pages2k

PAGES 2019: 0-30N Proxies

alayham — Wed, 15 Sep 2021 17:01:54 +0000

PAGES 2019: 0-30N Proxies alayham Wed, 09/15/2021 - 19:01

Next, the PAGES2019 0-30N latband. Their CPS reconstruction (CPS) for the 0-30N latband (extracted from the global reconstruction) looks almost exactly the same as reconstructions for the 0-30S and 30-60S latbands. However, none of the actual proxies in this latband look remotely like the latband reconstruction, as I’ll show below. In the course of examining […]

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PAGES 2019: 0-30N Proxies

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Climate Audit

PAGES19: 0-30S

alayham — Thu, 02 Sep 2021 20:07:39 +0000

PAGES19: 0-30S alayham Thu, 09/02/2021 - 22:07

In a Climategate email. Keith Briffa famously sneered at Michael Mann’s claim that a temperature reconstruction could represent a hemisphere, including the tropics, by regressing a “few poorly temperature respresentative tropical series” against “any other target series” – even the trend of Mann’s own “self-opinionated verbiage” as follows: I am sick to death of Mann […]

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PAGES19: 0-30S

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Climate Audit

PAGES2019: 30-60S

alayham — Thu, 26 Aug 2021 19:24:53 +0000

PAGES2019: 30-60S alayham Thu, 08/26/2021 - 21:24

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 […]

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PAGES2019: 30-60S

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Climate Audit

PAGES19 Asian Tree Ring Chronologies

alayham — Sun, 15 Aug 2021 20:25:31 +0000

PAGES19 Asian Tree Ring Chronologies alayham Sun, 08/15/2021 - 22:25

About 20% of the PAGES 2019 proxies are 50 Asian tree ring chronologies, all of which were originally published as chronologies in PAGES (2013). At the time, none of these series (and certainly not in these digital versions, had ever been published in technical literature, peer reviewed or otherwise. Nothing in the Supplementary Information to […]

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PAGES19 Asian Tree Ring Chronologies

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Climate Audit

Milankovitch Forcing and Tree Ring Proxies

alayham — Tue, 02 Mar 2021 22:05:03 +0000

Milankovitch Forcing and Tree Ring Proxies alayham Tue, 03/02/2021 - 23:05

Mar 2, 2021. This post was written in 2015 but, for some reason, I didn’t publish it at the time. Seems just as valid today as when it was written. Esper et al 2012, Orbital Forcing of Tree Ring Data pdf SI, is one of the few paleoclimate articles in past decade which really […]

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Milankovitch Forcing and Tree Ring Proxies

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Climate Audit

PAGES2K (2017): Antarctic Proxies

alayham — Fri, 01 Feb 2019 20:56:18 +0000

PAGES2K (2017): Antarctic Proxies alayham Fri, 02/01/2019 - 21:56

A common opinion (e,g, Scott Adams) is that the “other proxies”, not just Mann’s stripbark bristlecone tree rings, establish Hockey Stick. In today’s post, I’ll look at PAGES2K Antarctic data – a very important example since Antarctic isotope data (Vostok) is used in the classic diagram used by Al Gore (and many others) to illustrate […]

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PAGES2K (2017): Antarctic Proxies

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Climate Audit

PAGES2K: North American Tree Ring Proxies

alayham — Wed, 24 Oct 2018 18:57:02 +0000

PAGES2K: North American Tree Ring Proxies alayham Wed, 10/24/2018 - 20:57

The PAGES (2017) North American network consists entirely of tree rings. Climate Audit readers will recall the unique role of North American stripbark bristlecone chronologies in Mann et al 1998 and Mann et al 2008 (and in the majority of IPCC multiproxy reconstructions). In today’s post, I’ll parse the PAGES2K North American tree ring networks […]

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PAGES2K: North American Tree Ring Proxies

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Climate Audit

PAGES2K (2017) – South America Revisited

alayham — Sun, 07 Oct 2018 21:54:16 +0000

PAGES2K (2017) – South America Revisited alayham Sun, 10/07/2018 - 23:54

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 […]

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PAGES2K (2017) – South America Revisited

