In one of the more intriguing aspects of his study of global climate change over the past three millennia, Loehle (2004) presents a graph of the Sargasso Sea and South African temperature records of Keigwin (1996) and Holmgren et al. (1999, 2001) that reveals the existence of a major spike in surface air temperature that began sometime in the early 1400s. This abrupt and anomalous warming pushed the air temperatures of these two records considerably above their representations of the peak warmth of the 20th century, after which they fell back to pre-spike levels in the mid-1500s, in harmony with the work of McIntyre and McKitrick (2003), who found a similar period of higher-than-current temperatures in their reanalysis of the data employed by Mann et al. (1998, 1999) in the creation of their controversial "hockeystick" temperature history of the Northern Hemisphere, which gives no hint of the existence of this post-medieval high-temperature regime.
In another study that reveals the existence of this period of higher-than-current warmth, which we have dubbed the "Little" Medieval Warm Period, D'Arrigo et al. (2004) developed a maximum latewood density (MXD) chronology for the period 1389 to 2001, based on cores obtained from white spruce trees growing near the treeline on the eastern Seward Peninsula of Alaska, a portion of which data (1909-1950) were calibrated against May-August temperatures measured at Nome and then used to convert the entire MXD chronology to warm-season temperatures. In viewing their final result, it can readily be seen there was a two-decade period of close-to-20th-century-warmth in the mid-1500s that was preceded by a decade of warmth in the latter part of the 1400s that was actually greater than that of the mid-20th century.
In a subsequent study from the same region, D'Arrigo et al. (2005) derived a new tree-ring width data set from 14 white spruce chronologies covering the years 1358-2001. These data were then combined with additional tree-ring width chronologies from northwest Alaska to produce two versions of a much longer data series that extended all the way back to AD 978 . The first chronology was created using traditional methods of standardization (STD), which do not perform well in capturing multi-decadal or longer climate cycles, while the second chronology utilized the regional curve standardization (RCS) method, which better preserves low-frequency variations at multi-decadal time scales and longer. With respect to the STD- and RCS-derived temperature histories, each of them revealed, in the words of D'Arrigo et al., "several intervals of persistent above-average growth that broadly coincide with the timing of the late Medieval Warm Period." However, the warming is much more pronounced in the RCS chronology, where the greatest warmth occurred in the early to middle 1200s, with lesser peaks in the early to middle 1100s and early 1400s (the "Little" Medieval Warm Period).
Additional evidence for this previously unheralded warm period was obtained by Silenzi et al. (2004). Working with Vermetid reefs on the northwest coast of Sicily, they obtained oxygen isotopic data that they interpreted in terms of sea surface temperature (SST) variations. These data indicated that in the early to mid-1500s, SSTs in this region were warmer than they are currently. Likewise, Gray et al. (2004) developed a reconstruction of the leading mode of low-frequency North Atlantic (0-70°N) SST variability, known as the Atlantic Multidecadal Oscillation (AMO), for the period 1567-1990. Based on tree-ring records from regions known to border on strong centers of AMO variability, including eastern North America, Europe, Scandinavia and the Middle East, this record too displayed an intense warm phase, in this case between 1580 and 1596, the unmatched strength of which is clearly evident in reconstructed North Atlantic SST anomalies.
Many other studies have found much the same thing. Helama et al. (2002), for example, reconstructed midsummer temperatures for the last 7500 years using the long Scots pine ring-width chronology from northern Finland that was derived by Eronen et al. (2002). Their record revealed that the 20th century was indeed warm compared to the mean of the entire period (about 0.6°C warmer). However, there were three other hundred-year periods that were warmer still, the latter of which (AD 1500-1600) falls within the general time frame of what we call the "Little" Medieval Warm Period.
In a novel paper published in Nature, Chuine et al. (2004) used recorded dates of grape harvests in Burgundy, France to reconstruct mean spring-summer (April-August) air temperatures for that location on a yearly basis from 1370 to 2003, employing what they call "a process-based phenology model developed for the Pinot Noir grape." The resulting temperature history is significantly correlated with mean summer air temperatures deduced from tree rings in central France, the Burgundy portion of a spatially-distributed multi-proxy temperature reconstruction, as well as observed summer air temperatures in Paris, central England and the Alps. The thermal interconnectedness of these sites gives the new temperature history an important regional significance, the most intriguing aspect of which is the existence of much warmer-than-present air temperatures at various times in the past (most notably from the late 1300s through the early 1400s and over a large portion of the 1600s), where there are not even any hints of elevated warmth in the hockeystick record of Mann et al., but where several of the other records reviewed in this Summary indicate the existence of the "Little" Medieval Warm Period.
