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This post is a response to a 7-part set of posts by Andy May ("Petrophysicist") about AGW and climate models. May's climate blog can be found here: https://andymaypetrophysicist.com/?s=Climate+Model+Bias As well as my response (below) there is also a further response I created after May posted a "rebuttal" to my post. My further response can be seen here: http://planetarycfo.weebly.com/bloghome/response-to-mays-rebuttal-of-my-critique-of-his-multi-part-blog-on-agw May's original claims, in summary, are: Part 1: “climate models used for the AR5 and AR6 reports do not reproduce the important North Atlantic Ocean Oscillation (“NAO”)” Part 2: “A 30,000-foot look at the history of human-caused climate change modelling" Part 3: “Evidence that the IPCC has ignored possible solar influence on climate” Part 4: “ The IPCC ignores evidence that changes in convection and atmospheric circulation patterns in the oceans and atmosphere affect climate change on multidecadal times scales” Part 5: “storminess (extreme weather) was higher in the Little Ice Age, aka the “pre-industrial”" Part 6: “ bias in the IPCC AR6 WGII report on the impact, adaptation, and vulnerability to climate change” Part 7: “… and [bias] in their report on how to mitigate climate change” Let’s look in more detail to see if there is anything of significance in May’s claims. May’s Part 1 – “climate models and NAO” (North Atlantic Oscillation) He correctly identifies that all models have inaccuracies and uncertainties. But he fails to mention how such uncertainties are reduced, limited or constrained over time. For an appreciation of this aspect of model improvements over time, see: https://climate.nasa.gov/news/2943/study-confirms-climate-models-are-getting-future-warming-projections-right/ and the following, explaining how scientists handle “models running hot”: https://www.carbonbrief.org/guest-post-how-climate-scientists-should-handle-hot-models/ Andy May’s core criticism of climate models appears to be his claim that: “evidence of bias I found in the CMIP6 models and the AR6 report … we will show, new climate science discoveries, since 1990, are not explained by the IPCC models, do not show up in the model output, and newly discovered climate processes, especially important ocean oscillations, are not incorporated into them.” The only example he offers for this is North Atlantic Oscillation. So, presumably, he must consider that this is the strongest example of what he is claiming. For his claim to be a successful challenge to the results from climate models regarding the impacts of human activities on long-term climate change trends, May would have to demonstrate:
The main scientific work May references to support his claims on this is: Eade, Scaife et al - 2022 – “Quantifying the rarity of extreme multi-decadal trends: how unusual was the late twentieth century trend in the North Atlantic Oscillation” from which: “[this paper is about] … the chance of seeing larger positive trends in the NAO than have occurred in historical observations, specifically the maximum seen for the 31-winter window 1963–1993… (CMIP5+6) historical simulations have very rarely (around 1 in 200 chance) simulated maximum trends greater than the observed maximum… Stochastic model best fts to the observed NAO suggest an unlikely chance (around 1 in 20) for there to be maximum 31-year NAO trends as large as the maximum observed since 1860. This suggests that current climate models do not fully represent important aspects of the mechanism for low frequency variability of the NAO”. Eade et al (2022) uses as a base timescale the years 1963 – 1993. This immediately raises concerns, because in studying climate, scientists usually use base timeframes much longer than 30 years. 30 years is considered a bare minimum for most aspects of climate science. But that doesn’t mean that a 30 year data baseline is invalid. As a point of information, longer timeframe data about the NAO can be seen in Hannah (2017) “North Atlantic Oscillation”, going as far back as 1899 in fact. The following excerpt is particularly relevant: “The NAO is the primary variation in barometric pressure variation over the North Atlantic that affects the weather and climate of much of Europe. It is subject to internal variability or chaos in the climate system but is also influenced by slowly varying climatic forcing factors including anthropogenic greenhouse warming and solar and volcanic variability, which makes the NAO inherently predictable—at least in part—on a timescale of up to at least several months. Between the 1960s and 1990s the NAO was becoming more positive, but since then this trend has tended to reverse.” There are three matters of importance in that excerpt. Firstly, they do not indicate that the NAO has any impact on global climate