Background
The authors write that "pre-industrial climate reconstructions during the mid to late Holocene provide the necessary information for understanding natural variation in the climate system prior to anthropogenic changes in the atmosphere, hydrosphere, and land use," which information is a prerequisite for determining whether late 20th-century warming was natural or man-induced by the burning of fossil fuels.

What was done
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).

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.

Reference
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.