Consensus messaging, an update

If you’re a regular follower of this blog, you’ll know that some of the most active threads have concerned the scientific consensus about climate change and, more specifically, the issue of consensus messaging. Recently, a new book has been released that covers Contemporary Climate Change Debates and includes this as one of the topics. It has a chapter that asks [i]s emphasising consensus in climate science helpful for policymaking? and involves a debate between John Cook – who thinks it is – and Warren Pearce – who thinks it isn’t.

I’ve published a couple of consensus papers with John Cook, so will acknowledge a bias. However, I did like his chapter and though his point that [c]limate communication is not a zero sum game was well made. Clearly, how we communicate and what we focus on should depend on the circumstances and the audience. Consensus messaging shouldn’t always be the focus of communication strategies, but neither should it simply be dismissed.

Warren Pearce’s response suggests that there shouldn’t be a focus on the consensus because it’s narrow and human values are important. I agree that human values are important, but I think he’s wrong to suggest that it’s narrow. The scientific consensus is simply that humans are causing climate change and it essentially underpins this entire topic.

Warren argues that the consensus tells us nothing about the future of climate change, such as human and non-human impacts, policy options or the range of human values and cultures which interact with local climates. It may not tell us anything directly, but how can we possibly discuss these issues if those involved don’t accept that humans are causing climate change?

As examples of important topics that are unrelated to the scientific consensus, Warren highlights the disagreement about carbon budgets and debates about discount rates. Well, carbon budgets are entirely based on limiting human caused climate change and the debate about discount rates is associated with estimates of the future cost of human caused climate change discounted to today. How can one possibly engage in discussions about these topics if there isn’t an acceptance that humans are causing climate change?

So, I think Warren’s argument doesn’t make any sense. I agree that human values are important, and that there are many other aspects of this issue that are very important. However, I really don’t see how it’s possible to address these other issues if there isn’t a general acceptance of the scientific consensus that humans are causing climate change.

Of course, it’s always possible that I’m missing some subtlety in Warren’s argument. If so, maybe someone who understands it can try and explain it in the comments. My impression is that those who oppose consensus messaging either don’t understand the consensus, or don’t accept it. I would, however, be more than willing to be convinced otherwise if someone were willing to put some effort into explaining the argument in more detail.

Links:
Is Emphasising Consensus In Climate Science Helpful For Policymaking? – Chapter with John Cook’s and Warren Pearce’s articles about consensus messaging. I should have added that it’s worth reading John’s article to see some of the other arguments against what is presented in Warren’s article.

Zharkova et al.: an update

Last year I wrote a couple of posts about a paper by Valentina Zharkova and colleagues, which suggested that global warming was partly due to the Earth moving closer to the Sun as the Sun moves around the Solar System barycentre. If you read my posts, the comments, and this pubpeer thread you’ll notice a number of people pointing out that this was wrong; the Earth-Sun distance doesn’t change as the Sun goes around the Solar System barycentre.

Recently, an editorial response and comments from two reviewers have been released, as has a response from the authors. They’ve also released an erratum. You can find some discussion of this on Geoff Sharp’s post.

Unsurprisingly, the editorial response and the recviewers agree with the criticisms of the paper. The editor says:

There is a legitimate, and based on my reading, completely correct concern about the Earth-Sun distance calculation.

Similarly, reviewer 1 says:

Earth is orbiting about the sun and the barycentric motion of the sun is irrelevant for computing/predicting Earth’s temperature. In a simpler term, the Sun is very close to being the prime focus of the Earth orbit.

In their erratum, the authors acknowledge that they were wrong to suggest that the Earth-Sun distance changed as the Sun goes around the Solar System barycentre. However, they then go on to claim that there is still quite a substantial change in the Earth-Sun distance on century timescales. However, as pointed out in Geoff Sharp’s post, what they’ve done is to consider only the perihelion distance. This is the distance between the Sun and the Earth when the Earth is closest to the Sun. It’s well known that there is a slow change in the eccentricity of the Earth’s orbit; it goes from being almost circular, to being slightly elliptical.

This does indeed then lead to a slow change in the perhelion distance. However, there is an equivalent, but opposite, change in the aphelion distance, so that the average distance remains unchanged. This is illustrated in the figure in this comment. So, despite acknowledging a pretty fundamental error when discussing the motion of the Sun around the Solar System barycentre, they’ve gone on to make another pretty fundamental error.

