We begin our brief review with the study of Hemer and Harris (2003), who extracted a sediment core from beneath the Amery Ice Shelf, East Antarctica, at a point that is currently about 80 km landward of the location of its present edge. In analyzing the core's characteristics over the past 5700 ¹⁴C years, the two scientists observed a peak in absolute diatom abundance in general, and the abundance of Fragilariopsis curta in particular -- which parameters, in their words, "are associated with increased proximity to an area of primary production, such as the sea-ice zone" -- at about 750 ¹⁴C yr B.P., which puts the time of maximum Ice Shelf retreat in close proximity to the historical time frame of the Medieval Warm Period.

Khim et al. (2002) likewise analyzed a number of properties of a sediment core removed from the eastern Bransfield Basin just off the northern tip of the Antarctic Peninsula, including grain size, total organic carbon content, magnetic susceptibility, biogenic silica content, ²¹⁰Pb geochronology and radiocarbon (¹⁴C) age, all of which data clearly depicted, in their words, the presence of the "Little Ice Age and Medieval Warm period, together with preceding climatic events of similar intensity and duration," demonstrating that the same millennial-scale climatic oscillation that reverberates throughout the region of the North Atlantic is also manifest in the Southern Ocean.

At about the same time, Hall and Denton (2002) mapped the distribution and elevation of surficial deposits along the southern Scott Coast of Antarctica in the vicinity of the Wilson Piedmont Glacier, which runs parallel to the coast of the western Ross Sea from McMurdo Sound north to Granite Harbor. The chronology of the raised beaches they studied was determined from more than 60 ¹⁴C dates of incorporated organic materials they had previously collected from hand-dug excavations (Hall and Denton, 1999); and the record the dates helped define demonstrated that near the end of the Medieval Warm Period, "as late as 890 ¹⁴C yr BP," as Hall and Denton describe it, "the Wilson Piedmont Glacier was still less extensive than it is now [our italics]," demonstrating that the climate of that period was in all likelihood considerably warmer than it is currently.

Noon et al. (2003) used oxygen isotopes preserved in authigenic carbonate retrieved from freshwater sediments of Sombre Lake on Signy Island (60°43'S, 45°38'W) in the Southern Ocean to construct a 7000-year history of that region's climate. This work revealed that the general trend of temperature at the study site has been downward. Of most interest to us, however, is the millennial-scale oscillation of climate that is apparent in much of the record. This climate cycle is such that approximately 2000 years ago, after a thousand-year gap in the data, Signy Island experienced the relative warmth of the last vestiges of the Roman Warm Period, as delineated by McDermott et al. (2001) on the basis of a high-resolution speleothem δ¹⁸O record from southwest Ireland. Then comes the Dark Ages Cold period, which is also contemporaneous with what McDermott et al. observe in the Northern Hemisphere, after which the Medieval Warm Period appears at the same point in time and persists for the same length of time that it does in the vicinity of Ireland, whereupon the Little Ice Age sets in just as it does in the Northern Hemisphere. Finally, there is an indication of late 20th-century warming, but with still a long way to go before conditions comparable to those of the Medieval Warm Period are achieved.

Two years later, Castellano et al. (2005) derived a detailed history of Holocene volcanism from the sulfate record of the first 360 meters of the Dome Concordia ice core that covered the period 0-11.5 kyr BP, after which they compared their results for the past millennium with similar results obtained from eight other Antarctic ice cores. Before doing so, however, they normalized the results at each site by dividing its several volcanic-induced sulfate deposition values by the value produced at that site by the AD 1816 Tambora eruption, in order to reduce deposition differences among sites that might have been induced by differences in local site characteristics. This work revealed that most volcanic events in the early last millennium (AD 1000-1500) exhibited greater among-site variability in normalized sulphate deposition than was observed thereafter.