In another pertinent paper, Bartholy et al. (2004) meticulously codified and analyzed historical records collected by Antal Rethly (1879-1975), a Hungarian meteorologist who spent the greater portion of his long professional career assembling over 14,000 historical records related to the climate of the Carpathian Basin. With respect to the temperature history they thereby derived, they report 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." In addition, they say that "a warm episode in the 16th century [was] detected in both annual- and seasonal-scale analysis of the 50-year distribution of warm and cold conditions," which would again be the "Little" Medieval Warm Period.
Back in North America, Luckman and Wilson (2005) updated a regional temperature history, originally developed in 1997, using new tree-ring data from the Columbia Icefield region of the Canadian Rockies. The new update also employed different standardization techniques, including the regional curve standardization method that better captures low frequency variability (centennial- to millennial-scale) than that reported in the initial study. In addition, the new data set added over one hundred years to the chronology, which now covers the period AD 950-1994. This tree-ring record was found to explain 53% of May-August maximum temperature variation observed in the 1895-1994 historical data and was thus considered a good proxy indicator of such temperatures. Based on this relationship, the record showed considerable decadal- and centennial-scale temperature variability, where generally warmer conditions prevailed during the 11th and 12th centuries, between about 1350-1450 (the "Little" Medieval Warm Period) and from about 1875 through the end of the record. Of more than passing interest is the fact that the warmest summer of this record occurred in 1434, when it was 0.23°C warmer than the next warmest summer, which occurred in 1967.
Focusing on a different climatic parameter, but one that is highly correlated with temperature, Blundell and Barber (2005) utilized plant macrofossils, testate amoebae and degree of humification as proxies for environmental moisture conditions to develop a 2800-year "wetness history" from a peat core extracted from Tore Hill Moss, a raised bog in the Strathspey region of Scotland. The most clearly defined and longest interval of sustained dryness of this entire record stretches from about AD 850 to AD 1080, coincident with the well known Medieval Warm Period, while the most extreme wetness interval occurred during the depths of the last stage of the Little Ice Age, which was one of the coldest periods of the entire Holocene. Of most interest to the subject of this Summary, however, is the period of relative dryness centered on about AD 1550, which corresponds to the "Little" Medieval Warm Period and implies the existence of significant warmth at that time.
In a somewhat different study reminiscent of the repeat photography project of Idso and Idso (2000), Munroe (2003) replicated and analyzed six photographs taken in 1870 near the subalpine forest-alpine-tundra ecotone in the northern Uinta Mountains of Utah, USA, in an attempt to quantify the redistribution of vegetation that occurred there between the end of the Little Ice Age and the current stage of the Modern Warm Period. After achieving this objective, he used his findings to infer the nature of regional climate change over the last 130 years; but before concluding he directed his attention to what he describes as "downed logs, in situ stumps, and upright delimbed boles on the north side of Bald Mountain [that] indicate a treeline up to 60 m higher than the modern level," which he determined, on the basis of the modern atmospheric lapse rate, "corresponds to an increase of mean July temperature of 0.4°C."
With respect to these subfossil relics, Munroe writes that many of them "have been severely abraded by windblown ice, giving the impression of considerable antiquity," noting that "similar wood from elsewhere in the Rocky Mountains has been taken as evidence of higher treeline during the early Holocene climatic optimum, or 'altithermal' (Carrara et al., 1991)." However, he reports that a sample cut from one of the stumps was radiocarbon dated to only about 1550, and that "the actual germination of the tree may have occurred a century or more before AD 1550," which places the warm period indicated by the subfossil wood in approximately the same time interval as the warm periods identified in all of the prior studies we have discussed. What is more, Munroe remarks that "a higher treeline in the northern Uintas shortly before AD 1550 is consistent with contemporaneous evidence for warmer-than-modern climates in the southwestern United States (Dean, 1994; Petersen, 1994; Meyer et al., 1995; Pederson, 2000)."
