Reproduced from CO2Science.org

Little Medieval Warm Period -- Summary
by Sherwood, Keith, and Craig Idso

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, 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 978 AD. 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 (0-70N) 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 0.6C 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 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 0.23C 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 0.4C."

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)."

Last of all, in a study that provides additional 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.

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, of course, we would be remiss in not making the obligatory 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, and, therefore, that 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. Consequently, this biologically-beneficent trace gas of the atmosphere should 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 at work in the area of global climate change.


Last updated 27 July 2005

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