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Climate Audit

PAGES 2017: Arctic Lake Sediments

alayham — Sat, 22 Jul 2017 18:58:55 +0000

PAGES 2017: Arctic Lake Sediments alayham Sat, 07/22/2017 - 20:58

Arctic lake sediment series have been an important component of recent multiproxy studies. These series have been discussed on many occasions at Climate Audit (tag), mostly very critical. PAGES 2017 (and related Werner et al 2017) made some interesting changes to the Arctic lake sediment inventory of PAGES 2013, which I’ll discuss today.
Some prior Climate Audit criticisms have resulted in withdrawal or major changes or lingering controversy, including:

the controversy over Mann’s use of Korttajarvi (Finland) varve thickness and XRD proxies (tag) due to two factors: recent disturbance of varves by agricultural runoff and road construction; use of the data upside-down to the interpretation of the author. While Mann denied any error and refused to correct, Kaufman et al 2009, who had followed Mann’s incorrect orientation, grudgingly conceded the error after it had been pointed out at Climate Audit and issued a corrigendum inverting the orientation of the series (also reversing the orientation of four other series (of 23) in the corrigendum;
the orientation of the Hvitarvatn varve thickness series in PAGES 2013 was immediately criticized at Climate Audit in April 2013 (here). Subsequently, McKay and Kaufman (2014) conceded the error and issued an amended version of their Arctic reconstruction, but, like Mann, refused to issue a corrigendum to the original article. [Correction July 29, 2017: on October 7, 2014, immediately after publication of McKay and Kaufman 2014, I wrote to Nature, pointing out that McKay and Kaufman 2014 primarily addressed errors in PAGES 2013 (not new information) and urged that they issue a proper Corrigendum to PAGES 2013. In November 2015, over a year later, PAGES 2013 issued a belated Corrigendum. Neither Nature nor the authors notified me of this and I was unaware of this until a comment in this thread.]
almost immediately after publication of PAGES 2013, I pointed out that their Igaliku series (which had a huge hockey stick) was contaminated in its modern portion by agricultural runoff and erosion. I recall that Nick Stokes vigorously denied that this constituted an error. McKay and Kaufman 2014 partially responded to the problem by truncating a few data points towards the end (but not fully extinguishing the problem.)
I observed that the Kepler Lake (Alaska) d18O series was structurally similar to the Mount Logan (Alaska) ice core d18O series, about which I had criticized its ex post exclusion by PAGES 2013 on spurious grounds – the series went the “wrong” way and was excluded due to “regional” effects. Instead of re-instating Mount Logan (according to principled ex ante criteria), PAGES (McKay and Kaufman) instead excluded Kepler Lake.

Other prior CA criticisms of Arctic sediment proxies included:

inhomogeneity of the Iceberg Lake varve thickness series (tag) due to changing physical configuration of the moraine-bounded lake;
arbitrary exclusion of the early (pre-730) portion of the Blue Lake varve thickness series (tag), where the alleged inhomogeneity was simply that the climate was believed to be “warm with precipitation inferred to be higher than during the twentieth century”, not a physical disturbance.

Some of these issues had material impact on underlying reconstructions e.g. the notorious Mann et al 2008 “no-dendro” series or Kaufman et al 2009 (here).

McKay and Kaufman 2014 (PAGES)

McKay and Kaufman 2014 discontinued the following three series used in PAGES 2013 on the grounds that there was “insufficient evidence that they were sensitive to temperature”

the Kepler Lake (Alaska) d18O series mentioned above;
a particle-size series from East Lake, Southampton Island in the Canadian Arctic Archipelago;
a diatom (Fragilariopsis cylindrus) series from Holsteinborg Dyb in west Greenland.

They grudgingly inverted the Hvitarvatn series.
Sections of the following two sediment records were truncated on the grounds that the record was not “sensitive to temperature” during these intervals:

Blue Lake varve thickness prior to AD730;
Igaliku pollen accumulation rate after AD1920

A 50-year dating error in the Lone Spruce (Alaska) BSi series was fixed.
PAGES 2017
Four more PAGES 2013 Arctic sediment series were rejected in PAGES 2017, while two series used prior to PAGES 2013 (but not in PAGES 2013) were re-instated.
The following four PAGES 2014 Arctic sediment series were rejected in PAGES 2017:

the Iceberg Lake (Alaska) varve series (previously criticized at CA tag) because of its “unclear relation to temperature”
the truncated Korttajarvi X-ray density (XRD, a form of grey scale) see CA tag, originally used in Mann et al 2008 and Kaufman et al 2009, now also said to have an “unclear relation to temperature”
the previously truncated Igaliku series (CA tag)now removed in its entirety due to its “unclear relation to temperature”
the Lone Spruce (Alaska) BSi series (introduced in PAGES 2013, but only mentioned in passing in CA posts) was also removed due to its “unclear relation to temperature”.

