## A couple of highlights

Since I haven’t had much chance to write anything recently, I thought I would briefly advertise a couple of papers that may be of interest to my regular readers. One is by Clare Marie Flynn and Thorsten Mauritsen and is [o]n the Climate Sensitivity and Historical Warming Evolution in Recent Coupled Model Ensembles and compares the CMIP5 and CMIP6 models ensembles.

The CMIP6 ensemble suggests a shift towards a higher equilibrium climate sensitivity (ECS), when compared with the CMIP5 ensemble. The Flynn & Mauritsen paper illustrates that this can’t be due to chance, suggesting that the CMIP6 mean ECS is indeed highly unusual. Consistent with the paper I discussed here, they seem to find that the shift in ECS is mostly due to an increase in the shortwave cloud feedback, mostly in the Southern extratropics. Even though there is a shift to a higher ECS, they also find that none of the models with a Transient Climate Response (TCR) above 2.5oC matches the post-1970s warming. This seems broadly consistent with the results from the paper I discussed in this post.

Probability distribution of the greenhouse gas attributable effective climate sensitivity for the periods 1862-2012 (top) and 1955-2012 (bottom). [Credit: Tokarska et al. (2020)]

The other paper I thought I would highlight is Observational constraints on the effective climate sensitivity from the historical period, by Kasia Tokarska and colleagues. They make use of detection and attribution techniques to derive the surface air temperature and ocean warming that can be attributed directly to greenhouse gas increases. They then use this, together with an energy balance model, to infer the effective climate sensitivity (which they refer to as ${S}_{his{t}_{GHG}}$). As shown in the figure on the right, they find a 5-95% range of 1.3oC – 3.1oC for the period 1862-2012, and 1.7oC – 4.6oC for the period 1955-2012.

For the two time periods, the median values are 2.0oC (1862-2012) and 2.8oC (1955-2012). However, they do highlight that [O]ur estimate of ${S}_{his{t}_{GHG}}$ is lower than the documented ECS of some climate models (e.g. CMIP5 multi-model mean ECS of 3.22oC; Forster et al 2013), including that of some used in the analysis. However, it is well understood that time-dependent feedbacks might render ${S}_{his{t}_{GHG}}\,$lower than $S$ at equilibrium. This is because lower values for ${S}_{his{t}_{GHG}}$ than $S$ at equilibrium can be explained by the effects of changing strength of the feedbacks at higher levels of warming.

That was all I was really going to say. Both papers are open access, so I’d encourage those who are interested to read them in more detail.

## Zharkova et al. – retracted

Just a quick post to highlight that the Zharkova et al. paper, that I’ve discussed in a couple of previous posts, has now been retracted. The retraction notice is here. There’s a Retraction Watch post, which also includes a link to an author response. The authors who’ve objected (which is not all of them) seem to be suggesting that the editor’s interpretation of what they were saying in the paper was not a fair reflection of what was being said. This is slightly odd, given that both my email exchange with the lead author, and the lengthy discussion in this Pubpeer thread, suggests that the editor has correctly interpreted what was said in the paper.

In addition to the above, there is a New Scientist article by Adam Vaughan, and Gavin Schmidt has a new Realclimate post asking [w]hy are o many solar climate papers flawed?

I’m always a little uncomfortable about suggesting that papers should be retracted. I think it should mostly be because of some kind of scientific misconduct; we can still learn from papers that end up being wrong. We also have to be careful of papers being retracted because what they say is inconvenient. However, in this case the error is so fundamental that there is little value in such a paper being part of the literature. As far as I can tell, the journal went through a fairly rigorous re-evaluation of the paper and decided that they had no confidence in what was being presented; even the authors’ response to the updated reviews had basic errors, as Michael Brown points out in this Retraction Watch comment. I think the retraction is the correct outcome.

