Stewart, M.M., Larocque-Tobler, I. and Grosjean, M. 2011. Quantitative inter-annual and decadal June-July-August temperature variability ca. 570 BC to AD 120 (Iron Age-Roman Period) reconstructed from the varved sediments of Lake Silvaplana, Switzerland. Journal of Quaternary Science 26: 491-501.
What was done
Working with a piston core that was extracted during the winter of 2005-2006 from Lake Silvaplana (46°26'56"N, 9°47'33"E) in the eastern Swiss Alps, the authors developed a June-July-August (JJA) temperature history of the surrounding region based on biogenic silica (BSi) and chironomid data they obtained from the core.
What was learned
Stewart et al. report that "temperatures reconstructed from ca. 570 BC to AD 120 were warmer than today (AD 1950-2000; 9.8°C)," with the JJA temperatures of the warmest portion of this interval (ca. 570-351 BC) being approximately 11.2°C, after which they say they also "found moderately warm temperatures for the Roman Period (11°C, ca. 15 BC to AD 120)." Using the first of our World Temperature Databases -- the GHCN Surface Temperature database -- we calculate a mean linear temperature increase of 1.0°C between AD 1950 and 2000 for the Lake Silvaplana region, such that the temperature at the end of the AD 1950-2000 period would be 0.5°C more than the mean value of that period (9.8°C), which yields a peak Current Warm Period temperature of 10.3°C. And thus we see that the peak temperature of the Roman Warm Period (11°C) was about 0.7°C warmer than the peak temperature of the Current Warm Period, while the peak temperature of the earlier 570-351 BC period (11.2°C) was approximately 0.9°C warmer than the peak temperature of the Current Warm Period.
What it means
These findings clearly demonstrate that for this particular part of the planet, there is nothing unusual, unnatural or unprecedented about late-20th-century/early-21st-century warmth. And adding this result to the similar results that have been obtained for a great many other "parts of the planet," it is beginning to look like the Roman Warm Period, much like the Medieval Warm Period, was probably warmer than it has been recently over most of the planet. And when it is remembered that today's atmospheric CO2 concentration is currently close to 45% greater than it was back in the Roman Warm Period, and some 40% greater than it was during the Medieval Warm Period, the warmer temperatures of those two earlier periods appear even more impressive ... which makes one want to ask: Why is it so much colder nowadays?