Paper Reviewed

Esper, J., Duthorn, E., Krusic, P.J., Timonen, M. and Buntgen, U. 2014. Northern European summer temperature variations over the Common Era from integrated tree-ring density records. Journal of Quaternary Science 29: 487-494.

In a paper published in the Journal of Quaternary Science, Esper et al. (2014) write that tree-ring chronologies of maximum latewood density (MXD) "are most suitable to reconstruct annually resolved summer temperature variations of the late Holocene." And working with what they call "the world's two longest MXD-based climate reconstructions" - those of Melvin et al. (2013) and Esper et al. (2012) - they combined portions of each to produce a new-and-improved summer temperature history for northern Europe that stretches all the way "from 17 BC to the present." And what did they thereby learn?

As the international team of researchers from the Czech Republic, Finland, Germany, Greece, Sweden and Switzerland describes it, this history depicts "a long-term cooling trend of -0.30°C per 1,000 years over the Common Era in northern Europe" (see figure below). Most important of all, however, they note that their temperature reconstruction "has centennial-scale variations superimposed on this trend," which indicate that "conditions during Medieval and Roman times were probably warmer than in the late 20th century," when the previously-rising post-Little Ice Age mean global air temperature hit a ceiling of sorts above which it has yet to penetrate.



Northern Europe summer (June, July, August) temperature reconstruction. Data shown in°C with respect to the 1961-1990 mean. Adapted from Esper et al. (2014). Northern Europe summer (June, July, August) temperature reconstruction. Data shown in°C with respect to the 1961-1990 mean. Adapted from Esper et al. (2014).

And so we continue to collect ever more real-world evidence for the fact, that there is nothing unusual, unnatural or unprecedented about the Earth's current level of warmth.

References

Esper, J., Frank, D.C., Timonen, M., Zorita, E., Wilson, R.J.S., Luterbacher, J., Holzkamper, S., Fischer, N., Wagner, S., Nievergelt, D., Verstege, A. and Buntgen, U. 2012. Orbital forcing of tree-ring data. Nature Climate Change 2: 862-866.

Melvin, T.M., Grudd, H. and Briffa, K.R. 2013. Potential bias in 'updating' tree-ring chronologies using Regional Curve Standardization: reprocessing the Tornetrask maximum-latewood-density data. The Holocene 23: 364-373.