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PAGES2K (2017) – South America Revisited

The most recent large-scale compilation of proxy records over the past two millennia is PAGES (2017).  They made a concerted effort to archive data (to the credit of Julien Emile-Geay), archiving 692 series, but they perpetuated most other sins within the field.  Rather than abjuring ex post screening, it carried ex post screening to extremes never previously contemplated: tree ring chronologies with negative correlations to temperature are now banished from view altogether. However, its self-professed quality control did not exclude stripbark bristlecone chronologies, which continue to populate the network.

In keeping with my preference to look at regions and proxy types before worrying too much about aggregates, I looked at their South American network, which is an update of the South American network of PAGES2K (2013), which I discussed a few days after publication here.  There were major changes between 2013 and 2017 networks, which were not elucidated in the later study, but which will be discussed in today’s article. The changes illustrate the profound problems with the tree ring chronologies and lake sediment series which make up the vast majority of data in PAGES 2017 and similar studies.

The PAGES2K (2013) South American Network 

The PAGES2K (2013) South American network consisted of 23 proxies:

  • two ice core proxies from a single site (Quelccaya, Peru);
  • one lake sediment proxy (a reflectance indicator from Aculeo, Chile);
  • one ocean sediment proxy (Mg/Ca from the Cariaco Basin, offshore Venezuela);
  • four instrumental series as supposed proxies for instrumental temperature
  • 15 tree ring series. These 15 series were screened from the larger tree ring network of Neukom and Gergis (2012), which had 63 series, which, in turn, had been selected from a larger roster of unknown size using unknown procedures.

I discussed this network a few days after publication, pointing out some serious problems which had been overlooked in the hasty review of PAGES2K (2013) by Nature after it had been rejected by Science. The hasty review was required because IPCC AR6 had cited PAGES2K then in review, not anticipating that it would be rejected. Ironically, Michael Mann was one of the reviewers who recommended rejection.

  • I observed that the PAGES2K use of the very standard Quelccaya d18O series (used in most multiproxy series since Jones et al 1998) was upside-down to its use by all other authors – an error that ought to have been picked up and corrected before publication;
  • I criticized the use of the four instrumental records as supposed proxies for temperature observing that this “seems to be peeking at the answer if the “skill” of the early portion of the reconstruction is in any way assessed on the ability of the network (including instrumental) to estimate instrumental temperature”. This seems so obvious that it is hard to imagine any serious climate scientist using instrumental temperature data in a proxy network, except that the practice has been encountered much too often, including Mann et al 1998.
  • I observed that “one-third of the tree ring series are inverted” and asked whether this was “an ex ante relationship or mere ex post correlation?”. Perhaps the longest standing dispute between Climate Audit and authors relied upon by IPCC is over ex post screening or ex post orientation – both practices being condemned at Climate Audit since its earliest days.

I’ve also long spoken against the use of singleton proxies in multiproxy studies intended for policy reliance on the grounds that replicability across multiple sites ought to be insisted on, before inclusion in a multiproxy study. The Laguna Aculeo indicator – relative absorption band depth (RABD) centred in 660-670 nm said to measure “total sedimentary chlorin” – was then relatively unique; a rare example in a marine sediment here.) Values of the index were not even reported in its data archive – only the temperature reconstruction.

The PAGES 2017 Network

Eighteen of the 23 series in the 2013 network were rejected in 2017; only five were retained. Of these five, one series (Quelccaya d18O) was used in the opposite orientation to the 2013 network. Needless to say, the PAGES2K 2017 authors did not disclose that they reversed the orientation of the series from the earlier study. This was the second PAGES2K 2013 series where the authors recognized that their original use was upside down: I had also criticized their upside-down use of the Hvitarvatn, Iceland series, which they grudgingly corrected in a later publication and even more grudgingly (after some sneering on my part) and much later issued a corrigendum.

The disposition of the 2013 network is shown in the table below.

