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Roman Warm Period (Europe - Northern) -- 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 tolerate 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 phenomena 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, focusing on Northern Europe.

Jiang et al. (2002) used diatom assemblages from a high-resolution core extracted from the seabed of the north Icelandic shelf to reconstruct a 4600-year history of mean summer sea surface temperature in that general region. In doing so, they found that the warmest temperature of the record (~8.1C) occurred near its beginning about 4400 years before present (BP). Thereafter, the climate cooled, fitfully over the next 1700 years, but more consistently over the final 2700 years. In fact, most of the data of this final period are well described by a steadily declining linear relationship. There is, however, one data point at about 1500 years BP (during the Roman Warm Period) that rises above this line by ~0.5C and another at about 1350 years BP (during the Dark Ages Cold Period) that falls below the line by ~0.5C. Then comes a departure centered on about 850 years BP (during the Medieval Warm Period), when the temperature rises by more than 1C above the line describing the long-term downward trend. Last of all, the most recent data point (during the Current Warm Period) has a value of ~6.3C.

These findings clearly indicate that the past 2700 years have witnessed a significant deterioration of the climate in the vicinity of the north Icelandic shelf, as the region has moved ever further away from the benign weather of the Roman Warm Period. After the planet's descent into the Dark Ages Cold Period, however, the Icelandic record depicts a nearly complete recovery during the middle of the Medieval Warm Period; but the warmth of this period soon gave way to the rapid cooling that produced the Little Ice Age, which brought mean summer sea surface temperatures down by ~2.2C from what they were at the peak of the Medieval Warm Period. Clearly, it's time for a little warmth once again; and the results of this study suggest that the region surrounding Iceland is going to need a whole lot of it to return to its former "glory days" of both the Medieval and Roman Warm Periods.

Working nearby, on Iceland itself, Olafsdottir et al. (2001) simulated the spatial relationship between temperature change and potential vegetation cover there over the period of the Holocene, evaluating their results against palynological and geomorphological data. This work revealed that during the Holocene Climatic Optimum, vegetation may have covered about 60% of the land. By the time the Roman Warm Period began to wane about 2300 years ago, however, a vegetative decline commenced that continued until the Medieval Warm Period reversed the decline for about 400 years. The appearance of the Little Ice Age, however, resulted in "an unprecedented low potential for vegetation for the Holocene that lasted c. 600 years, i.e., between AD c. 1300 and 1900," which suggests that the Roman Warm Period was likely the most vegetation-friendly (i.e., warmest) period of the post Climatic Optimum era.

Berglund (2003) identified several periods of expansion and decline of human cultures in Northwest Europe and compared them with a history of reconstructed climate "based on insolation, glacier activity, lake and sea levels, bog growth, tree line, and tree growth." This work revealed "a positive correlation between human impact/land-use and climate change." More specifically, in the latter part of the record where both cultural and climate changes were best defined, there was, in Berglund's words, a great "retreat of agriculture" centered on about AD 500, which led to "reforestation in large areas of central Europe and Scandinavia." He additionally notes that "this period was one of rapid cooling indicated from tree-ring data (Eronen et al., 1999) as well as sea surface temperatures based on diatom stratigraphy in [the] Norwegian Sea (Jansen and Koc, 2000), which can be correlated with Bond's event 1 in the North Atlantic sediments (Bond et al., 1997)." And, of course, the climatic state from which this cooling began was the agriculturally-friendly Roman Warm Period.

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 this period then enabled them to produce a 7400-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, consists of the last two millennia, which the six researchers 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 that "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 additional warmer and colder periods. And in a telling commentary on climate-alarmist claims to the contrary, Grudd et al. report that "the relatively warm conditions of the late twentieth century do not exceed those reconstructed for several earlier time intervals." In fact, the warmth of many of the earlier warm intervals significantly exceeded the warmth of the late 20th century.

