Review from CO2Science.org, last updated 16 July 2014:
Medieval Warm Period in Central Europe -- Summary
Was there really a global Medieval Warm Period? The IPCC used to acknowledge there was; but they have long since changed their view on the subject. Mounting evidence, however, suggests they were wrong to do so; and in this summary, new and important data from Central Europe that support their original belief are described and discussed.
Filippi et al. (1999) obtained stable isotope data (delta 18O and delta 13C) from bulk carbonate and ostracode calcite in a radiocarbon-dated sediment core removed from Lake Neuchatel in the western Swiss Lowlands at the foot of the Jura Mountains, which they used to reconstruct the climatic history of that region over the past 1500 years. And in doing so, they determined that mean annual air temperature dropped by about 1.5°C during the transition from the Medieval Warm Period (MWP) to the Little Ice Age (LIA). In addition, they state that "the warming during the 20th century does not seem to have fully compensated the cooling at the MWP-LIA transition" and that during the Medieval Warm Period, mean annual air temperatures were "on average higher than at present."
Working nearby in the Czech Republic, Bodri and Cermak (1999) derived individual ground surface temperature histories from the temperature-depth logs of 98 separate boreholes. This work revealed, in their words, "the existence of a medieval warm epoch lasting from 1100-1300 AD," which they describe as "one of the warmest postglacial times." They also noted that during the main phase of the Little Ice Age, from 1600-1700 AD, "all investigated territory was already subjected to massive cooling," and that "the observed recent warming may thus be easily a natural return of climate from the previous colder conditions back to a 'normal'."
After the passing of four additional years, Niggemann et al. (2003) saw the publication of the results of their study of petrographical and geochemical properties of three stalagmites found in the B7-Cave of Sauerland, Northwest Germany, from which they developed a climatic history that covered the prior 17,600 years. All three of these records, in their words, "resemble records from an Irish stalagmite (McDermott et al., 1999)," which has also been described by McDermott et al. (2001). With respect to their own records, for example, the four researchers explicitly note that they provide evidence for the existence of the Little Ice Age, the Medieval Warm Period and the Roman Warm Period, which also implies the existence of what McDermott et al. (2001) called the Dark Ages Cold Period that separated the Medieval and Roman Warm Periods, as well as the unnamed cold period that preceded the Roman Warm Period. And the wealth of corroborative information contained in these records (and many others) clearly suggests that there is nothing unusual, unprecedented or unexpected about the 20th-century warming that ushered in the Current Warm Period.
Moving ahead one additional year, Bartholy et al. (2004) noted, as background for their study of the issue at hand, that Antal Rethly (1879-1975) was a meteorologist, professor and director of the National Meteorological and Earth Magnetism Institute of Hungary, who spent the greater portion of his long professional life collecting over 14,000 historical records related to the climate of the Carpathian Basin and ultimately publishing a four-volume set of books about them in the Hungarian language that contain approximately 2500 pages (Rethly, 1962, 1970; Rethly and Simon, 1999). And building upon this immense foundation of pertinent materials, they meticulously codified and analyzed all of the historical records collected by Rethly, noting that "in order to provide regional climate scenarios for any particular area, past climate tendencies and climatological extremes must be analyzed." In this context, therefore, and with respect to temperature, the three Hungarian scientists reported that "the warm peaks of the Medieval Warm Epoch and colder climate of the Little Ice Age followed by the recovery warming period can be detected in the reconstructed temperature index time series." And so, once again, we find additional substantial evidence for the existence of the Medieval Warm Period, which many of the world's climate alarmists refuse to acknowledge as ever occurring.
One year later, in an important study of a precisely dated δ18O record with better than decadal resolution that they derived from a stalagmite recovered from Spannagel Cave in the Central Alps of Austria, Mangini et al. (2005) developed a highly-resolved record of temperature at high elevation (approximately 2500 meters above sea level) during the past 2000 years, based on a transfer function they derived from a comparison of their δ18O data with the reconstructed temperature history of post-1500 Europe that was developed by Luterbacher et al. (2004).
The lowest temperatures of the past two millennia, according to the new record, occurred during the Little Ice Age (AD 1400-1850), while the highest temperatures were found in the Medieval Warm Period (MWP: AD 800-1300). More specifically, Mangini et al. say that the highest temperatures of the MWP were "slightly higher than those of the top section of the stalagmite (1950 AD) and higher than the present-day temperature." In fact, at three different points during the MWP, their data indicate temperature spikes in excess of 1°C above present (1995-1998) temperatures.