  • the second Quelccaya series (accumulation) – which had also been used in Mann et al 1998 – was rejected as being a “hydroclimate proxy”. They did not explain how it had passed the supposedly rigorous protocols of PAGES 2013.
  • the Cariaco ocean sediment series was exported to their Ocean proxy network. (Fair enough).
  • the four instrumental series were rejected as proxies with the laconic explanation that they were “instrumental data” (thus, complying with one of my 2013 criticisms)
  • they rejected the five tree ring series which had been assigned (ex post) negative orientations. From a statistical perspective, ex post screening of series (which met ex ante criteria) on grounds of negative correlation is just as pernicious as ex post orientation. This is no real improvement
  • they rejected one tree ring series due its failure to meet an internal consistency statistic (EPS). It is unclear why this wasn’t picked up in 2013
  • they rejected six tree ring series as being too short (less than 300 years). I agree with this policy: if one’s objective is to compare modern temperatures to (say) medieval temperatures, introduction of such short proxies results in inhomogeneity which ought to be avoided. (This sensible 300-year policy was unfortunately ignored in PAGES2017 Ocean network.)

The disposition of the 2013 network is shown below.

 

New Proxies

There were three “new” proxies: one tree ring series and two lake sediment series. In addition, two tree ring series were updated.

New Tree Ring Proxies

The”new” tree ring series (CAN Composite 15) had, like the other series, been in the Neukom and Gergis 2012 network. For some reason, it had been screened out of the PAGES 2013 network, but now determined to meet the PAGES2K criteria after all. Of the original 63(!) tree ring chronologies in the Neukom and Gergis 2012, only four(!) made their way into the PAGES2017 network.  I do not believe for a minute that these four tree ring chronologies are unique thermometers. A more likely interpretation is that their satisfaction of proxy criteria was fortuitous and that they are no more trustworthy as thermometers than the excluded chronologies. Nor did any of these four chronologies reach back to the medieval period: their start dates ranged from 1435 to 1636, start dates, long after the medieval period.

Interestingly, the fresh data in the two updated tree ring series further illustrates the ineffectiveness of these South American tree ring chronologies as temperature proxies, as shown in the plots of Central Andes 6 (CAN 6) and Central Andes 9 (CAN 9) below.

 

CAN9, which is barely over 300 years long, has high values in mid-20th century, but declines in the last half of the 20th century despite temperatures increase. Its late 20th century decline continues into the 21st century, where values have reverted to the long-term mean. Similarly CAN6 has had little longterm change, but had a late 20th century spike, but has regressed to low values subsequently.

A more plausible interpretation of the data is that these four series were selected ex post because their 20th century values were somewhat higher than values in earlier centuries, but are not magic thermometers.

Laguna Chepical

Only one of the two new lake sediment series purports to show elevated and increasing 20th century levels: the Laguna Chepical (de Jong et al 2013) series. But closer examination of the data shows that the modern portion of this lake sediment series, like the notoriously contaminated Korttajarvi series of Mann et al 2008 and the equally contaminated (but less notorious) Igaliku series of PAGES2K (2013), is also compromised by man-made construction.  The original authors (de Jong et al) argued that man-made construction did not compromise lake sediment reflectance as a climate proxy, but, when held up to sunshine, their argument is flimsy.

Laguna Chepical is located in central Chile (32S) at high altitude (3055 m), approximately 130 km north of Santiago. The authors measured reflectance at relatively high resolution, from which they selected the ratio of reflectance at 570 nm to reflectance at 630 nm (R570/R630), interpreted as indicative of the clay mineral content in the lake sediments. They observed a strong decrease in this ratio during the 20th century (for which instrumental temperature data was available). Summer temperatures increased during this period. A simple correlation calculation was said to show that R570/R630 was “strongly and significantly negatively correlated with summer temperatures.”  The authors proposed the following explanation:

We reason that cool summers, associated with late lake ice break-up and hence relatively long periods of ice cover, favor the settling of very fine particles in the lake, which leads to increased clay contents in the sediments.

But there’s a catch: around 1885, just prior to the calibration period, there was a ten-fold (!) increase in sediment accumulation rate. This can be seen in a comparison of the two plots shown below: top – R570_R630 versus depth; bottom – “temperature”, a linear transformation of R570_R630, to year. The two red arrows show two pairs of matching points. The layer at ~20 cm of core (right arrow) is dated to ~1885 AD, while the layer at ~41 cm (left arrow) is dated to ~440 AD. In other words, the top ~20 cm of core was accumulated in ~115 years of time, but it had taken ~1445 years to accumulate the prior ~20 cm of core. The rate of modern accumulation is more than ten(!) times greater than the rate of accumulation in the previous 15 centuries.

It is more or less certain that an order-of-magnitude increase in sediment accumulation rate in modern period is due to some sort of man-made land disturbance, rather than climate.  For example, modern period increases in sediment accumulation at Korttajarvi and Igaliku were due to local land disturbance (construction, agriculture), not climate. Failing to recognize this led to embarrassing mistakes in Mann et al 2008 and PAGES2K (2013) respectively.