trends. It’s clear the impacts are on weather patterns in Northern Europe. Secondly, it notes that human drivers affect it. Thirdly, it notes that the rising trend of the 30 years to the 1990s has reversed in more recent decades, ie the trend is for reducing/declining NAO currently. None of these aspects are mentioned by May. There are lots of references to the North Atlantic Oscillation in IPCC reports, where it is noted to be a regional (rather than a global) phenomenon. It should be noted that the above sources do not specifically establish any support for any of the points 1 to 4 above. My conclusion on May’s part 1 – a cherry-picked strawman red herring, presented with very poor scholarship. May’s Part 2 – “A 30,000-foot look at the history of human-caused climate change modelling” After skipping lightly through a history of climate modelling, May then references: Lewis, N. (2023, May). "Objectively combining climate sensitivity evidence." Climate Dynamics, 60, 3139- 3165. which: “… produced an ECS range of 1.75-2.7 … (5-95%ile)”. Lewis’ range for ECS is lower than the IPCC’s range of 2.0 – 5.0 (from AR6 – see below), making him an outlier. But note that the two ranges overlap, so they might both be right. Lewis’ work does not invalidate the reported range in AR6. It just claims that the range should be shifted downwards a little. Because Lewis’ paper is so recent (it post-dates IPCC AR6) it will be interesting to see if it affects any subsequent estimated range for ECS in the IPCC’s next report. Another source of information about ECS estimates can be found in Knutti (2017) “Beyond Equilibrium Climate Sensitivity”, which shows many estimates from dozens of credible studies. These include some outliers at the high end of the range, as well as some outliers at the low end of the range. Lewis (2023) is the sole source of evidence May claims supports his claim that “AR6 subjectively modeled ECS is biased high”.  May throws in an unsubstantiated claim: “… the idea that CO2-caused warming feedback changes radically over geologically short periods of 150 years is a very desperate reach.” He does this without backing it up with any scientific evidence, only his criticisms of climate models. He does not mention that climate models are only one of many lines of evidence supporting the IPCC’s conclusion, in AR6 WG1 (2021) that: “It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred.” May then makes another unsubstantiated claim, without any evidence to support it: “The proportion of the warming that is due to human activities is unknown, but as we have seen in this post, it is very unlikely to be either zero or 100%, it falls in the middle somewhere, and may be small.” In fact, the warming from human drivers is very unlikely to be small, as shown in the following diagram from IPCC AR6 WG1 (2021), which indicates that human drivers are highly likely to be the main driver of warming since industrialisation. AR6 references many lines of credible evidence for this (not just climate models).  My conclusion on May’s part 2 – inconclusive on ECS, which will be subject to proper review and assessment by the IPCC in due course, which is the right way to proceed. May makes spurious claims unsupported by evidence. May’s Part 3 – “solar activity” May says: “… the IPCC still assumes the Sun delivers a nearly constant amount of energy to Earth over periods of a few hundred years, constant enough that it has no impact on our climate … and … they work with annual averages” May challenges the IPCC's work on this, and the core of his argument seems to be work by Irvine, B. (2023, December 20). "A Thought Experiment; Simplifying the Climate Riddle on warming at the surface of the oceans." on warming at the surface of the oceans. Unfortunately, Irvine’s 2023 work on this is not published in a peer-reviewed journal, so no credence can be placed on it. However, his earlier (2014) work was published in a peer reviewed source.” May said in his part 3: “The difference in the surface warming effect can be a factor of three or more, Watt-per-Watt, relative to a change in greenhouse gas back-radiation. Evidence that Bob Irvine’s hypothesis is correct includes… “ The details of that hypothesis and discussion are beyond my level of expertise, and so I’ll make no further comment about it here, except to say I’d welcome any views from credible people with more expertise than mine. Presumably the IPCC had an opportunity to assess Irvine's 2014 work on this, but has not yet done so for his 2023 (unpublished) work. May uses arguments such as: "The evidence that solar variability affects Earth’s climate is huge and well established in the literature as reviewed by Douglas Hoyt and Kenneth Schatten in their excellent book The Role of the Sun in Climate Change, and by Joanna Haigh in her report, Solar Influences on Climate" As I state