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The Roman Warm Period vs. the Current Warm Period
Reference
Chen, L., Zonneveld, K.A.F. and Versteegh, G.J.M. 2011. Short term climate variability during the "Roman Classical Period" in the eastern Mediterranean. Quaternary Science Reviews 30: 3880-3891.

Background
The authors write that "although it is obvious that anthropogenic activities influence climate, natural processes steer climate as well (Stott et al., 2000; Carslaw et al., 2002)," and they say that "this has resulted in intensive discussions in the academic community and general society alike about to what extent these natural processes might be responsible for the present change of climate," and they state, in this regard, that "one of the most intriguing questions within the climate debate is if the present temperature rise is unique in the late Holocene or if there have been pre-industrial time intervals where comparable climatic perturbations occurred," noting that "one of these time intervals where historical records suggest that climate conditions might have been similar to today is the so called 'Roman Warm Period' (~200 BC - AD 400)."

What was done
In light of these considerations, and in order "to obtain insight into the character and potential forcing of short-term climatic and oceanographic variability in the southern Italian region during the 'Roman Classical Period' (60 BC - AD 200)," Chen et al. developed high-resolution climatic and environmental reconstructions "based on a dinoflagellate cyst record from a well dated site in the Gulf of Taranto located at the distal end of the Po River discharge plume."

What was learned
The three researchers determined that the dinoflagellate cyst warm/cold ratio suggests that sea surface temperature (SST) was "relatively high and stable between 60 BC and AD 200," which they say is suggestive of "slightly higher SST than today." In fact, they say that the association that is observed between 60 BC and AD 90 is equivalent to modern regions that are characterized by higher SST than those in the present day Gulf of Taranto. And noting that "Versteegh et al. (2007) showed that SST in the region is strongly related to local air temperature," they go on to suggest that the region's air temperature "might have been warmer during the Roman Period as well."

To buttress this conclusion, Chen et al. indicate that "similar results of ameliorative climate conditions during the Roman Period have also been suggested by other studies in the Mediterranean region." As an example, they note that based on an investigation of stalagmites in the southeast Alps, "Frisia et al. (2005) conclude that the 'Roman Classical Period' temperatures were similar or slightly higher than those of today with the highest temperatures reached between around 400 BC and 0 AD," and they state that the relatively high temperatures reconstructed during the Roman Period "are consistent with a reduced glacier extent in the Alps," citing Holzhauser et al. (2005) and Giraudi (2009). In addition, they state that based on pollen records in Georgia, "a maximum phase of warming in the Holocene is reconstructed between 100 BC and AD 200," citing Kvavadze and Connor (2005).

What it means
With respect to the southern region of Italy, it would appear that the relatively high temperatures of today are not unique. In fact, they may well be somewhat lower than those that prevailed there during the Roman Warm Period. And these findings suggest that the non-unique warmth of our day need not be attributed to a unique phenomenon, such as the historical increase in the atmosphere's CO2 concentration that has resulted from mankind's burning of fossil fuels.

References
Carlaw, K.S., Harrison, R.G. and Kirkby, J. 2002. Cosmic rays, clouds, and climate. Science 298: 1732-1737.

Frisia, S., Borsato, A., Spotl, C., Villa, I.M. and Cucchi, F. 2005. Climate variability in the SE Alps of Italy over the past 17,000 years reconstructed from a stalagmite record. Boreas 34: 445-455.

Giraudi, C. 2009. Late Holocene glacial and periglacial evolution in the upper Orco Valley, northwestern Italian Alps. Quaternary Research 71: 1-8.

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.

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.

Stott, P.A., Tett, S.F.B., Jones, G.S., Allen, M.R., Mitchell, J.F.B. and Jenkins, G.J. 2000. External control of 20th century temperature by natural and anthropogenic forcings. Science 290: 2133-2137.

Versteegh, G.J.M., de Leeuw, J.W., Taricco, C., and Romero, A. 2007. Temperature and productivity influences on U-37(K') and their possible relation to solar forcing of the Mediterranean winter. Geochemistry Geophysics Geosystems 8: 10.1029/2006GC001543.

Reviewed 29 February 2012