Unskeptical Challenges to Hockey Stick Temperature Reconstructions

Comparison of Loehle 2008, Pages 2K and HadCRUT5

In 2007, Loehle published a multiproxy study[1] estimating global temperature over the last 2000 years. It's distinctive feature was that it removed all tree ring proxies, leaving only 18 proxies to estimate global temperatures. According to the abstract the paper shows a warm MWP compared to today. "The mean series shows the Medieval Warm Period (MWP) and Little Ice Age (LIA) quite clearly, with the MWP being approximately 0.3°C warmer than 20th century values at these eighteen sites."


Loehle 2007 Temperature Reconstruction

Admittedly this reconstruction looks very different from the "hockey stick" reconstructions from Mann and other authors. However, Loehle's reconstruction contained several errors in the way the dates were handled, and once discovered, a revised version of this paper was published in 2008.[2] "Several errors in data handling in that report have come to light, leading to the need for this report, which corrects these errors." In the corrected version of the paper, data only goes through 1935. "With the corrected dating, the number of series for which data is available drops from 11 to 8 in 1935, so that subsequent values of the reconstruction would be based on less than half the total number of series, and hence would have greatly decreased accuracy. Accordingly, the corrected estimates only run from 16 AD to 1935 AD, rather than to 1980 as in Loehle (2007)." Then it says that the MWP was "0.412 Deg C above the last reported value at 1935."

The data for Loehle 2008 is online, so it's relatively easy to compare the above corrected reconstruction with the instrumental record and other multiproxy studies, like Pages 2K. Since Loehle's paper was only able to use 29-year averages, I added 29-year smoothing to Pages 2K[3] and HadCRUT5, which are both annual. The mean of Pages 2K reconstruction almost entirely fits within the uncertainty envelope for Loehle 2008, but Loehle's reconstruction is far more variable than Pages 2K. This is almost certainly due to the arbitrary reduction of proxies in Loehle's reconstruction which biases GMST toward the Northern Hemisphere.


Proxy Locations in Loehle 2008

But even in Loehle's reconstruction with 29-year smoothing, temperatures have risen over 1.5 C since the LIA, and they are currently warmer than at any point in the MWP. Not only that, but warming rates are greater than at any point in the last 2000 years. This paper does very little to undermine reconstructions of global temperatures that show a hockey stick shape. This paper actually just a wavier hockey stick than Pages2K.

So in 2009, when Joanne Nova published her Skeptics Handbook II,[4] it would do no good to discuss the corrected Loehle 2008 paper. Instead, she included a graph of Loehle 2007 with its errors and her annotations, along with a photo of a tombstone for the hockey stick. She then informs us that according to this graph, "Temperatures were higher 1000 years ago, & cooler 300 years ago. We started warming long before cars and power stations were invented. There’s little correlation with CO2 levels."


JoNova's Use of Loehle 2007

JoNova used the uncorrected Loehle 2007 in an attempt to promote the "real shape" of the last 2000 years and correct the hockey stick paper from Mann et al 1998 (MBH98).[5] This paper, published in Nature, was controversial when published, not so much because there was anything wrong with it, but because it made current warming unique and seemingly unprecedented over the last 2000 years. Steve McIntyre attempted to publish a criticism of it in Nature, but it was refused publication in Nature, buthe was able to get a version of the paper published in another journal.[6]. JoNova describes this paper's findings as, "McIntyre showed that the statistics were so flawed that you could feed in random data, and still make the same hockey stick shape nine times out of ten. Mann had left out some tree rings he said he’d included. If someone did a graph like this in a stock prospectus, they would be jailed." JoNova then referred to MBH98 as an "audacious fraud." However, Wahl and Ammann's independent analysis of MBH98 found his study to be robust[7], and further studies have replicated the hockey stick.[9] McIntyre's criticisms of MBH98 have also been sufficiently answered.[8] But this didn't stop JoNova from calling Mann's paper "statistical trickery" and posting McIntyre's "corrections" to the graph.


JoNova's Annotation of Mann's Hockey Stick

At some point I may provide a summary of the wild claims made about MBH98. Here, though, I'm more interested in pointing out how people who call themselves "skeptics" frequently fail at being skeptical; they instead apply a double standard in favor of their desired view of climate change. JoNova promoted Loehle 2007's graph and ignored its corrections, even though these errors were found, corrected and published in Loehle 2008 before JoNova's handbook was published. However, even though McIntyre's criticisms have been found to be flawed, and event hough an independent analysis found MBH98 to be robust, she did publish McIntyre's "corrections" to MBH98 and called it "statistical trickery" and an "audacious fraud." This is a clear case of motivated reasoning in which a erroneous position is uncritically accepted and a  statistically robust position is uncritically rejected.


References:


[1] Loehle, C. (2007). A 2000-Year Global Temperature Reconstruction Based on Non-Treering Proxies. Energy & Environment, 18(7), 1049–1058. https://doi.org/10.1260/095830507782616797

[2] Loehle, Craig, and J. Huston McCulloch. “Correction to: A 2000-Year Global Temperature Reconstruction Based on Non-Tree Ring Proxies.” Energy & Environment, vol. 19, no. 1, Jan. 2008, pp. 93–100, doi:10.1260/095830508783563109.
https://www.asc.ohio-state.edu/mcculloch.2/AGW/Loehle/Loehle_McC_E&E_2008.pdf

[3] PAGES2k Consortium. A global multiproxy database for temperature reconstructions of the Common Era. Sci Data 4, 170088 (2017). https://doi.org/10.1038/sdata.2017.88

[4] JoNova. Skeptics Handbook II.
https://joannenova.com.au/globalwarming/skeptics-handbook-ii/the_skeptics_handbook_II-sml.pdf

[5] Mann, M., Bradley, R. & Hughes, M. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392, 779–787 (1998). https://doi.org/10.1038/33859


[6] McIntyre, S., and McKitrick, R. (2005), Hockey sticks, principal components, and spurious significance, Geophys. Res. Lett., 32, L03710, doi:10.1029/2004GL021750.

[7] Wahl, E.R., Ammann, C.M. Robustness of the Mann, Bradley, Hughes reconstruction of Northern Hemisphere surface temperatures: Examination of criticisms based on the nature and processing of proxy climate evidence. Climatic Change 85, 33–69 (2007). https://doi.org/10.1007/s10584-006-9105-7

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