What was done
The authors located fossil oyster beds within the Tappan Zee area of the Hudson River estuary (New York, USA) via chirp sub-bottom and side-scan sonar surveys, after which they retrieved sediment cores from the sites that provided shells for radiocarbon dating.

What was learned
Carbotte et al. report that "oysters flourished during the mid-Holocene warm period," when they note that "summertime temperatures were 2-4°C warmer than today (e.g., Webb et al., 1993; Ganopolski et al., 1998)."  Thereafter, they found that the oysters "disappeared with the onset of cooler climate at 4,000-5,000 cal. years BP," but that they "returned during warmer conditions of the late Holocene," which they specifically identify as the Roman and Medieval Warm Periods as delineated by Keigwin (1996) and McDermott et al. (2001), explicitly stating that "these warmer periods coincide with the return of oysters in the Tappan Zee."  Unfortunately, they report that their shell dates suggest a final "major demise at ~500-900 years BP," which timing they describe as being "consistent with the onset of the Little Ice Age," noting further that within nearby Chesapeake Bay, "Cronin et al. (2003) report a sustained period of cooler springtime water temperatures (by ~2-5°C) during the Little Ice Age relative to the earlier Medieval Warm Period."  Last of all, they add that "similar aged fluctuations in oyster presence are observed within shell middens elsewhere along the Atlantic seaboard," citing results obtained from Maine to Florida.

What it means
This intriguing study of the periodic establishment and demise of oyster beds in the Hudson River estuary and elsewhere along the east coast of the United States paints a clear picture of alternating multi-century warm and cold intervals over the past two millennia that is vastly different from the 1000-year-long "hockeystick" temperature history of Mann et al. (1998, 1999) and the 2000-year-long history produced by Mann and Jones (2003), wherein Northern Hemispheric and global mean temperatures experience essentially no low-frequency variability until the advent of the 20th century, when temperatures are portrayed as rising dramatically, allowing them to claim that 20th-century warming was driven by anthropogenic CO2 emissions.  With respect to this contention, however, they are most certainly wrong, as papers that tell essentially the same climatic story as that of the Carbotte et al. study are published almost weekly in the scientific literature, as one can readily verify by perusing our website archives; and this ever-growing mountain of real-world evidence clearly shows that late 20th-century warmth was by no means unique over the past two millennia, having been equaled, and often surpassed, at various times throughout both the Medieval Warm Period of a thousand years ago and the Roman Warm Period of two thousand years ago, during both of which epochs the air's CO2 concentration was fully 100 ppm less than what it is today, signifying that what we could call the Modern Warm Period is nothing more than the most recent high-temperature phase of this natural non-CO2-driven millennial-scale oscillation of earth's climate.

References
Cronin, T.M., Dwyer, G.S., Kamiya, T., Schwede, S. and Willard, D.A.  2003.  Medieval warm period, Little Ice Age and 20th century temperature variability from Chesapeake Bay.  Global and Planetary Change 36: 17-29.

Ganopolski, A., Kubatzki, C., Claussen, M., Brovkin, V. and Petoukhov, V.  1998.  The influence of vegetation-atmosphere-ocean interaction on climate during the mid-Holocene.  Science 280: 1916-1919.

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

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1998.  Global-scale temperature patterns and climate forcing over the past six centuries.  Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1999.  Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations.  Geophysical Research Letters 26: 759-762.

Mann, M.E. and Jones, P.D.  2003.  Global surface temperatures over the past two millennia.  Geophysical Research Letters 30: 10.1029/2003GL017814.

McDermott, F., Mattey, D.P. and Hawkesworth, C.  2001.  Centennial-scale Holocene climate variability revealed by a high-resolution speleothem δ¹⁸O record from SW Ireland.  Science 294: 1328-1331.

Webb III, T., Bartlein, P.J., Harrison, S.P. and Anderson, K.H.  1993.  Vegetation, lake levels, and climate in eastern North America for the past 18000 years.  In: Wright, H.E., Kutzbach, J.E., Webb III, T., Ruddiman, W.F., Street-Perrott, F.A. and Bartlein, P.J. (Eds.)  Global Climates Since the Last Glacial Maximum, University of Minnesota Press, Minneapolis, Minnesota, USA, pp. 415-467.