## Growth?

Just over a year ago, I wrote a post about limits to growth that focussed on an article written by Michael Liebreich. I found his argument particularly silly as it seemed to suggest that the economy could grow until the Sun died. Yesterday I came across another article about limits to growth that also included the lifetime of the Sun, but this time in a slightly more nuanced way; it was more attempting to satirise degrowth arguments than actually suggest that growth was only limited by the Sun’s lifetime.

The article in question is actually a review – by Ted Nordhaus – of Vaclv Smil’s book Growth. Smil discusses his book in this Guardian article, where he is quoted as saying

Without a biosphere in a good shape, there is no life on the planet. It’s very simple. That’s all you need to know. The economists will tell you we can decouple growth from material consumption, but that is total nonsense. The options are quite clear from the historical evidence. If you don’t manage decline, then you succumb to it and you are gone. The best hope is that you find some way to manage it.

Nordhaus’ response suggests that even though economic growth and development has lead to environmental damage, this doesn’t necessarily imply that the solution is degrowth. According to Nordhaus, economic growth can lead to more efficient use of resources and will, eventually, slow. Consequently, environmental impacts might actually increase if we were to actively follow a path of degrowth. His argument is that economic growth would eventually lead us to limiting our impact on the environment. Eventually we would develop alternatives that would allow us to no longer exploit the resources that we had been relying on.

This is where I find the argument somewhat unfortunate. For example, he highlights that [w]e “saved” the whales only after we had hunted many global populations to extirpation, and developed better substitutes for most of the resources we depended upon them for. It may be true that when commerical whaling (mostly) stopped, we had viable alternatives. However, I think these had existed for quite some time; we could have stopped well before we actually did. I don’t think it’s the case that the emergence of alternatives is a prime reason for limiting our impact on the environment. It seems much more likely that we do so when the impact is so severe that there is little economic benefit to continued exploitation.

I haven’t really developed strong views with regards to growth versus de-growth. I’m well aware that economic growth has brought many benefits, and that there are many who have yet to benefit. I’m certainly very uncomfortable with the idea that we might actively develop a pathway that essentially prevents others from experiencing the benefits that many of us have experienced. However, economic growth is clearly associated with resource exploitation that is often not sustainable.

Whatever you may think of Smil’s overall argument, he’s certainly correct that the biosphere is crucial to our survival. Even though we can’t avoid exploiting the environment, maybe we could try harder to prioritise sustainability, rather than assuming that alternatives will always appear just in the nick of time.

Growth – From Microorganisms to Megacities – by Vaclav Smil.
Vaclav Smil: ‘Growth must end. Our economist friends don’t seem to realise that’ – Vaclav Smil interview in the Guardian.
Must Growth Doom the Planet? – Ted Nordhaus’s review of Vaclav Smil’s book.
Limits to Growth – my previous post about limits to growth.
The Secret of Eternal Growth – Michael Liebreich’s article about eternal growth.

## Debate about communicating tipping points

The new book about Contemporary Climate Change Debates, that I discussed in this post, includes a debate about whether or not ‘tipping point[s]’ [are] helpful for describing and communicating possible climate futures? James Annan suggests that the answer is “no”, while Michel Crucifix suggests that it’s “yes”.

James discusses his essay briefly in this post and you can download a copy of his essay here. I have read Michel’s essay, but I don’t have a copy that I can post publicly. I think James is right that they were somewhat talking past each other.

I share some of James’ concerns about introducting tipping points into the public narrative. People do sometimes seem to confuse tipping points and a runaway, which is what motivated me to write this post. For clarity, a tipping point is a threshold beyond which some part of the climate system changes (tips), irreversibly, into a new state. A runaway, on the other hand, is a state where the outgoing flux is limited, resulting in substantial surface warming, with the system only returning to energy balance when the temperature has increased by 100s of K. The latter is simply not possible in our current state.

Another issue with tipping points is that we don’t know if they are truly irreversible; if we could start to artificially draw down atmospheric CO2 might some then reverse? Additionally, the timescale over which they manifest themselves is typically long, in many cases centuries. Hence, they don’t necessarily imply a need for urgent action.

