Does the Loehle reconstruction prove the Midieval Warm Period was hotter than today?

Question

Tomcat recently cited a Loehle and McClulloch analysis in which they basically took all prior reconstructions which didn't use tree rings (there were 18 of them), smoothed and standardized them, then averaged them together.

Paper available here:

http://www.ncasi.org/publications/Detail.aspx?id=3025

The paper concluded that MWP temperatures were ~0.3°C warmer than "20th century values". Since 20th century values varied by 0.7°C, I'm not sure what that means. Unfortunately, the paper was also fraught with problems as discussed here:

http://thatstrangeweather.blogspot.com/2007/11/loehle-reconstruction.html

http://www.realclimate.org/index.php/archives/2007/12/past-reconstructions/

McCulloch issued a correction of some of these errors:

http://www.econ.ohio-state.edu/jhm/AGW/Loehle/SupplementaryInfo.pdf

If you look at his final plot on page 12, you'll see that the weighted and culled plots show the MWP peak 0.1°C warmer than 1935, which was 0.55°C cooler than today.

Answer 1

The conclusion of the paper was,

"the MWP peak remains 0.07°C above the end of the 20th-century values, though the difference is not significant."

Some observations:

- The scientist himself concludes that "the difference is not significant," so why would a skeptic misrepresent it as significant?

- The study averaged 29 years of 20th century temperatures to represent it, so the latest data point compared with the MWP is 1992.  Who in their right mind would claim that 1992 is a proxy for current temperatures?  At best he has shown that the temperatures were arguably close 15+ years ago.

- The earth has warmed a lot since 1992.  Averaging in the previous 14 years distorts the data downward and therefore skews (biases) the study and waters down the argument.

- The study used only 18 points around the globe.  A couple of days ago skeptics complained that several thousand was an insufficient record to show global warming, now 18 is sufficient to disprove it?  To make a point at all they need to have their cake and eat it too.

- The study mixes all different types of temperature estimation available, but selectively discards tree ring records.  Anyone can argue just about anything if you selectively cherry pick only the data that agrees with your conclusion.

- Are they really accurate to .07 degrees?  Pollen counts in ice cores are accurate and adequate as temperature measurements, to an accuracy of 7 hundredths of a degree?  The author says not.

- Are those different types of historical estimates really comparable to each other?  Only to show general trends perhaps, but surely not to estimate specific exact temperatures.

- The study doesn't seem to explain any supposed mechanisms involved.  

The skeptics representing this study as showing something say "It was warmer during the Medieval Warm Period!"  So what?  I've never seen a skeptic explain what mechanism caused that so they can claim to have proven something about the causes or trajectory of global warming, or whether or not it's anthropogenic.

As the author puts it, "It must be emphasized, of course, that this result is based on limited data."

The author's actual conclusion, corrected in 2008 with the help of skeptic poster child Steve McIntyre (an energy exploration industry insider who apparently claims he is not involved in the oil industry) is:

"The main significance of the results here is not the details of every wiggle, which are probably not reliable, but the overall picture of the 2000 year pattern showing the MWP and LIA timing and curve shapes. Future studies need to acquire more and better data to refine this picture."

In other words, according to the author, forget the data points as exact temperatures, but see that the Medieval Warm Period and the Little Ice Age do seem to show up in the various types of records he looked at.  Did we really need a scientific study to tell us that?  No, we already knew that one was warm and the other was cold, and approximately when they happened.  

Kudos to the author though if he gets his funding for "Future studies... to acquire more and better data to refine this picture."

So what can we deduce that his future studies will be?

One of Loehle's previous papers is:

"Geologic Methane as a Source for Post-Glacial CO2 Increases: The Hydrocarbon Pump Hypothesis," C. Loehle (Environ. Res. Div., Argonne Nat. Lab., Argonne IL 60439), Geophys. Res. Lett., 20(14), 1415-1418, July 23, 1993.

"Using a simple dynamic model, the hydrocarbon pump, evaluates the hypothesis that historical CO2 levels could have been governed by releases of methane from clathrates and as natural gas. This is likely; confirming evidence is presented."

So his interest in confirming past temperatures may be to develop his hypothesis that these sorts of warmings in the past may be related to seabed melting and methane releases... which can cause significant subsequent greenhouse gas warming.  

Add man's greenhouse gasses to that, as we have for the past 150 years, and we're simply reinforcing the earth's demonstrated tendency to warm in response to increased greenhouse gasses.  Let's hope Loehle gets his funding to further study it.

EDIT -

The revised 2008 version of the paper (thanks Dana) clarifies that as many as 18 temperature series were used, but as few as 11, and those dropped to 8 available after 1935, so the reconstruction ends there.  So do we all agree that 11 points constitutes an accurate world climate model?  I'll go with the auther and advocate more study.