Mangini et al. additionally reported that their temperature reconstruction compares well with reconstructions developed from Greenland ice cores (Muller and Gordon, 2000), Bermuda Rise ocean-bottom sediments (Keigwin, 1996), and glacier tongue advances and retreats in the Alps (Holzhauser, 1997; Wanner et al., 2000), as well as with the Northern Hemispheric temperature reconstruction of Moberg et al. (2005). Considered together, therefore, they say these several data sets "indicate that the MWP was a climatically distinct period in the Northern Hemisphere," emphasizing that "this conclusion is in strong contradiction to the temperature reconstruction by the IPCC, which only sees the last 100 years as a period of increased temperature during the last 2000 years."
In a second severe blow to IPCC dogma, Mangini et al. found "a high correlation between δ18O and δ14C, that reflects the amount of radiocarbon in the upper atmosphere," and they wrote that this correlation "suggests that solar variability was a major driver of climate in Central Europe during the past 2 millennia." In this regard, they further report that "the maxima of δ18O coincide with solar minima (Dalton, Maunder, Sporer, Wolf, as well as with minima at around AD 700, 500 and 300)," and that "the coldest period between 1688 and 1698 coincided with the Maunder Minimum." Also, in a linear-model analysis of the percent of variance of their full temperature reconstruction that is individually explained by solar and CO2 forcing, they found that the impact of the sun was fully 279 times greater than that of the air's CO2 concentration, noting that "the flat evolution of CO2 during the first 19 centuries yields almost vanishing correlation coefficients with the temperature reconstructions."
Clearly, the IPCC-endorsed hockeystick temperature history of Mann et al. (1998, 1999) does not reflect the true thermal history of the Northern Hemisphere over the past thousand years, nor does the hockeystick temperature history of Mann and Jones (2003) reflect the true thermal history of the world over the past two millennia. In addition, both sets of studies, as well as the IPCC itself, appear to be focusing on the wrong instigator of climate change over these periods, i.e., CO2 in lieu of solar activity.
Appearing in the same year as Mangini et al.'s paper was the paper of Büntgen et al. (2005), who - using the regional curve standardization technique applied to ring-width measurements from both living trees and relict wood - developed a 1052-year summer (June-August) temperature proxy from high-elevation Alpine environments in Switzerland and the western Austrian Alps (between 46°28' to 47°00'N and 7°49' to 11°30'E). This temperature history revealed the presence of warm conditions from the beginning of the record in AD 951 up to about AD 1350, which the five researchers associated with the Medieval Warm Period. Thereafter, temperatures declined and an extended cold period (the Little Ice Age) ensued, which persisted until approximately 1850 ... with one brief exception for a few short decades in the mid- to late-1500s, when there was an unusually warm period, the temperatures of which were only exceeded at the beginning and end of the 1052-year record, i.e., during the Medieval and Current Warm Periods.
Concomitantly, Holzhauser et al. (2005) "for the first time," in their words, presented high-resolution records of variations in glacier size in the Swiss Alps together with lake-level fluctuations in the Jura mountains, the northern French Pre-Alps and the Swiss Plateau in developing a 3500-year climate history of west-central Europe, beginning with an in-depth analysis of the Great Aletsch glacier, which is the largest of all glaciers located in the European Alps.
Near the beginning of the time period studied, the three researchers reported that "during the late Bronze Age Optimum from 1350 to 1250 BC, the Great Aletsch glacier was approximately 1000 m shorter than it is today," noting that "the period from 1450 to 1250 BC has been recognized as a warm-dry phase in other Alpine and Northern Hemisphere proxies (Tinner et al., 2003)." Then, after an intervening unnamed cold-wet phase, when the glacier grew in both mass and length, they say that "during the Iron/Roman Age Optimum between c. 200 BC and AD 50," which is perhaps better known as the Roman Warm Period, the glacier again retreated and "reached today's extent or was even somewhat shorter than today." Next came the Dark Ages Cold Period, which they say was followed by "the Medieval Warm Period, from around AD 800 to the onset of the Little Ice Age around AD 1300," which latter cold-wet phase was "characterized by three successive [glacier length] peaks: a first maximum after 1369 (in the late 1370s), a second between 1670 and 1680, and a third at 1859/60," following which the glacier began its latest and still-ongoing recession in 1865. In addition, they say that written documents from the fifteenth century AD indicate that at some time during that hundred-year interval "the glacier was of a size similar to that of the 1930s," which latter period in many parts of the world was as warm as, or even warmer than, it is today, in harmony with a growing body of evidence which suggests that a "Little" Medieval Warm Period manifested itself during the fifteenth century within the broader expanse of the Little Ice Age.
Data pertaining to the Gorner glacier (the second largest of the Swiss Alps) and the Lower Grindelwald glacier of the Bernese Alps tell much the same story, as Holzhauser et al. report that these glaciers and the Great Aletsch glacier "experienced nearly synchronous advances" throughout the study period.