Credit: Schellnhuber et al. (2016)

However, it is certainly the case that there are thresholds beyond which certain systems may start to undergo changes that might prove very difficult to reverse. As the figure on the right (from Schellnhuber et al. 2016) illustrates, it may already be too late to avoid the death of most tropical coral reefs. Similarly, we may already be close to the point where the Greenland and West Antarctic ice sheets start to melt irreversibly, and where we may lose the summer Arctic sea ice and Alpine glaciers.

If we warm as much as 4C, then we risk the loss of the Amazon and Boreal forests and could irreversible change weather patters in the Sahel. Beyond 4C, we risk the irreversible loss of permafrost, the East Antartic ice sheet may start irreversibly melting, and we could lose winter Arctic sea ice.

So, some may be triggered relatively soon, while others are unlikely to be triggered unless climate sensitivity is much higher than we expect, or we end up emitting much more than currently seems likely. Therefore, we do have to be careful of how we introduce these into the public narrative, but I see no real reason why we shouldn’t do so.

We typically motivate climate action on the basis of warming leading to climate impacts that will become increasingly severe if we fail to limit our emissions. I don’t think the existence of possible tipping points specifically changes what we should do; it is simply a further indication that we should be doing our best to limit how much we emit. Although the impact of crossing a tipping point may only manifest itself on long timescales, it does seem clear that if we do cross some of these thresholds, reversing these changes will be extremely difficult. Just like we may want to give ourselves a reasonable chance of avoiding the impacts of, for example, >2C of warming, we may also want to avoid discovering if this level of warming could also trigger irreversible changes in some parts of the climate system.

## But RCPs

Just as I thought I was out the ClimateBall Gods pull me back in. The “but RCP” flythe club got the best of me. For the time lost I found talking points for my Bingo. More on this project in due time. Here are the main ones:

1. 8.5 is bollocks
2. 8.5 is not BAU
3. The IPCC calls 8.5 “BAU”
4. The IPCC uses it as such
5. Centuries or millennia separate 8.5 and when we might see 8.5W/m2
6. We never were on an 8.5 path
7. Only using 8.5 is bad science
8. Without 8.5, there is no huge alarm
9. Don’t present a < 1% scenario like the IPCC does
10. It is not about blame

RCP stands for Representative Concentration Pathway. The acronym is usually followed with the numbers 2.6, 4.5, 6, or 8.5. Stating numbers will suffice in what follows. BAU stands for business-as-usual and IPCC for Intergovernmental Panel on Climate Change.

Now, for the experimental part of the post. One short paragraph for each talking point. I defer to ClimateBall authorities as much as I can. With your feedback I will revise the responses. I will also add secondary talking points in the comments.

One important caveat. “But RCPs” should not deflect from the main takeaway: global warming will continue until CO2 emissions reach zero. We will need to adapt to warming levels of 1.5C, likely more [Glen]. The answers start with a variation on the main take away, parry the contrarian deflection, correct the misleading information or the falsity, and repeat the main takeaway. A truth sandwich if you please, with a relevant salad, a clarifying soup, and a call to action dessert.

***

§1. It is common knowledge that getting to 8.5 is unlikely . It has been introduced to depict a relatively conservative business as usual case with low income, high population and high energy demand due to only modest improvements in energy intensity [Keywan & alii]. Things changed since 2011, and some might suggest that 3C is the new BAU. In effect, every dollar spent on mitigation now makes RCP8.5 emissions even more unlikely [Justin]. Scenario selection is not so important for impacts in the next decade or two [Glen]. If you don’t like 8.5, add 10 years [Gernot]. If you think that 8.5 is bollocks, well, that’s your unarticulated opinion.

§2. While 8.5 emissions are not BAU, the 8.5 concentration pathway can still arise from a lower emissions scenario if feedbacks are strong [Richard]. We also need to distinguish between emissions pathways and warming outcomes, which depend on emissions, carbon cycle feedbacks, and climate sensitivity [Zeke]. Without getting to net zero a radiative forcing of 8.5W/m2 will happen eventually [SteveE].

