Earlier this week, in Facts against the Mann, we noted how some ice core data cast doubt on Michael E. Mann’s recent claim that:

“overall warmth of the globe and northern hemisphere today is substantially greater than during Medieval time”

This was made in the context of an invective filled rebuttal in the Nevada City, CA Union newspaper, over a column questioning the validity of Mann’s work, citing McIntyre and McKittrick’s work. Sauer wrote:

McIntyre and McKitrick’s work led to a U.S. Senate investigation that debunked Mann’s hockey stick theory that 1998 was the hottest year in the last 1,000. The Senate investigation also found the study of the social networking of the paleoclimatology world showed how close it was and how often a small group of scientists both co-wrote and peer-reviewed papers for each other. In addition, no statisticians were ever involved in their research work or peer review articles.

In response, Mann hauled out the usual tired old hate-filled libel:

Mr. Sauer parroted baseless talking points that have their origin in fossil-fuel industry-funded climate change denial propaganda, not honest scientific discourse.

Source: http://www.theunion.com/opinion/columns/18308425-113/michael-mann-columnist-peddles-discredited-fossil-fuel-industry

Jim McIntosh writes on the WUWT Facebook page:

[A] recent paper suggests tree ring proxies (which underpin Mann’s Hockey Stick Graph that largely kicked off the global warming scare) have underestimated past temperatures, which creates the misimpression that current warming is unique.

Recently published hard science. And bound to never be reported in the media.

” In other words, the new methodology allowed the researchers to capture the low-frequency climatic signals that were systematically eliminated in the MXD data sets. Thus, as a consequence, earlier warm periods during the late 14th and 15th, and 17th centuries “appear warmer” and “have been retained” by this new method, leading the team of six researchers to conclude that “late 20th century warming has not been unique within the context of the past 750 years.”

From the Esper et al. 2015 paper, note that the black line represents the instrumental record. The reconstruction, in red, shows Medieval Warming Period is clearly seen to have temperatures greater than the current modern period. As for the divergence of the instrumental record against the reconstruction, it is important to note that the trees sensing temperature aren’t near human habitation as most of our observing thermometers are, but rather in the Pyrenees mountains.

Here is the paper by Esper. et al. 2015, bold mine.

LONG-TERM SUMMER TEMPERATURE VARIATIONS IN THE PYRENEES FROM DETRENDED STABLE CARBON ISOTOPES

Abstract:

Substantial effort has recently been put into the development of climate reconstructions from tree-ring stable carbon isotopes, though the interpretation of long-term trends retained in such timeseries remains challenging. Here we use detrended δ13C measurements in Pinus uncinata treerings, from the Spanish Pyrenees, to reconstruct decadal variations in summer temperature back to the 13th century. The June-August temperature signal of this reconstruction is attributed using decadally as well as annually resolved, 20th century δ13C data. Results indicate that late 20th century warming has not been unique within the context of the past 750 years. Our reconstruction contains greater amplitude than previous reconstructions derived from traditional tree-ring density data, and describes particularly cool conditions during the late 19th century. Some of these differences, including early warm periods in the 14th and 17th centuries, have been retained via δ13C timeseries detrending — a novel approach in tree-ring stable isotope chronology development. The overall reduced variance in earlier studies points to an underestimation of pre-instrumental summer temperature variability derived

from traditional tree-ring parameters.

Full paper here: https://www.blogs.uni-mainz.de/fb09climatology/files/2012/03/Esper_2015_Geochron.pdf

What I find most interesting is their graph showing how they calibrated the δ13C data against the instrumental temperature record. While not a perfect match, with R values being rather low, the reconstruction does seem to capture the trend effectively.

It seems to me, that δ13C analysis is a better match than tree ring widths by themselves for determining past temperatures, though like anything to do with tree growth, there are many other factors determining growth as outlined in Liebig’s Law of the Minimum. Hence, uncertainty will likely always be large.

[added] I wonder if Mann’s Bristlecone Pine tree core samples from MBH98 still survive, and what they might say if a δ13C analysis like Esper did was run on them?

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