With respect to what was responsible for the millennial-scale climatic oscillation that produced the alternating periods of cold-wet and warm-dry conditions that fostered the similarly-paced cycle of glacier growth and retreat, the Swiss and French scientists report that "glacier maximums coincided with radiocarbon peaks, i.e., periods of weaker solar activity," which in their estimation "suggests a possible solar origin of the climate oscillations punctuating the last 3500 years in west-central Europe, in agreement with previous studies (Denton and Karlen, 1973; Magny, 1993; van Geel et al., 1996; Bond et al., 2001)." And to underscore that point, they concluded their paper by stating that "a comparison between the fluctuations of the Great Aletsch glacier and the variations in the atmospheric residual 14C records supports the hypothesis that variations in solar activity were a major forcing factor of climate oscillations in west-central Europe during the late Holocene."
And because the current warmth of the study region has not yet resulted in a shrinkage of the Great Aletsch glacier equivalent to what it experienced during the Bronze Age Optimum of a little over three thousand years ago, or what it experienced during the Roman Warm Period of two thousand years ago, there is nothing unusual or "unprecedented," as climate alarmists often claim, about the region's current warmth. In addition, our modern warmth is occurring at just about the time one would expect it to occur, in light of the rather consistent time intervals that have separated prior warm nodes of the millennial-scale climatic oscillation that produced them, which further suggests that our current warmth, like that of prior Holocene warm periods, is likely solar-induced, which pretty much leaves CO2 "out in the cold," as far as being responsible for twentieth-century global warming is concerned.
Also with a paper published in the same year were Chapron et al. (2005), who - while noting that "millennial-scale Holocene climate fluctuations have been documented by lake level fluctuations, archaeological and palynological records for many small lakes in the Jura Mountains and several larger peri-alpine lakes" - sought to learn more about the pervasive climatic oscillation behind this phenomenon by documenting the Holocene evolution of Rhone River clastic sediment supply in Lake Le Bourget via sub-bottom seismic profiling and multidisciplinary analysis of well-dated sediment cores. And this work revealed, as they describe it, that "up to five 'Little Ice Age-like' Holocene cold periods developing enhanced Rhone River flooding activity in Lake Le Bourget [were] documented at c. 7200, 5200, 2800, 1600 and 200 cal. yr BP," and that "these abrupt climate changes were associated in the NW Alps with Mont Blanc glacier advances, enhanced glaciofluvial regimes and high lake levels." They also noted that "correlations with European lake level fluctuations and winter precipitation regimes inferred from glacier fluctuations in western Norway suggest that these five Holocene cooling events at 45°N were associated with enhanced westerlies, possibly resulting from a persistent negative mode of the North Atlantic Oscillation."
Situated between these Little Ice Age-like periods would have been Current Warm Period-like conditions. The most recent of these prior warm regimes (the Medieval Warm Period) would thus have been centered somewhere in the vicinity of AD 1100, while the next one back in time (the Roman Warm Period) would have been centered somewhere in the vicinity of 200 BC, which matches well with what is known about these warm regimes from many other studies. In addition, since something other than an increase in the atmosphere's CO2 concentration was obviously responsible for the establishment of these prior Current Warm Period-like regimes, it is reasonable to assume that another increase in that same "something" - and not the coincidental rise in the air's CO2 content - was likely responsible for ushering in the Current Warm Period.
One year later, Robert et al. (2006) analyzed assemblages of minerals and microfossils from a sediment core taken from the Berre coastal lagoon in southeast France (~ 43.44°N, 5.10°E) in an effort to reconstruct environmental changes in that region over the past 1500 years. The results of their analyses revealed three distinct climatic intervals: (1) a cold period that extended from about AD 400 to 900, (2) a warm interval between about AD 980 and 1370, and (3) a cold interval that peaked during the 16th and 17th centuries, which climatic intervals correspond, respectively, to the well-known Dark Ages Cold Period, Medieval Warm Period (MWP) and Little Ice Age.
Most significantly, the team of eight researchers also found evidence of a higher kaolinite content in the sediment core during the MWP, which suggests, in their words, "increased chemical weathering in relation to higher temperatures and/or precipitation." In addition, they discovered that the concentration of microfossils of the thermophilic taxon Spiniferites bentorii also peaked during the same time interval; and this finding provides additional evidence that the temperatures of that period were likely higher than those of the recent past.