In yet another study that provides indirect evidence for the existence of this century-scale "Little" Medieval Warm Period, Fleitmann et al. (2004) developed a stable isotope history from three stalagmites in a cave in Southern Oman that provided an annually-resolved 780-year record of Indian Ocean monsoon rainfall. Over the last eight decades of the 20th century, when global temperatures rose dramatically as the earth emerged from the Little Ice Age and entered the Modern Warm Period, this record reveals that Indian Ocean monsoon rainfall declined dramatically; and it indicates that the other most dramatic decline coincided with the major temperature spike that is evident in the temperature histories discussed above.
"And the beat goes on ..."
Pla and Catalan (2005) analyzed chrysophyte cyst data collected from 105 lakes in the Central and Eastern Pyrenees of northeast Spain to produce a Holocene history of winter/spring temperatures in this part of the world. This work revealed a significant oscillation in winter/spring temperatures in which the region's climate alternated between warm and cold phases over the past several thousand years. Of particular note were the Little Ice Age, Medieval Warm Period, Dark Ages Cold Period and Roman Warm Period, the warmest of which intervals was the Medieval Warm Period, which started around AD 900 AD and was about 0.25°C warmer than it is currently. Following the Medieval Warm Period, temperatures fell to their lowest values of the entire record (about 1.0°C below present), whereupon they began to warm, but remained below present-day values until the early 19th and 20th centuries, with one exception. A significant warming was observed between 1350 and 1400, when temperatures rose a full degree Celsius to a value about 0.15°C warmer than the present, during what we refer to as ... drum roll ... the Little Medieval Warm Period!
Proceeding a little faster, Chen et al. (2005) studied the chemical composition of sediments deposited in Lake Erhai (25°35'-25°58'N, 100°05'-100°17'E), which is the largest fault lake in the western Yunnan Province of China. More specifically, they applied Principal Component Analysis to the concentrations of 21 major and minor elements found in the sediments, thereby deriving historical variations in temperature and precipitation over the period AD 1340-1990. In doing so, they found an initial period (1340-1550) of relatively high temperature and low rainfall - the Little MWP.
Sharma et al. (2005) used ð13C values of Sphagnum remains from peat deposits located along a sequence of beach ridges of Lake Superior in North America to reconstruct changes in regional water balance from about 1000 to 3500 years BP, after which they compared their findings with water level reconstructions of adjacent Lake Michigan derived by Baedke and Thompson (2000) from sedimentological studies. In doing so they found maxima of Sphagnum ð13C values in peat deposits developed from 3400 to 2400 years BP and from 1900 to 1400 years BP, which closely match two periods of Lake Michigan high-water stands evident in the lake level record of Baedke and Thompson. These two periods coincide with the cooler climatic conditions that prevailed on either side of the Roman Warm Period, the most recent of which is the well known Dark Ages Cold Period. This latter cold high-water period was then followed by a period of low water and declining ð13C values, which coincide with the well known Medieval Warm Period that ultimately gave way to the Little Ice Age. Thereafter, there are no more ð13C data; but the lake level data reveal a third low-level stand of Lake Michigan from about 600 to 500 years BP that coincides nicely with the Little Medieval Warm Period.
Using the regional curve standardization technique applied to ring-width measurements of both living trees and relict wood, Büntgen et al. (2005) 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 exercise revealed the presence of warm conditions from the beginning of the record in AD 951 up to about AD 1350, which the authors associated with the Medieval Warm Period. Thereafter, temperatures declined and an extended cold period known as the Little Ice Age ensued and persisted until approximately 1850 ... with one brief exception. For a few short decades in the mid- to late-1500s, there was an uncharacteristically warm episode, 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. And, of course, this would be our Little MWP.
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 report 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 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. Most interestingly, however, 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 the ever-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.
Weckstrom et al. (2006) developed a high-resolution quantitative history of temperature variability over the past 800 years, based on analyses of diatoms found in a sediment core retrieved from a treeline lake (Lake Tsuolbmajavri) located in Finnish Lapland. The result, in their words, "depicts three warm time intervals around AD 1200-1300, 1380-1550 and from AD 1920 until the present." Of these intervals, they say they "associate the warmth of the 13th century with the termination phase of the Medieval Warm Period and the rapid post-1920 temperature increase with the industrially induced anthropogenic warming," the last decade of which climate alarmists typically tout as having been the warmest such period of the last two millennia. Most interestingly, however, Weckstrom et al.'s data indicate that the peak warmth of the AD 1200-1300 termination phase of the MWP was about 0.15°C warmer than the peak warmth of the post-1920 period. Yet even more interesting is the fact that the peak warmth of the AD 1380-1550 period -- which we refer to as the Little Medieval Warm Period -- was warmer still, at 0.25°C above the peak warmth of the post-1920 period.
