Review of Sean Rush’s thesis titled : “CLIMATE CHANGE: THE PAST, THE PRESENT AND THE FUTURE”

​The thesis is undated, but the most recent references are 2019 and from reading the text, it’s clear that it pre-dates the most recent IPCC reports published in 2021 – 2022.
 
Early in the document, Rush makes:
“a plea to policy makers to ignore extreme and unsupported views in policy making”.
On further inspection, it appears that, by this, Rush is referring to extreme catastrophist views, not extreme sceptic views, about Anthropogenic Global Warming (“AGW”).
 
He doesn’t say much more than the above quoted excerpt, about what might be described as outliers at the high end of estimates about the speed, extent and impacts of AGW on climate changes and their impacts on human societies.
However, he does feature in his thesis a large number of people (and their views) who are outliers at the other end of the spectrum, who might be described as AGW sceptics.  People like Patrick Michaels, Richard Lindzen, John Christy, Roger Pielke, Michael Shellenberger, Bjorn Lomborg.  
 
Rush notes (about the work of Professor Lamb, who died 26 years ago now):
“limitations on relying on a 25-year-old text are acknowledged in the fast-moving world of climate science.” 
But he does draw heavily from Lamb’s work at various places in his thesis.
 
After describing work on climate sensitivity (sensitivity to a doubling of CO2), Rush says:
“There is broad agreement that a doubling of carbon dioxide will, on its own, result in around 1°C increase in global average temperature.” 
That is true but misleading because he doesn’t, in the same paragraph, describe, or give a number showing, the additional impacts of feedbacks on increases in global average temperature.  This is an important distinction to make, because the 1C number Rush gives is, realistically, meaningless, because it doesn’t tell the reader by how much global average temperature rises as a result of all the impacts of CO2 (and other Greenhouse Gas (“GHG”)) emissions from human activities, both directly and indirectly (through resulting feedbacks).

Feedbacks address the consequences of such a warming from CO2 on the climate system where most agree that the knock-on effects will result in additional changes (in real time) that will add further warming, albeit some of them with time-lags.”
​
As per IPCC AR6 WG1 (2021) the range of likely estimates of “Equilibrium Climate Sensitivity” (ECS)  (ie including feedbacks) is about 2 to 4, with values outside this range being considered extremely unlikely.
That IPCC report has all but eliminated values below 2 as extreme outliers. Even AR5 (the report Rush cites in his thesis) had a likeliest range of 1 to 6, but the more recent report has narrowed that range and increased the lower bound, so an overemphasis on low values for ECS in his thesis seems even more out of touch, in retrospect, than it would have been at the time it was written. 
There’s more information about the progression of this range in ECS values over time in IPCC reports in the following analysis from Carbon Brief, where you can see the ranges of ECS estimates have narrowed in the most recent IPCC report (AR6, 2021) in which ECS estimates below about 2 have been all but eliminated as being extremely unlikely:
https://www.carbonbrief.org/in-depth-qa-the-ipccs-sixth-assessment-report-on-climate-science/
 
 
 
 

Rush says:
“In summary, there has never been much doubt that adding carbon dioxide to the atmosphere results in a warmer atmosphere. The debate has always been about how much more warmth would emerge and whether it would be noticeable amongst the variable weather systems with which humans and the rest of the biosphere regularly grapple.”
 
In fact, the emergence of the human-driven warming “signal” from the “variable weather systems” and  natural drivers of warming has become clearer over recent decades, as shown by this graphic from IPCC AR6 (2021).

    ​Rush doesn’t mention this important feature of the recent IPCC reports.  However, this is another example where retrospect is a tough lens on those it is seen through.  This signal of the divergence of the human driven global warming signal from the natural variability “noise” has become stronger in the IPCC AR6 report, which was not available at the time Rush wrote his thesis.  So perhaps we should cut him a little slack on this point.
 
 
Rush talks about various natural climate drivers:
“What is clear is that over the very long-time scales, climate fluctuates as a consequence of periodic changes in the Earth’s distance from the Sun. Known as Milankovitch cycles they include: the shape of Earth’s orbit, known as eccentricity; the angle Earth’s axis is tilted with respect to Earth’s orbital plane, known as obliquity; the direction Earth’s axis of rotation is pointed, known as the precession of the equinoxes. Figure 2.1 shows how these processes appear to have more influence on surface temperatures over the long term than the presence of GHGs.”
His main source for Figure 2.1 is Davis, W.J. 2017. “The relationship between atmospheric carbon dioxide concentration and global temperature for the last 425 million years”.
 