About this same time, Joerin et al. (2006) wrote that "the exceptional trend of warming during the twentieth century in relation to the last 1000 years highlights the importance of assessing natural variability of climate change." Why? Because it is essential to be able to determine, by comparison, if there is anything unusual, unnatural, or unprecedented about the past century's increase in temperature, which is the way in which the world's climate alarmists typically describe 20th-century global warming. Thus, in their quest to accomplish this objective, the three Swiss researchers examined glacier recessions in the Swiss Alps over the past ten thousand years based on radiocarbon-derived ages of materials found in proglacial fluvial sediments of subglacial origin, focusing on subfossil remains of wood and peat. And combining their results with earlier data of a similar nature, they were able to construct a master chronology of Swiss glacier fluctuations over the course of the Holocene. So what did they find?
First of all, Joerin et al. reported discovering that "alpine glacier recessions occurred at least 12 times during the Holocene," once again demonstrating the reality of the millennial-scale oscillation of climate that has reverberated throughout glacial and interglacial periods alike as far back in time as scientists have searched for the phenomenon. And as a result of this finding, it is clear that 20th-century global warming was not unusual. It was merely the latest example of what has been the normthroughout hundreds of thousands of years.
Second, they determined that glacier recessions have been decreasing in frequency since approximately 7000 years ago, and especially since 3200 years ago, "culminating in the maximum glacier extent of the 'Little Ice Age'." Consequently, the significant warming of the 20th century cannot be considered strange, since it represents a climatic rebounding from thecoldest period of the current interglacial, which interglacial just happens to be the coldest of the last five interglacials, according to Petit et al. (1999). And when the earth has been that cold for a few centuries, it is not unnatural to expect that, once started, the scheduled warming would be quite significant.
Third, the last of the major glacier recessions in the Swiss Alps occurred between about 1400 and 1200 years ago, according to Joerin et al.'s data; but it took place between 1200 and 800 years ago, according to the data of Holzhauser et al. (2005) for the Great Aletsch Glacier. Of this discrepancy, Joerin et al. say that given the uncertainty of the radiocarbon dates, the two records need not be considered inconsistent with each other. What is more, their presentation of the Great Aletsch Glacier data indicates that the glacier's length at about AD 1000 - when there was fully 100 ppm less CO2 in the air than there is today - was just slightly less than its length in 2002, suggesting that the peak temperature of the Medieval Warm Period likely was slightly higher than the peak temperature of the 20th century. And, consequently, 20th-century warming has likely not been unprecedented over the past millennium; and there is thus no compelling reason to attribute it to anthropogenic CO2 emissions, for the temperature increase of the past hundred or so years has been simply a run-of-the-mill consequence of cyclically-recurring forces of nature that have manifested themselves again and again and again throughout earth's history at millennial-scale intervals.
Contemporaneously, Buntgen et al. (2006) developed an annually-resolved mean summer (June-September) temperature record for the European Alps, covering the period AD 755-2004 and based on 180 recent and historic larch (Larix deciduaMill.) maximum latewood density series, which were created via the regional curve standardization method that preserves interannual to multi-centennial temperature-related variations. Among a number of other things, notable features of this history were the high temperatures of the late tenth, early thirteenth, and twentieth centuries and the prolonged cooling from ~1350 to 1700, or as they described it: "warmth during medieval and recent times, and cold in between." Also of great interest, they reported that the coldest decade of the record was the 1810s, and that even though the record extended all the way through 2004, the warmest decade of the record was the 1940s. In addition, they observed that "warm summers seemed to coincide with periods of high solar activity, and cold summers vice versa." And, finally, they report that comparing their newest temperature record with other regional- and large-scale reconstructions "reveals similar decadal to longer-term variability," causing them to conclude - in the final sentence of their paper - that based upon their findings and the similar findings many others, "the twentieth-century contribution of anthropogenic greenhouse gases and aerosol remains insecure."
Hearkening back a year or so, and extending the work of Mangini et al. (2005), who had developed a 2000-year temperature history of the central European Alps based on an analysis of δ18O data obtained from stalagmite SPA 12 of Austria's Spannagel Cave, Vollweiler et al. (2006) used similarly-measured δ18O data obtained from two adjacent stalagmites (SPA 128 and SPA 70) within the same cave to create a master δ18O history covering the last 9000 years, which Mangini et al. (2007) compared with the Hematite-Stained-Grain (HSG) history of ice-rafted debris in North Atlantic Ocean sediments developed by Bond et al. (2001), who had reported that "over the last 12,000 years virtually every centennial time-scale increase in drift ice documented in our North Atlantic records was tied to a solar minimum."
In pursuing this course of action, Mangini et al. found an incredibly good correspondence between the peaks and valleys of their δ18O curve and the HSG curve of Bond et al., concluding that (1) "the excellent match between the curves obtained from these two independent data sets gives evidence that the δ18O signal recorded in Spannagel cave reflects the intensity of the warm North Atlantic drift, disproving the assumption that the Spannagel isotope record is merely a local phenomenon," and, therefore, that (2) their δ18O curve "can reasonably be assumed to reflect non-local conditions," implying it has wide regional applicability.
