Political Climate
Jul 20, 2012
Tree rings suggest Roman world was warmer than thought

by Fred Pearce

How did the Romans manage to grow grapes in northern England when most climate studies suggest the weather was much cooler then? We may now have an answer: it wasn’t that cold at all.

Long-term temperature reconstructions often rely on the width of tree rings: they assume that warmer summers make for wider rings. Using this measure, it seems that global temperatures changed very little over the past two millennia. Such studies are behind the famous “hockey stick” graph, created by Michael Mann of Pennsylvania State University in University Park, which shows stable temperatures for a millennium before the 20th century.

Jan Esper of Johannes Gutenberg University in Mainz, Germany, thinks that at least some of those tree rings actually show something else: a long-term cooling trend that lasted right up until the Industrial Revolution. The trend came about because of reduced solar heating caused by changes to the Earth’s orbit known as Milankovitch wobbles, says Esper. His results suggest the Roman world was 0.6 C warmer than previously thought enough to make grape vines in northern England a possibility.

Esper and his colleagues say that warmer summers do not necessarily make tree rings wider but they often make them denser. He studied the density of tree rings in hundreds of northern Scandinavian trees and found that they showed evidence of a gradual cooling trend that began around 2000 years ago.

The finding fits with other proxies for temperature - such as the chemical make-up of air trapped in glaciers and the organic remains in ancient lake sediments - which have also suggested a cooling trend.

Esper’s study is the latest to indicate that temperatures were less stable than originally thought. In 2009, Darrell Kaufman of Northern Arizona University at Flagstaff published evidence, using a range of proxies, that indicated a cooling in the Arctic for most of the past 2000 years (Science, DOI: 10.1126/science.1173983). Esper’s findings suggest that the cooling trend was even stronger than Kaufman concluded.

The finding does not change our understanding of the warming power of carbon dioxide. In fact, it shows that human CO2 emissions have interrupted a long cooling period that would ultimately have delivered the next ice age. (Have not heard that one before, stated so explicitly.  BB)

Esper says temperature reconstructions will have to be redone because past studies probably underestimated temperatures during the medieval warm period and other warm periods going back to Roman times. The further back in time, the greater the underestimate would be.

But others have doubts. Mann argues that Esper’s tree-ring measurements come from high latitudes and reflect only summer temperatures. “The implications of this study are vastly overstated by the authors,” he says.

Journal reference: Nature Climate Change, DOI: 10.1038/nclimate1589


Dear Mr. Pearce

Yesterday, whilst waiting for my wife to join me in Marks and Spencer, I chanced to glance at the latest edition of New Scientist, in which your contribution appears. I confess that, both for reasons of time and from a sense of disenchantment anyway with this populist rag, I did little more than skim read. However, I think that enough was gleaned legitimately to allow for comment.

The thrust of your piece was that the handle of the hockey stick was, in fact, correct. Somebody’s recent study of tree rings had indicated that, whilst they might or might not be wider or narrower, much could be deduced from their density. From this it had been concluded that the past two thousand years had seen a warm temperature continuum of remarkable consistency up to about the middle of the 19th century, say. This, in turn, explained why our forebears had been able to cultivate vines as far North as York. Thence, the conclusion seemed to be drawn that Mann had been right all along in claiming a perilous temperature increase from roughly the advent of the Industrial Revolution, the blade.

Of course, MacIntyre’s undermining of Mann’s little frolic was not so much based upon scientific grounds although, God knows, there wasn’t much science to it, as upon his blithe disregard for statistical rigour. You make no mention of this. Why, pray? Neither do you make any mention of Briffa’s equally egregious pseudo-science directed at sustaining the original Mann fiction. You also overlook circumstances that are richly supported by multi-stranded evidence.

Examples? Well, how about the fact that: even now viniculture is not pursued with much success in Northern latitudes in this country - or, indeed as far as I know, in any other, save where local topography is helpful, as in Germany, say; however warm the Medieval Warm Period may or may not have been, it was most surely followed by several centuries of real and readily perceptible cooling, to wit the Little Ice Age. For this latter, of course, there is a wealth of evidence not merely scientific but from the arts and historical records as well. Actually, in passing, the same may be said of the preceding MWP.

Furthermore, for somebody seeking to make the case that you appear to be advancing, the happenstance of both the MWP and of LIA is acknowledged both by NASA and the IPCC, not to mention that bastion of scientific rectitude, The Royal Society. So I think that it should be taken as accepted fact albeit, most assuredly, not specifically because it carries the endorsement of those particular bodies.

Would you not agree?

The point, of course, is that, if this be so, then where is Mann’s temperature continuum?

As a practising journalist, I presume that you would at least like to make some claim to being influenced by a measure of respect for objectivity. One wonders why then you continue so compulsively to try to defend the indefensible, and give credence to a manifest scientific mountebank.


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