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1400 Years of Climate Variability in the Gulf of Mexico
Reference
Richey, J.N., Poore, R.Z., Flower, B.P. and Quinn, T.M. 2007. 1400 yr multiproxy record of climate variability from the northern Gulf of Mexico. Geology 35: 423-426.

Background
The authors note that the variability of the hemispheric temperature reconstructions of Mann and Jones (2003) over the past one to two thousand years are "subdued (<=0.5C)," and that their low-amplitude reconstructions contrast "with several individual marine records that indicate that centennial-scale sea surface temperature (SST) oscillations of 2-3C occurred during the past 1-2 k.y. (i.e., Keigwin, 1996; Watanabe et al., 2001; Lund and Curry, 2006; Newton et al., 2006)," just as they also contrast with "tree-ring and multiproxy reconstructions designed to capture multicentennial-scale variability (e.g., Esper et al., 2002; Moberg et al., 2005)," which further suggests that "the amplitude of natural climate variability over the past 1 k.y. is >0.5C."

What was done
Richey et al. say that "a continuous decadal-scale resolution record of climate variability over the past 1400 years in the northern Gulf of Mexico was constructed from a box core recovered in the Pigmy Basin, northern Gulf of Mexico [2711.61'N, 9124.54'W]," based on climate proxies derived from "paired analyses of Mg/Ca and δ18O in the white variety of the planktic foraminifer Globigerinoides ruber and relative abundance variations of G. sacculifer in the foraminifer assemblages."

What was learned
The researchers report that "two multi-decadal intervals of sustained high Mg/Ca indicate that Gulf of Mexico sea surface temperatures (SSTs) were as warm or warmer than [our italics] near-modern conditions between 1000 and 1400 yr B.P.," while "foraminiferal Mg/Ca during the coolest interval of the Little Ice Age (ca. 250 yr B.P.) indicate that SST was 2-2.5C below modern SST." In addition, they found that "four minima in the Mg/Ca record between 900 and 250 yr. B.P. correspond with the Maunder, Sporer, Wolf, and Oort sunspot minima."

What it means
Solid empirical evidence continues to accumulate, demonstrating that the Medieval Warm Period (MWP) was an interval of elevated global temperatures that were equally as warm as, or even warmer than, those of the Current Warm Period, and that the MWP's elevated warmth was likely solar-induced, which suggests that the Current Warm Period may well be deriving its warmth from the same source, as it is likely nothing more than the most recent manifestation of the warm node of this ever-recurring climate cycle.

References
Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2254.

Keigwin, L.D. 1996. The Little Ice Age and Medieval Warm Period in the Sargasso Sea. Science 274: 1504-1508.

Lund, D.C. and Curry, W. 2006. Florida current surface temperature and salinity variability during the last millennium. Paleoceanography 21: 10.1029/2005PA001218.

Mann, M.E. and Jones, P.D. 2003. Global surface temperature over the past two millennia. Geophysical Research Letters 30: 1820-1823.

Moberg, A., Sonechkin, D.M., Holmgren, K., Datsenko, N.M. and Karlen, W. 2005. Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433: 613-617.

Newton, A., Thunell, R. and Stott, L. 2006. Climate and hydrographic variability in the Indo-Pacific Warm Pool during the last millennium. Geophysical Research Letters 33: 10.1029/2006GL027234.

Watanabe, T., Winter, A. and Oba, T. 2001. Seasonal changes in sea surface temperature and salinity during the Little Ice Age in the Caribbean Sea deduced from Mg/Ca and 18O/16O ratios in corals. Marine Geology 173: 21-35.

Reviewed 31 October 2007