The SI to PAGES 2017 also contained the following lengthy quality-control comment on Hvitarvatn, which had been a source of PAGES 2013 embarrassment:

Based on studies of glacier mass balance and glaciology in Iceland (e.g. Bjornsson; Flowers), Icelandic glacier fluctuations are dominantly controlled by changes in melt season temperature. Glacier fluctuations influence the production and transport of eroded material and the eventual deposition of this sediment in a downstream basin (i.e. a proglacial lake). .. On short timescales (seasonal, annual, inter-annual), changes in sediment accumulation can be driven by many factors and we can all agree that identifying individual controls is messy. But on longer timescales (for example, centennial timescales, … I would argue strongly that changes in sediment accumulation are driven by changes in glacier size. This is laid out in Larsen et al., 2011 QSR. We subsequently expanded on this initial study to: 1) include the whole Holocene (Larsen et al., 2012 QSR attached, which demonstrates a clear “8.2ka-event” signal and subsequent Neoglacial onset), and 2) by measuring varve thickness in multiple cores along a lake transect and tying the core data to seismic stratigraphy (Larsen et al. 2013 EPSL attached). This latter work demonstrates that the trends in sediment accumulation are consistent and observed throughout the lake basin. Given the available data, I feel comfortable summarizing as follows: Icelandic glacier fluctuations are dominantly controlled by summer temperature. On longer timescales, fluctuations of the Langjokull ice cap can be reconstructed from changes in mean varve thickness at glacial lake Hvitarvatn.
Previous comment: QC failed: article states “sediment flux to Hvítárvatn is dominantly controlled by the integrated rate of sediment production by erosion beneath Langjökull, modulated on annual to decadal timescales by the efficiency of the subglacial fluvial sediment delivery system.”, variability function of proximity, absolute values function of sediment availability. This is _not_ temperature!; QQ by PF not passed

The two paragraphs seem inconsistent. They did not discuss or attempt to explain the similarity of Hvitarvatn and Big Round varve thickness series.
The following two series were re-instated:

Soper Lake, a varve thickness series used in Mann et al 2008. It was too short for either Kaufman et al 2009 (1000 years required) or PAGES 2013 (minimum AD1500 start). However, it has a pronounced Hockey Stick shape over the shorter period. I think that the policy of Kaufman et al (restricting to long series) is by far the better way to achieve interpretable results and regard this re-instatement as quality deterioration.
Hallett Lake, which like Lone Spruce, was an Alaska BSi series (one with slightly lower resolution). It had been used in Kaufman et al 2009 but dropped in PAGES 2013. It’sfar from obvious why one Alaskan BSi series is now held to be temperature sensitive and not the other – especially when the opposite was concluded in PAGES 2013.

The Hallett Lake BSi series extends back to the mid-Holocene and is shown below to keep changes in the last millennium in perspective. The Hallett BSi series does indeed have somewhat of a Hockey-Stick shape (which might have explained why it was preferred to Lone Spruce BSi), but on a Holocene scale, the blade of the stick is inconsequential – a point that can be missed when multiproxy techniques first convert series to SD units.

PAGES 2017 introduced two new low-resolution alkenone series, both by D’Andrea:

Lake E, a Greenland Lake adjacent to the Braya So alkenone series already in PAGES2013. (The original author even combined the two series in a version).
Kongressvatnet in Svalbard (16.1 years)

Werner et al (CPD 2017)

Werner et al (CPD 2017) is an Arctic reconstruction by PAGES2017 authors. Strangely, it rejects three more Arctic sediment series, each of which had been involved in previous controversy:

it rejected Hvitarvatn (a non-HS series) on the grounds that its “annual and centennial signal inconsistent”. It didn’t say how it arrived at this conclusion.
it rejected Blue Lake (which had very elevated first millennium values) on the grounds that it had a “very nonlinear response, short overlap with instrumental, unclear interpretation”.
it rejected Lehmilampi on the grounds that “exact interpretation unclear from original article”. Yet it retained Nautajarvi from the same authors, even though its “darksum” series is, if anything, harder to interpret.