§3. It’s also important to call them “concentration pathways” as they’re named since over a decade ago, and to realise that over a decade ago it was already the extreme case with also bio-physical assumptions that differ from the other three RCPs. They never were “business as usual” [Joris]. Most climate modelling studies that use RCP8.5 are using the scenario in the form defined in terms of concentrations (the amount of CO2 & other GHGs bulding up in the atmosphere), not the form defined in terms of emissions (the amount humans are releasing) [Richard]. While the public and politicians at large discuss if and how we can get on a 2.6, to talk semantics about if we should rename “BAU” may be well intended, but is absurd [August].

§4. It would be incorrect to claim that the IPCC used 8.5 as BAU. First, 8.5 has a much faster rise in emissions than 1970-2010 [Richard]. Second, AR5 explicitly said “the term BAU has fallen out of favour because the idea of business as usual in century-long socio-economic projections is hard to fathom” [AT]. Scenarios without additional efforts to constrain emissions (’baseline scenarios’) lead to pathways ranging between 6.0 and 8.5 [IPCC]. Even the family of SSP5 baseline scenarios don’t all end up at 8.5 w/m^2 [Zeke].

§5. The idea that we might see 8.5W/m2 in centuries or millennia is bollocks. The remaining carbon budget in SSP5-8.5 is 7700 GtCO2, so 192 years of current emissions. If emissions increase, we reach it faster. Hence why it’s super important countries meet their Paris agreement commitments. For instance, in the high-end of the current policies estimates you’d get to 8.5 w/m^2 concentrations by 2150, assuming constant emissions of ~65 GtCO2 after 2100 [Zeke]. Its all based on MAGICC model runs. The 8.5 w/m^2 scenario being used in CMIP6 is the SSP5 REMIND Baseline [Zeke].

§6. While we may dispute the likeliness of getting to 8.5W/m2 in 2100, we are indeed in an 8.5 path [Kathryn]. Despite its long term aggressiveness, to date our cumulative emissions are closest to RCP 8.5 [Bob]:

§7. RCP8.5 is popular because trying it first is the best use of finite computational resources. If an effect can’t be found in 8.5, then there’s no point in trying the lower RCPs. However, if 2.6 is tried first, and there’s no effect, it says nothing about the higher RCPs [PaulW]. Anyone saying that 8.5 is a standalone forecast is at best in error; those who know the facts should be held to a higher standard [Bill].

§8. The fact remains that 5C is the baseline warming [Zeke]. Five degrees less is what separates us from the ice age [Gavin]. The claim that without 8.5 there is no “huge alarm” implies that *any” lower value would not be a huge alarm [me]. There will be ONE takeaway from this whole “but RCPs” thing, for most people in policy, business & media, and it will be this: “We can worry less about the effects of climate change!” Great work, guys. Really good stuff [Kate].

§9. If the IPCC could attribute a valid statistic to scenarios, they would make predictions, not scenarios. The that there is a 1% probability to 8.5 certainly doesn’t come from the IPCC [PaulS].

§10. Some may pretend that “but RCP” isn’t about blame. Yet they can’t prevent themselves from appealing to INTEGRITY, credibility or whatnot. (Examples on demand.) The following meme format should reveal how our usual suspects are squirreling with “but RCPs”:

• Tired – arguing about 8.5
• Wired – working toward 2.6 [Costa]
• Inspired – 2.6 is just a milestone, not a final goal [Jonathan]
• Bored – let’s just get a policy in place that seriously limits emissions and drives toward a broad international emissions trading system [Kevin].

Srsly, the obsession with RCPs is misplaced [mt]. Who cares about “but RCPs” if the conclusion is that we need to stop using fossil fuels no matter what scenario [Somite]. Getting to carbon zero is key in every way. In any event, those who still care about the “but RPCs” ClimateBall fight could take a look at Pietro’s synopsis:

https://pitmonticone.github.io/rcp85-debate/

Posted in ClimateBall, ClimateBall Bingo | 194 Comments

## feedbacks, runaway, and tipping points

There’s been some discussion on Twitter about feedbacks, runaways, and tipping points. The issue is that some seem to confuse these and sometimes imply that we could cross thresholds where we’ll undergo a runaway. I thought I would briefly try to explain these terms.

