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Roman Warm Period (Europe - Central) -- Summary
Climate alarmists contend that the degree of global warmth over the latter part of the 20th century was greater than it has been at any other time over the past one to two millennia, because this contention helps support their claim that what they call the "unprecedented" temperatures of the past few decades were CO2-induced. Hence, they cannot stomach the thought that the Medieval Warm Period of a thousand years ago could have been just as warm as, or even warmer than, it has been recently, especially since there was so much less CO2 in the air a thousand years ago than there is now. Likewise, they are equally loath to admit that temperatures of the Roman Warm Period of two thousand years ago may also have rivaled, or exceeded, those of the recent past, since atmospheric CO2 concentrations at that time were also much lower than they are today. As a result, climate alarmists rarely even mention the Roman Warm Period, as they are happy to let sleeping dogs lie. In addition, they refuse to acknowledge that these two prior warm periods were global in extent, claiming instead that they were local phenomenon restricted to lands surrounding the North Atlantic Ocean. In another part of our Subject Index we explore these contentions as they apply to the Medieval Warm Period. In this Summary, we explore them as they pertain to the Roman Warm Period, beginning with Central Europe.

McDermott et al. (2001) derived a δ18O record -- with a time resolution they say is "approximately an order of magnitude better than in the North Atlantic cores that record evidence for quasi-periodic (1475 500 year) ice rafting during the Holocene" -- from a stalagmite found in southwestern Ireland's Crag Cave, after which they compared this record with the δ18O records from the GRIP and GISP2 ice cores from Greenland. This exercise provided, in the words of the three researchers, evidence for "centennial-scale δ18O variations that correlate with subtle δ18O changes in the Greenland ice cores, indicating regionally coherent variability in the early Holocene." They additionally note that the Crag Cave data "exhibit variations that are broadly consistent with a Medieval Warm Period at ~1000 200 years ago and a two-stage Little Ice Age, as reconstructed by inverse modeling of temperature profiles in the Greenland Ice Sheet." Also evident in the Crag Cave data were the δ18O signatures of the earlier Roman Warm Period and Dark Ages Cold Period, which comprised the prior such cycle of climate in that region. Based on these findings, the three researchers concluded that the coherent δ18O variations in the records from both sides of the North Atlantic "indicate that many of the subtle multicentury δ18O variations in the Greenland ice cores reflect regional North Atlantic margin climate signals rather than local effects." And, of course, their data confirm the reality of the Medieval-Warm-Period/Little-Ice-Age cycle, as well as the preceding and even more strongly expressed Roman-Warm-Period/Dark-Ages-Cold-Period cycle, demonstrating there is nothing unusual -- or unprecedented, as climate alarmists are fond of saying -- about the global warming of the past century or so.

On the mainland, Niggemann et al. (2003) studied petrographical and geochemical properties of three stalagmites found in Sauerland's B7-Cave in northwest Germany, from which they developed a climatic history of that region covering the last 17,600 years. These records resembled those of McDermott et al.; and Niggemann et al. explicitly noted 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 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.

In another important mainland study, Holzhauser et al. (2005) "for the first time," in their words, developed 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 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 better known as the Roman Warm Period, the glacier again retreated and "reached today's extent or was even somewhat shorter [our italics] 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 state 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)."

To emphasize this latter point, Holzhauser et al. concluded their paper by noting 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." All we would add to these conclusions is that because the current warmth of the study region has not yet resulted in a shrinkage of the Great Aletsch glacier equal 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 like to claim, about the region's current warmth. In addition, we note that the warmth of the modern era is occurring at just about the time one would expect it to occur, in light of the 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.

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," Chapron et al. (2005) sought to learn more about the pervasive climatic oscillation responsible for these observations by documenting the Holocene evolution of Rhone River clastic sediments in Lake Le Bourget via sub-bottom seismic profiling and multidisciplinary analysis of well-dated sediment cores. This work revealed 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 report 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 45N 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 we know 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.

Most recently, 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) located at the southern slopes of the Austrian Central Alps just above the present tree-line, in order to develop 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. Of most interest to us, in this regard, was Schmidt et al.'s finding that "spring-temperature anomalies during Roman and Medieval times equaled or slightly exceeded [our italics] the modern values and paralleled tree-line and glacier fluctuations," indicative of their broad range of applicability.

In summary, ever more data of ever greater diversity continue to testify of the reality of the millennial-scale climatic oscillation that has reverberated throughout the Holocene in Central Europe and elsewhere. And it is important to note that the wealth of information contained in these many records suggests there is nothing unusual, unprecedented or unexpected about the 20th-century warming that has ushered in the Current Warm Period, which may well have not yet attained the level of warmth experienced during the Roman Warm Period.

References
Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294: 2130-2136.

Chapron, E., Arnaud, F., Noel, H., Revel, M., Desmet, M. and Perdereau, L. 2005. Rohne River flood deposits in Lake Le Bourget: a proxy for Holocene environmental changes in the NW Alps, France. Boreas 34: 404-416.

Denton, G.H. and Karlen, W. 1973. Holocene climate variations - their pattern and possible cause. Quaternary Research 3: 155-205.

Holzhauser, H., Magny, M. and Zumbuhl, H.J. 2005. Glacier and lake-level variations in west-central Europe over the last 3500 years. The Holocene 15: 789-801.

Magny, M. 1993. Solar influences on Holocene climatic changes illustrated by correlations between past lake-level fluctuations and the atmospheric 14C record. Quaternary Research 40: 1-9.

McDermott, F., Mattey, D.P. and Hawkesworth, C. 2001. Centennial-scale Holocene climate variability revealed by a high-resolution speleothem δ18O record from SW Ireland. Science 294: 1328-1331.

Niggemann, S., Mangini, A., Richter, D.K. and Wurth, G. 2003. A paleoclimate record of the last 17,600 years in stalagmites from the B7 cave, Sauerland, Germany. Quaternary Science Reviews 22: 555-567.

Schmidt, R., Kamenik, C. and Roth, M. 2007. Siliceous algae-based seasonal temperature inference and indicator pollen tracking ca. 4,000 years of climate/land use dependency in the southern Austrian Alps. Journal of Paleolimnology 38: 541-554.

Tinner, W., Lotter, A.F., Ammann, B., Condera, M., Hubschmied, P., van Leeuwan, J.F.N. and Wehrli, M. 2003. Climatic change and contemporaneous land-use phases north and south of the Alps 2300 BC to AD 800. Quaternary Science Reviews 22: 1447-1460.

van Geel, B., Buurman, J. and Waterbolk, H.T. 1996. Archaeological and palaeoecological indications of an abrupt climate change in the Netherlands and evidence for climatological teleconnections around 2650 BP. Journal of Quaternary Science 11: 451-460.

Last updated 26 March 2008