Davis is an outlier who suggests no causation between CO2 and global average temperatures.  The vast majority of climate scientists agree, in contrast, that there is a strong causation between CO2 and temperatures in the current warming since the industrial revolution.
 
Also, Davis’s work on this, including the same chart Rush uses as his Figure 2.1, is specifically criticised in detail here:
https://fightingdeniers.quora.com/There-is-urgent-need-to-find-that-CO%E2%82%82-is-uncorrelated-to-climate
from which:
“this piece of work [from Davis] probably is not a great step forward for science. I doubt whether sound frequency analysis - other than cherry-picked by restricting the computed time domain - could be performed on such data, not to mention that it is well known in paleoclimatology that there are other triggers on climate besides CO₂, such as Milanković, continental drift, the dimmer young sun, etc., which were all not considered in Davis’ calculations. Davis depicts a new version of the Hieb graph… “
 
That type of graph (the Hieb graph), and how to make a better version than the one produced by Davis, is discussed here:
https://www.realclimate.org/index.php/archives/2014/03/can-we-make-better-graphs-of-global-temperature-history/
 
See also:
https://skepticalscience.com/argument.php?a=66&p=2
 
Davis 2017 is not referenced in IPCC AR6 WG1 (2021), which should be a cautionary warning when assessing its relevance or credibility in the context of discussions about AGW in recent decades.
 
Rush says:
“… little evidence that ancient climate changes have been caused directly by changes in GHG values, it is widely accepted that an initial temperature increase (caused for example, by orbital changes) may be amplified by the increase in GHGs - possibly as much as doubling the initial temperature increase.”
He cites Petit et al, 2001 (actually published in 1999) to support this statement.
While Petit says something about likely causes of some ancient climate changes, when it comes to applying that learning to climate change in modern times, he concludes:
“CO2 and CH4 concentrations are strongly correlated with Antarctic temperatures; this is because, overall, our results support the idea that greenhouse gases have contributed significantly to the glacial–interglacial change. This correlation, together with the uniquely elevated concentrations of these gases today, is of relevance with respect to the continuing debate on the future of Earth’s climate.”
 
Rush presents various examples of “abrupt” climate changes from the geological past.  However, he doesn’t include any compelling evidence that they are global climate changes and that the changes occurred at a speed as fast as the current global average temperature anomaly in the most recent few decades (with the exception of asteroid strikes). 
In that section of his thesis, Rush cites:
 
Alley R.B et al Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event (1993)
 
and:
Alley R.B., The Younger Dryas cold interval as viewed from central Greenland, Quaternary Science Reviews 19 (2000)
 
Richard Alley’s work has frequently been misused by sceptics to suggest conclusions that Alley himself does not support.  Eg see:
https://skepticalscience.com/argument.php?p=3&t=102&&a=434
from which:
“The last word goes to Richard Alley, who points out that however interesting the study of past climate may be, it doesn’t help us where we’re heading:
"Whether temperatures have been warmer or colder in the past is largely irrelevant to the impacts of the ongoing warming. If you don’t care about humans and the other species here, global warming may not be all that important; nature has caused warmer and colder times in the past, and life survived. But, those warmer and colder times did not come when there were almost seven billion people living as we do. The best science says that if our warming becomes large, its influences on us will be primarily negative, and the temperature of the Holocene or the Cretaceous has no bearing on that. Furthermore, the existence of warmer and colder times in the past does not remove our fingerprints from the current warming, any more than the existence of natural fires would remove an arsonist’s fingerprints from a can of flammable liquid. If anything, nature has been pushing to cool the climate over the last few decades, but warming has occurred.””
 
One phenomenon Rush cites is Dansgaard-Oeschger events and:
“The Bølling–Allerød interstadial (“Bolling warm period”) commenced about 14,500 years ago … approximate 3 – 5 ° C warming of North Atlantic intermediate depths… What made this warming so remarkable was not only its abrupt onset,… “

But D-O events are not global ones.
The following is from:
http://web.archive.org/web/20071109170246/http://tamino.wordpress.com/2007/09/13/unstoppable-hot-air/
“they’re deliberately misleading when they refer to them as “global warmings similar to ours…” They aren’t similar to the present warming, and they’re not global…   when the northern hemisphere warms during a D-O event, the southern hemisphere cools. Seidov and Maslin refer to this as the “bipolar climate see-saw.”  ”
http://web.archive.org/web/20071109170246/http://tamino.wordpress.com/2007/09/13/unstoppable-hot-air/
 
See also:
https://skepticalscience.com/500-scientists-global-warming-consensus.htm
 
Rush claims:
“What is clear from these data are that climate has fluctuated far more rapidly and to greater extremes than is currently being experienced.”
Strictly, what he says is true, but only in relation to regional climate changes, or global ones involving asteroid strikes, which are (of course) near-instantaneous.
His comments about those regional events and very rapid (asteroid-related) events is arguably disingenuous when anyone attempts to assess their relevance for applying them to recent global warming.
 
