See CO2 Science:
What was learned
Based on graphical representations of their reconstructed SSTs pertaining to five intervals of the Roman Warm Period (RWP) - which the three researchers identify as occurring between 2500 and 1600 years Before Present - it is evident that fully one-third of the peak summer SSTs were either equal to, or greater than, the mean summer SST of the period AD 1961-1990, leading them to conclude that the mean summer temperature of that portion of the RWP "was similar to the late 20th century."
What it means
Once again - see Roman Warm Period in our Subject Index - we have another manifestation of the fact that in numerous places around the world, it was just as warm - or warmer - some two millennia ago than it has been recently, when there was 30% less CO2 in the air than there is today. And this observation strongly suggests that the warmth of today is likely due to something other than mankind's CO2 emissions.
Wang, T., Surge, D. and Mithen, S. 2012. Seasonal temperature variability of the Neoglacial (3300-2500 BP) and Roman Warm Period (2500-1600 BP) reconstructed from oxygen isotope ratios of limpet shells (Patella vulgata), Northwest Scotland. Palaeogeography, Palaeoclimatology, Palaeoecology 317-318: 104-113.