Answer 2

I think people view this whole issue from the wrong perspective.  If there was indeed a MWP on the order McCulloch suggests (which isn't as big as some seem to think), that doesn't make things better, but worse.  Because mans added radiative forces are still there, but now they are riding on top of potentially larger natural variations than models had been predicting.  Thus, if the same natural process that caused the MWP occurred simultaneously with the human forcing we'd see a larger temperature increase than expected.

With that said, my answer to your question would be no.  This paper doesn't prove MWP was hotter than today.

People who only read the headlines or spin about Mann's paper and the congressional testimony regarding it, miss the important parts.  Here's some of that testimony:

http://www.pewclimate.org/what_s_being_d...

JAY GULLEDGE, Ph.D., Senior Fellow Pew Center On Global Climate Change

[QUOTE]If you take nothing else from my testimony, please take these three points:

1.    The scientific evidence of significant human influence on climate is strong and would in no way be weakened if there were no Mann hockey stick.

2.   The scientific debate over the Medieval Warm Period (MWP) has been gradually evolving for at least 20 years.  The results of the Mann hockey stick simply reflect the gradual development of thought on the issue over time.

3.   The impact of the McIntyre and McKitrick critique on the original Mann paper, after being scrutinized by the National Academy of Science, the Wegman panel and a number of meticulous individual research groups, is essentially nil with regard to the conclusions of the Mann paper and the 2001 IPCC assessment. [END QUOTE]

Here's an excellent presentation of Dr. Gerald North (chair of the National Academies of Science committee to evaluate the so-called hockey-stick report):

http://www.met.tamu.edu/people/faculty/d...

Here's the NAS report in brief:

http://dels.nas.edu/dels/rpt_briefs/Surf...

[QUOTE]

The basic conclusion of Mann et al. (1998,1999) was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1,000 years. This conclusion has subsequently been supported by an

array of evidence that includes both additional large-scale surface temperature reconstructions and pronounced changes in a variety of local proxy indicators, such as melting on icecaps and the retreat of glaciers around the world, which in many cases appear to be unprecedented during at least the last 2,000 years.[END QUOTE]

Edit:

Elmore - I recognize your writing Nancy ;-) You really should take the time to educate yourself further before making such bold assertions (with no supporting evidence, I might add). You should actually read and listen to my links before repeating misleading information about Mann's paper.  His basic conclusions were upheld by the National Academies of Science committee given the task to look into it. So your arguement is with them, not me.

Answer 3

A. Anything published by the journal Energy & Environment usually isn't reliable. The journal is not considered a scientific or academic journal and has long been criticize for publishing substandard papers.

http://pubs.acs.org/subscribe/journals/e...

B. The best assessment of past reconstructed temperature is from a report titled “Surface Temperature Reconstructions for the Last 2,000 Years,” researched and published by the National Research Council at a request from Congress.

http://www.nap.edu/catalog.php?record_id...

C. Paleoclimatology is interesting, and can help us to understand the effects of climate change. But our understanding of recent anthropogenic (manmade) global warming comes not from reconstructed temperatures, but from an understanding of basic physics and observations of climate change over the past decades.

Note to the person above me (opps, he erased his answer)…the notion that the Medieval Warm Period was warmer than the 20th century was the result of a misunderstanding. The notion began with this graph, (page 84) http://books.google.com/books?id=0Nucx3u... by Lamb and was reprinted by the IPCC in their First Assessment Report. The graph, unfortunately, was never labeled by the IPCC as “estimated temperatures for Central England”. The IPCC republished the graph to illustrate that climate could change, and it was never, not by Lamb or by the IPCC meant to be an estimation of global reconstructed temperatures.

The first attempt to reconstruct global temperatures was done by Mann, et al in 1998 (Hockey Stick Graph). Since Mann, temperature reconstructions for the past two millennia have been published at least a dozen of times. This research has been summarized quite well by the National Research Council (see link under B).

Edit:

Here is an interesting tangent for Elmore J on the “But they used to grow grapes in England” argument... An ancient Roman fort named Vindolanda sits next to Hadrian's Wall in Northern England. Vindolanda is famous because, during excavations, spectacular wooden writing tablets where found with messages written in ink. Well, to make a long story short, it appears that wealthy Romans imported their wine from Gaul. So grapes and wine might well have been grown and produced in England during the past, but this does not necessarily mean it was good wine. What does this say about the climate?

Answer 4

I am certainly not convinced the midieval warm period is warmer than today but there certainly has been a lot of variation.  As a geologist, time scales of 10s of years are not generally very important.  Over the long term clearly there have been some ups and downs since we came out of the last perid of glaciation 10,000 years or so ago.  We are clearly in an up period much to our huge benefit.  Be thankful you are not living in a period your great great great great (etc for about 500 greats) grandfather had to endure.

Answer 5

I believe that a branch of science with millions of variables that has been studied only a few decades is too immature to draw any meaningful conclusions.  The planet has warmed and cooled well beyond today's fluctuations many times during the planet's existence, and we just don't know enough to say why.