In their reconstruction, they elected not to use 9 series on the grounds that they lacked annual resolution: six chironomid/midge reconstructions (Hamptrask, Lake 4, Pieni-Kauro, Hudson, Moose, Screaming Lynx), three alkenone series (Braya So, Kongressvatnet, Lake E) and the one remaining BSi series (Hallett Lake).
The net result is that the sediment series in Werner et al 2009 reverted back to five series from Kaufman et al 2009 (Donard, Big Round from Baffin Island; C2 and Lower Murray from Ellesmere Island; truncated Nautajarvi, Finland) plus the short Soper Lake (Ellesmere Island) series to help with the HS.

Taking Inventory

The inventory flows of Arctic sediment proxies are summarized below. 32 “different” (non-isomorphic) series were introduced in the four studies as “temperature sensitive”. (For the purposes of this inventory, flipped versions are treated as one series.) 16 of the 32 series were rejected in a subsequent study as not being “temperature sensitive” after all. This is a very high casualty rate given original assurances on the supposed carefulness of the original study. The casualty rate tended to be particularly high for series which had a high medieval or early portion (e.g. Haukadalsvatn, Blue Lake).

Not only is the number of surviving series (16) a discouraging proportion of the opening inventory, but even the opening inventory was itself culled from a much larger population of lake sediment series. Making matters worse, because the inventory of proxies changes only slightly from study to study, the networks of sediments in each study are not independent, as opposed to slight variations. Because data snooping and ex post selection are endemic, little to no credence can be given to the (very slight) HS displayed by any composite. (This is not to say that the “true” answer is a non-HS, only that the answer is tainted.)
In trying to get a more informed understanding of Arctic proxies, I’ve found it helpful to examine Arctic lake sediments over a Holocene perspective and to examine multiple series (e.g. varve thickness, magnetic susceptibility, BSi) from a single site at the same time. I think that it is most helpful to work with proxies which are available at multiple sites (PAGES too often uses singletons). I also think that it is best to work out from the best understood sites – a principle used in the mineral exploration industry. I have some notes on some sites that have particularly interested me (Hvitarvatn, Big Round in particular) and will try to write them up some time.

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https://climateaudit.org/2017/07/22/pages-2017-arctic-lake-sediments/

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Climate Audit

Was early onset industrial-era warming anthropogenic, as Abram et al. claim?

alayham — Wed, 31 Aug 2016 17:12:25 +0000

Was early onset industrial-era warming anthropogenic, as Abram et al. claim? alayham Wed, 08/31/2016 - 19:12

