Roman Warming and Medieval Warming warmer than current warm period

Sherwood, Keith and Craig Idso at CO2 Science report:

Northern Scandinavian Temperatures: It's a Whole New Ball Game
Volume 15, Number 30: 25 July 2012

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

In a game-changing paper published in the online version of Nature Climate Change, Esper et al. (8 July 2012) provide j45iconvincing evidence that both the Medieval and Roman Warm Periods of 1000 and 2000 years ago, respectively, were warmer than the Current Warm Period has been to date, in spite of the fact that today's atmospheric CO2 concentration is some 40% greater than it was during those two earlier periods. 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).

Read more at CO2 Science

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).