Yes, there are rapid events in geological history, as rapid as (or more rapid than) current global average temperature change, but only if you either:

a)   Include asteroid impacts (which are very rapid), or
b)   Include events that are rapid but only occurred locally or regionally, not globally

or both, which appears to be what Rush has done.
 
Rush says:
“none of the literature reviewed identified increases or decreases in greenhouse gases as a primary reason for these abrupt shifts…   None of the reviewed climate literature addressed extinction or other catastrophic events arising from the above noted abrupt climate changes”

Rush does not indicate the range of literature he reviewed before making that statement.
Has he perhaps cherry picked his literature review, or in his selection of climatic events affecting, for example, mass extinction events?  Another major omission?

There is evidence of mass extinction events caused by climate changes that were less rapid (but more global) than the ones Rush cites in his thesis.  Eg the PETM.

Rush says:
“None of the known 5 mass extinction events occurred during the periods reviewed herein. Given the magnitude of the warming (and cooling) events noted above and the survival of species, including humans, some comfort might be derived that the current warming will not result in a 6th mass extinction event – at least not exclusively from climate change. Consequently, it is possible that human and other parts of the biosphere are resilient against past, and accordingly future, climate changes.”
 
His line of reasoning exhibits a logical fallacy.  He suggests, in effect, that “because x, y and z (non-global) events were fast (occurring in a few decades) and did not cause rapid mass extinctions, therefore a fast event happening now cannot cause a mass extinction”

What he overlooks is that there were several mass extinction events that occurred over longer timeframes than a few decades.  It's entirely feasible, therefore, that current rapid climate change, when assessed over a timescale of a few more decades, or centuries, might be the cause of another mass extinction event.  It cannot be ruled out from the events in geological history commented on by Rush.

Rush says (based on IPCC AR5, 2014):
“IPCC’s low confidence that substantial disruptions in human and natural systems will occur in the near to medium term. It also points towards the resilience of human and other species to adapt to climate changes. That resilience needs to be tested in the modern context given the now global dependence human civilisation has on stable climate. But that vulnerability might also be balanced with the advances in technology now available.”

However, from the more recent AR6 WG1 (2021):
“It is unequivocal that human activities have heated our climate.  Recent changes are rapid, intensifying, and unprecedented over centuries to thousands of years… ”
 AR6 WG1 deals mainly with the physical science, so does not say much about impacts on human societies.  However, AR6 WG2 includes this aspect:
https://www.ipcc.ch/report/ar6/wg2/
from which:
"Human-induced climate change, including more frequent and intense extreme events, has caused widespread adverse impacts and related losses and damages to nature and people, beyond natural climate variability. Some development and adaptation efforts have reduced vulnerability. Across sectors and regions the most vulnerable people and systems are observed to be disproportionately affected. The rise in weather and climate extremes has led to some irreversible impacts as natural and human systems are pushed beyond their ability to adapt. (high confidence)"

 
 
Rush says:
“… it is plausible that some of the global warming since 1850 could be a recovery rather than a direct result of human activities.”
He provides no reference to support this assertion.  However, in any case it is rebutted here, in relation to the work of Akasofu, who made a similar claim:
https://skepticalscience.com/akasofu-LIA-recovery.htm
 
Rush, commenting on CO2 and temperatures from 1850 to modern times, says:
“However, the fluctuations in global temperatures seem to be somewhat at odds with the rise in atmospheric carbon dioxide (Figure 3.5) which started rising exponentially from 1960. Natural variability is deemed to be responsible for the oscillations surrounding the underlying warming trend associated with additional carbon dioxide.”
He doesn’t support his assertion about natural variations in that paragraph with a reference to any scientific peer-reviewed published work.  Instead, he simply includes a chart from Our World In Data.
 
He doesn't mention at that point the cooling impacts of human aerosols in the mid 20th Century, which, as well as natural variability (not just oscillations) helps explain the degree of ‘noise’ around the relationship between CO2, other GHGs and global warming since the industrial revolution.
For more explanation, see:
https://skepticalscience.com/aerosols-global-warming.htm
 
This is another important omission in that section of Rush’s thesis (although aerosols are given attention in later sections, without relating them back to the earlier chart and comments about natural “oscillations”).
 
