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The MWP and LIA in the Ross Sea Region of Antarctica
Reference
Bertler, N.A.N., Mayewski, P.A. and Carter, L. 2011. Cold conditions in Antarctica during the Little Ice Age -- Implications for abrupt climate change mechanisms. Earth and Planetary Science Letters 308: 41-51.

Background
For many years, certain climatologists claimed that the Little Ice Age (LIA) and Medieval Warm Period (MWP) were neither global phenomena nor strong enough where they did occur to have a discernable influence on global air temperature, which tended to make the global warming of the latter part of the 20th century appear highly unusual and, therefore, more easily associated with anthropogenic CO2 emissions than it otherwise would have been. Thus, it is quite significant if evidence is found for these two climatic phenomena as far away from the North Atlantic region as Antarctica.

What was done
The authors obtained new deuterium (δD) data from the Ross Sea region of Antarctica that they acquired via analysis of the top fifty meters of a 180-meter-long ice core that had been extracted from the ice divide of Victoria Lower Glacier in the northernmost McMurdo Dry Valleys, which they converted to temperature data by means of a temperature-isotope relationship developed by Steig et al. (1998) from data obtained from the Taylor Dome ice core record.

What was learned
Bertler et al. report that they identified three distinct time periods in their record: the last 150 years of the Medieval Warm Period (AD 1140 to 1287), the Little Ice Age (AD 1288 to 1807), and the Modern Era (AD 1808 to 2000). And with respect to the Little Ice Age, they note that summer temperatures at the core site were 2°C colder than those of the Modern Era (ME), while they write that "the McMurdo Dry Valleys were 0.35°C warmer during the MWP than during ME, accompanied by warmer conditions in the Ross Sea."

What it means
The three researchers say their results suggest that "the McMurdo Dry Valleys experienced warm summers, with increased snow accumulation and higher sea surface temperatures in the Ross Sea/Southern Ocean, perhaps accompanied by less sea-ice." And they report that "a magnetic susceptibility record from Palmer Deep marine core (PD92 30MS) also supports warmer MWP conditions, this time in Drake Passage (Domack and Mayewski, 1999)." In addition, they write that their data are "consistent with an Antarctic-wide assessment of LIA climate," noting that "New Zealand, which has direct climatic and oceanic links with Antarctica, also displays a cooling that began around AD 1250-1350 and peaked ~AD 1500 to 1650 before recovering at the end of the 19th century (Lorrey et al., 2008)," further noting that southernmost South America also underwent a Little Ice Age climate shift, citing the studies of Koch and Kilian (2005) and Lamy et al. (2001). And with such discoveries continuing to be made at the bottom of the world, there can be little doubt that the warm-cold-warm oscillation of the MWP-LIA-ME has been a truly global phenomenon.

References
Domack, E.W. and Mayewski, P.A. 1999. Bi-polar ocean linkages: evidence from late-Holocene Antarctic marine and Greenland ice-core records. The Holocene 9: 247-251.

Koch, J. and Kilian, R. 2005. "Little Ice Age" glacier fluctuations, Gran Campo Nevado, southernmost Chile. The Holocene 15: 20-28.

Lamy, F., Hebbein, D., Rohl, U. and Wefer, G. 2001. Holocene rainfall variability in southern Chile: a marine record of latitudinal shifts of the Southern Westerlies. Earth and Planetary Science Letters 185: 369-382.

Lorrey, A., Williams, P., Salinger, J., Martin, T., Palmer, J., Fowler, A., Zhao, J. and Neil, H. 2008. Speleothem stable isotope records interpreted within a multi-proxy framework and implications for New Zealand palaeoclimate reconstruction. Quaternary International 187: 52-75.

Steig, E.J., Brook, E.J., White, J.W.C., Sucher, C.M., Bender, M.L., Lehman, S.J., Morse, D.L., Waddington, E.D. and Clow, G.D. 1998. Synchronous climate changes in Antarctica and the North Atlantic. Science 282: 92-95.

Reviewed 9 November 2011