Past 1,000 years at the Poles
From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
Hemer and Harris (2003) extracted a sediment core from beneath the Amery Ice Shelf, East Antarctica, at a point that is currently about 80 km landward of the location of its present edge. In analyzing the core’s characteristics over the past 5,700 14C years, the two scientists observed a peak in absolute diatom abundance in general, and the abundance of Fragilariopsis curta in particular—which parameters, in their words, “are associated with increased proximity to an area of primary production, such as the sea-ice zone”—at about 750 14C yr B.P., which puts the time of maximum Ice Shelf retreat in close proximity to the historical time frame of the Medieval Warm Period.
Khim et al. (2002) likewise analyzed a sediment core removed from the eastern Bransfield Basin just off the northern tip of the Antarctic Peninsula, including grain size, total organic carbon content, magnetic susceptibility, biogenic silica content, 210Pb geochronology, and radiocarbon (14C) age, all of which data clearly depicted, in their words, the presence of the “Little Ice Age and Medieval Warm period, together with preceding climatic events of similar intensity and duration.”
Hall and Denton (2002) mapped the distribution and elevation of surficial deposits along the southern Scott Coast of Antarctica in the vicinity of the Wilson Piedmont Glacier, which runs parallel to the coast of the western Ross Sea from McMurdo Sound north to Granite Harbor. The chronology of the raised beaches they studied was determined from more than 60 14C dates of incorporated organic materials they had previously collected from hand-dug excavations (Hall and Denton, 1999); the record the dates helped define demonstrated that near the end of the Medieval Warm Period, “as late as 890 14C yr BP,” as Hall and Denton describe it, “the Wilson Piedmont Glacier was still less extensive than it is now,” demonstrating that the climate of that period was in all likelihood considerably warmer than it is currently.
Noon et al. (2003) used oxygen isotopes preserved in authigenic carbonate retrieved from freshwater sediments of Sombre Lake on Signy Island (60°43’S, 45°38’W) in the Southern Ocean to construct a 7,000-year history of that region’s climate. This work revealed that the general trend of temperature at the study site has been downward. Of most interest to us, however, is the millennial-scale oscillation of climate that is apparent in much of the record. This climate cycle is such that approximately 2,000 years ago, after a thousand-year gap in the data, Signy Island experienced the relative warmth of the last vestiges of the Roman Warm Period, as delineated by McDermott et al. (2001) on the basis of a high-resolution speleothem δ18O record from southwest Ireland. Then comes the Dark Ages Cold period, which is also contemporaneous with what McDermott et al. observe in the Northern Hemisphere, after which the Medieval Warm Period appears at the same point in time and persists for the same length of time that it does in the vicinity of Ireland, whereupon the Little Ice Age sets in just as it does in the Northern Hemisphere. Finally, there is an indication of late twentieth century warming, but with still a long way to go before conditions comparable to those of the Medieval Warm Period are achieved.
Two years later, Castellano et al. (2005) derived a detailed history of Holocene volcanism from the sulfate record of the first 360 meters of the Dome Concordia ice core that covered the period 0-11.5 kyr BP, after which they compared their results for the past millennium with similar results obtained from eight other Antarctic ice cores. Before doing so, however, they normalized the results at each site by dividing its several volcanic-induced sulfate deposition values by the value produced at that site by the AD 1816 Tambora eruption, in order to reduce deposition differences among sites that might have been induced by differences in local site characteristics. This work revealed that most volcanic events in the early last millennium (AD 1000-1500) exhibited greater among-site variability in normalized sulphate deposition than was observed thereafter.
Citing Budner and Cole-Dai (2003) in noting that “the Antarctic polar vortex is involved in the distribution of stratospheric volcanic aerosols over the continent,” Castellano et al. say that assuming the intensity and persistence of the polar vortex in both the troposphere and stratosphere “affect the penetration of air masses to inland Antarctica, isolating the continental area during cold periods and facilitating the advection of peripheral air masses during warm periods (Krinner and Genthon, 1998), we support the hypothesis that the pattern of volcanic deposition intensity and geographical variability [higher values at coastal sites] could reflect a warmer climate of Antarctica in the early last millennium,” and that “the re-establishment of colder conditions, starting in about AD 1500, reduced the variability of volcanic depositions.”
