Reference
Cunningham, L.K., Austin, W.E.N., Knudsen, K.L., Eiriksson, J., Scourse, J.D., Wannamaker Jr., A.D., Butler, P.G., Cage, A.G., Richter, T., Husum, K., Hald, M., Andersson, C., Zorita, E., Linderholm, H.W., Gunnarson, B.E., Sicre, M.-A., Sejrup, H.P., Jiang, H. and Wilson, R.J.S. 2013. Reconstructions of surface ocean conditions from the northeast Atlantic and Nordic seas during the last millennium. The Holocene 23: 921-935.
Background
The authors write that "marine evidence for climate change spanning the last millennium is extremely limited when compared with terrestrial records available from the Northern Hemisphere," noting that "this deficiency was highlighted by the IPCC (AR4)," wherein it was written by Jansen et al. (2007) that (1) "there are important limitations due to a lack of ... ocean records," and that (2) "this assessment [the AR4] would be improved with extensive networks of proxy data that run right up to the present day." Well, as of the publication of Cunningham et al. (2013), that deficiency is no longer as great as it was back at the time of the publication of the IPCC's AR4; and we are much the better for it.
What was done
Using ten proxy sea surface temperature (SST) records - six from the Norwegian Margin, three from the North Icelandic Shelf and one from the Scottish Margin - Cunningham et al. prepared a 1,000-year SST record spanning the period from AD 1000 to 2000, which revealed, in their words, that "the 'Medieval Climate Anomaly' warming was most pronounced before AD 1200, with a long-term cooling trend apparent after AD 1250." And what did the new record reveal about the uniqueness - or not! - of the region's late-20th-century Current Warm Period?
What was learned
The twenty researchers report that "in recent decades temperatures have been similar to those inferred for the 'Medieval Climate Anomaly'." However, their graphical representations (three versions of them) of the one-thousand-year period clearly indicate that the peak warmth of the Current Warm Period has actually been a couple tenths of a degree C less than that of the earlier Medieval Warm Period.
What it means
As has been found to be the case in so many land-based assessments of the relative warmth of the Medieval and Current Warm Periods, there is nothing unusual, unnatural or unprecedented about earth's current level of warmth ... although it could be thought to be somewhat unusual in that even with the 120-ppm increase in the atmosphere's CO2 concentration that occurred over the last 1000 years, it is still not as hot now as it was back in the Medieval Warm Period.
Reference
Jansen, E., Overpeck, J., Briffa, K.R., Duplessy, J.-C., Joos, F., Masson-Delmotte, V., Olago, D., Otto-Bliesner, B., Peltier, W.R., Rahmstorf, S., Ramesh, R., Raynaud, D., Rind, D., Solomina, O., Villalba, R. and Zhang, D. 2007. Palaeoclimate. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M. and Miller, H.L. (Eds.). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom, pp. 433-497.