elsewhere, citing a book does not constitute peer-reviewed science. Additionally, May seems to 'shoot himself in the foot' by referencing Joanna Haigh's "Solar Influences on Climate" because that report includes the following quote, which contradicts what May is trying to claim: "... the results of statistical studies of global temperature records concur with those from climate models that, while increases in solar activity probably contributed 7-30% of the global warming apparent over the century leading up to the 1960s, the warming in the latter part of the 20th century is almost entirely due to the increasing concentrations of greenhouse gases from human activity ... [looking forward] ... even if solar activity were to [decline and] reach the record low levels seen in the 17th century Maunder Minimum (implying a reduction in the solar radiation absorbed, averaged over the globe, of 0.2-0.6 Wm-2), it would only partially offset the increased climate warming projected through the uncontrolled anthropogenic emission of greenhouse gases (equivalent to a trapping of heat energy of around 4 Wm-2 over the next century)." May then says: “… gaps in our knowledge of the mechanisms impede the acceptance that multi-centennial or multi-millennial solar changes can influence our climate… empirical and theoretical evidence that solar output and/or solar energy input to the Earth’s climate system varies significantly over periods of a few hundred years... This is not to say that human greenhouse gases have no effect, it is likely that they do have some effect, but evidence suggests that natural influences, like the Modern Solar Maximu and ocean oscillations, play a significant role also “ May concludes: “… [that] solar variability is responsible for all or part of modern global warming… the IPCC reports and the CMIP models do not consider or investigate this possibility” That is incorrect. IPCC reports cover solar variability, citing credible sources to support it’s assessments. For example, IPCC AR6 WG1 (2021) says: “The AR5 … concluded that the best estimate of radiative forcing due to TSI [solar radiation] changes for the period 1750–2011 was 0.05–0.10 W m–2 (medium confidence), and that TSI very likely changed by –0.04 [–0.08 to 0.00] W m–2 between 1986 and 2008… A new reconstruction of solar irradiance extends back 9 kyr based upon updated cosmogenic isotope datasets and improved models for production and deposition of cosmogenic nuclides (Poluianov et al., 2016), and shows that solar activity during the second half of the 20th century was in the upper decile of the range. TSI features millennial-scale changes with typical magnitudes of 1.5 [1.4 to 2.1] W m–2 (C.-J. Wu et al., 2018). Although stronger variations in the deeper past cannot be ruled out completely (Egorova et al., 2018; Reinhold et al., 2019), there is no indication of such changes having happened over the last 9 kyr. ” This evidence directly contradicts May’s claim. Many of the people whose work May cites to support his claims in this part raise red flags, as they have a track record of failing to provide credible and persuasive evidence, or even promulgating misinformation,eg: McKitrick & Christy – see: https://skepticalscience.com/skeptic_John_Christy.htm https://www.realclimate.org/index.php/archives/2019/06/absence-and-evidence/ Soon, Connolly and Connolly – see: From: https://en.wikipedia.org/wiki/Willie_Soon#:~:text=Scientific%20career&text=Climate%20scientists%20such%20as%20Gavin,opposed%20to%20climate%2Dchange%20legislation. “Climate scientists such as Gavin Schmidt of the Goddard Institute for Space Studies have refuted Soon's arguments, and the Smithsonian does not support his conclusions. He is nonetheless frequently cited by politicians opposed to climate-change legislation.” And: https://www.realclimate.org/index.php/archives/2023/09/as-soon-as-possible/ https://www.realclimate.org/index.php/archives/2024/03/more-solar-shenanigans/ Scafetta – see: https://www.realclimate.org/index.php/archives/2023/09/the-scafetta-saga/ In fact, May’s list of references reads like a list of outlier skeptics whose work on this has been debunked many times. May’s comment about ocean oscillations is a reference to his discussion about North Atlantic Oscillations in his part 1, which I have addressed earlier. He introduced no new evidence about any ocean oscillations in part 3. My conclusion about May’s part 3 – smoke and mirrors, but no credible, substantive evidence to support his claims in this part. May’s Part 4 – “convection and atmospheric circulation” May’s core claim in this part seems to be: "It is unclear exactly how the temperature gradient and meridional transport interact, but clearly, they are the main drivers of global climate change at all time scales". To make this very clear, an important point is that May is suggesting those