Climate alarmists contend that the degree of global warmth over the latter part of the 20th century, and continuing to the present day, was greater than it was 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 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 studies conducted in lands surrounding the Mediterranean Sea.
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 the 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 work 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 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. Thus, Martinez-Cortizas et al. concluded, and rightly so, that "for the past 4000 years ... the Roman Warm Period and the Medieval Warm Period were the most important warming periods."
Four years later, 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. By so doing, they found "an alternation of three relatively cold periods with three relatively warm episodes." In order of their occurrence, these periods were described by Desprat et al. 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)." Commenting on their findings, Desprat et al. offered the opinion that "solar radiative budget and oceanic circulation seem to be the main mechanisms forcing this cyclicity in NW Iberia," noting that "a millennial-scale climatic cyclicity over the last 3000 years is detected for the first time in NW Iberia paralleling global climatic changes recorded in North Atlantic marine records (Bond et al., 1997; Bianchi and McCave, 1999; Chapman and Shackelton, 2000)." And this body of findings suggests that the establishment of the Current Warm Period over the course of the past century or so may have been nothing more than the most recent manifestation of this naturally-recurring phenomenon.
After two more years had passed, Kvavadze and Connor (2005) analyzed various sets of data pertaining to the ecology, pollen productivity and Holocene history of Zelkova carpinifolia, a Tertiary-relict tree whose pollen is almost always accompanied by elevated concentrations of the pollen of other thermophilous taxa; and because Zelkova carpinifolia requires heat and moisture during the growing period, they say that the discovery of fossil remains of the species in Holocene sediments "can be a good indicator of optimal climatic conditions." More specifically, they indicate that "Western Georgian pollen spectra of the Subatlantic period show that the period began in a cold phase, but, by 2200 cal yr BP, climatic amelioration commenced," noting that "the maximum phase of warming [was] observed in spectra from 1900 cal yr BP," which interval of warmth was Georgia's contribution to the Roman Warm Period.
A cooler phase of climate, during the Dark Ages Cold Period, "occurred in Western Georgia about 1500-1400 cal yr BP," according to the two scientists; but it too was followed by another warm period "from 1350 to 800 years ago," which was, of course, the Medieval Warm Period. During portions of this latter warm epoch, they report that tree lines "migrated upwards and the distribution of Zelkova broadened." In addition, they present a history of Holocene oscillations of the upper tree-line in Abkhasia -- derived by Kvavadze et al. (1992) -- that depicts slightly greater-than-1950 elevations during a portion of the Medieval Warm Period and much greater extensions above the 1950 tree-line during parts of the Roman Warm Period, which observations imply much warmer conditions than what prevailed there around AD 1950, which was the "present" of Kvavadze and Connor's study.
Working contemporaneously, Pla and Catalan (2005) analyzed chrysophyte cyst data they collected from 105 lakes located within the Central and Eastern Pyrenees of northeast Spain to produce a Holocene history of winter/spring temperatures. A significant oscillation was evident in this thermal reconstruction 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, once again, the subject of this summary: the Roman Warm Period.
Last of all, we come to the paper of Garcia et al. (2007), who introduced the report of their work by noting 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 proceeded 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." In doing so, the six Spanish researchers were able to identify 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. significantly advanced the likelihood that the classical climatic oscillations described for the Late Holocene -- of which the Roman Warm Period is a prime example -- were indeed both real and global in scope, as well as not-CO2-induced, which means that earth's current level of warmth need not be CO2-induced as well.
Bianchi, G.G. and McCave, I.N. 1999. Holocene periodicity in North Atlantic climate and deep-ocean flow south of Iceland. Nature 397: 515-517.
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., Goņi, 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.
IPCC. 2001. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change.
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., 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-Muņoz, 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.Last updated 26 May 2010