Hormes et al. (2004) identified and dated periods of soil formation in moraines in the Kebnekaise mountain region of Swedish Lapland in the foreground of the Nipalsglaciaren, after which they compared the climatic implications of their results with those of other proxy climate records derived throughout other areas of northern and central Scandinavia. The chief result of these efforts was that two periods of soil formation were identified (2750-2000 and 1170-740 cal yr BP), which spans of time coincide nearly perfectly with the Roman and Medieval Warm Periods delineated by McDermott et al. (2001) in the δ18O record they developed from a stalagmite in southwestern Ireland's Crag Cave. Hormes et al. additionally report that during the periods when the soil formation processes they discovered took place, "the glacier was most likely in a position similar to today, and climate conditions were also similar to today."

With respect to their identification of soil formation during the Roman Warm Period, Hormes et al. describe similar prior findings of contemporaneous soil formation at Svartisen glacier between 2350 and 1990 cal yr BP by Karlen (1979), Austre Okstindbreen glacier between 2350 and 1800 cal yr BP by Griffey and Worsley (1978), and Austre Okstindbreen glacier between 2750 and 2150 by Karlen (1979). In addition, they note that the pine tree-based temperature history of northern Fennoscandia developed by Grudd et al. (2002) "discloses a spike +2C higher than today's around 2300 cal yr BP," and that "the lacustrine records in Lapland and Finland are also consistent with supposition of a warmer climate than at present before 2000 cal yr BP and cooler temperatures before 2450 cal yr BP (Rosen et al., 2001; Seppa and Birks, 2001; Shemesh et al., 2001; Hammarlund et al., 2002; Heikkila and Seppa, 2003)."

Utilizing plant macrofossils, testate amoebae and degree of humification as proxies for environmental moisture conditions, Blundell and Barber (2005) developed a 2800-year "wetness history" from a peat core extracted from Tore Hill Moss, a raised bog in the Strathspey region of Scotland. Based on the results they obtained from the three proxies they studied, the two researchers derived a relative wetness history that begins 2800 years ago and extends all the way to AD 2000. The most clearly defined and longest interval of sustained dryness of this entire history 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. Preceding the Medieval Warm Period, their hydro-climate reconstruction reveals a highly chaotic period of generally greater wetness that corresponds to the Dark Ages Cold Period, as well as dryness peaks representing the Roman Warm Period and two other periods of relative dryness. In addition, the correlation this study demonstrates to exist between relative wetness and warmth in Scotland strongly suggests that the temperature of the late 20th century was nowhere near the highest of the past two millennia in that part of the world. In fact, it suggests there were five other periods over the past 2800 years that were considerably warmer.

Linderholm and Gunnarson (2005) utilized the well replicated period of 1632 BC to AD 2000 of the Jmtland multi-millennial tree-ring width chronology derived from living and subfossil Scots pines sampled close to the present tree-line in the central Scandinavian Mountains as a proxy for summer temperatures. Several periods of anomalously warm and cold summers were noted throughout this record: (1) 550 to 450 BC (Roman Warm Period), when summer temperatures were the warmest of the entire record, exceeding the 1961-1990 mean by more than 6C, (2) AD 300 to 400 (Dark Ages Cold Period), which was "the longest period of consecutive cold summers," averaging 1.5C less than the 1961-1990 mean, (3) AD 900 to 1000, a warm era corresponding to the Medieval Warm Period, and (4) AD 1550 to 1900, a cold period known as the Little Ice Age.

Last of all, Allen et al. (2007) analyzed pollen characteristics within sediment cores retrieved from a small unnamed lake located near the coast of Nordkinnhalvoya, Finnmark, Norway, after which they used the results of this effort to construct a climatic history of the area over the course of the Holocene. In doing so, they discovered that "regional vegetation responded to Holocene climatic variability at centennial-millennial time scales." More specifically, they report identifying "the most recent widely documented cooling event, the Little Ice Age of ca 450-100 cal BP," the "Dark Ages cool interval, a period during which various other proxies indicate cooling in Fennoscandia and beyond," which they place at 1600-1100 cal BP, the "Medieval Warm Period that separated the latter two cool intervals," and "the warm period around two millennia ago during which the Roman Empire reached its peak," which, of course, was the Roman Warm Period.

In view of these several research findings, it should be obvious that the Roman Warm Period was a very real feature of northern European climatic history, and that it likely was even warmer than the Current Warm Period has been to date. Furthermore, since all of that prior warmth occurred at times when the atmosphere's CO2 concentration was more than 100 ppm less than it is today, there is no compelling reason to believe that the lesser warmth of today has anything at all to do with the air's current much-higher CO2 content.