In the context of climate change, external factors that can lead to warming are typically called forcings. This would be things like changes to the solar flux, volcanic eruptions, and our release of greenhouse gases into the atmosphere. Feedbacks are then responses to this externally driven warming that either act to amplify, or suppress, the warming. Some of these are fast, such as water vapour and clouds, while others are slower, such as changes to vegetation or ice sheets. Some are also negative and quite strong (such as the Planck response). This means that even though the overall effect of these feedbacks is to amplify the externally-driven warming, it is limited (the negative feedbacks eventually balance the the effect of the change in forcing and the resulting positive feedbacks). For example, if we were to double atmospheric CO2, we’d expect to eventually warm by about 3oC.

A runaway, on the other hand, typically refers to what happened on Venus. Essentially, virtually all of the CO2 was released into the atmosphere, the warming was so substantial that any liquid water evaporated and was eventually lost to space, most atmopsheric molecules lighter than CO2 were also lost to space, and the surface warmed by many 100s of oC. On the Earth, such a runaway is simply not possible, because most of the carbon, that can then form CO2, is locked up in the lithosphere. We can’t emit enough CO2, either through anthropogenic influences or naturally, to undergo a runaway.

Finally, a tipping point refers to us crossing some threshold where the climate system changes (tips), irreversibly, into a new state. There is the possibility of a global tipping point, but this is seen as very unlikely. However, it is possible that we could cross thresholds where some parts of the system undergo essentially irreversible changes. Examples would be melting of the West Antarctic Ice Sheet, the Greenland Ice Sheet, Amazon rain forest die-off, release of carbon from the permafrost, and the disappearance of summer Arctic sea ice.

If we were to cross any of these tipping threshold, then the changes would further amplify the warming (through either releasing additional CO2, or methane, or changing the albedo) and – in the case of the ice sheets – would lead to substantial sea level rise. There are a few things to bear in mind, though. The timescales are typically long; if we cross a tipping threshold it will still take a long time (centuries) for the full effect to manifest. Also, we don’t have a particularly good idea of where these thresholds might lie; we could already have crossed some, or might not do so unless we were to warm substantially. Additionally, there is still debate as to whether or not some of these are truly irreversible; if we could artificially draw down atmospheric CO2 would some, like Arctic summer sea ice, then reverse?

So, one does have to be slightly careful as to what one implies about tipping points and their significance. On that note, I was going to highlight a recent paper by James Annan that might initiate an interesting discussion about using tipping points in the public narrative. However, this has got long enough, so I will leave that to another post.

Update: Something I meant to stress, is that even though crossing some tipping point might lead to irreversible changes that would amplify our warming, it is still limited. There is only so much that these effects can change the albedo, and there is a limit to how much CO2, or methane, that they could release.

Update 2: If you want to read a more detailed description of the runaway greenhouse effect there is a very good Skeptical Science post by Chris Colose. There is a formal aspect to a runaway that I hadn’t appreciated, but was made aware of by MarkR’s comment. It involves a condensable greenhouse gas (such as water vapour) in equilibrium with a surface reservoir, accumulating in the atmosphere so that it eventually limits the outgoing longwavelength flux. If the incoming flux exceeds this, then energy will accumulate without there being any way to balance this. The surface will then undergo runaway warming until the entire surface reservoir of the condensable greenhouse gas has been depleted.

For water vapour, this longwavelength limit would occur at about 310W/m^2, which – given our albedo of 0.3 – is currently greater than the flux we’re receiving from the Sun. This essentially means that water vapour in the atmosphere is in a regime where it condenses, rather than accumulates. We, therefore, can’t undergo such a runaway in our current state, and no level of CO2 emissions will trigger it.