Having established this important point, Mangini et al. next focused on why their δ18O curve "displays larger variations for the last 2000 years than the multi-proxy record in Europe, which is mainly derived from tree-ring data" and "from low resolution archives (Mann et al., 1998, 1999; Mann and Jones, 2003)." The most probable answer, in their words, "is that tree-rings rather record the climate conditions during spring and summer," whereas both the HSG and δ18O curves "mirror winter-like conditions, which are only poorly recorded in tree-rings."
One important consequence of these differences is that whereas the Mann et al. and Mann and Jones data sets do not reflect the existence of the Medieval Warm Period and Little Ice Age, the Spannagel Cave data do. And applying the calibration curve derived for SPA 12 by Manginni et al. (2005) to the new δ18O curve, it can readily be determined that the peak temperature of the Medieval Warm Period was approximately 1.5°C higher than the peak temperature of the Current Warm Period. In addition, the new data set of Manginni et al. (2007) confirms the inference of Bond et al.'s finding that over the last 12,000 years virtually every centennial-scale cooling of the North Atlantic region "was tied to a solar minimum," demonstrating that the data sets of Mann et al. and Mann and Jones fail to capture the full range of temperature variability over the past two millennia. And as a result, the new data set clearly depicts the existence of both the Little Ice Age and Medieval Warm Period, the latter of which is seen to have been substantially warmer over periods of centuries than the warmest parts of the 20th century, almost certainly as a result of enhanced solar activity, and in spite of the fact that the air's CO2 concentration during the Medieval Warm Period was more than 100 ppm less than it is today. And so there is every reason to believe that the global warming of the past century was neither unprecedented nor CO2-induced. Rather, it appears to have been nothing special and solar-induced.
Hard at work in the same year, Schmidt et al. (2007) combined spring and autumn temperature anomaly reconstructions based on siliceous algae and pollen tracers found in a sediment core extracted from an Alpine lake (Oberer Landschitzsee; 47°14'52" N, 13°51'40" E) located at the southern slopes of the Austrian Central Alps just slightly above the present tree-line, with the goal of developing a 4000-year climatic reconstruction that they subsequently compared with (1) a similar time-scale reconstruction from another lake in the drainage area, (2) local historical records, and (3) other climate proxies on Alpine and Northern Hemispheric scales. And in pursuing their goal, they found that "spring-temperature anomalies during Roman and Medieval times equaled or slightly exceeded the modern values and paralleled tree-line and glacier fluctuations," indicative of their broad range of applicability. As for the timing of the Medieval Warm Period, they identified "warm phases similar to present between ca. 850-1000 AD and 1200-1300 AD," which they say were "followed by climate deterioration at ca 1300 AD, which culminated during the Little Ice Age." Hence, their data placed the possibly-slightly-warmer-than-present Medieval Warm Period as occurring between AD 850 and 1300.
One year later, Schmidt et al. (2008) analyzed sediment grain size, as well as the concentrations of major and trace elements and minerals found in a sediment core recovered from an Austrian alpine lake, Oberer Landschitzsee (47°14'52" N, 13°51'40" E), which covered the past 4,000 years, together with autumn and spring temperature anomalies and ice-cover estimated from selected pollen markers and a diatom and chrysophyte cyst thermistor-based regional calibration dataset, in order to recreate the surrounding region's late-Holocene climate and land-use history. This work identified the Roman Warm Period (300 BC to AD 400) and the Medieval Warm Period (AD 1000 to AD 1600), as well as the fact that "spring temperature anomalies during Roman and Medieval times equaled or slightly exceeded the modern values." Also of significance was their detection of two other warm periods - 1800 to 1300 BC and 1000 to 500 BC - as well as the cooler periods that were sandwiched between them, including the Little Ice Age that occurred between the Medieval Warm Period and the Current Warm Period. In addition, they were able to ascertain that "four waves of alpine land use were coupled mainly with warm periods."
Based on these findings and those of many others, it is clear that there is a well-established millennial-scale oscillation of climate that has reverberated throughout glacial and interglacial times alike, which has alternately brought the planet relatively warmer and colder climatic conditions, independent of any changes in the air's CO2 content; and the results of this study bear further witness to this fact. They also indicate that in the vicinity of Oberer Landschitzsee, the two warm periods that preceded the Current Warm Period (when the atmosphere's CO2 concentration was fully 100 ppm less than it is today) were at least as warm as - or even slightly warmer than - it is at present. Hence, it is likely that earth's current run-of-the-millwarmth is totally unrelated to its much higher atmospheric CO2 concentration and is instead but a manifestation of this natural climatic cycle.