When one re-examines the original publication (De Jong et al 2013 ), one finds that they reported a man-made intervention at the precise time when sedimentation rates increased so dramatically:

A small creek with episodic flow enters the lake in the northwestern side and has formed a small, shallow delta. Additional sediment inflow likely occurs during snow melting from the surrounding slopes to the N, E and W. …  An outflow is located in the SW. Since ca. AD 1885, this outflow was dammed and regulated (A. Espinoza, personal communication, 2006). [my bold]

The order-of-magnitude increase in sediment accumulation in the core clearly results from the dam, rather than increase in temperature (the sediment accumulation increase is a local phenomenon). Within this enormous increase in sedimentation rate, there is a noticeable increase in clay mineral content (measured by the fall in R570_630 reflectance values from ~0.90 to ~0.82) to levels which were essentially unprecedented in the previous three millennia. It seems logical that the increase in clay mineral content is a by-product of this dramatic  increase in sedimentation, as opposed to the speculative connection to cool summers and late ice break-up proposed to the authors.

The authors purported to dismiss any connection between the construction of the earth dam in 1885 and the subsequent increase in clay mineral content in lake sediments as follows:

An additional, potentially important environmental variable was the construction of the earth dam in AD 1885. However, as indicated by cluster analyses, the construction of a low (ca. 2 m) earth dam and the subsequent relatively small increase in maximum lake depth did not significantly affect most of the sediment properties measured with VIS-RS scanning and had no influence on the R570/R630 values. Therefore, the reconstruction of summer temperatures based on calibration-in-time, which was developed for the period after dam building, is also valid back in time. [my bold]

Unfortunately, the authors failed to provide any statistics or other supporting data for this assertion. I don’t know how “cluster analyses” could possibly show that the construction of the dam in 1885 had “no influence on the R570/R630 values”, which, after all, fell to unprecedented levels following dam construction and subsequent ten-fold increase in sediment accumulation rate. I don’t believe that it is possible to draw such a conclusion from “cluster analyses”.  Also, speaking strongly against the assumption of non-impact of dam construction is the following statement in Meyer et al 2017:

The main prerequisite for its [VIS‐RS scanning] successful application is that temporal variation in lake hydrology over the period of interest has not appreciably affected sedimentation dynamics at the core site, since major changes in sediment texture and organic content are likely to create confounding effects in the VIS‐RS signature.

That condition was obviously not met at Laguna Chepical.

Laguna Escondida

The other new South American proxy in PAGES2017 is from Laguna Escondida in northern Patagonia (45S) from Elbert et al 2013. It is a temperature estimate from biogenic silica flux (mg/cm^2*yr). Biogenic silica % and/or flux is measured quite commonly in paleoclimate lake sediment studies, but is not commonly used as a temperature proxy. It measures productivity of diatoms. BSi was used used in one other PAGES2017 proxy, Hallett Lake, Alaska (as percentage, rather than flux). The Hallett Lake series had been previously discussed at Climate Audit, where I noted that its very elevated early values had been chopped off for no apparent reason other than that they were elevated. Another location with BSi measurements is Hvitarvatn , Iceland, a site discussed on several occasions at Climate Audit; its varve thickness measurements were used in PAGES2K.  This series had high medieval values, with a decreasing trend to the modern period. It lacks the strong HS-blade of Laguna Chepical discussed above.

 

Summary

The eight PAGES2017 series are summarized in a consistent panel plot below for the period 1000 on.

The tree ring component of this network is, more or less, a reductio ad absurdum of tree ring chronologies as useful temperature proxues: only four of 63 original tree chronologies have sufficient Hockey Stick-ness to be retained in the network, with even these poor remnants reverting to the mean in the 21st century updates. There is negligible similarity between the three lake sediment series, each of which uses a different indicator, though similar measurements appear to have been taken for all three sites. The only series with a meaningful HS (Chepical) appears to result from construction of a dam in 1885AD, rather than from increased temperature. This leaves the Quelccaya ice core series – which was a staple of temperature reconstructions as early as 1998 and, which, ironically, was used upside down in PAGES2K (2013), corrected in PAGES 2017 without disclosure/admission of the earlier error.

All in all, a rather pathetic show by PAGES2K.

Source: 
Climate Audit

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