Working with a core of 2.5 meters length, which they sampled at intervals of 2 cm in the upper 1 meter and at intervals of 5 cm below that depth, Martinez-Cortizas et al. (1999) derived a record of mercury deposition in the peat bog of Penido Vello in northwest Spain that extends to 4000 radiocarbon years before present, which they analyzed for a number of parameters. This work revealed, in their words, "that cold climates promoted an enhanced accumulation and the preservation of mercury with low thermal stability, and warm climates were characterized by a lower accumulation and the predominance of mercury with moderate to high thermal stability." Based on these findings and further analyses, they derived a temperature history for the region that they standardized to the mean temperature of the most recent 30 years of their record. This protocol revealed that the mean temperature of the Medieval Warm Period in northwest Spain was 1.5°C warmer than it was over the 30 years leading up to the time of their study, and that the mean temperature of the Roman Warm Period was fully 2°C warmer. Even more impressive was their finding that several decadal-scale intervals during the Roman Warm Period were more than 2.5°C warmer than the 1968-98 period, while an interval in excess of 80 years during the Medieval Warm Period was more than 3°C warmer. These observations thus led Martinez-Cortizas et al. to conclude that "for the past 4000 years ... the Roman Warm Period and the Medieval Warm Period were the most important warming periods."

Desprat et al. (2003) studied the climatic variability of the last three millennia in northwest Iberia via a high-resolution pollen analysis of a sediment core retrieved from the central axis of the Ria de Vigo in the south of Galicia. In doing so, they learned that over the past 3000 years there was "an alternation of three relatively cold periods with three relatively warm episodes." In order of occurrence, these periods are described by the three researchers as the "first cold phase of the Subatlantic period (975-250 BC)," which was "followed by the Roman Warm Period (250 BC-450 AD)," which was followed by "a successive cold period (450-950 AD), the Dark Ages," which "was terminated by the onset of the Medieval Warm Period (950-1400 AD)," which was followed by "the Little Ice Age (1400-1850 AD), including the Maunder Minimum (at around 1700 AD)," which "was succeeded by the recent warming (1850 AD to the present)." In addition, they report that "solar radiative budget and oceanic circulation seem to be the main mechanisms forcing this cyclicity in NW Iberia," which they describe as "paralleling global climatic changes recorded in North Atlantic marine records (Bond et al., 1997; Bianchi and McCave, 1999; Chapman and Shackelton, 2000)."

Also paralleling the findings of Desprat et al. were those of Pla and Catalan (2005), who analyzed chrysophyte cyst data collected from 105 lakes in the Central and Eastern Pyrenees of northeast Spain. Their work led to the production of a Holocene history of winter/spring temperatures in this region, revealing a significant climatic oscillation that alternated between warm and cold phases. Of particular note, in this regard, were the Little Ice Age, Medieval Warm Period, Dark Ages Cold Period and, once again, the Roman Warm Period.

Further to the east in European Georgia, Kvavadze and Connor (2005) focused their attention on Zelkova carpinifolia (a Tertiary-relict mesophilous tree that requires warm growing conditions), noting that "the discovery of fossil remains in Holocene sediments can be a good indicator of optimal climatic conditions." Pursuing such a course, they found that Western Georgian pollen spectra indicated that the Subatlantic period began about 2580 cal yr BP in a cold phase, but that by 2200 cal yr BP "climatic amelioration commenced," with "the maximum phase of warming observed in spectra from 1900 cal yr BP," which interval of warmth was Georgia's contribution to the Roman Warm Period. Thereafter, a cooler phase of climate, during the Dark Ages Cold Period, "occurred in Western Georgia about 1500-1400 cal yr BP," and it too was followed by another warm period "from 1350 to 800 years ago," which was the Medieval Warm Period.

During portions of this latter time interval, the two researchers report that tree lines "migrated upwards and the distribution of Zelkova broadened." What is more, they present a history of Holocene oscillations of the upper tree-line in Abkhasia -- derived by Kvavadze et al. (1992) -- which depicts greater-than-present elevations during a portion of the Medieval Warm Period and much greater extensions above the current tree-line during parts of the Roman Warm Period. Then, following the Medieval Warm Period, they report that "subsequent phases of climatic deterioration (including the Little Ice Age) ... saw an almost complete disappearance of Zelkova from Georgian forests." Based on these observations, therefore, it would appear that the Medieval Warm Period and especially the Roman Warm Period likely were significantly warmer than what it was in European Georgia during the latter part of the 20th century.