Describing this phenomenon in terms of what it implies, Castellano et al. say “this warm/cold step could be like a Medieval Climate Optimum-like to Little Ice Age-like transition.” They additionally cite Goosse et al. (2004) as reporting evidence from Antarctic ice-core δD and δ18O data “in support of a Medieval Warming-like period in the Southern Hemisphere, delayed by about 150 years with respect to Northern Hemisphere Medieval Warming.” The researchers conclude by postulating that “changes in the extent and intra-Antarctic variability of volcanic depositional fluxes may have been consequences of the establishment of a Medieval Warming-like period that lasted until about AD 1500.”
A year later, Hall et al. (2006) collected skin and hair (and even some whole-body mummified remains) from Holocene raised-beach excavations at various locations along Antarctica’s Victoria Land Coast, which they identified by both visual inspection and DNA analysis as coming from southern elephant seals, and which they analyzed for age by radiocarbon dating. By these means they obtained data from 14 different locations within their study region—which they describe as being “well south” of the seals’ current “core sub-Antarctic breeding and molting grounds”—that indicate that the period of time they denominate the Seal Optimum began about 600 BC and ended about AD1400, the latter of which dates they describe as being “broadly contemporaneous with the onset of Little Ice Age climatic conditions in the Northern Hemisphere and with glacier advance near [Victoria Land’s] Terra Nova Bay.”
In describing the significance of their findings, the US, British, and Italian researchers say they are indicative of “warmer-than-present climate conditions” at the times and locations of the identified presence of the southern elephant seal, and that “if, as proposed in the literature, the [Ross] ice shelf survived this period, it would have been exposed to environments substantially warmer than present,” which would have included both the Roman Warm Period and Medieval Warm Period.
More recently, Williams et al. (2007) presented methyl chloride (CH3Cl) measurements of air extracted from a 300-m ice core that was obtained at the South Pole, Antarctica, covering the time period 160 BC to AD 1860. In describing what they found, the researchers say “CH3Cl levels were elevated from 900-1300 AD by about 50 ppt relative to the previous 1000 years, coincident with the warm Medieval Climate Anomaly (MCA),” and that they “decreased to a minimum during the Little Ice Age cooling (1650-1800 AD), before rising again to the modern atmospheric level of 550 ppt.” Noting that “today, more than 90% of the CH3Cl sources and the majority of CH3Cl sinks lie between 30°N and 30°S (Khalil and Rasmussen, 1999; Yoshida et al., 2004),” they say “it is likely that climate-controlled variability in CH3Cl reflects changes in tropical and subtropical conditions.” They go on to say that “ice core CH3Cl variability over the last two millennia suggests a positive relationship between atmospheric CH3Cl and global [our italics] mean temperature.”
As best we can determine from the graphical representation of their data, the peak CH3Cl concentration measured by Williams et al. during the MCA is approximately 533 ppt, which is within 3 percent of its current mean value of 550 ppt and well within the range of 520 to 580 ppt that characterizes methyl chloride’s current variability. Hence, we may validly conclude that the mean peak temperature of the MCA (which we refer to as the Medieval Warm Period) over the latitude range 30°N to 30°S—and possibly over the entire globe—may not have been materially different from the mean peak temperature so far attained during the Current Warm Period.
This conclusion, along with the findings of the other studies we have reviewed of the climate of Antarctica, suggests there is nothing unusual, unnatural, or unprecedented about the current level of earth’s warmth.
Dahl-Jensen et al. (1998) used temperature measurements from two Greenland Ice Sheet boreholes to reconstruct the temperature history of this portion of the earth over the past 50,000 years. Their data indicate that after the termination of the glacial period, temperatures steadily rose to a maximum of 2.5°C warmer than at present during the Holocene Climatic Optimum (4,000 to 7,000 years ago). The Medieval Warm Period and Little Ice Age were also documented in the record, with temperatures 1°C warmer and 0.5-0.7°C cooler than at present, respectively. After the Little Ice Age, they report that temperatures once again rose, but that they “have decreased during the last decades.” These results thus clearly indicate that the Medieval Warm Period in this part of the Arctic was significantly warmer than current temperatures.
Wagner and Melles (2001) also worked on Greenland, where they extracted a 3.5-m-long sediment core from a lake (Raffels So) on an island (Raffles O) located just off Liverpool Land on the east coast of Greenland, which they analyzed for a number of properties related to the past presence of seabirds there, obtaining a 10,000-year record that tells us much about the region’s climatic history. Key to the study were biogeochemical data that, in the words of the researchers, reflect “variations in seabird breeding colonies in the catchment which influence nutrient and cadmium supply to the lake.”
Wagner and Melles’ data reveal sharp increases in the values of the parameters they measured between about 1100 and 700 years before present (BP), 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 (Dark Ages Cold Period) of little to no bird presence. Thereafter, their data suggest another absence of birds during what they call “a subsequent Little Ice Age,” which they note was “the coldest period since the early Holocene in East Greenland.”