Inching another year closer to the present, Millet et al. (2009) wrote that "among biological proxies from lake sediments, chironomid [non-biting midge] assemblages are viewed as one of the most promising climatic indicators," and that "the accuracy of chironomid assemblages for the reconstruction of Lateglacial temperatures is now broadly demonstrated." Thus, they developed a new chironomid-based temperature record from Lake Anterne (northern French Alps) that covered the past two millennia, compared that reconstruction with other late-Holocene temperature records from Central Europe, and addressed the question of whether previously described centennial-scale climate events such as the Medieval Warm Period or the Little Ice Age can be detected in this new summer temperature record, noting that "at a hemispheric or global scale the existence of the LIA and MWP have been questioned."
The six scientists reported that evidence was indeed found "of a cold phase at Lake Anterne between AD 400 and 680, a warm episode between AD 680 and 1350, and another cold phase between AD 1350 and 1900," and they said that these events were "correlated to the so-called 'Dark Age Cold Period' (DACP), the 'Medieval Warm Period' and the 'Little Ice Age'." In addition, they noted that "many other climate reconstructions across western Europe confirm the existence of several significant climatic changes during the last 1800 years in Central Europe and more specifically the DACP, the MWP and the LIA." Last of all, they reported that the reconstructed temperatures of the 20th century failed to show a return to MWP levels of warmth, which failure, however, they attributed to a breakdown of the chironomid-temperature relationship over the final century of their 1800-year history.
One year later, Gasiorowski and Sienkiewicz (2010) inferred the thermal conditions of Smreczynski Staw Lake (49°12'N, 19°51'E) in the Tatra Mountains of southern Poland via analyses of the distributions of various cladocera, chironomid and diatom species they identified and quantified in a sediment core they had extracted from the center of the lake in the spring of 2003, which contained sediments that had accumulated there over the prior 1500 years. This work revealed the presence of "a diverse ecosystem at the beginning of [the] record, ca. AD 360-570," which period of time has typically been assigned to the Dark Ages Cold Period. Thereafter, however, they found that from AD 570 to 1220 "environmental conditions were better," and that various cold-water taxa were "totally absent." And they write that the younger section of this zone - approximately its upper third (AD 850-1150), which contained the highest concentration of warm-waterChironomus species - "can be correlated with the Medieval Warm Period."
Next came the Little Ice Age, which was the focal point of their study, extending all the way to the start of the 20th century, after which relative warmth once again returned, persisting to the present. And based on the Chironomusconcentrations of this portion of their record (the Current Warm Period or CWP), their data suggested that the peak warmth of the CWP and the earlier MWP were about the same. Once again, therefore, we have another paleoclimate record that displays the millennial-scale oscillation of climate that reverberates throughout the Holocene and about as far back in time as researchers have looked for it. And once again we have another demonstration of the fact that the peak warmth of the late 20th-century and the early 21st-century has not been as unprecedented as the world's climate alarmists have typically claimed it to be.
Near simultaneously, Larocque-Tobler et al. (2010) wrote that to better describe the amplitude of temperature change during the last millennium, "new records to increase the geographic coverage of paleoclimatic information are needed," and that "only by obtaining numerous high-resolution temperature records will it be possible to determine if the 20th century climate change exceeded the natural pre-industrial variability of European climate." Thus, to help achieve this important goal, they proceeded to obtain another such temperature record spanning the last millennium via an analysis of fossil chironomids (non-biting midges), which they identified and quantified in four sediment cores extracted from the bed of Lake Silvaplana (46°26'56"N, 9°47'33"E) in the Upper Engadine (a high-elevation valley in the eastern Swiss Alps).
This work revealed, as they described it, that "at the beginning of the record, corresponding to the last part of the 'Medieval Climate Anomaly' (here the period between ca. AD 1032 and 1262), the chironomid-inferred mean July air temperatures were 1°C warmer than the climate reference period (1961-1990)," which would also make them warmer than most subsequent temperatures as well. And in looking at their graphs of 20- and 50-year running means, it can be seen that the peak mean warmth of the Medieval Warm Period exceeded that of the Current Warm Period by about 0.5°C in the case of 20-year averages and 1.2°C in the case of 50-year averages. And thus it was that the five researchers concluded that "based on the chironomid-inferred temperatures, there is no evidence that mean-July air temperature exceeded the natural variability recorded during the Medieval Climate Anomaly in the 20th century at Lake Silvaplana," while noting that similar results "were also obtained in northern Sweden (Grudd, 2008), in Western Europe (Guiot et al., 2005), in a composite of Northern Hemisphere tree-ring reconstructions (Esper et al., 2002) and a composite of tree rings and other archives (Moberg et al., 2005)."