In one final study from the Iberian Peninsula, Garcia et al. (2007) state that "despite many studies that have pointed to .. the validity of the classical climatic oscillations described for the Late Holocene (Medieval Warm Period, Little Ice Age, etc.), there is a research line that suggests the non-global signature of these periods (IPCC, 2001; Jones and Mann, 2004)." Noting that "the best way to solve this controversy would be to increase the number of high-resolution records covering the last millennia and to increase the spatial coverage of these records," they proceed to do just that.

Working with a number of sediment cores retrieved from a river-fed wetland that is flooded for approximately seven months of each year in Las Tablas de Daimiel National Park, south central Iberian Peninsula, Spain, Garcia et al. employed "a high resolution pollen record in combination with geochemical data from sediments composed mainly of layers of charophytes alternating with layers of vegetal remains plus some detrital beds" to reconstruct "the environmental evolution of the last 3000 years." This work revealed the existence of five distinct climatic stages: "a cold and arid phase during the Subatlantic (Late Iron Cold Period, < B.C. 150), a warmer and wetter phase (Roman Warm Period, B.C. 150-A.D. 270), a new colder and drier period coinciding with the Dark Ages (A.D. 270-900), the warmer and wetter Medieval Warm Period (A.D. 900-1400), and finally a cooling phase (Little Ice Age, > A.D. 1400)."

Noting that "the Iberian Peninsula is unique, as it is located at the intersection between the Mediterranean and the Atlantic, Europe and Africa, and is consequently affected by all of them," Garcia et al.'s findings significantly advance the likelihood that, as they describe it, "the classical climatic oscillations described for the Late Holocene (Medieval Warm Period, Little Ice Age, etc.)" -- where "etc." clearly includes the Roman Warm Period -- were indeed both real and global in scope. Consequently, the establishment of the Current Warm Period over the course of the past century would appear to have been nothing more than the most recent cold-to-warm transition of this ever-recurring natural climatic oscillation. Therefore, this latter spate of modern global warming would also appear to have been a totally independent phenomenon that was completely unrelated to the concurrent historical increase in the air's CO2 content.

References
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Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., de Menocal, 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.

Chapman, M.R. and Shackelton, N.L. 2000. Evidence of 550-year and 1000-year cyclicities in North Atlantic circulation patterns during the Holocene. The Holocene 10: 287-291.

Desprat, S., Goñi, M.F.S. and Loutre, M.-F. 2003. Revealing climatic variability of the last three millennia in northwestern Iberia using pollen influx data. Earth and Planetary Science Letters 213: 63-78.

Garcia, M.J.G., Zapata, M.B.R., Santisteban, J.I., Mediavilla, R., Lopez-Pamo, E. and Dabrio, C.J. 2007. Vegetation History and Archaeobotany 16: 241-250.

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Jones, P.D. and Mann, M.E. 2004. Climate over past millennia. Reviews of Geophysics 42: 10.1029/2003RG000143.

Kvavadze, E.V., Bukreeva, G.F. and Rukhadze, L.P. 1992. Komp'iuternaia Tekhnologia Rekonstruktsii Paleogeograficheskikh Rekonstruksii V Gorakh (na primere golotsena Abkhazii). Metsniereba, Tbilisi.

Kvavadze, E.V. and Connor, S.E. 2005. Zelkova carpinifolia (Pallas) K. Koch in Holocene sediments of Georgia - an indicator of climatic optima. Review of Palaeobotany and Palynology 133: 69-89.

Martinez-Cortizas, A., Pontevedra-Pombal, X., Garcia-Rodeja, E., Novoa-Muñoz, J.C. and Shotyk, W. 1999. Mercury in a Spanish peat bog: Archive of climate change and atmospheric metal deposition. Science 284: 939-942.

Pla, S. and Catalan, J. 2005. Chrysophyte cysts from lake sediments reveal the submillennial winter/spring climate variability in the northwestern Mediterranean region throughout the Holocene. Climate Dynamics 24: 263-278.