I don’t know what the outcome of this is yet. It’s the authors who have released the editorial response, reviewers comments, and their erratum. Their erratum is still not published and it may be that it goes through further review. Would certainly hope so.

Another CMIP6 climate sensitivity constraint

I thought I would follow up yesterday’s post with one that highlights another paper that looks at CMIP6 climate sensitivity. It’s a paper by Femke Nijsse, and colleagues, and considers [a]n emergent constraint on Transient Climate Response from simulated historical warming in CMIP6 models. The basic idea is to use the warming since the 1970s to constrain the transient climate response (TCR).

Credit: Nijsse et al. (2020)

The figure on the right illustrates the basic constraint. The light grey vertical band is the observed warming since 1975. The darker grey horizontal band shows the range of TCR values from models that satisfy this constraint. This result suggests a likely TCR range of 1.5K to 2.2K, with a best estimate of 1.8K.

Credit: Nijsse et al. (2020)

The figure below shows the TCR plotted against the ECS, for each of the models considered. Models with TCR values between 1.5K and 2.2K, can have ECS values that range from ~2K, to ~5K. So, although this analysis might suggest a better constrained TCR, it doesn’t particularly constrain the ECS.

Something I did wonder (which maybe someone else can clarify) is if this implied anything with respect to carbon budget estimates. Carbon budgets are typically estimated using the Transient Climate Response to Cumulative Emissions (TCRE). According to this paper (H/T Femke Nijsse) the TCRE for the CMIP6 simulations is 1.8K ± 0.4K per 1000 GtC. The reason this can then be used to estimate the carbon budget is that there is essentially no warming committment; global temperatures are expected to stabilise when emissions cease.

However, I think (although I’m not sure) that this does depend somewhat on the TCR-to-ECS ratio, which is typically thought to be about 0.6. However, if it’s possible that the ECS is more than twice the TCR, would we still expect global temperatures to stabilise when emissions cease? If not, would this then imply that carbon budget estimates could be too low? I don’t know the answer, so will stop there.

Climate sensitivity in CMIP6 GCMs

Anyone who is aware of what’s going on in climate science should have heard that the latest generation of climate models, known as CMIP6, seem to be suggesting a somewhat higher climate sensitivity than suggested by the previous CMIP5 models. The Equilibrium Climate Sensitivity (ECS) from CMIP5 models varied from 2.1K to 4.7K with a mean of 3.3K, while for the CMIP6 models it goes from 1.8K to 5.6K with a mean of 3.9K. In addition, 10 of the 27 CMIP6 GCMs having ECS values above 4.5K.

Thanks to a Twitter thread from Gavin Schmidt (which may have been the result of a tweet from Oliver Bothe) I’ve been made aware of a new paper from Mark Zelinka et al. that diagnoses the causes of higher climate sensitivity in CMIP6 models. According to their analysis, the main reason for the difference between the CMIP5 and CMIP6 ECS values is an enhanced SW low cloud feedback, mostly in the Southern extratropics (latitudes poleward of 30^o). The idea being that there is a reduction in low level clouds in these regions which leads to an increase in the absorbed SW solar flux.

Credit: Zelinka et al. (2020)

The figure on the right illustrates the CMIP6 (orange) and CMIP5 (blue) SW low cloud feedback, and their difference (black). It clearly shows an increase poleward of 30S in the CMIP6 models, when compared to the CMIP5 models. As I understand it, this isn’t the only reason for the higher ECS values in the CMIP6 GCMs (they also suggest a sightly larger change in forcing due to a doubling of CO₂) but it does seem to be the dominant factor.

Of course, the suggestion of an increased ECS in CMIP6 GCMs does not tell us that the ECS is indeed higher in the real world. The paper seems to suggest that the positive low cloud extratropical feedback is consistent with observations and theory. However, this doesn’t preclude that there could still be an error in some feedbacks that has yet to be established. The paper did, however, conclude with an interesting point. The enhanced low cloud SW feedback occurs in a region with efficient ocean heat uptake. If I understand this (which I may not) this might indicate that the efficient ocean heat uptake could be masking some surface warming while the climate is still changing (i.e., before reaching equilibrium).

I don’t know really know how to conclude this. It seems that there’s a growing understanding of why the CMIP6 GCMs suggest a higher ECS than the CMIP5 GCMs. This could reflect that the real world ECS is also higher than we had expected, but it’s still too early to really tell. I rather hope that it is not.

