The Roman Warm Period in Southwest Scotland
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.
Working with shells of the common European limpet (Patella vulgata) that they collected from Croig Cave, an archaeological site on the Isle of Mull in the Hebrides Islands west of mainland Scotland, Wang et al. developed a high-resolution sea surface temperature (SST) history based on measured δ18O values using the calibration procedure developed by Fenger et al. (2007).
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."
Once again 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.
Fenger, T., Surge, D., Schone, B. and Milner, N. 2007. Sclerochronology and geochemical variation in limpet shells (Patella vulgata): a new archive to reconstruct coastal sea surface temperature. Geochemistry, Geophysics, Geosystems 8: 10.1029/2006GC001488.