) plus surface air temperature data from adjacent Jan Mayen Island, Comiso et al. (2001) determined that the ice phenomenon was "a relatively smaller feature several decades ago," due to the warmer temperatures that were prevalent at that time.  In addition, they report that observational evidence from Jan Mayen Island indicates that temperatures there actually cooled at a rate of 0.15 ± 0.03°C per decade during the past 75 years.

Concluding our discussion of this final aspect of Greenland's temperature history, we note that in a study of three coastal stations in southern and central Greenland that possess almost uninterrupted temperature records between 1950 and 2000, Chylek et al. (2004) discovered that "summer temperatures, which are most relevant to Greenland ice sheet melting rates, do not show any persistent increase during the last fifty years."  In fact, working with the two stations with the longest records (both over a century in length), they determined that coastal Greenland's peak temperatures occurred between 1930 and 1940, and that the subsequent decrease in temperature was so substantial and sustained that current coastal temperatures "are about 1°C below their 1940 values."  Furthermore, they note that "at the summit of the Greenland ice sheet the summer average temperature has decreased at the rate of 2.2°C per decade since the beginning of the measurements in 1987."  Hence, as with the Arctic as a whole, it would appear that Greenland has not experienced any net warming over the most dramatic period of atmospheric CO2 increase on record.  In fact, it has cooled during this period … and cooled significantly.

At the start of the 20th century, however, Greenland was warming, as it emerged, along with the rest of the world, from the depths of the Little Ice Age.  What is more, between 1920 and 1930, when the atmosphere's CO2 concentration rose by a mere 3 to 4 ppm, there was a phenomenal warming at all five coastal locations for which contemporary temperature records are available.  In fact, in the words of Chylek et al., "average annual temperature rose between 2 and 4°C [and by as much as 6°C in the winter] in less than ten years."  And this warming, as they note, "is also seen in the ¹⁸O/¹⁶O record of the Summit ice core (Steig et al., 1994; Stuiver et al., 1995; White et al., 1997)."

In commenting on this dramatic temperature rise, which they call the great Greenland warming of the 1920s, Chylek et al. conclude that "since there was no significant increase in the atmospheric greenhouse gas concentration during that time, the Greenland warming of the 1920s demonstrates that a large and rapid temperature increase can occur over Greenland, and perhaps in other regions of the Arctic, due to internal climate variability such as the NAM/NAO [Northern Annular Mode/North Atlantic Oscillation], without a significant anthropogenic influence."  These facts thus led them to speculate that "the NAO may play a crucial role in determining local Greenland climate during the 21st century, resulting in a local climate that may defy the global climate change."

In further contemplating the results of the study of Chylek et al., it is clear that the entire history of anthropogenic CO2 emissions since the inception of the Industrial Revolution has had no discernable impact on Greenland air temperatures, while the studies described in our Editorials of 10 Mar 2004 and 17 Mar 2004 demonstrate pretty much the same thing for the entire Arctic and Antarctic regions of the globe.  Hence, it can readily be appreciated that there is absolutely no substance to the climate-alarmist claim that earth's polar regions provide evidence for an impending CO2-induced warming of any magnitude anywhere.  What these many studies of the temperature history of Greenland do depict is long-term oscillatory cooling ever since the Climatic Optimum of the mid-Holocene, when it was perhaps 2.5°C warmer than it is now, within which cooling trend is included the Medieval Warm Period, when it was about 1°C warmer than it is currently, and the Little Ice Age, when it was 0.5 to 0.7°C cooler than now, after which temperatures rebounded to a new maximum in the 1930s, only to fall steadily thereafter.

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