Climate Audit

The 30-60N latitude band gets lots of attention in paleoclimate collections – probably more proxies than the rest of the world combined. The 30-60S latitude band is exactly the same size, but it is little studied. It is the world of the Roaring Forties and Furious Fifties, a world that is almost entirely ocean. The only land is New Zealand, Tasmania and the southern coast of Australia facing Antarctica, the tip of South Africa and the narrow part of South America: southern Chile and Argentina. But 96% or so is ocean.

No Ocean Proxies

Although the 60-30S is almost entirely ocean, PAGES 2019 did not use a single ocean proxy in its data. They used only eight series (out of 19 PAGES 2017). Seven tree ring series: two from New Zealand (both less than 500 years), three from Tasmania (one long, two less than 500 years), two from southern South America (both less than 500 years) and one weird lake sediment from Chile (a “singleton” proxy using pigments in the sediments).

Only One Long Proxy

Only one proxy in the network has values prior to AD750 and only two proxies have values prior to AD1450. Thus, the only information directly comparing medieval and modern values comes from these two proxies: Mt Read, Tasmania (a series used as long ago as Mann et al 1998 and Jones et al 1998) and many times since and the Laguna Aculeo pigment series – neither of which have shapes remotely similar to the PAGES2K 60-30S latband reconstruction – see below. (The latband reconstruction was calculated from the enormous file at NOAA here).

Take a look at the underlying data (converted to SD Units) – more commentary below.

Comments on the PAGES 60-30S HS

Quite aside from many issues about the PAGES2019 selection of 60-30S proxies, obvious questions arise about how they derived their latband reconstruction.

• the blade of the reconstruction HS goes from -1 sigma in early 20th century to more than 4 sigma in 2000. Yet there is no comparable deviation in any of the underlying proxies. The three South American proxies and the long Mt Read, Tasmania tree ring chronology don’t have anything like a blade; the four short tree ring chronologies (two Tasmania and two New Zealand) have increase sharply in 20th century, but not enough to yield the PAGES 2019 HS. (These tree chronologies have been selected from a much larger candidate populaion – a screening process that already imparts a serious bias.)
• the only 30-60S proxy with a value in the year 2000 is Mount Read, which has a value of ~1 sigma. Yet the PAGES 2019 30-60S (CPS) reconstruction has a value of over 4 sigma. How did they do that?
• PAGES 2019 provide code for the generation of figures from reconstructions, but didn’t archive the code for the generation of the reconstructions. (At least in the links provided in any of the articles.) So it’s impossible to precisely diagnose what’s going on.
• although PAGES proclaim the importance of public archiving as a selection criterion, only one of the tree ring chronologies (the long Mount Read chronology) can be firmly associated with ITRDB measurement data archives. Both South American tree ring chronologies derive from lead author Neukom’s calculation on unarchived South American data. D’Arrigo’s 1995 data remains unarchived, as does the Duncan New Zealand data. At the time of the PAGES 2013 publication of the two Allen tree ring chronologies from Tasmania, no relevant measurement data was archived; since then, Allen has archived measurement data from Tasmania, but PAGES 2019 doesn’t contain any citation.
• the Laguna Aculeo series is a purported temperature reconstruction from pigments. At present, there are no other similar temperature reconstructions, leaving this series as a sort of ad hoc singleton.

High-Resolution Ocean Proxies

But most of all, given that the 60-30S latband is almost entirely (~96%) ocean, it seems bizarre that PAGES 2019 did not use any ocean core proxies, especially since there are physical formulas for estimating SST from alkenone or Mg/Ca measurements. Any conversion of tree ring widths to temperature in deg C is the result of ad hoc statistical fitting, not a universal formula. Alkenone values have been measured all over the modern ocean and nicely fit known ocean temperatures. In addition, alkenone values for ocean cores going back to deeper time (even to the Miocene) give a consistent and reproducible narrative. So there’s a lot to like about them as a candidate for a “good” proxy.

While there are numerous high-resolution (10 year resolution) alkenone and Mg/Ca measurements in the North Atlantic with values through the last millennium and up to the present, to my knowledge, there were not any such series as of PAGES 2013 or PAGES 2017. (In my opinion, IPCC AR5 ought to have noted this and suggested that this deficiency be remedied.)

PAGES 2017 included three ocean core proxy series in the 30-60S, all from offshore Chile. Their resolutions ranged from 24 to 83 years. There are some thus far undiscussed puzzles in the PAGES 2017 version of these series – as, in each case, modern values available in the underlying archive series were deleted. In each case, unsurprisingly, the effect of the deletion was to hide a decline. I will discuss this series below.

Subsequent to PAGES2017, the very first high-resolution (less than 10 years) 30-60S ocean core alkenone (or Mg/Ca) proxy was published: MD07-3093. [Collins, JA et al. (2019): Centennial-scale SE Pacific sea surface temperature variability over the past 2300 years. Paleoceanography and Paleoclimatology link.] It has values dated from 372 BC to 1992 AD, with a resolution of 5.4 years. The appearance of the high-resolution MD07-3093 is obviously very different – even opposite – to the PAGES 2019 reconstruction.

The three 30-60S ocean cores that were in PAGES 2017 (and dropped from PAGES 2019) are shown below. In each case, I’ve compared the NOAA or Pangaea original archive data (black) to the PAGES 2017 version (red). In each case, the PAGES2017 version was shortened by removal of a few closing values: CF7-33 was shortened from AD1874 close to AD1784; GeoB 3313-1 shortened from AD1884 to AD1650; and GeoB 7186-3 shortened from AD1938 to AD1900.

Although each of these series is lower resolution than the new MD07-3093 series, they tell the same story: a 1-1.5 deg C decrease in temperatures from the first millennium to the 20th century. And MD07-3093 indicates that the decrease has been maintained into the late 20th century (at least in these offshore Chile ocean core datasets.)

Conclusion

Given that the 60-30S latband is almost entirely ocean, it seems logical that IPCC and PAGES2K should use data from ocean proxies to estimate past temperature in this latitude band. But this isn’t what they’ve done. Instead, they’ve purported to estimate past temperature from a few scattered tree ring chronologies, only one of which reaches earlier than AD1850; and an idiosyncratic singleton pigment series. Ironically, the only 30-60S proxy series in PAGES 2019 that reaches back into the first millennium – the Mount Read, Tasmania tree ring series – was used by Mann et al 1998-1999, Jones et al 1998 and numerous other supposedly “independent” multiproxy studies. Neither of the two series reaching back to the medieval period permit the conclusion that modern period is warmer than medieval period. Caveat: I’m not saying that it isn’t; only that this data doesn’t show it, let alone support the big-bladed HS cited by IPCC. High-resolution alkenone measurements from ocean cores offshore Chile show a consistent decrease in ocean temperatures over the past two millennia that is neither reported nor discussed by IPCC (or PAGES 2019).

To be clear, some of the technical articles on 30-60S ocean core proxies by specialist authors are truly excellent and far more magisterial than the IPCC mustered, in particular, several articles on offshore Chile. Here are a few:

Mohtadi et al, 2007. Cooling of the southern high latitudes during the Medieval Period
and its effect on ENSO link

Killian and Lamy 2012. A review of Glacial and Holocene paleoclimate records from southernmost Patagonia (49-55degS) link

Collins et al 2019. Centennial‐Scale SE Pacific Sea Surface Temperature Variability Over the Past 2,300 Years link