mechanisms as "main drivers of [modern] global climate change" , as an alternative to human drivers such as emissions of greenhouse gases from the burning of fossil fuels. He expands on his hypothesis: “Atmospheric circulation and convection do play a role in global climate change since they affect the speed and efficiency of meridional heat transport, which helps determine the equator-to-pole temperature gradient and the residence time of thermal energy in the climate system… circulation patterns are very important in regional precipitation patterns, which are very poorly understood, and poorly represented in climate models.” May’s main source is Scotese on temperature gradients between equator and poles. He claims: “Scotese’s work suggests that the “normal” surface temperature for the Earth over the past 500 million years is about 19-20 degrees, thus our surface temperature today is well below normal for Earth … based on a model created by making 100 maps of ancient Köppe climatic belts around the world, each map represents the estimated paleoclimate of a five-million-year period, so the maps cover the past 500 million years.” Apart from the obvious challenge of how to apply any findings to changes in global temperatures in timescales of decades in the modern era when the data he worked with represented five-million-year timescales, May's use of Scotese’s work on this to make some spurious claims about residence time of thermal energy, heat loss to space from the poles and to apply it to a discussion of modern global warming has generally been rebutted. See: https://www.realclimate.org/index.php/archives/2014/03/can-we-make-better-graphs-of-global-temperature-history/ from which: “Scotese is an expert in reconstructions of continental positions through time and in creating his ‘temperature reconstruction’ he is basically following an old-fashioned idea (best exemplified by Frakes et al’s 1992 textbook) that the planet has two long-term stable equilibria (‘warm’ or ‘cool’) which it has oscillated between over geologic history. This kind of heuristic reconstruction comes from the qualitative geological record which gives indications of glaciations and hothouses, but is not really adequate for quantitative reconstructions of global mean temperatures. Over the last few decades, much better geochemical proxy compilations with better dating have appeared (for instance, Royer et al (2004)) and the idea that there are only two long-term climate states has long fallen by the wayside.” It is curious that May refers to some of Scotese's work, since Scotese has recently authored a paper in which he clearly recognises the important role of CO2 in driving global warming . The following is from: Scotese et al (2021) "Phanerozoic Paleotemperatures: The Earth’s Changing Climate during the Last 540 million years": "The Earth has entered a “super-interglacial”. The injection of CO2 into the atmosphere as a consequence of the burning of fossil fuels has warmed the Earth more than 1˚C and will continue to warm the Earth for another 300 years (~2300 CE)... We have estimated the amount of future global warming using a straightforward carbon budget model that predicts the changing amount of atmospheric CO2... Using these carbon budget equations, a dynamic model was constructed that predicts the amount of global warming during the next 300 years. Given that the Global Average Temperature (GAT) in 2000 was 14.5˚C (58˚ F), the concentration of atmospheric CO2 was 369 ppm, this model predicts that in 2200 the concentration of atmospheric CO2 will be ~777 ppm and the global temperature will rise about 5˚ C from 14.5˚C (58˚F) to 19.5˚C (67˚ F)." May’s core conclusion in this part is: “If we look at [Earth] as a real, dynamic planet with a circulating atmosphere and ocean, [that is] a part of the climate system that the CMIP models do not model well.” alongside his statement, that is at best an unproven hypothesis of May's, that: "It is unclear exactly how the temperature gradient and meridional transport interact, but clearly, they are the main drivers of global climate change at all time scales" He rests his case on the assertion, based on Scotese’s work, that: “… the thermal energy (heat) transported to the low humidity poles is more easily sent to space than it would be in the tropics” and that this somehow disproves or replaces the well-evidenced knowledge that, as Scotese himself says: "The Earth has entered a “super-interglacial”. The injection of CO2 into the atmosphere as a consequence of the burning of fossil fuels has warmed the Earth more than 1˚C and will continue to warm the Earth for another 300 years (~2300 CE)" May's evidence base to support his outlier hypotheses is scant to say the least, and relies almost totally on a geological perspective by Scotese and work on heat transfer around the globe, without comparing