The Raffels So data also show 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 measurements are not as great as those obtained from the earlier Medieval Warm Period, which indicates that higher temperatures prevailed during the period from 1,100 to 700 years BP than what has been observed over the most recent hundred years.
A third relevant Greenland study was conducted by Kaplan et al. (2002), who derived a climatic history of the Holocene by analyzing the physical-chemical properties of sediments obtained from a small lake in southern Greenland. They determined that the interval from 6,000 to 3,000 years BP was marked by warmth and stability, but that the climate cooled thereafter until its culmination in the Little Ice Age. From 1,300-900 years BP, however, there was a partial amelioration during the Medieval Warm Period, which was associated with an approximate 1.5°C rise in temperature.
In a non-Greenland Arctic study, Jiang et al. (2002) analyzed diatom assemblages from a high-resolution core extracted from the seabed of the north Icelandic shelf to reconstruct a 4,600-year history of mean summer sea surface temperature at that location. Starting from a maximum value of about 8.1°C at 4,400 years BP, the climate was found to have cooled fitfully for about 1,700 years and then more consistently over the final 2,700 years of the record. The most dramatic departure from this long-term decline was centered on about 850 years BP, during the Medieval Warm Period, when the temperature rose by more than 1°C above the line describing the long-term downward trend to effect an almost complete recovery from the colder temperatures of the Dark Ages Cold Period, after which temperatures continued their descent into the Little Ice Age, ending with a final most recent value of approximately 6.3°C. These data also clearly indicate that the Medieval Warm Period in this part of the Arctic was significantly warmer than it is there now.
Moore et al. (2001) analyzed sediment cores from Donard Lake, Baffin Island, Canada, producing a 1,240-year record of average summer temperatures for this Arctic region. Over the entire period from AD 750-1990, temperatures averaged 2.9°C. However, anomalously warm decades with summer temperatures as high as 4°C occurred around AD 1000 and 1100, while at the beginning of the thirteenth century, Donard Lake witnessed “one of the largest climatic transitions in over a millennium,” as “average summer temperatures rose rapidly by nearly 2°C from 1195-1220 AD, ending in the warmest decade in the record” with temperatures near 4.5°C.
This rapid warming of the thirteenth century was followed by a period of extended warmth that lasted until an abrupt cooling event occurred around 1375 and made the following decade one of the coldest in the record. This event signaled the onset of the Little Ice Age, which lasted for 400 years, until a gradual warming trend began about 1800, which was followed by a dramatic cooling event in 1900 that brought temperatures back to levels similar to those of the Little Ice Age. This cold regime lasted until about 1950, whereupon temperatures warmed for about two decades but then tended downwards again all the way to the end of the record in 1990. Hence, in this part of the Arctic the Medieval Warm Period was also warmer than it is there currently.
Grudd et al. (2002) assembled tree-ring widths from 880 living, dead, and subfossil northern Swedish pines into a continuous and precisely dated chronology covering the period 5407 BC to AD 1997. The strong association between these data and summer (June-August) mean temperatures of the last 129 years of the period then enabled them to produce a 7,400-year history of summer mean temperature for northern Swedish Lapland.
The most dependable portion of this record, based upon the number of trees that were sampled, consisted of the last two millennia, which the authors say “display features of century-timescale climatic variation known from other proxy and historical sources, including a warm ‘Roman’ period in the first centuries AD and a generally cold ‘Dark Ages’ climate from about AD 500 to about AD 900.” They also note that “the warm period around AD 1000 may correspond to a so-called ‘Mediaeval Warm Period,’ known from a variety of historical sources and other proxy records.” Lastly, they say “the climatic deterioration in the twelfth century can be regarded as the starting point of a prolonged cold period that continued to the first decade of the twentieth century,” which “Little Ice Age,” in their words, is also “known from instrumental, historical and proxy records.” Going back further in time, the tree-ring record displays several more of these relatively warmer and colder periods. They report that “the relatively warm conditions of the late twentieth century do not exceed those reconstructed for several earlier time intervals.”
Seppa and Birks (2002) used a recently developed pollen-climate reconstruction model and a new pollen stratigraphy from Toskaljarvi—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. As they describe it, 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) and in northern Fennoscandia (Korhola et al., 2000).” In addition, they report there is “a clear correlation between [their] 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.” Finally, 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 twentieth century.