On climate sensitivity, Rush states:
“ scientists agree, that if all else is held constant (e.g. clouds, aerosols, humidity), a doubling of CO2 should produce about 1°C of direct warming (Held and Soden (2000); Randall et al (2007), Strangeways (2011); Lindzen (2007)).”

I think that is, again, an estimate for “direct effects”, ie excluding feedbacks.  What’s more important is the ECS, showing the impacts of all known feedbacks.  As shown earlier, values below about 2 for ECS are extremely unlikely, ie the true value is likely to be more than double the number cited by Rush.
Progression in ECS range estimates can be seen in Knutti (2017) “Beyond Equilibrium Climate Sensitivity” and these ranges have been narrowed in the latest IPCC report (AR6), as reported by Carbon Brief, and values below about 2 have been all but ruled our as extremely unlikely.

Rush is right to point out:
“If ECS is less than 2°C versus more than 4°C, then the conclusions regarding the causes of 20th century warming and the likely amount of 21st century warming, and resultant climate changes are substantially different.”
However, the current best estimate for ECS is about 3.

Rush’s main cited source for the discussion of ECS is Patrick Michaels, about whom Skepticalscience.com lists many examples of misinformation:
https://skepticalscience.com/Patrick_Michaels_arg.htm
 
More about Michaels here:
https://climateinvestigations.org/patrick-michaels/
 
Rush uses a chart (figure 4.2) , citing the source as:
“Climate Change – the facts 2014”
 
which doesn’t appear in his references section.
The actual source publication is a book called “Climate Change: the facts” edited by Alan Moran, published in 2015, more specifically a chapter in the book entitled “Why climate models are failing”, by Patrick J. Michaels
 
A better source for information about ECS is Knutti (2017) “Beyond Equilibrium Climate Sensitivity”, from which the following shows a much broader range of ECS estimates from a wider range of sources:

​It appears that Michaels selected a much smaller number of estimates to include in his cited work, and those were all estimates with lower bounds at the lower end of the ranges considered and reported by the IPCC.  It’s not clear why he has done this, and the basis on which he selected the ECS estimates he reviewed, except by introducing that data by saying:
“Beginning in 2011, an increasing number of papers and modelling experiments began to appear in the literature indicating, either from models or from historical observations, that the sensitivity of the paradigmatic family of GCMs is too high… Figure 2 graphically summarises the various results in these papers…”

Michaels titles his chart “Climate sensitivity estimates from new research beginning in 2011 compared with the assessed range given in the IPCC Fifth Assessment Report (AR5) and the collection of climate models used in the IPCC AR5”
 
If we look at Knutti (2017) “Beyond Equilibrium Climate Sensitivity”, we see that there are quite a few ECS estimates since 2011 that could be argued to support the opposite proposition; that, in contrast to what Michaels claims, the IPCC estimated range of ECS, ie the climate sensitivity in the General Circulation Models, is too low.

Rush doesn’t mention this in his thesis.  As a result, his thesis is out of balance on this aspect and it’s possible that a reader might be misled into thinking that the chart from Michaels is showing all the work on ECS since 2011, not just the “outlier” work showing ECS estimates with a very low lower bound.
 
Re “the hiatus”, Rush says:
“Knight et al (2009) acknowledge the trend for January 1999 to December 2008 is +0.07 (± 0.07°C) per decade —much less than the 0.18°C per decade recorded between 1979 and 2005 and the 0.2°C per decade that had been expected in the following decade per AR4.”

The misuse of the so-called hiatus, in terms of claims by sceptics that it shows ‘global warming has halted’, is rebutted here:
https://skepticalscience.com/global-cooling-january-2007-to-january-2008.htm
from which:
“A common claim amongst climate skeptics is that the Earth has been cooling recently. 1998 was the first year claimed by skeptics for 'Global Cooling'. Then 1995 followed by 2002. Skeptics have also emphasized the year 2007-2008 and most recently the last half of 2010.  To find out whether there is actually a 'cooling trend,' it is important to consider all of these claims as a whole, since they follow the same pattern. In making these claims, skeptics cherrypick short periods of time, usually about 20 years or less… ”

Rush does say:
“Whilst some would have the scientific community declare an end to global warming as a consequence of the hiatus, the fact that a relatively short period of the late 20th Century/early 21st Century did not follow modelled trends, is not fatal to their utility.”
 
Rather than being “fatal” to AGW, the hiatus has been shown to be a minor, small-term blip in an otherwise strongly evidenced upward rising trend.
The following is from NASA, showing the supposed “hiatus” circled in red.

