McKibben: Last Week’s Climate Report “landed … with a gentle plop”

I have been reliably informed by many people over the years that I am “a lazy sod with a short attention span”.

As such from the TAR (2001) to AR5 (2013) I only looked at the WG-I assessment reports, and found increasing discrepancies between the “alarmist” contents of the SPM and the mostly good “scientific” content of the main (WG-I) reports.

The main reports were full of “(90%) confidence intervals” and “error ranges”, with frank discussions about the “uncertainty” surrounding many issues.

The SPMs were not …

With the AR6 cycle of documents, I actually took the time to at least do a “quick skim / fast scroll through” of the WG-II and WG-III assessment reports.

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… a disconnect between the dire words of the report and the WG1 part of the report …

What was released on Monday of last week by the IPCC was not the “Full volume” of “The (Synthesis) Report” (AR6 SYR), it was the SPM to the SYR.

Note that on Friday a “Longer Report” was also released on the IPCCs “AR6 Synthesis Report : Climate Change 2023” webpage (direct link).
After closer reading this actually appears to be a midway between what were called “Technical Summary” sections of the Working Group assessment reports and a “Longer SPM, with added hysteria and hyperbole” document.

From the (AR6) WG-I report, AKA “The Physical Science Basis” (section 1.6.1.4, “The likelihood of reference scenarios, scenario uncertainty and storylines”, on page 239) :

Among the five core scenarios used most in this report, SSP3-7.0 and SSP5-8.5 are explicit ‘no-climate-policy’ scenarios (Gidden et al., 2019; Cross-Chapter Box 1.4, Table 1), assuming a carbon price of zero. These future ‘baseline’ scenarios are hence counterfactuals …

Note that the (new, CMIP6) SSP3-7.0 and SSP5-8.5 “emission pathways” neatly bracket the (old, CMIP5) RCP8.5 pathway.

From the WG-II, or “Adaptation”, report (sub-section “AR6 WGI Reference Periods, Climate Projections and Global Warming Levels” of Cross-Chapter Box CLIMATE, “Climate Reference Periods, Global Warming Levels, and Common Climate Dimensions”, on page 136) :

The plausibility of emissions levels as high as the emissions scenario conventionally associated with the RCP8.5 and SSP5–8.5 concentration pathways has been called into question since AR5, as has the emissions pathway feasibility of the low scenarios (Hausfather and Peters, 2020; Rose and and M. Scott, 2020). However, these views are contested (Schwalm et al., 2020, for RCP8.5), and it is important to realise that emissions scenarios and concentration pathways are not the same thing, and higher concentration pathways such as RCP8.5 could arise from lower emissions scenarios if carbon cycle feedbacks are stronger than assumed in the integrated assessment models (IAMs) used to create the standard scenarios (Booth et al., 2017).

From the WG-III, or “Mitigation”, report I think that FAQ 3.3, “How plausible are high emissions scenarios, and how do they inform policy?”, on page 386, is worth copying in its entirety :

IAMs are used to develop a wide range of scenarios describing future trajectories for greenhouse gas emissions based on a wide set of assumptions regarding socio-economic development, technological changes, political development and climate policy. Typically, the IAM-based scenarios can be divided into (i) reference scenarios (describing possible trajectories in the absence of new stringent climate policies) and (ii) mitigation scenarios (describing the impact of various climate policy assumptions). Reference scenarios typically result in high emissions and, subsequently, high levels of climate change (in the order of 2.5°C–4°C during the 21st century). The purpose of such reference scenarios is to explore the consequences of climate change and act as a reference for mitigation scenarios. The possible emission levels for reference scenarios diverge from stabilising and even slowly declining emissions (e.g., for current policy scenarios or SSP1) to very high emission levels (e.g., SSP5 and RCP8.5). The latter leads to nearly 5°C of warming by the end of the century for medium climate sensitivity. Hausfather and Peters (2020) pointed out that since 2011, the rapid development of renewable energy technologies and emerging climate policy have made it considerably less likely that emissions could end up as high as RCP8.5. This means that reaching emissions levels as high as RCP8.5 has become less likely. Still, high emissions cannot be ruled out for many reasons, including political factors and, for instance, higher than anticipated population and economic growth. Climate projections of RCP8.5 can also result from strong feedbacks of climate change on (natural) emission sources and high climate sensitivity (AR6 WGI Chapter 7). Therefore, their median climate impacts might also materialise while following a lower emission path (e.g., Hausfather and Betts 2020). All-in-all, this means that high-end scenarios have become considerably less likely since AR5 but cannot be ruled out. High-end scenarios (like RCP8.5) can be very useful to explore high-end risks of climate change but are not typical “business-as-usual” projections and should therefore not be presented as such.

WG-I : If you look at all of “the scientific literature” it is clear that RCP8.5 is CUB (*).

WG-II (and WG-III) : Yes RCP8.5 is CUB (*) … but we’re going to use any “outlier” studies we can find to justify scaring people anyway.

WG-III (1) : RCP8.5 is “very useful” for our political goals.
WG-III (2) : When it comes to mitigating CO2 emissions “higher than anticipated” population and economic growth should be considered as “Bad Things”.

(*) CUB = Complete and Utter B*ll*cks.

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Going back to the WG-II, “Adapation”, report again we find (in section 1.1.4, “What is New in the History of Interdisciplinary Climate Change Assessment”, on page 131) :

First, this AR6 assessment has an increased focus on risk- and solutions-frameworks. The risk framing can move beyond the limits of single best estimates or most-likely outcomes and include high-consequence outcomes for which probabilities are low or in some cases unknown (Jones et al., 2014; Mach and Field, 2017). …

Second, emphases on social justice and different forms of expertise have emerged (Section 1.4.1.1, 17.5.2). As climate change impacts and implemented responses increasingly occur, there is heightened awareness of the ways that climate responses interact with issues of justice and social progress. In this report, there is expanded attention to inequity in climate vulnerability and responses, the role of power and participation in processes of implementation, unequal and differential impacts, and climate justice. The historic focus on scientific literature has also been increasingly accompanied by attention to and incorporation of Indigenous knowledge, local knowledge, and associated scholars (Section 1.3.2.3, Chapter 12).

For both the WG-II and WG-III teams : “Science ? We don’t need no steeeenking science !”