Moving ahead one more year, Magny et al. (2011) wrote that "present-day global warming has provoked an increasing interest in the reconstruction of climate changes over the last millennium (Guiot et al., 2005; Jones et al., 2009)," which time interval, as they describe it, is "characterized by a succession of distinct climatic phases, i.e. a Medieval Warm Period (MWP) followed by a long cooler Little Ice Age (LIA) and finally by a post-industrial rapid increase in temperature," which is generally referred to as the initial phase of the Current Warm Period (CWP). And in a study designed to compare the temperatures of these two periods, the six scientists, working at Lake Joux (46°36'N, 6°15'E) at an altitude of 1006 meters above sea level (a.s.l.) in the Swiss Jura Mountains, employed a multi-proxy approach with pollen and lake-level data to develop a 1000-year history of the mean temperature of the warmest month of the year (MTWA, which was July at Lake Joux), based on the Modern Analogue Technique, which procedure is described by them as "a commonly used and accepted method for the reconstruction of Lateglacial and Holocene climate oscillations from continental and marine sequences," citing the confirming works of Guiot et al. (1993), Cheddadi et al., (1997), Davis et al. (2003), Peyron et al. (2005), Kotthoff et al. (2008) and Pross et al. (2009).
At the conclusion of their analyses, Magny et al. wrote that their data "give evidence of the successive climate periods generally recognized within the last 1000 years," which they described as "a MWP between ca. AD 1100 and 1320, (2) a LIA which, in the Joux Valley, initiated as early as ca. AD 1350 and ended at ca. AD 1870, and (3) a last warmer and drier period," which is generally referred to as the beginning of the Current Warm Period (CWP).
"Considering the question of present-day global warming on a regional scale," in the words of Magny et al., "the increase in MTWA by ca. 1.6°C observed at Laoura (1100 m a.s.l., near the Joux basin) for the period 1991-2008, when compared to the reference period 1961-1990, still appears to be in the range of the positive temperature anomaly reconstructed at Lake Joux ca. AD 1300 during the late MWP." And they note that "meteorological data observed at La Brevine (1043 m a.s.l., also near the Joux basin) suggest a similar pattern with an increase in MTWA by 1°C over the period 1991-2008" relative to 1961-1990. Yet both of these late-20th/early-21st century temperature increases fall significantly short of that reached during the MWP, when the temperature at Joux Lake exceeded that of the 1961-1990 reference period by fully 2.0°C.
In light of these findings, it would thus appear that the peak warmth of the MWP at Lake Joux exceeded that of the CWP at that location by something on the order of 0.4-1.0°C, in harmony with similar findings obtained at numerous other locations around the world. And these observations clearly indicate that temperatures even warmer than those of the present can occur with much less CO2 in the air than there is today, suggesting that there is no compelling reason to attribute earth's current level of warmth to the atmosphere's current higher CO2 concentration.
Appearing in print during the same year as Magny et al.'s paper was the report of Moschen et al. (2011), who wrote that "currently, there is specific interest in climate change during our historical past and in the human impact on past and future climate and ecosystem dynamics," and they said that "in this context, the reconstruction of decadal to centennial scale natural climate variability is of importance to estimate to what extent human activities contribute to the recent warming trend observable at a regional and global scale." And so they went on to present "a high resolution reconstruction of local growing season temperature anomalies at Durres Maar, Germany [50°52'N, 6°53'E], spanning the last two millennia," which was "derived from a stable carbon isotope time series of cellulose chemically extracted from Sphagnum leaves (δ13Ccellulose) separated from a kettle-hole peat deposit of several meters thickness," where the temperature reconstruction was based on the temperature dependency of Sphagnum δ13CCcellulose observed in calibration studies.
Lasting from the 4th to the 7th century AD, the five researchers identified a cold phase with below-average temperature, "in accordance with the so-called European Migration Period," which has also come to be known as the Dark Ages Cold Period. Thereafter, they state that "during High Medieval Times above-average temperatures are obvious." In fact, the peak warmth of this Medieval Warm Period, which looks from the graph of their data to run from about AD 830 to AD 1150, was approximately 2.8°C greater than the peak warmth of the Current Warm Period in terms of individual anomaly points, while it was approximately 2.7°C greater in terms of 60-year running means. And between these two warm periods, the Little Ice Age could be seen to hold sway.
In terms of Moschen et al.'s stated purpose of hoping to illustrate "to what extent human activities contribute to the recent warming trend observable at a regional and global scale," based on what types of natural climate changes have occurred over the past two millennia, it would have to be concluded that human activities have contributed absolutely nothing in the way of warming, as it was much warmer at Durres Maar, Germany, back in the good old (High Medieval) days, when there was far less CO2 in the air than there is today.