References
Allen, J.R.M., Long, A.J., Ottley, C.J., Pearson, D.G. and Huntley, B. 2007. Holocene climate variability in northernmost Europe. Quaternary Science Reviews 26: 1432-1453.

Berglund, B.E. 2003. Human impact and climate changes -- synchronous events and a causal link? Quaternary International 105: 7-12.

Blundell, A. and Barber, K. 2005. A 2800-year palaeoclimatic record from Tore Hill Moss, Strathspey, Scotland: the need for a multi-proxy approach to peat-based climate reconstructions. Quaternary Science Reviews 24: 1261-1277.

Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I. and Bonani, G. 1997. A pervasive millennial-scale cycle in North Atlantic Holocene and glacial climates. Science 278: 1257-1266.

Eronen, M., Hyvarinen, H. and Zetterberg, P. 1999. Holocene humidity changes in northern Finnish Lapland inferred from lake sediments and submerged Scots pines dated by tree-rings. The Holocene 9: 569-580.

Griffey, N.J. and Worsley, P. 1978. The pattern of neoglacial glacier variations in the Okstindan region of northern Norway during the last three millennia. Boreas 7: 1-17.

Grudd, H., Briffa, K.R., Karlen, W., Bartholin, T.S., Jones, P.D. and Kromer, B. 2002. A 7400-year tree-ring chronology in northern Swedish Lapland: natural climatic variability expressed on annual to millennial timescales. The Holocene 12: 657-665.

Hammarlund, D., Barnekow, L., Birks, H.J.B., Buchardt, B. and Edwards, T.W.D. 2002. Hoolocene changes in atmospheric circulation recorded in the oxygen-isotope stratigraphy of lacustrine carbonates from northern Sweden. The Holocene 12: 339- 351.

Heikkila, M. and Seppa, H. 2003. A 11,000-yr palaeotemperature reconstruction from the southern boreal zone in Finland. Quaternary Science Reviews 22: 541-554.

Hormes, A., Karlen, W. and Possnert, G. 2004. Radiocarbon dating of palaeosol components in moraines in Lapland, northern Sweden. Quaternary Science Reviews 23: 2031-2043.

Jansen, E. and Koc, N. 2000. Century to decadal scale records of Norwegian sea surface temperature variations of the past 2 millennia. PAGES Newsletter 8(1): 13-14.

Jiang, H., Seidenkrantz, M-S., Knudsen, K.L. and Eiriksson, J. 2002. Late-Holocene summer sea-surface temperatures based on a diatom record from the north Icelandic shelf. The Holocene 12: 137-147.

Karlen, W. 1979. Glacier variations in the Svartisen area, northern Norway. Geografiska Annaler 61A: 11-28.

Linderholm, H.W. and Gunnarson, B.E. 2005. Summer temperature variability in central Scandinavia during the last 3600 years. Geografiska Annaler 87A: 231-241.

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.

Olafsdottir, R., Schlyter, P. and Haraldsson, H.V. 2001. Simulating Icelandic vegetation cover during the Holocene: Implications for long-term land degradation. Geografiska Annaler 83A: 203-215.

Rosen, P., Segerstrom, U., Eriksson, L., Renberg, I. and Birks, H.J.B. 2001. Holocene climatic change reconstructed from diatoms, chironomids, pollen and near-infrared spectroscopy at an alpine lake (Sjuodjijaure) in northern Sweden. The Holocene 11: 551-562.

Seppa, H. and Birks, H.J.B. 2001. July mean temperature and annual precipitation trends during the Holocene in the Fennoscandian tree-line area: pollen-based climate reconstruction. The Holocene 11: 527-539.

Shemesh, A., Rosqvist, G., Rietti-Shati, M., Rubensdotter, L., Bigler, C., Yam, R. and Karlen, W. 2001. Holocene climatic changes in Swedish Lapland inferred from an oxygen isotope record of lacustrine biogenic silica. The Holocene 11: 447-454.

Last updated 23 April 2008