In setting the stage for their paradigm-altering work, the twelve researchers - hailing from Finland, Germany, Scotland and Switzerland - write that "solar insolation changes, resulting from long-term oscillations of orbital configurations (Milankovitch, 1941), are an important driver of Holocene climate," referencing the studies of Mayewski et al. (2004) and Wanner et al. (2008). In addition, they state that this forcing has been "substantial over the past 2000 years, up to four times as large as the 1.6 W/m² net anthropogenic forcing since 1750," as suggested by the work of Berger and Loutre (1991). And on the basis of "numerous high-latitude proxy records," as they describe it, they note that "slow orbital changes have recently been shown to gradually force boreal summer temperature cooling over the common era," citing Kaufman et al. (2009).

Fast-forwarding to the present, Esper et al. describe how they developed "a 2000-year summer temperature reconstruction based on 587 high-precision maximum latewood density (MXD) series from northern Scandinavia," which feat was accomplished "over three years using living and subfossil pine (Pinus sylvestris) trees from 14 lakes and 3 lakeshore sites above 65°N, making it not only longer but also much better replicated than any existing MXD time series." Then, after calibrating the pine MXD series against regional June-July-August mean temperature over the period 1876-2006, they obtained their final summer temperature history for the period stretching from 138 BC to AD 2006, as depicted in the graph below.

Figure 1. The summer (June-July-August) temperature reconstruction of Esper et al. (2012), adapted from their paper.

As determined from the relationship depicted in the figure above, Esper et al. calculate a long-term cooling trend of -0.31 ± 0.03°C per thousand years, which cooling they say is "missing in published tree-ring proxy records" but is "in line with coupled general circulation models (Zorita et al., 2005; Fischer and Jungclaus, 2011)," which computational results portray, as they describe it: substantial summer cooling over the past two millennia in northern boreal and Arctic latitudes.

"These findings," as the European researchers continue, "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 data may underestimate pre-instrumental temperatures including warmth during Medieval and Roman times," although they suggest that the impacts of the omitted long-term trend in basic tree-ring data may "diminish towards lower Northern Hemisphere latitudes, as the forcing and radiative feedbacks decrease towards equatorial regions."

And so it is that the question for our day ought to be: Why was much of the CO2-starved world of Medieval and Roman times decidedly warmer (by about 0.3 and 0.5°C, respectively) than it was during the peak warmth of the 20th century? Clearly, the greenhouse effect of atmospheric CO2 - if it has not been grossly over-estimated - must currently be being significantly tempered by some unappreciated CO2- and/or warming-induced negative-feedback phenomenon (possibly of biological origin) to the degree that the basic greenhouse effect of earth's rising atmospheric CO2 concentration cannot fully compensate for the decrease in solar insolation experienced over the past two millennia as a result of the "long-term oscillations of orbital configurations" cited by Esper et al. (2012).

Sherwood, Keith and Craig Idso

References
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