Today appears to be global warming day here at Hit & Run. As some astute H&R commenters have noted, Nature Climate Change has just published a new study by German climate researchers that peers deep into the entrails (OK the heartwood) of Scandinavian trees to figure out how temperatures have trended since the 2nd century BC.
The abstract of the article, Orbital Forcing of Tree Ring Data, sums up:
Solar insolation changes, resulting from long-term oscillations of orbital configurations, are an important driver of Holocene climate. The forcing is substantial over the past 2,000 years, up to four times as large as the 1.6 W m−2 net anthropogenic forcing since 1750, but the trend varies considerably over time, space and with season. Using numerous high-latitude proxy records, slow orbital changes have recently been shownto gradually force boreal summer temperature cooling over the common era. Here, we present new evidence based on maximum latewood density data from northern Scandinavia, indicating that this cooling trend was stronger (−0.31 °C per 1,000 years, ±0.03 °C) than previously reported, and demonstrate that this signature is missing in published tree-ring proxy records. The long-term trend now revealed in maximum latewood density data is in line with coupled general circulation models, indicating albedo-driven feedback mechanisms and substantial summer cooling over the past two millennia in northern boreal and Arctic latitudes. These findings, together with the missing orbital signature in published dendrochronological records, suggest that large-scale near-surface air-temperature reconstructions relying on tree-ring data may underestimate pre-instrumental temperatures including warmth during Medieval and Roman times.
This graph summarizes their findings:
"This figure we calculated may not seem particularly significant," says [Jan] Esper [one of the researchers], "however, it is also not negligible when compared to global warming, which up to now has been less than 1°C. Our results suggest that the large-scale climate reconstruction shown by the Intergovernmental Panel on Climate Change (IPCC) likely underestimate this long-term cooling trend over the past few millennia."
The folks over at RealClimate welcome the new results but suggest that other reconstructions of tree ring data outside of the northern latitudes will find that orbital forcing (sunlight hitting the surface) actually has a far smaller effect on the global temperature trend than that reported in the new study. University of Pennsylvania climatologist Michael Mann (the creator of the "hockey-stick" which did not find evidence for a significant Medieval Waming period) tells The New Scientist: "The implications of this study are vastly overstated by the authors."
Time will tell if that is so.