A guest post by Nic Lewis

Introduction
A recent PAGES 2k Consortium paper in Nature,[i] Abram et al., that claims human-induced, greenhouse gas driven warming commenced circa 180 years ago,[ii] has been attracting some attention. The study arrives at its start dates by using a change-point analysis method, SiZer, to assess when the most recent significant and sustained warming trend commenced. Commendably, the lead author has provided the data and Matlab code used in the study, including the SiZer code.[iii]
Their post-1500 AD proxy-based regional reconstructions are the PAGES2K reconstructions, which have been discussed and criticized on many occasions at CA (see tag), with the Gergis et al 2016 Australian reconstruction substituted for the withdrawn version. I won’t comment on the validity of the post-1500 AD proxy-based regional reconstructions on which the observational side of their study is based – Steve McIntyre is much better placed than me to do so.
However, analysis of those reconstructions can only provide evidence as to when sustained warming started, not as to whether the cause was natural or anthropogenic. In this post, I will examine and question the paper’s conclusions about the early onset of warming detected in the study being attributable to the small increase in greenhouse gas emissions during the start of the Industrial Age.
The authors’ claim that the start of anthropogenic warming can be dated to the 1830s is based on model simulations of climate change from 1500 AD on.[iv] A simple reality check points to that claim being likely to be wrong: it flies in the face of the best estimates of the evolution of radiative forcing. According to the IPCC 5th Assessment [Working Group I] Report (AR5) estimates, the change in total effective radiative forcing from preindustrial (which the IPCC takes as 1750) to 1840 was –0.01 W/m2, or +0.01 W/m2 if changes only in anthropogenic forcings, and not solar and volcanic forcings, are included. Although the increase in forcing from all greenhouse gases (including ozone) is estimated to be +0.20 W/m2 by 1840, that forcing is estimated to be almost entirely cancelled out by negative forcing, primarily from anthropogenic aerosols and partly from land use change increasing planetary albedo.[v] Total anthropogenic forcing did not reach +0.20 W/m2 until 1890; in 1870 it was still under +0.10 W/m2.
It is not credible that a negligible increase of 0.01 W/m2 would have had any measurable effect on ocean or land temperatures globally; it is doubtful that an increase of 0.1 W/m2 would do so. Even a change of 0.20 W/m2 would have affected global mean surface temperature (GMST) by less than 0.1°C. Moreover, anthropogenic aerosol and land use change forcing was concentrated in the Northern hemisphere extratropics and the tropics, with little in the Southern extratropics, which would suggest that the onset of positive net anthropogenic forcing (and hence anthropogenic warming) in the tropics and Northern hemisphere extratropics would have been delayed until circa 1870. Yet it is in those regions that Abrams et al. find the earliest onset of anthropogenic warming.
It is possible that AR5 best estimates overstate the strength of anthropogenic aerosol forcing, with the result that total anthropogenic forcing became positive enough to have a measurable impact on temperatures at an earlier date than if those estimates were correct. However, if so, that is good news. It would imply that climate sensitivity – both transient and equilibrium – is lower than current best estimates based on AR5 forcing values suggest, since the observed warming over the industrial era would then have been produced by a larger increase in forcing.
The influence of volcanism on the diagnosed early onset dates of warming
How, then, are the study’s results to be explained? The study’s change point analysis, whilst interesting, seems to me an unsuitable method of detecting the onset of anthropogenic warming. One would expect to find a change of slope in global temperature somewhere – depending on natural internal climate variability – around the late 1830s,. That is because, although anthropogenic forcing did not become significant until much later in the 19th century, there was a big change in average volcanic forcing at that time. Over the industrial period (taken as 1750–2015) as a whole, on the AR5 best estimate basis volcanic forcing averaged –0.4 W/m2. Whilst it averaged close to that level during the last four decades of the 18th century, during the first four decades of the 19th century there was heavy volcanic activity,[vi] with (according to IPCC AR5 estimates) forcing averaging –1.0 W/m2. By contrast, volcanic forcing over the next four decades was small, averaging only –0.1 W/m2. The resulting +0.9 W/m2, change in average forcing is several times larger than the total change in anthropogenic forcing over the whole of the 19th century.
I would expect that pattern of forcing to produce, on a decadal mean basis, a depressed GMST level from circa 1800 to the late 1830s followed by a recovery. In the 1880s, when volcanic activity was again high (Krakatau), one might expect an interruption in the upward movement. However, Atlantic multidecadal variability is thought to have commenced an extended upswing during the 1830s, peaking in the late 1800s.[vii] That would itself have produced a warming trend during that period, particularly over the Northern hemisphere. Multicentennial internal climate variability might also have had a role. Moreover, by the late 1800s anthropogenic forcing had become non-negligible at 0.25 W/m2 per AR5, so would have produced a weak warming trend on its own (although not a strong enough one to counter the effects of Atlantic – and maybe Pacific – multidecadal variability becoming negative during the first two decades of the 20th century).
Warming from the late 1830s to the late 19th century due to recovery from the heavy volcanism earlier in the century and the upswing in Atlantic multidecadal variability would have been superimposed on a slow trend of recovery in surface temperature from the Little Ice Age (LIA), as the ocean interior warmed after the end of the particularly cold four hundred year period from (according to the paper) AD 1400–1800 – a process with a similarly long timescale. The resulting temporal pattern fits the global reconstructions of surface air and sea temperature trends shown in their Figure 1c and 1d, and would account for a change point being found in the 1830s or 1840s. But it does not at all imply that anthropogenic forcing had any measurable influence before the late 1800s. The change of GMST slope circa the late 1830s, and the rise in GMST from then until the late 1800s, was likely almost entirely due to natural factors.
Abram et al. claim that diagnosis using SiZer is little affected by temporary cooling episodes, such as those produced by heavy volcanic activity. This claim is based on the results of analyses of noisy synthetic temperature time series in which gradual warming commences at a known time. They show that a decade-long downwards excursion in temperature starting 25 or 50 years before the actual warming commencement date brings forward the diagnosed warming commencement date by only five to twenty years (Extended Data Figure 3a). However, heavy volcanic activity extended over the first four decades of the 19th century, not for a single decade. Also, the assumed AR(1) annual temperature time series autocorrelation of 0.1 looks light, certainly for sea surface temperatures, where the instrumental record suggests a figure of 0.4 or so would be more realistic. Moreover, their assumed ratio of 100-yr trend to 2σ noise of 1:0.5 looks optimistic to me. In the light of these factors, it seems plausible that the cooling produced by heavy volcanism during 1800–1839 might have brought forward the diagnosed second quarter of the 19th century warming commencement dates by up to several decades from when they would otherwise have been diagnosed.
Abram et al.’s evidence for early warming being anthropogenic
The paper’s conclusions about an anthropogenic origin of the early warming onset are based on evidence summarised in their Figure 3, reproduced here as Figure 1.

