Researchers from the University of Bergen and the University of Colorado studied marine sediment cores from the Norwegian continental margin. They were able to get accurate dates from lead isotope dating of interspersed volcanic rocks in the core. They examined oxygen-18 isotopes from the calcium carbonate in the shells of planktonic foraminifera to reconstruct temperature. (Oxygen-18 is a proxy for temperature, see NASA’s Earth Observatory explanation of the method here.)
This allowed the researchers to come up with what they call “near surface water summer temperature.” for the past 2,000 years. The following graph depicts their temperature reconstruction:
The graph shows that the current warm period is cooler than the Medieval and Roman warm periods. The researchers report a statistically significant correlation with the Gleissberg solar cycle. This is more evidence that the forces of natural variability overcome the effect, if any, of anthropogenic carbon dioxide emissions, and it shows that current warming is neither unprecedented nor unusual.
Sejrup, H.P., Haflidason, H. and Andrews, J.T. 2011. A Holocene North Atlantic SST record and regional climate variability. Quaternary Science Reviews 30: 3181-3195. Abstract here.