it with energy balance and energy budget approaches such as those used in many well-evidenced and supported climate models. My conclusion on May’s part 4 – an inconclusive attempt to generate scientific doubt about the usefulness of climate modelling, using work that has been found to be inadequate. He uses known uncertainties in modelling regional climate patterns to try to discredit climate models as a piece of evidence supporting AGW. Climate models are based on solid evidence and provide accurate enough results to inform policy making about AGW and impacts of climate change at a global level. Regional level analyses are improving all the time, and already recognise known uncertainties in regional climate patterns. May has not provided sufficiently robust evidence to successfully show any significant level of doubt about this. May’s Part 5 – “storminess (extreme weather) was higher in the Little Ice Age” May’s core claim in this part appears to be: “… weather is more extreme when the planet is colder, meridional transport is strong, and the temperature gradient is steep” He cites a book written by himself and Marcel Crok. That does not constitute peer-reviewed science. His other main source for this part is Zhongwei Yan, Professor at the Institute of Atmospheric Physics of the Chinese Academy of Sciences, China. He is shown as one of the contributors in: Weijie Zhao (2020) “Extreme weather and climate events in China under changing climate” In that work, however, Yan says things that contradict what May is claiming. Eg Yan says, in answer to a question about global warming: “Greater temperature rise leads to stronger impacts. In China, heat waves are increasing; severe rainfalls are increasing; light rainfalls are decreasing. The increase of extreme heat waves and rainfalls means that the meteorological factors for both drought and flood are also increasing” In the middle of this section, May suddenly introduces a claim about clouds: “The effect of clouds and their variability on climate change is still largely unknown and remains the largest source of uncertainty in calculating the effect of greenhouse gases on our climate. The IPCC cannot tell if the net effect of increasing cloud cover warms or cools the Earth. Because cloud formation and destruction cannot be modelled, their effect is ignored or simply “parameterized”” Apart from IPCC, he references work by Mototaka Nakamura, who, in addition to the referenced work, has also produced a short piece published on a number of climate denial websites such as WattsUpWithThat. According to Media Bias Fact Check: https://mediabiasfactcheck.com/watts-up-with-that/
So, it appears the sources May cites in this part have been used by AGW denial websites. Although this isn't conclusive, it rings warning bells for anyone wanting to fact check or credibility-check referenced works by Nakamura. While it is true that clouds are a source of uncertainty, those uncertainties are estimated in IPCC reports (included in the “well-mixed greenhouse gases” category in the IPCC diagram of drivers I included earlier), and their estimates are based on properly cited scientific works referenced in their reports. May does not introduce any substantially new evidence on the topic of clouds. In this part, he recycles some spurious claims from previous parts without introducing any new evidence. For example, he claims: “The evidence that solar variability affects Earth’s climate is huge” (I addressed this in my comments about his part 3) and: “the IPCC/CMIP6 climate models run much too hot relative to observations” (I addressed this by referencing a Carbon Brief article in my comments about his part 1) In summary, this part is a hotch-potch of various unsubstantiated claims, which don’t stand up to even a peremptory scrutiny and don’t even form a logically related and coherent line of argument. May’s Part 6 – “climate model bias” Here, May focusses on IPCC WG2 (impacts). This is a topic where there is genuine ongoing debate, in policy circles as well as the scientific community. There is potential for useful contributions to be made in working out how fast the impacts from AGW will increase, what damages will be caused, what policy responses and actions are to be recommended and what the economic paybacks will be, for current and future generations. However, before tackling the subject, May leads with a backward swipe at the IPCC, saying: “… their assumptions about natural warming, especially the impact of solar variability, are very controversial”. He is referring to supposed assumptions behind the findings of the IPCC in AR6 WG1 that: “It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred.” And the use of such supposed “assumptions” and results in future scenarios of GHG emissions and impacts, in IPCC AR6 WG2. I dealt with May’s comments and claims of this type