Links:
An emergent constraint on Transient Climate Response from simulated historical warming in CMIP6 models, by Femke Nijsse, Peter Cox, and Mark Williamson (2020).
Carbon-concentration and carbon-climate feedbacks in CMIP6 models, and their comparison to CMIP5 models, by Vivek Arora et al. (2020).

2019: A year in review

The end of another year, so time to do another round-up of this year’s posts. My main impression of 2019, unfortunatey, is that the climate debate is moved from disagreements with people who either deny climate change or the need to do anything, to disagreements between people who broadly agree. Maybe we’re simply incapable of not finding something to fight about. In the spirit of trying to be positive, I’ll try to make 2020 a year where I find reasons to agree with people.

Anyway, below is my summary of 2019.

My most read post in January was one about what bothers, and confuses, me about climate change, which probably still applies. There was also an interesting discussion about the Hawkmoth effect.

In February we had a discussion of survivor bias (an issue I need to think about a little more) and also highlighted how difficult it was to have common ground with those who don’t even seem to accept the basis for the discussion.

March saw me attempt a Bayesian climate sensitivity analysis, but also saw the beginnings of the now infamous RCP8.5 debate. There was also one of Willard’s interviews, this one with Jonathan Gilligan.

April was a quiet month. My most active post was one about whether or not STS was trivial, and we also saw the second part of Jonathan Gilligan’s interview.

May was also a quiet month, with a brief discussion of the impact of 4^oC of warming and a discussion of why models aren’t really failed hypotheses.

June saw a recursion of our discussion of Kooninisms, a joint post with Eric Winsberg about extreme weather event attribution, and Willard’s interview with Rachel, who played a key role in developing the moderation policies here.

The key post in July was one that discussed a silly Scientific Reports paper, my responses to which received quite a lot of media coverage.

The dominant issue in August was the continuing discussion about RCP8.5, which I found rather frustrating.

September was quite active, with a discussion of Solar radiation management, two posts about Pat Frank’s nonsense, and an open thread about IAMs.

In October I highlighted how I was stepping outside my comfort zone, and we had quite an active discussion of the economic impacts of climate change.

November included a post about methane, an issue that I think is still not all that well understood. It was also the 10^th anniversary of Climategate.

Post in December were dominated by the never-ending RCP8.5 debate, which I think illustrated that people can have different perspectives and don’t also get the difference between stocks and flows.

There you have it, my quick review of the blog’s 2019 posts. Some topics I found quite interesting to write about, and that I hope some found interesting to read. I hope everyone has a good New Year and all the best for 2020.

Links:
You ain’t ever gonna burn my heart out – A year in Stoats, which reminded me that I should really have thanked everyone who commented, and all those who lurk. So, a belated thank you to everyone.

Listen to the (political) science

I’ve been meaning to post a review of 2019, but wanted to first comment on something else. I quite often see criticism of how some people approach the issue of climate change. For example, in the Guardian yesterday, there was an article by a political scientist suggesting that even though we may have now defeated climate denial, it looks like there may be a new form of climate denial. Specifically, this new form of climate denial is the failure to recognise the complexity of policy making, and how our values might influence how we would approach this issue.

The suggestion in the article is that the environmental movement has actively suppressed attempts to consider these difficult questions, and its elitism has has done more to invite populist backlashes than to further its own goals..

What I find confusing about these criticisms is that they seem to conflate activism with policy making. Activists have agendas; they’re trying to get policy makers to engage with something that they regard as important. Their message will often be intentionally simplistic. It’s not really their job to work out how to implement some policy or even if we should actually do so. Presumably it’s also their values that are driving their activism; why should they be expected to show awareness of other people’s values? Policy makers may well need to take this all into account, but it’s not obvious why we should expect activists to do so.

To be clear, it may well be advantageous for activists to have an awareness of the political process and to aim for inclusion, rather than exclusion. I’m also certainly not suggesting that they should be openly dismissive of other people’s values, it’s just not clear why it should influence their activism. Other people/groups are perfectly entitled to have their own agendas.

In the context of climate change, the criticism often revolves around a sense that some regard this as being an entirely technological/scientific issue. For example, it’s wrong to say listen to the science because science can’t actually tell us what to do. This is, of course, true; science simply provides information. What we do with that information will be influenced by many other factors. However, this is an obvious simplification that is motivated by a desire for people to take the scientific information into account when making decisions. It’s not a literal suggestion that we should simply listen to the science and all will be clear.