Citing Budner and Cole-Dai (2003) in noting that "the Antarctic polar vortex is involved in the distribution of stratospheric volcanic aerosols over the continent," Castellano et al. say that assuming the intensity and persistence of the polar vortex in both the troposphere and stratosphere "affect the penetration of air masses to inland Antarctica, isolating the continental area during cold periods and facilitating the advection of peripheral air masses during warm periods (Krinner and Genthon, 1998), we support the hypothesis that the pattern of volcanic deposition intensity and geographical variability [higher values at coastal sites] could reflect a warmer climate of Antarctica in the early last millennium," and that "the re-establishment of colder conditions, starting in about AD 1500, reduced the variability of volcanic depositions."

Describing this phenomenon in terms of what it implies, Castellano et al. say "this warm/cold step could be like a Medieval Climate Optimum-like to Little Ice Age-like transition." We agree, noting they additionally cite Goosse et al. (2004) as reporting evidence from Antarctic ice-core δD and δ¹⁸O data "in support of a Medieval Warming-like period in the Southern Hemisphere, delayed by about 150 years with respect to Northern Hemisphere Medieval Warming." Hence, the ten researchers conclude their report by postulating that "changes in the extent and intra-Antarctic variability of volcanic depositional fluxes may have been consequences of the establishment of a Medieval Warming-like period that lasted until about AD 1500."

A year later, Hall et al. (2006) collected skin and hair (and even some whole-body mummified remains) from Holocene raised-beach excavations at various locations along Antarctica's Victoria Land Coast, which they identified by both visual inspection and DNA analysis as coming from southern elephant seals, and which they analyzed for age by radiocarbon dating. By these means they obtained data from fourteen different locations within their study region -- which they describe as being "well south" of the seals' current "core sub-Antarctic breeding and molting grounds" -- that indicate that the period of time they denominate the Seal Optimum began about 600 BC and ended about AD1400, the latter of which dates they describe as being "broadly contemporaneous with the onset of Little Ice Age climatic conditions in the Northern Hemisphere and with glacier advance near [Victoria Land's] Terra Nova Bay."

In describing the significance of their findings, the US, British and Italian researchers say they are indicative of "warmer-than-present climate conditions" at the times and locations of the identified presence of the southern elephant seal, and that "if, as proposed in the literature, the [Ross] ice shelf survived this period, it would have been exposed to environments substantially warmer than present," which would have included both the Roman Warm Period and Medieval Warm Period.

Most recently, Williams et al. (2007) presented methyl chloride (CH3Cl) measurements of air extracted from a 300-m ice core that was obtained at the South Pole, Antarctica, covering the time period 160 BC to AD 1860. In describing what they found, the researchers say that "CH3Cl levels were elevated from 900-1300 AD by about 50 ppt relative to the previous 1000 years, coincident with the warm Medieval Climate Anomaly (MCA)," and that they "decreased to a minimum during the Little Ice Age cooling (1650-1800 AD), before rising again to the modern atmospheric level of 550 ppt." Noting that "today, more than 90% of the CH3Cl sources and the majority of CH3Cl sinks lie between 30°N and 30°S (Khalil and Rasmussen, 1999; Yoshida et al., 2004)," they say "it is likely that climate-controlled variability in CH3Cl reflects changes in tropical and subtropical conditions." In fact, they go so far as to state that "ice core CH3Cl variability over the last two millennia suggests a positive relationship between atmospheric CH3Cl and global [our italics] mean temperature."

As best we can determine from the graphical representation of their data, the peak CH3Cl concentration measured by Williams et al. during the MCA is approximately 533 ppt, which is within 3% of its current mean value of 550 ppt and well within the range of 520 to 580 ppt that characterizes methyl chloride's current variability. Hence, we may validly conclude that the mean peak temperature of the MCA (which we refer to as the Medieval Warm Period) over the latitude range 30°N to 30°S -- and possibly over the entire globe -- may not have been materially different from the mean peak temperature so far attained during the Current Warm Period. And this conclusion, along with the findings of the other studies we have reviewed, suggests there is nothing that is unusual, unnatural or unprecedented about the current level of earth's warmth, which further suggests that the historical increase in the atmosphere's CO2 concentration may not have had anything to do with concomitant 20th-century global warming.