Figure 1. Reproduction of Figure 3 of Abram et al. 2016
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I will discuss each source of evidence separately, in the reverse order from where it appears in their Figure 3.
Evidence in Abrams et al. Figure 3d
The paper’s authors comment that:
“Naturally forced climate cooling may have helped to set the stage for the widespread onset of industrial-era warming in the tropical oceans and over Northern Hemisphere landmasses during the mid-nineteenth century.”
which is very true, but then go on to say:
“Simulations suggest that recovery from volcanic cooling is not an essential requirement for reproducing the mid-nineteenth-century onset of industrial-era warming. Multi-model experiments forced with only greenhouse gases capture regional onsets for sustained industrial-era warming that are consistent with the tropical ocean and Northern Hemisphere continental reconstructions (Fig 3d).”
However, this is irrelevant. It is unsurprising that model simulations forced only with greenhouse gases, as used in their Figure 3d, produce sustained warming from quite early in the industrial period. But in fact the historical forcing that produced the warming was the net sum of greenhouse gas (GHG) forcing and other forcings, which were mainly negative. As I stated, the AR5 best estimate of the sum total of historical forcings is almost zero in 1840, and only rises slowly thereafter. On a rolling 5-year average basis, estimated global total forcing doesn’t exceed 0.1 W/m2 – enough to produce, within a decade, a very small warming of 0.02 to 0.05°C – until 1901. However, GHG forcing reaches a more material level of 0.2 W/m2 in the mid-1840s – at which point total anthropogenic forcing is only 0.05 W/m2.
Evidence in Abrams et al. Figure 3c
The only anthropogenic forcing included in CSIRO-Mk3L simulations used here is from GHG. Various combinations of natural forcings were employed in addition. Accordingly, the same objections – regarding the omission of negative non-GHG anthropogenic forcing – apply here as to the Figure 3d evidence.
Evidence in Abrams et al. Figure 3b
The LOVECLIM simulations used here are stated to include “Full forcing” ones as well as GHG-only, solar only and volcanic only forcing simulations. However, Full forcing includes only GHG, land use change, solar and volcanic forcing. It does not include aerosol forcing, the dominant anthropogenic negative forcing. Accordingly, essentially the same objections apply here as to the Figure 3c and 3d evidence, albeit with slightly less force since the minor negative land use change forcing was included. Perhaps reflecting land use change forcing, in the Northern hemisphere the diagnosed warming commencement dates with Full forcing are generally several decades later than those found using the PAGES 2k reconstruction and the multimodel ensemble used for Figure 3a.
Evidence in Abrams et al. Figure 3a
This is really the crux of the evidence supplied by Abrams et al. as to an anthropogenic cause of early onset warming. The regional warming onset dates from simulations given in their Table 1, which they claim are reasonably consistent with the dates they derive from their proxy reconstructions, are based entirely on the multimodel last-millennium climate simulations to which their Figure 3a relates. They claim that these simulations, by ten models, employed full radiative forcings, both natural and anthropogenic. However, this statement appears to be incorrect.
The simulations used cover 850–2005 AD in two parts: 1000 year CMIP5 last-millennium (past1000) simulations with forcings that are consistent with the PMIP3 protocol, extended by CMIP5 historical period simulations covering 1850–2005. There are two problems with using this data.
First, the forcings PMIP3 specifies for use in last-millennium climate simulations excludes anthropogenic aerosols.[viii] That will result in anthropogenic forcing, and warming, reaching a non-negligible level at a much earlier point in the industrial period than if all anthropogenic forcings were included at AR5 best estimate values. For about half the ensemble of ten (nine in practice) models used, it appears that the last-millennium climate simulations were continued to 2005 using the same set of PMIP3 forcings, producing a quasi-historical period simulation covering 1859–2005.
Secondly, it appears that for the remainder of the ensemble of models, the main CMIP5 historical period (1850–2005) model simulations, rather than continuations of the past1000 simulations, were used to extent the past1000 PMIP3-protocol simulations. That is important since, like the past1000 simulations, the main CMIP5 historical runs were initialised by branching off from preindustrial control runs. The ocean will be substantially cooler at the end of the past1000 simulations, following the long cold 1400–1840 period, than when the historical simulations were branched off the preindustrial control simulations (which generally do not have any volcanic forcing). As a result of this discontinuity, for such models there would be an immediate jump in temperatures (ignoring any influence of internal variability) between 1849 and 1850. Allowing for a 5–20 year early bias of the SiZer change point date in the presence of a burst of previous negative forcing (Extended Data Figure 3a), such as caused by the heavy volcanism over the first four decades of the 19th century, such jumps could account for finding warming onset dates in the 1830s.
Although the inclusion of negative anthropogenic aerosol forcing in the main CMIP5 historical simulations would result in them warming at a slightly slower rate after the initial jump in 1850 than if PMIP3-protocol forcings were included, the growth in negative anthropogenic aerosol forcing between preindustrial and 1850 is, unlike the rise in GHG, generally ignored in the CMIP5 historical simulations, so aerosol forcing would start from a zero base in 1850 and so have a negligible effect over the following few decades. Moreover, two of the models involved only included direct aerosol forcing – which per AR5 accounts for only about half total aerosol forcing– in their main CMIP5 historical simulations.
To illustrate my point, I accessed the data used and extracted the model simulated temperatures for Europe, one of the regions that has a diagnosed mid-19th century warming onset from the proxy-based reconstructions, with onset diagnosed later in the 19th century from the multi-model reconstructions. Figure 2 shows the various time series, averaged pentadally in order to reduce annual climate noise.

