Background
The authors write that "solar insolation changes, resulting from long-term oscillations of orbital configurations (Milankovitch, 1941) are an important driver of Holocene climate (Mayewski et al., 2004; Wanner et al., 2008)," noting that this forcing "is substantial over the past 2,000 years, up to four times as large as the 1.6 W/m² net anthropogenic forcing since 1750 (IPCC, 2007)." And very importantly, they say that their "evaluation of long-term temperature reconstructions, even over the past 7,000 years from across northern Eurasia, demonstrates that TRW-[tree-ring width]-based records fail to show orbital signatures found in low-resolution proxy archives and climate model simulations."

What was done
As an alternative to the non-responding TRW data, Esper et al. spent a full three years developing "a 2,000-year summer [June, July, August] temperature reconstruction based on 587 high-precision maximum latewood density (MXD) series from northern Scandinavia," based on living and subfossil pine (Pinus sylvestris) trees associated with 14 lakes and 3 lakeshore sites located north of 65°N latitude.

What was learned
The twelve researchers report that over the period 138 BC - AD 2006, there was a regional cooling trend of -0.31±0.03°C per 1,000 years, which phenomenon, in their words, "is missing in published tree-ring records," but "is in line with coupled general circulation models (Zorita et al., 2005; Fischer and Jungclaus, 2011)," indicative of "albedo-driven feedback mechanisms and substantial summer cooling over the past two millennia in northern boreal and Arctic latitudes."

What it means
"These findings," according to Esper et al., "together with the missing orbital signature in published dendrochronological records, suggest that large-scale near-surface air-temperature reconstructions (Mann et al., 1999; Esper et al., 2002; Frank et al., 2007; Hegerl et al., 2007; Mann et al., 2008) relying on tree-ring [width] data may underestimate pre-instrumental temperatures including warmth during Medieval and Roman times," as they say is also the case with the study of Frank et al. (2010), while adding that the MXD data suggest that "large-scale summer temperatures were some tenths of a degree Celsius warmer during Roman times than previously thought."

References
Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253.

Fischer, N. and Jungclaus, J.H. 2011. Evolution of the seasonal temperature cycle in a transient Holocene simulation: Orbital forcing and sea-ice. Climate of the Past 7: 1139-1148.

Frank, D., Esper, J. and Cook, E.R. 2007. Adjustment for proxy number and coherence in a large-scale temperature reconstruction. Geophysical Research Letters 34: 10.1029/2007GL030571.

Frank, D.C., Esper, J., Raible, C.C., Büntgen, U., Trouet, V., Stocker, B. and Joos, F. 2010. Ensemble reconstruction constraints on the global carbon cycle sensitivity to climate. Nature 463: 527-530.

Hegerl, G.C., Crowley, T.J., Allen, M., Hyde, W.J., Pollack, H.N., Smerdon, J. and Zorita, E. 2007. Detection of human influence on a new, validated 1500-year temperature reconstruction. Journal of Climate 20: 650-666.

IPCC. 2007. Climate Change 2007: The Physical Science Basis (Solomon, et al., Eds). Cambridge University Press, Cambridge, United Kingdom.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.

Mann, M.E., Zhang, Z.H., Hughes, M.K., Bradley, R.S., Miller, S.K., Rutherford, S. and Ni, F. 2008. Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia. Proceedings of the National Academy of Sciences USA 105: 13,252-13,257.

Mayewski, P.A., Rohling, E.E., Stager, J.C., Karlen, W., Maasch, K.A., Meeker, L.D., Meyerson, E.A., Gasse, F., van Kreveld, S., Holmgren, K., Lee-Thorp, J., Rosqvist, G. Rack, F., Staubwasser, M., Schneider, R.R. and Steig, E.J. 2004. Holocene climate variability. Quaternary Research 62: 243-255.

Milankovitch, M. 1941. Kanon der Erdbestrahlung und seine Anwendung auf das Eiszeitenproblem. Koniglich Serbische Akademie.

Wanner, H., Beer, J., Butikofer, J., Crowley, T.J., Cubasch, U., Fluckiger, J., Goosse, H., Grosjean, M., Joos, F., Kaplan, J.O., Kuttel, M., Muller, S.A., Prentice, I.C., Solomina, O., Stocker, T.F., Tarasov, P., Wagner, M. and Widmann, M. 2008. Mid- to late Holocene climate change: An overview. Quaternary Science Reviews 27: 1791-1828.

Zorita, E., Gonzlez-Rouco, F., von Storch, H., Montavez, J.P. and Valero, F. 2005. Natural and anthropogenic modes of surface temperature variations in the last thousand years. Geophysical Research Letters 32: 10.1029/2004GL021563.