in my comments about his part 1. It’s worth pointing out that the IPCC’s findings on the basics of AGW and its impacts around the world are generally not controversial, except among those dismissive of AGW or its impacts. What is controversial is what’s called the “damage function”, ie how fast and how large the impacts are going to be in the future, and the extent to which current actions are already avoiding some of those damages and future technology solutions might further mitigate emissions and reduce impacts. A good source on this is Keen (2023), which I talk about and reference in a blog post here: http://planetarycfo.weebly.com/bloghome/what-agw-damage-curve-do-you-prefer Moving on, the following seems to be May’s core claim in his part 6: “WGII only considers the problems of climate change and not the benefits, [and therefore] reveals their bias and invalidates their analysis” And May goes on to claim: “… while many studies anticipate problems in the future, they also predict a future where humanity is better educated, better fed, longer lived, healthier, with less poverty, and less conflict. This is simply continuing a trend that has been underway for many decades. O’Neill reports that currently there are 700-800 million people at risk of hunger globally. By 2050, even including the possible effects of 2°C of warming, that number will fall to 250 million… Currently the world’s economy is growing between 2 and 3% per year and this is not expected to change much in the future. Looking ahead at a possible 2.5°C of warming in the next century or so, economists anticipate between a positive net climate change impact of about 2% and negative net impact of about 2.5% on global GDP.” There is certainly a strong debate about the extent of net impacts of AGW in the coming decades to centuries. This is perhaps one of the few remaining topics on AGW where there is a genuine debate to be had, because the basic facts of AGW have been established beyond any reasonable doubt (despite the attempted sowing of FUD by anti-AGW disinformation propagandists): We know that:
What’s up for debate is how much we should do, how fast, and using what proportion of global GDP, and how quick the economic “payback” occurs, in terms of net benefits obtained (including future climate change damages avoided through our actions). An example is the Paris 1.5 or 2.0 targets, and Global Net Zero by 2050. Very few economists predict positive net benefits from AGW, projected from current Business As Usual scenarios. May challenges what now constitutes “Business As Usual” scenarios, and makes a sensible comment: “To be fair, the IPCC does not call SSP5-8.5 business-as-usual, that label is used by others, presumably because that is what it [was] called in the first report in 1990.” The highest emitting scenarios are seen by most commentators these days as representing extreme backlash or reversal scenarios, ie representing much rowing-back on existing international commitments and policies. They are seen as unrealistic because the world has committed to Paris 1.5/2.0 and Global Net Zero by 2050. May goes on to say: “WGII often uses the biased and too hot WGI models as input to maximal and implausible emissions scenarios to do their modeled climate impact projections… The whole idea of using scenarios is to investigate the full range of possible outcomes, not cherry-pick the model input to manufacture a desired outcome, a problem often called reporting bias.” I think he’s being very harsh here. It’s clear in AR6 WG2 (2022) that the IPCC sets out in many sections the full range of future emissions scenarios, not just the highest emitting ones. Although those highest emitting scenarios are no longer considered to represent BAU, they are still valuable as a warning not to back-pedal on, or delay, important actions addressing AGW. And one aspect that the IPCC has been criticised about is that little account is taken of the possibilities of climate tipping points being triggered, which could cause discontinuities in rates of change and could set the world back onto pathways of damage even more serious than those high-emitting scenarios. May cherry-picks what he claims are some apparent inconsistencies between WG1 and WG2 regarding extreme weather events, eg cyclones, heatwaves and flooding in the USA, and claims this shows inconsistencies between WG1 and WG2. What it appears to come down to is that what May claims is “… cherry pick locations and events” in WG2, is what most people would see as showing regional variability in how the AGW impacts play out over time. May refers back to a chart of Hadcrut data in his part 2, purporting to show: “Arrhenius’ conclusions were mostly accepted as the world warmed from 1908 to 1945, but when the world cooled from 1945 to 1976, while CO2 was still increasing, the hypothesis fell into disrepute. The