Figure 2: Multimodel simulated surface temperature in Europe (pentads starting in year shown)
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Results from the four models that appear to have generated simulations over 1850–2005 by continuing the last millennium simulations are shown by dashed lines; the thick red line shows their mean. None of these models included anthropogenic aerosol forcing. Results from the five models that appear to have generated simulations over 1850–2005 by branching from the (warmer) preindustrial control simulation are shown by solid lines; the thick black line shows their mean.[ix]
Although difficult to see in the figure, the mean of the discontinuous models shows a jump from the 1840-49 average to the 1850–55 average temperature, of 0.23°C. By contrast, the mean for the continuous models shows an increase of only 0.05°C. I have chosen these periods as they are the longest ones straddling the start of 1850 transition from the last millennium simulations to the 1850–2005 ones that are unaffected by contemporaneous or very recent volcanic activity. Repeating the exercise for temperatures in Asia and North America, where the warming onsets dates diagnosed from the simulations straddled the 1840s, showed a mean excess for the discontinuous models, in the difference between the mean 1840-49 and 1850–55 averages, of 0.17°C, similar to that for Europe.[x] The much larger increase for the discontinuous models between the mean temperatures for these periods very much supports my argument that their historical simulations started from a warmer state than their past1000 simulations ended in.
Conclusions
It appears that the claim in Abrams et al. that the diagnosed early onset – about 180 years ago in some regions – of industrial-era warming is of anthropogenic origin is based on inappropriate evidence that does not substantiate that claim, which is very likely incorrect. Most of the evidence given for the anthropogenic origin claim, which is entirely model-simulation based, ignores the industrial era increase in aerosol forcing, the dominant negative (cooling) anthropogenic forcing; the remaining evidence appears to be invalidated by a simulation discontinuity in 1850. The only evidence provided that includes even the post 1850 increase in anthropogenic aerosol forcing – half of the Figure 3a multi-model ensemble simulations – is affected by the simulations from 1850 on being started with the ocean significantly warmer than it was in 1849.
Recovery from the heavy volcanism earlier in the century and an upswing in Atlantic multidecadal variability, superimposed on a slow trend of recovery in surface temperature from the LIA as the ocean interior warmed after the end of the particularly cold four hundred year period from AD 1400–1800, appears adequate to account for warming from the late 1830s to the final quarter of the 19th century. It is unlikely that anthropogenic forcing, estimated to be very low until the 1870s, played any part in warming before then. The heavy volcanism in the first four decades of the 19th century likely caused the warming onset dates diagnosed from the proxy data, at least, to be up to several decades earlier than they would have been in its absence.
Ironically, should the study’s finding of anthropogenic warming starting as early as circa the 1830s be correct, it would imply that anthropogenic aerosol forcing is weaker than estimated in IPCC AR5, and therefore that observational estimates of climate sensitivity (both transient and equilibrium) based on AR5 forcing values need to be revised downwards. That is because total anthropogenic forcing would only have become positive enough to have had any measurable impact on temperatures in the 1830s if AR5 best estimates significantly overstate the strength of anthropogenic aerosol forcing.
Nicholas Lewis August 2016

