What was done
Working with a core of 2.5 meters length, which they sampled at intervals of 2 cm in the upper 1 meter and at intervals of 5 cm below that depth, the authors derived a record of mercury deposition in the peat bog of Penido Vello in northwest Spain (43°32'N, 7°34'W) that extends to 4000 radiocarbon years before the present, which they analyzed for a number of parameters.  This work revealed, in their words, "that cold climates promoted an enhanced accumulation and the preservation of mercury with low thermal stability, and warm climates were characterized by a lower accumulation and the predominance of mercury with moderate to high thermal stability."  Based on these findings and further analyses, they derived a temperature history for the region that they standardized to the mean temperature of the most recent 30 years of their record.

What was learned
The five scientists determined that the mean temperature of the Medieval Warm Period in northwest Spain was 1.5°C warmer than it was over the 30 years leading up to the time of their study, and that the mean temperature of the Roman Warm Period was 2°C warmer.  Even more impressive was their finding that several decadal-scale intervals during the Roman Warm Period were more than 2.5°C warmer than the 1968-98 period, while an interval in excess of 80 years during the Medieval Warm Period was more than 3°C warmer.

What it means
Martinez-Cortizas et al. conclude, and rightly so, that "for the past 4000 years ... the Roman Warm Period and the Medieval Warm Period were the most important warming periods."  And, of course, we hardly need to note that these conclusions totally repudiate (for this small part of the world, at least) the drastically different claims of Mann et al. (1998, 1999) and Mann and Jones (2003), as well as the "consensus" judgment of the Intergovernmental Panel on Climate Change, while many other studies repudiate the IPCC's position in many other parts of the world (see Medieval Warm Period and Roman Warm Period in our Subject Index).