pattern of global average surface temperature change, often used as the primary metric of “climate change” is illustrated in figure 1” The relevant reference is the following, although the chart May shares does not appear in it: Morice et al (2012) “Quantifying uncertainties in global and regional temperature change using an ensemble of observational estimates: The HadCRUT4 data set” (yet again, poorly cited by May without giving a proper reference and not attributing the source of the chart, so this is guesswork). But in part 6 May uses this as his source for his claim that: “AR6 WGI finds that since 1950 there has been an increase in the number of hot days and heatwaves, but as figure 1 in part 2 shows the world was cooling in 1950” In fact, it is not generally disputed that there was a slight cooling for a time from about 1950, because aerosol cooling was masking AGW at that time (a finding not mentioned by May). To put this in a wider context, see the diagram below from AR6 WG1, where that slight cooling from 1950 for a few years can be seen.  However, even so, May appears to have chosen to chart only one set of temperature measurements, rather than an ensemble (which is the more generally accepted method, to provide a more valid result). Therefore, it’s possible that the temperature data May has used accentuates the very slight fall in global average temperature for the few years from about 1950. The important point to note is that the statement that: “AR6 WGI [2021] finds [globally] that since 1950 there has been an increase in the number of hot days and heatwaves” is entirely consistent with the diagram above, given the longer timeframe than just the timespan around 1950 which May refers to. This is an example of a cherry-pick by May, and a rather poor one at that. May cites results of economic assessments (but without citing a specific work for this, other than Lomborg, who is not an economist and O’Neill, who appears not to have published relevant peer-reviewed work on this subject). May cites examples of low estimates of AGW damage in terms of percentage of GDP. Many mainstream economists are apt to underestimate damages from AGW, especially because they take little or no account of climate tipping points and they use high discount rates to minimise the calculated damages in the future. Some other economists calculate the damages from AGW, going forward, as being significant (nearly 100% of GDP) by the end of this century, under some scenarios of future greenhouse gas emissions. For example, see Keen (2023) “Loading the dice against pension funds” for a fuller treatment of this subject. Suffice it to say that estimates of small incremental damages of a percentage or two of global GDP by 2100 are increasingly being seen as gross underestimates of net damages, especially given that triggering of single or multiple climate tipping points cannot be ruled out. For ease of referencing , here is a link to my relevant blog post: http://planetarycfo.weebly.com/bloghome/what-agw-damage-curve-do-you-prefer May says: “Bjorn Lomborg reports that human welfare will likely increase 450% in the 21st century and damages due to climate change might reduce this to 434%” So, we see that Lomborg is one of his main influences on this topic. That’s not surprising, since May seems to have caught some of Lomborg’s bad habits (poor quality evidence, non-credible sources, cherry picking, strawmanning etc). There are numerous works debunking Lomborg’s many pieces of misinformation about climate change and its impacts. For an example, see Bob Ward’s 2020 review “A closer examination of the fantastical numbers in Bjorn Lomborg’s new book”, here: https://www.lse.ac.uk/granthaminstitute/news/a-closer-examination-of-the-fantastical-numbers-in-bjorn-lomborgs-new-book/ Lomborg is not a credible source on climate change. May cites O’Neill 2023., about whom DeSmog says: https://www.desmog.com/brendan-o-neill/ “O’Neill identifies as a “Marxist Libertarian” and began his career in journalism at Living Marxism, a publication of the Revolutionary Communist Party, a now-defunct Trotskyist group of which he was a member… O’Neill wrote an article for Spiked which downplayed the significance of climate change, stating: “the notion that climate change is an End Times event, rather than a practical problem that can be solved with tech, especially the rollout of nuclear power, is little more than the prejudice of Malthusian elites who view the very project of modernity as an intemperate expression of speciesist supremacy by mankind”” So, we see that O’Neill is a techno-optimist when it comes to AGW. It is true that, as May cites: “Climate change explains a portion of long-term increases in economic damages of hurricanes (limited evidence, low agreement)” That is an example of May’s quote mining – finding a quote showing an item