Noting that temperature changes in high latitudes are (1) sensitive indicators of global temperature changes, and that they can (2) serve as a basis for verifying climate model calculations, Naurzbaev et al. (2002) developed a 2,427-year proxy temperature history for the part of the Taimyr Peninsula of northern Russia that lies between 70°30’ and 72°28’ North latitude, based on a study of ring-widths of living and preserved larch trees, noting further that “it has been established that the main driver of tree-ring variability at the polar timber-line [where they worked] is temperature (Vaganov et al., 1996; Briffa et al., 1998; Schweingruber and Briffa, 1996).” In doing so, they found that “the warmest periods over the last two millennia in this region were clearly in the third [Roman Warm Period], tenth to twelfth [Medieval Warm Period] and during the twentieth [Current Warm Period] centuries.”
With respect to the second of these periods, they emphasize that “the warmth of the two centuries AD 1058-1157 and 950-1049 attests to the reality of relative mediaeval warmth in this region.” Their data also reveal three other important pieces of information: (1) the Roman and Medieval Warm Periods were both warmer than the Current Warm Period has been to date, (2) the “beginning of the end” of the Little Ice Age was somewhere in the vicinity of 1830, and (3) the Current Warm Period peaked somewhere in the vicinity of 1940. All of these observations are at odds with what is portrayed in the thousand-year Northern Hemispheric “hockey stick” temperature history of Mann et al. (1998, 1999) and its thousand-year global extension developed by Mann and Jones (2003).
Knudsen et al. (2004) documented climatic changes over the past 1,200 years by means of high-resolution multi-proxy studies of benthic and planktonic foraminiferal assemblages, stable isotopes, and ice-rafted debris found in three sediment cores retrieved from the North Icelandic shelf. This work revealed that “the time period between 1200 and around 7,800 cal. years BP, including the Medieval Warm Period, was characterized by relatively high bottom and surface water temperatures,” after which “a general temperature decrease in the area marks the transition to … the Little Ice Age.” They also note that “minimum sea-surface temperatures were reached at around 350 cal. BP, when very cold conditions were indicated by several proxies.” Thereafter, they say “a modern warming of surface waters … is not [our italics] registered in the proxy data,” and that “there is no clear indication of warming of water masses in the area during the last decades,” even in sea surface temperatures measured over the period 1948-2002.
Grinsted et al. (2006) developed “a model of chemical fractionation in ice based on differing elution rates for pairs of ions … as a proxy for summer melt (1130-1990),” based on data obtained from a 121-meter-long ice core they extracted from the highest ice field in Svalbard (Lomonosovfonna: 78°51’53”N, 17°25’30”E), which was “validated against twentieth-century instrumental records and longer historical climate proxies.” This history indicated that “in the oldest part of the core (1130-1200), the washout indices [were] more than 4 times as high as those seen during the last century, indicating a high degree of runoff.” In addition, they report they have performed regular snow pit studies near the ice core site since 1997 (Virkkunen, 2004) and that “the very warm 2001 summer resulted in similar loss of ions and washout ratios as the earliest part of the core.” They then state that “this suggests that the Medieval Warm Period in Svalbard summer conditions [was] as warm (or warmer) as present-day, consistent with the Northern Hemisphere temperature reconstruction of Moberg et al. (2005).” In addition, they conclude that “the degree of summer melt was significantly larger during the period 1130-1300 than in the 1990s,” which likewise suggests that a large portion of the Medieval Warm Period was significantly warmer than the peak warmth (1990s) of the Current Warm Period.
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 11 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 say—citing seven other studies—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.”
They found “the twelfth and thirteenth centuries were relatively warm,” with their data indicating 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 twentieth century, which has also been shown to be the case for Donard Lake (66.25°N, 62°W) by Moore et al. (2001).
Mazzarella and Scafetta (2011) examined monthly North Atlantic Oscillation (NAO) from 1659 to the present to argue that the time-integrated record of the NAO is "a reliable global climate proxy," comparing its oscillations with "those observed in the European historical record of middle latitude aurorae (Krivsky and Pejml, 1998) to claim that a 60-year oscillation exists in the global climate and likely has an astronomical origin, as previously proposed (Scafetta, 2010)." This natural oscillation was in its warm phase from 1970-2000, most likely contributing largely to the global warming during this period.
The studies reviewed above indicate that the Arctic—which climate models suggest should be sensitive to greenhouse-gas-induced warming—is still not as warm as it was many centuries ago during portions of the Medieval Warm Period, when there was much less CO2 and methane in the air than there is today. This further suggests the planet’s more modest current warmth need not be the result of historical increases in these two greenhouse gases.
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