References
Budner, D. and Cole-Dai, J. 2003. The number and magnitude of large explosive volcanic eruptions between 904 and 1865 A.D.: Quantitative evidence from a new South Pole ice core. In: Robock, A. and Oppenheimer, C. (Eds.), Volcanism and the Earth's Atmosphere, Geophysics Monograph Series 139: 165-176.

Castellano, E., Becagli, S., Hansson, M., Hutterli, M., Petit, J.R., Rampino, M.R., Severi, M., Steffensen, J.P., Traversi, R. and Udisti, R. 2005. Holocene volcanic history as recorded in the sulfate stratigraphy of the European Project for Ice Coring in Antarctica Dome C (EDC96) ice core. Journal of Geophysical Research 110: 10.1029/JD005259.

Goosse, H., Masson-Delmotte, V., Renssen, H., Delmotte, M., Fichefet, T., Morgan, V., van Ommen, T., Khim, B.K. and Stenni, B. 2004. A late medieval warm period in the Southern Ocean as a delayed response to external forcing. Geophysical Research Letters 31: 10.1029/2003GL019140.

Hall, B.L. and Denton, G.H. 1999. New relative sea-level curves for the southern Scott Coast, Antarctica: evidence for Holocene deglaciation of the western Ross Sea. Journal of Quaternary Science 14: 641-650.

Hall, B.L. and Denton, G.H. 2002. Holocene history of the Wilson Piedmont Glacier along the southern Scott Coast, Antarctica. The Holocene 12: 619-627.

Hall, B.L., Hoelzel, A.R., Baroni, C., Denton, G.H., Le Boeuf, B.J., Overturf, B. and Topf, A.L. 2006. Holocene elephant seal distribution implies warmer-than-present climate in the Ross Sea. Proceedings of the National Academy of Sciences USA 103: 10,213-10,217.

Hemer, M.A. and Harris, P.T. 2003. Sediment core from beneath the Amery Ice Shelf, East Antarctica, suggests mid-Holocene ice-shelf retreat. Geology 31: 127-130.

Khalil, M.A.K. and Rasmussen, R.A. 1999. Atmospheric methyl chloride. Atmospheric Environment 33: 1305-1321.

Khim, B-K., Yoon, H.I., Kang, C.Y. and Bahk, J.J. 2002. Unstable climate oscillations during the Late Holocene in the Eastern Bransfield Basin, Antarctic Peninsula. Quaternary Research 58: 234-245.

Krinner, G. and Genthon, C. 1998. GCM simulations of the Last Glacial Maximum surface climate of Greenland and Antarctica. Climate Dynamics 14: 741-758.

McDermott, F., Mattey, D.P. and Hawkesworth, C. 2001. Centennial-scale Holocene climate variability revealed by a high-resolution speleothem ð¹⁸O record from SW Ireland. Science 294: 1328-1331.

Noon, P.E., Leng, M.J. and Jones, V.J. 2003. Oxygen-isotope (ð¹⁸O) evidence of Holocene hydrological changes at Signy Island, maritime Antarctica. The Holocene 13: 251-263.

Williams, M.B., Aydin, M., Tatum, C. and Saltzman, E.S. 2007. A 2000 year atmospheric history of methyl chloride from a South Pole ice core: Evidence for climate-controlled variability. Geophysical Research Letters 34: 10.1029/2006GL029142.

Yoshida, Y., Wang, Y.H., Zeng, T. and Yantosea, R. 2004. A three-dimensional global model study of atmospheric methyl chloride budget and distributions. Journal of Geophysical Research 109: 10.1029/2004JD004951.