Barron and Bukry (2007) derived high-resolution records of diatoms and silicoflagellate assemblages spanning the past 2000 years from analyses of sediment cores extracted from three sites on the eastern slope of the Gulf of California. In all three cores the relative abundance of Azpeitia nodulifera (a tropical diatom whose presence suggests higher sea surface temperatures) was found to be far greater during the Medieval Warm Period than at any other time over the 2000-year period studied. In addition, the first of the cores exhibited elevated A. nodulifera abundances from the start of the record to about AD 350 (during the latter part of the Roman Warm Period), as well as between AD 1520 and 1560 (during what we have denominated the Little Medieval Warm Period). And by analyzing radiocarbon production data, Barron and Bukry also determined that the changes in climate they identified likely were driven by solar forcing.
Working with the top 2 cm of a 20-cm-long stalagmite that was collected in November of 1995 from Shihua Cave near Beijing, China, Ku and Li (1998) obtained annually-resolved ð18O data covering the past five centuries. Based on their analyses of these and other pertinent data, they determined that fluctuations of the ð18O data over periods of less than ten years "reflect changes in precipitation, whereas on coarser time scales (>50 years), the stalagmite ð18O records temperature variations." This finding, in turn, led them to conclude that "the period AD 1620-1900 was cold and periods 1520-1620 and 1900-1994 were warm." And in comparing their graphical representations of these two warm periods, it appears that the earlier period -- which we have dubbed the Little Medieval Warm Period -- was probably just a tad warmer than it was over the last two decades of the 20th century, which climate alarmists typically claim was the warmest period of the last two millennia.
Last of all -- for a little while, at least -- Saenger et al. (2009) developed what they describe as "an absolutely dated and annually resolved record of sea surface temperature [SST] from the Bahamas [25.84°N, 78.62°W], based on a 440-year time series [1552-1991] of coral [Siderastrea siderea] growth rates," which they found to possess "an inverse correlation with instrumental SST," which was verified by "applying it to an S. siderea colony from Belize (17.50°N, 87.76°W)." This work revealed, in the words of the researchers who conducted it, that "temperatures were as warm as today from about 1552 [where their record begins, somewhere in the midst of the mini-warm period] to 1570, then cooled by about 1°C from 1650 to 1730 before warming until the present," which for their record was 1991. And in comparing 1991 warmth with that of the then present (2009), we find that the HadCRUT3 and Global Historical Climatology Network databases depict about a 0.3°C increase in temperature between 1991 and 2009; but the graph of Saenger et al.'s data shows their temperature history ending about 0.3°C short of its peak Little Medieval Warm Period value. Hence, Saenger et al.'s conclusion that "SSTs were as warm as present from 1552 to 1570" indeed appears to be correct.
In light of these several observations, of which there may well be many others that suggest the same thing, we wonder if the widely distributed warming that began somewhere in the vicinity of the 15th century and ended somewhere in the vicinity of the 16th century was an independent phenomenon or perhaps the "last hurrah" of the Medieval Warm Period before it relinquished control of earth's climate to the Little Ice Age. Whatever may be the case, it is beginning to look like the Medieval Warm Period proper and the earlier Roman Warm Period were not the only eras to exhibit surface air temperatures that equaled or eclipsed those of the 20th century. And we would be remiss in not making the observation that all of these warmer-than-present eras achieved their enhanced thermal status without any help from elevated atmospheric CO2 concentrations, which were fully 100 ppm less than they are today at those earlier times.
Consequently, whatever caused the warmth of those prior eras could well be maintaining the warmth of the present era, which relieves CO2 of that undeserved responsibility. As a result, the biologically-beneficent trace gas of the atmosphere must be presumed innocent of inducing any global warming until clearly proven to have done so, which, we believe, will never happen. Other much more powerful phenomena are likely to be the causative agents of modern -- and ancient --global climate change.
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