Expanding upon the work of some of their group two years earlier (Larocque-Tobler et al., 2010), Larocque-Tobler et al. (2012) note that "the climate of the last millennium is still controversial because too few high-resolution paleo-climate reconstructions exist to answer two key research questions," namely, (1) "Were the 'Medieval Climate Anomaly' (MCA) and the 'Little Ice Age' (LIA) of similar spatial extent and timing in Europe and in the Northern Hemisphere?" and (2) "Does the amplitude of climate change of the last century exceed the natural variability?"
Working with a lake sediment core extracted from the deepest point of Seebergsee (46°37'N, 7°28'E) in the northern Swiss Alps in AD 2005, the second Larocque-Tobler team employed chironomid head capsules preserved in the sediments to reconstruct mean July air temperatures for the past 1000 years, after which they compared their results to those of Larocque-Tobler et al. (2010) for another Swiss lake (Silvaplana in the eastern Alps), then to regional and European records of early instrumental data (Luterbacher et al., 2004; Auer et al., 2007; Bohm et al., 2010), as well as a composite of paleoclimate reconstructions from the Greater Alpine Region and to millennial scale climate reconstructions of the entire Northern Hemisphere (Mangini et al., 2005; Moberg et al., 2005; Osborn and Briffa, 2006), in order to address the two research questions that inspired their study and "to improve understanding of the climatic variability of the last millennium."
The six scientists' work revealed that the peak warmth of the MCA just prior to AD 1200 was approximately 0.9°C greaterthan the peak warmth near the end of their record, as best as can be determined from the graph of their data. And, therefore, as more and more palaeo-temperature data are acquired, the IPCC-endorsed "hockeystick" temperature record of Mann et al. (1999) - which gives little indication of the existence of the MCA and shows recent temperatures towering over those of that earlier time period - continues to fade slowly into oblivion, as it is repudiated by ever more real-world data. And it's not just the most recent data of Larocque-Tobler et al. that refute the IPCC's view of this matter; for the group of six says that their newest temperature history is "mirrored by the chironomid reconstruction from Silvaplana and the Greater Alpine Region composite of reconstructions." And they add that "several other reconstructions from the Northern Hemisphere also show [recent] warm inferred temperatures that were not as warm as the MCA."
Last of all comes the study of Niemann et al. (2012), who introduced the report of their most recent work, as so many before them had done, by noting that "the assessment of climate variations in Earth's history is of paramount importance for our comprehension of recent and future climate variability." And they stated that for this important purpose "geological archives containing climate-sensitive proxy indicators are used to reconstruct paleoclimate."
In taking this approach to the problem, Niemann et al. employed what they described as "a novel proxy for continentalmean annual air temperature (MAAT) and soil pH" that is "based on the temperature (T) and pH-dependent distribution of specific bacterial membrane lipids (branched glycerol dialkyl glycerol tetraethers - GDGTs) in soil organic matter," which technique derives from the fact that "microorganisms can modify the composition of their cellular membrane lipids to adapt membrane functionality to specific environmental parameters such as T and pH," as described by Hazel and Williams (1990) and Weijers et al. (2007), the latter of whom devised "transfer functions that relate the degree of the GDGT methylation (expressed in the Methylation index - MBT) and cyclisation (expressed in the cyclisation ratio - CBT) to mean annual air temperature." And this they did, using sediment cores that were collected in September 2009 and May 2010 from a small alpine lake (Cadagno) in the Piora Valley of south-central Switzerland, as well as soil samples taken from the surrounding catchment area.
As a result of these efforts, the nine Dutch and Swiss researchers reported that "major climate anomalies recorded by the MBT/CBT-paleothermometer" were "the Little Ice Age (~14th to 19th century) and the Medieval Warm Period (MWP, ~9th to 14th century)," which they say experienced "temperatures similar to the present-day values." And "in addition to the MWP," they state that their "lacustrine paleo T record indicates Holocene warm phases at about 3, 5, 7 and 11 kyr before present, which agrees in timing with other records from both the Alps and the sub-polar North-East Atlantic Ocean."
Thus it was that Niemann et al.'s study once again indicated that there is nothing unusual, unnatural or unprecedented about earth's current climate. And their findings of still other - and sometimes even warmer - such periods, going even further back in time, when the atmosphere's CO2 content was way less than it is currently, almost mandates that there is no rational reason or need to invoke anthropogenic-induced increases in the air's CO2 concentration as the cause of the planet's current level of non-unique run-of-the-mill warmth. The burning of fossil fuels has had nothing to do with it.
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Last updated 16 July 2014