What was done
The authors reviewed a set of twenty temperature logs derived from boreholes located at fourteen different sites in mainland Portugal in an attempt to reconstruct a five-century surface air temperature history for that part of the world.

What was learned
Seven of the borehole temperature logs were too "noisy" to use; while six displayed evidence of groundwater perturbations and were thus not usable for that reason.  Of the remaining seven logs, all depicted little temperature change over the first three centuries of record.  Thereafter, four of them exhibited warming trends that began about 1800 and peaked around 1940, one showed a warming that peaked in the mid-1800s, another was constant across the entire five centuries, and one actually revealed cooling over the last century.  From these observations the authors concluded that "the single inversions cannot be interpreted individually."

Correia and Safanda next performed a joint analysis of the seven borehole records, obtaining a warming of 0.5-0.6°C since the second half of the 18th century, which was followed by a cooling of 0.2°C.  They then compared the joint borehole record with the surface air temperature record directly measured at a meteorological station in Lisbon, about 150-200 km to the northwest.  From the beginning of the surface air temperature record in 1856 until 1949, the Lisbon data yielded a warming of 0.8°C; while the borehole record displayed a warming of only 0.3-0.4°C.

What it means
The two temperature histories could both be right; for they are separated from each other by a significant distance.  They could also both be wrong.  The Lisbon record, for example, could suffer from a number of the problems enumerated in our editorial The Global Surface Air Temperature Record Must Be Wrong; while the joint borehole record was only obtained after several individual records were rejected for various reasons and a number of simplifying assumptions were invoked in the analyses of the remaining records.  The authors recognize these problems, noting that the issue is not yet resolved and that much more work must be done to arrive at a satisfactory conclusion.