about a low confidence finding. A fuller picture is provided by using a fuller quote from the IPCC, as follows. The IPCC says, in AR6 WG1 (2021): “Climate change is already affecting every inhabited region across the globe, with human influence contributing to many observed changes in weather and climate extremes” And, also from IPCC AR6 WG1 (2021): “The attribution of observed changes in extremes to human influence (including greenhouse gas and aerosol emissions and land-use changes) has substantially advanced since AR5, in particular for extreme precipitation, droughts, tropical cyclones, and compound extremes (high confidence). There is limited evidence for windstorms and convective storms. Some recent hot extreme events would have been extremely unlikely to occur without human influence on the climate system.” The following diagram from the same report provides a nuanced picture of variability of these impacts by region. This does, however, offer opportunities for skeptics to cherry-pick data from regions where there is medium or low confidence in the effects, to support a skeptic view of AGW with partial or selective data, giving an unrepresentative view of the impacts of AGW at a global level. It appears that this is what May has done.  This is an area of ongoing research, eg in attribution studies.
For example, see: https://www.worldweatherattribution.org/ May finishes this part with a summary, including the following excerpt: “Just as WGI ignored the potential impact of solar variability and changes in meridional transport, WGII ignored the potential benefits of warming and additional atmospheric CO2. This invalidates the report. By ignoring the well-documented benefits of global warming and additional CO2, they clearly cannot assess the impact of climate change or our vulnerability to climate changes”. This is a hotch-potch of statements and inferences run together. Let’s unpack it a little: 1) “Just as WGI ignored the potential impact of solar variability and changes in meridional transport” I addressed this claim earlier and it is rebutted with evidence. 2) “WGII ignored the potential benefits of warming and additional atmospheric CO2” May has not provided any credible evidence of potential benefits of warming, just references to sources such as Lomborg, so his claim falls at the first hurdle. 3) “This invalidates the report” No it doesn’t. The findings are still valid, even if there were omissions of positives from AGW (which May hasn’t actually demonstrated with credible evidence). The onus is on others to make a properly evidenced case for benefits from AGW to set against the many negatives evidenced in WG2. 4) “By ignoring the well-documented benefits of global warming and additional CO2 …” May simply restates his claim – an example of “proof by blatant assertion” 5) “… they clearly cannot assess the impact of climate change or our vulnerability to climate changes” This is an assertion which he does not support with credible evidence. It is, to say the least, far from “clear”. May’s Part 7 – WGIII May’s core claim here appears to be: “It is premature to write a volume on methods of reducing emissions, until the emissions are shown to cause problems. So far additional emissions have done little except to make winters, nights, and higher latitudes warmer; and increase plant growth… a very questionable method of cost/benefit analysis, the “social cost of carbon,” or IAM analysis for their assessment of the costs and benefits of mitigating greenhouse gases.” While the subject of proper economic and financial evaluation of costs and benefits from tackling AGW is controversial and is an area of active research, it should also be noted that the IPCC has been heavily criticised for failing to take proper account of the fat-tail risks of triggering single or multiple climate tipping points. Also, the economic approach of William Nordhaus, which May relies heavily on in this part, has been counteracted by many economists (some of them even mainstream). An example is Keen (2023) “Loading the dice against pension funds” (discussed earlier). In short, May doesn’t introduce any significant new evidence in this part, but mostly recycles and repeats his claims from previous sections. Summary In summary, May's multi-part blog about AGW doesn't stand up well to scrutiny. He brings in many spurious and unsubstantiated claims about AGW and IPCC reports. However, let's not throw the baby out with the bathwater. The subjects he talks about, especially towards the end of the series, where he talks about economic impacts of AGW, do provide a useful prompt for us to review the genuinely debatable areas such as future AGW impact damage functions. This can be a useful line of further enquiry as we progress the transition to a decarbonised global economy.
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