Late-Holocene Climate of the Northeast Atlantic Ocean
Richter, T.O., Peeters, F.J.C. and van Weering, T.C.E. 2009. Late Holocene (0-2.4 ka BP) surface water temperature and salinity variability, Feni Drift, NE Atlantic Ocean. Quaternary Science Reviews 28: 1941-1955.
According to the authors, their research revealed "a general long-term cooling trend," but that "superimposed on this overall trend" were "partly higher temperatures and salinities from 180 to 560 AD and 750-1160 AD," which they say "may be ascribed to the Roman and Medieval Warm Periods, respectively," the latter of which was followed by the Little Ice Age (LIA) and what they describe as the "post-LIA recovery and, possibly, (late) 20th century anthropogenic warming."
Of this latter warming, they say that it "concurs with distinct continental-scale warming, consistently reaching unprecedented maximum temperatures after ~1990 AD." Their use of the word "unprecedented," however, is a bit misleading; for they subsequently state that "the SST increase over the last three decades does not, or not 'yet', appear unusual compared to the entire 0-2.4 ka record," and that "the warming trend over the second half of the 20th century has not yet reverted the late Holocene millennial-scale cooling." In fact, their data clearly indicate that the peak temperature of the Medieval Warm Period was approximately 2.2°C greater than the peak temperature of the late 20th century, and that the peak temperature of the Roman Warm Period was about 2.7°C greater than that of the late 20th century.
The fact that the warmest portions of the Roman and Medieval Warm Periods in the vicinity of the northeast Atlantic were so much warmer than the warmest portion of the Current Warm Period - and at times when the air's CO2 content was so much less than it is currently - strongly suggests that the atmosphere's CO2 concentration had little to no impact on the late-Holocene climatic history of that part of the planet.
So what was responsible for the oscillating temperatures of the surface waters of the northeast Atlantic Ocean? The three Dutch researchers say that "pervasive multidecadal- to centennial-scale variability throughout the sedimentary proxy records can be partly attributed to solar forcing and/or variable heat extraction from the surface ocean caused by shifts in the prevailing state of the North Atlantic Oscillation," as well as to "internal (unforced) fluctuations." Hence, there would appear to be little need to blame the current high (relatively speaking) atmospheric CO2 concentration for the current low (relatively speaking with respect to the entire 2400-year period) temperature of the northeast Atlantic.