​The more recent IPCC AR6 WG1 report (2021) gives more of an insight about the “hiatus”, now that there are a few more years of data to add to what was available at the time of the 2014 AR5 report:
“The observed slower increase in global surface temperature (relative to preceding and following periods) in the 1998–2012 period, sometimes referred to as ‘the hiatus’, was temporary (very high confidence). The increase in global surface temperature during the 1998–2012 period is also greater in the data sets used in the AR6 assessment than in those available at the time of AR5. Using these updated observational data sets and a like-for-like consistent comparison of simulated and observed global surface temperature, all observed estimates of the 1998–2012 trend lie within the very likely range of CMIP6 trends. Furthermore, the heating of the climate system continued during this period, as reflected in the continued warming of the global ocean (very high confidence) and in the continued rise of hot extremes over land (medium confidence). Since 2012, global surface temperature has risen strongly, with the past five years (2016–2020) being the hottest five-year period between 1850 and 2020 (high confidence). {2.3.1, 3.3.1, 3.5.1, Cross-Chapter Box 3.1}”
 
 
Rush says more about GCMs ("General Circulation Models"):
“Overall, the sophistication of GCMs and the integration of the laws of physics with super-computer power has undoubtedly enabled science to advance. But the GCMs are still a work in progress…  It remains to be seen if the CMIP6 suite of models have cured some of the errors that ‘plagued’ the CMIP5 suite. Nevertheless, they will undoubtedly provide a good tool to project likely temperature trends and broad changes to weather patterns.”
 
Perhaps, in informing his views, Rush puts too much store in the work of the late Professor Lamb, who held a minority view on the role of humans in causing climate change.  Eg, Rush says:
“At the time of his death (1997) Lamb had objected to the notion that any changes of climate which may be observed at that time, or in the nearer future, must be attributable to human activity because it was unproven and, outside urban and industrial areas, probably untrue…”
 
 
However, Rush does also say:
The author contacted a reviewer of Lamb 1995, Professor Trevor Davies, current Pro-Vice-Chancellor for Research, Enterprise and Engagement at the University of East Anglia. He commented:
“… As the evidence has continued to build over the last 25 years, especially in the area of “attribution”, as the observations (frequency of severe events, and their analysis, etc) have – by and large – matched the predictions, and we have understood much more the role of the oceans, I speculate – because he was open-minded and a good observationalist – that he would, today, be accepting of the reality and importance of rapid climate change due mainly to human activity...”
 
In principle Rush accepts AGW and the need to act to address it.  He says:
“This “resultant warming” manifested itself in the mid-1970s and although natural features, such as ENSO can account for temperature variability, the over-riding trend upwards has been broadly accepted as having a largely human origin …  Lamb 1995 commented: “in view of the increasing variety and scale of human ‘insults’ to the environment, there is clearly no room for complacency and every need for precautionary calculations and watchfulness.”  Sage advice”
 
 
But he is quite critical of the use of climate models to inform policy:
“The utility of GCMs for long term policy planning needs to be more closely scrutinised and the weaknesses in the accuracy of projections needs to be properly understood by policy makers. Projections should also be carefully caveated and distanced from some of the more extreme views that dominates the popular discourse.”
What he fails to follow through on is the consideration of the Precautionary Principle, in ensuring that public policy changes are not “too little, too late” to at least avoid the reasonably concerning risks, eg “fat-tail” risks from crossing climate tipping points.  Rush does not mention climate tipping points once in the thesis; another quite obvious omission of importance.
 
Overall, his thesis lacks balance.  It gives much more prominence to outlier views at the “AGW isn’t an issue” end of the spectrum than he does to outliers at what might be described as the “excessive catastrophism” end of the spectrum.  He is also very sparing in his descriptions of the outlier sources he uses, which might leave the reader being misled into thinking that those outlier views are more mainstream than they are.  The thesis also has a number of important omissions, eg:

• Emergence of the human-driven warming “signal” from the variable weather systems / natural drivers "noise"
• Climate tipping points
• Reasons for past mass extinction events, and their relevance to modern global warming
• Review of ECS estimate ranges with upper bounds at the higher end of the ranges reported by the IPCC
• Global warming impacts of all human activities, not just those that relate to CO2 emissions
• Use of the Precautionary Principle

If his thesis was titled something like “A review of some outlier / sceptic views about AGW and the role of CO2 emissions” and if it included accurate and comprehensive descriptions of those outlier / sceptic views (calling them such) then Rush could perhaps be said to be making a positive contribution to the literature about AGW.  However, in the absence of such titling, phrasing and description, all he has produced is a misleading, cherry-picked, slanted, biased, sceptic-oriented view of climate change.