What was done
Working with four sediment cores taken from Chesapeake Bay (CB) between 1996 and 2000, the authors reconstructed a 2200-year record of spring sea surface temperature (SST) "using the magnesium/calcium (Mg/Ca) proxy method as a paleothermometer (Chivas et al., 1986)."  For most of the record, which began at 200 BC, their sampling yielded one data point every eight years.  From 1700 AD to the end of the record, however, one data point was obtained every 1-3 years.

What was learned
Cronin et al. report that mean 20th century CB temperature was "not warmer than mean temperatures during MWP-I," the first stage of the Medieval Warm Period, which they delineate as occurring between 450 and 900 AD.  In addition, we note there were similar periods of equivalent warmth during the Roman Warm Period, between approximately 100 BC and 200 AD.

The warmest quarter-century period of all occurred between 1850 and 1875, after which temperatures plummeted, only to rise again about 1910.  This latter warming, which peaked in the 1940s and 50s, was more erratic than its immediate predecessor; and, therefore, its highest temperatures were not sustained for as long a period of time.  Then came another cooling, which was only briefly mitigated in the early 1980s (as close as we can tell from Cronin et al.'s graphs), after which temperatures dropped to some of their lowest levels of both the 19th and 20th centuries.

What it means
The Chesapeake Bay temperature history differs from the "hockeystick" temperature record championed by the world's climate alarmists in a number of important ways.  First of all, it demonstrates the existence of the Medieval Warm Period, which the hockeystick does not recognize, as well as the existence of the Roman Warm Period, to which time period the hockeystick does not extend.  Second, it demonstrates that these earlier warm periods, when the air's CO2 concentration was fully 100 ppm less than what it is today, were equally as warm as what it was, in the mean, over the course of the 20th century.  Third, it demonstrates that the warmest quarter-century of the entire 2200 years did not occur during the 20th century.  Fourth, and perhaps most amazingly, it demonstrates that the final few years of the CB record, which fall within the late 20th-century period that climate alarmists claim to have been the warmest period of the past two millennia, were actually some of the coldest years of the past quarter of a millennium.

Reference
Chivas, A.R., DeDeckker, P. and Shelley, J.M.G.  1986.  Magnesium content of non-marine ostracod shells: a new palaeosalinometer and palaeothermometer.  Palaeogeography, Palaeoclimatology, Palaeoecology 54: 43-61.