Actually it's not that bad, but the headline was too good to resist. I'm grateful to a reader for sending me a copy of the new temperature reconstruction from Moschen et al, recently published in Climate of the Past. The authors reconstruct temperatures from carbon isotope analysis of Sphagnum moss.
The temperature reconstruction is based on the Sphagnum δ13C cellulose/temperature dependency observed in calibration studies. Reconstructed GST anomalies show considerable centennial and decadal scale variability. A cold and presumably wet phase with below-average temperature is reconstructed between the 4th and 7th century AD which is in accordance with the so called European Migration Period, marking the transition from the Late Roman Period to the Early Middle Ages. At High Medieval Times, the amplitude in the reconstructed temperature variability is most likely overestimated; nevertheless, above-average temperatures are obvious during this time span, which are followed by a temperature decrease.
I did like the bit about the MWP being overestimated - the journal editor will have breathed a sigh of relief at that comment.
;)
The authors seem keen to downplay the MWP, emphasising the similarities to tree ring studies (they cite a northern hemisphere temperature reconstruction by Büntgen et al., which is the green line in the chart above).
The local [growing season temperature] anomalies show a remarkable agreement to northern hemispheric temperature reconstructions based on tree-ring datasets and are also in accordance with climate reconstructions on the basis of lake sediments, glacier advances and retreats, and historical datasets. Most notably, e.g., during the Early Middle Ages and at High Medieval Times, temperatures were neither low nor high in general. Rather high frequency temperature variability with multiple narrow intervals of below- and aboveaverage temperatures at maximum lasting a few decades are reconstructed.
I'm not sure about this - that sure looks like a medieval warm period to me. (They find no evidence of the Roman Warm Period and peat cutting has destroyed any record of the Little Ice Age.)
That said, I think I would also need some convincing about the validity of the proxy - some of it sounds a bit iffy to me.
Calibration studies have systematically investigated the relationship between climate parameters and the stable carbon isotope composition of cellulose (δ13C cellulose) from modern Sphagnum plants. Ménot and Burns (2001) found that in addition to atmospheric CO2 partial pressure, temperature and water availability play significant roles for their δ13C cellulose values. Regarding potential relationships between the δ13C cellulose of Sphagnum plant material and micro-climatological parameters, Skrzypek et al. (2007) report strong correlations between the δ13C cellulose values of Sphagnum and air temperature during the growing season. The major problem in the application of the Sphagnum δ13C cellulose to peat records in order to derive climatic signals arises from the finding that a significant offset exists between the stable carbon isotope composition of cellulose from different Sphagnum plant components (Loader et al., 2007; Moschen et al., 2009). Thus, physical separation of individual plant parts prior to isotope analyses is a necessity to avoid misinterpretations of stable isotope time series.
And you go on to read things like:
Due to the closed coupling of several environmental factors to air temperature, a (presumably indirect) dependency of the stable carbon isotope composition of Sphagnum cellulose on local air temperature has been assumed. To date there is no laboratory study on this relationship, however, the Sphagnum δ13C cellulose/temperature dependency has been proven in field studies (Ménot and Burns, 2001; Skrzypek et al., 2007).
I assume this means that they found a correlation and are assuming causation.
So, not entirely convinced, but it's interesting nevertheless.
The paper is: R. Moschen, N. Kühl, S. Peters, H. Vos, and A. Lücke. Temperature variability at Dürres Maar, Germany during the Migration Period and at High Medieval Times, inferred from stable carbon isotopes of Sphagnum cellulose, Clim. Past, 7, 1011–1026, 2011