Correction: 1 September 2016
Nerilie Abram helpfully advises me that the HadCM3 1850–2005 simulation was in fact a continuation of its last millennium simulation; this was not evident from the details given in the paper’s Extended Data Table 2. In addition, she says that a mean offset was applied to the historical portion of the MIROC-ESM simulation to avoid an artificial jump in this dataset. This does not seem to have been mentioned in the main text, Extended Data or Supplementary Information, and I do not know the amount of the offset. However, it now seems inappropriate to treat this model as belonging in the discontinuous category (although it is not entirely clear whether it can properly be categorised as continuous). The classifications of HadCM3 and MIROC-ESM in Figure 2 are accordingly incorrect, as are the heavy lines showing means for each category, which should be ignored.
With these two models categorised as continuous, the discontinuous category models show on average a 0.19°C jump in Europe, relative to the change for continuous models, from the 1840-49 mean to the 1850–55 mean temperature, similar to that previously derived. I have now calculated the median increase across all seven land regions for each set of models (continuous and discontinuous simulation ones). With HadCM3 and MIROC-ESM categorised as continuous, the overall median increase is 0.30°C higher for the discontinuous models than for the continuous models; this difference was almost the same, at 0.31°C, when they were treated as discontinuous models. Although the proportion of discontinuous models is now smaller, the problem with aerosol forcing being either ignored, either entirely or save as to its increase from its level in 1850 (possibly from the 1820s for HadCM3) remains.

[i] Abram NJ et al. (2016) Early onset of industrial-era warming across the oceans and continents. Nature, doi:10.1038/nature19082. In archive FAQ_&_supporting_docs.zip available at https://cloudstor.aarnet.edu.au/plus/index.php/s/4pQheVzMddCXwJN
[ii] Around the 1830s in the tropical oceans and the Arctic, and around the 1840s in North America, Asia and Europe.
[iii] http://www.ncdc.noaa.gov/paleo/study/20083
[iv] The past1000 CMIP5/PMIP3 simulations used commence in 800 AD, and are extended from 1849 to 2005, but Abram et al only use the 1500–1999 AD portion.
[v] Per AR5 best estimates, total aerosol forcing was over four times as strong as land use change forcing (which was also negative) throughout the 1800s.
[vi] Very high volcanic forcing of –1.0 W/m2 in the decade to 1810 was followed by extremely high forcing of –2.1 W/m2 in the next decade due to the eruption of Tambora in 1815, which balanced out with the volcano-free 1820s, and forcing again very high at –1.1 W/m2 in the fourth decade due to the 1835 Cosiguina eruption.
[vii] Delworth, T. L., and M. Mann (2000), Observed and simulated multidecadal variability in the Northern Hemisphere, Clim. Dyn., 16, 661–676, doi:10.1007/s003820000075. ( A smoothed version shown in Figure 2(d) of Chylek P et al. (2011) GRL 38, L13704, doi:10.1029/2011GL047501.)
[viii] PMIP3 also excludes other anthropogenic non-greenhouse gas forcings other than land use change.
[ix] The tenth model, Fgoals-s2, did not generate the requisite surface air temperature data.
[x] The average across the three regions of differences in mean 1840-49 and 1850–55 temperatures based on taking multi-model medians rather than means is even larger, at 0.28°C.

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