New Paper: Roman & Medieval Warm Periods Were Warmer Than Previously Thought

  • Date: 25/04/13
  • The Hockey Schtick
A paper published in Nature Climate Change finds prior temperature reconstructions from tree-rings “may underestimate pre-instrumental [pre-1850] temperatures including warmth during Medieval and Roman times.” Many reconstructions show temperatures during the Medieval and Roman periods were warmer than the present, and this study suggests they were even warmer than previously thought.

From the latest NIPCC Report: 
Tree-ring width vs. tree-ring latewood maximum density
Reference: 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.
According to Esper et al. (2012), ”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),” and 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).” Furthermore, 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.”

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.In discussing this new proxy, 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.”

“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.”

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