Reference
Moschen, R., Kuhl, N., Peters, S., Vos, H. and Lucke, A. 2011. Temperature variability at Durres Maar, Germany during the Migration Period and at High Medieval Times, inferred from stable carbon isotopes of Sphagnum cellulose. Climate of the Past 7: 1011-1026.
Background
The authors write that "currently, there is specific interest in climate change during our historical past and in the human impact on past and future climate and ecosystem dynamics," and they say that "in this context, the reconstruction of decadal to centennial scale natural climate variability is of importance to estimate to what extent human activities contribute to the recent warming trend observable at a regional and global scale."
What was done
Moschen et al. present "a high resolution reconstruction of local growing season temperature anomalies at Durres Maar, Germany [50°52'N, 6°53'E], spanning the last two millennia," which was "derived from a stable carbon isotope time series of cellulose chemically extracted from Sphagnum leaves (δ13Ccellulose) separated from a kettle-hole peat deposit of several meters thickness," where the temperature reconstruction was based on the temperature dependency of Sphagnum δ13Ccellulose observed in calibration studies.
What was learned
"From the 4th to the 7th century AD," in the words of the five researchers, a cold phase "with below-average temperature is reconstructed, which is in accordance with the so-called European Migration Period," which has also come to be known as the Dark Ages Cold Period. Thereafter, they state that "during High Medieval Times above-average temperatures are obvious." In fact, the peak warmth of this Medieval Warm Period, which looks from the graph of their data to run from about AD 830 to AD 1150, was approximately 2.8°C greater than the peak warmth of the Current Warm Period in terms of individual anomaly points, while it was approximately 2.7°C greater in terms of 60-year running means. And between these two warm periods, the Little Ice Age can be seen to hold sway.
What it means
In terms of Moschen et al.'s stated purpose of hoping to illustrate "to what extent human activities contribute to the recent warming trend observable at a regional and global scale," based on what types of natural climate changes have occurred over the past two millennia, it would have to be concluded that human activities have contributed absolutely nothing in the way of warming, as it was much warmer at Durres Maar, Germany, back in the "good old (High Medieval) days," when there was far less CO2 in the air than there is today.