My rather cynical view is that some of this criticism is driven by a sense that science has too prominent a role in this debate and that other important factors are not being considered. This may well be true, but there’s nothing stopping people from highlighting these other important issues. Semi-academic critiques that sound objective, but that are probably highly subjective, may not be the most effective way to do this.

Also, given that the slogan of one of the most successful, and prominent, climate activists is unite behind the science you’re going to have a hard time convincing me that a simple message that promotes the importance of the science, isn’t an effective way to highlight this issue.

Addendum:
I should say that having written this I do still find myself somewhat conflicted. I agree with much of what the Guardian article was saying. Even if we have defeated climate denial (which may be optimistic) the next stage will be very difficult. There isn’t an obvious and simple way to implement climate policy. There are plenty of different, and valid, views about how to proceed. We should think about things like personal freedom, distributive justice and respect for established traditions and ways of life. It isn’t simply a scientitific/technological issue; our values should, of course, play a role in how we perceive this issue.

However, it seems clear that there will be a difference between the simplistic messages that activists might use to promote their agendas, and the realities of how we then deal with these issues. Expecting activists to incorporate all this complexity into their messaging seems unrealistic, but then I’m not a political scientist.

The never-ending RCP8.5 debate

I think my New Year’s resolution is going to be to not talk about RCP8.5. However, I think I will briefly summarise the state of the never-ending debate. I thought we’d reached a bit of a breakthough when Zeke Hausfather and Justin Ritchie posted an article suggesting that [a] 3C world is now “Business as Usual”, but it seems to have degenerated once more. David Wallace-Wells also has a follow up article pointing out that our climate future doesn’t look as bad as it once looked. If you’re interested, Pietro Monticone has a summary of the state of the debate.

The basic idea is that current emission projections (from the International Energy Agency) suggest that emissions won’t rise much between now and 2040. If you then make some reasonable assumptions and project these to 2100, you find that we will probably follow something close to an RCP6 pathway which will probably lead to warming of between 1.9^oC and 4.4^oC, with a best estimate of around 3^oC. In other words, current policy suggests that business-as-usual is closer to RCP6, than to RCP8.5 (which has often been regarded as a business-as-usual pathway).

So, the basic message seems to be that we’re heading towards a world where the climate impacts might not be as apocalyptic as they could have been. Good news, in some sense. However, there are a great many uncertainties associated with these projections. Even though we may be heading towards a 3^oC world, we can’t rule out that we’ll still end up in a >4^oC world.

This is where I have some problems. Some are interpreting this as suggesting that we’ve essentially limited warming to ~3^oC, which completely ignores all the uncertainties associated with these projections. Similarly, some are arguing that we should pay no attention to studies that use RCP8.5, which completely ignores that we still can’t rule out levels of warming typically associated with RCP8.5 (>4^oC, for example). From a climate modelling perspective, RCP8.5 is simply a concentration/forcing pathway. Even if it’s now higher than is likely along business-as-usual pathway, it is still a useful pathway for investigating the higher levels of warming that are still possible.

I think it is good that we may have ruled out some of the worst case impacts, but we’re still potentially heading for a world that has warmed by more than 3^oC (potentially even 4^o – 5^oC). Also, despite the confidence of some energy analysts, we have still yet to peak global emissions. A number of quite high-profile climate scientists (Richard Betts and Ken Caldeira) are still pointing out that given the uncertainties associated with socio-economic projections, and potential carbon cycle feedbacks, we really can’t yet completely rule out an RCP8.5 concentration pathway.

I do think it’s become very unlikely that we will follow such a high concentration pathway, but I also think that this current narrative has been poorly framed, and that some will use it to argue that we don’t really need to do much more about climate change. I do worry that in 10 years time we’ll be having a similar discussion – “yes, emissions may have continued rising through the 2020s, but – trust us – they’re just about to peak”. I hope I’m wrong.

Links:
A 3C world is now “Business as Usual” – Breakthrough Institute article by Zeke Hausfather and Justin Ritchie.
We’re Getting a Clearer Picture of the Climate Future — and It’s Not as Bad as It Once Looked – Article by David Wallace Wells.
RCP8.5 issues & comments – summary by Pietro Monticone.
Your Hot Take on Climate Models Is Just Another Political Opinion and That’s Okay – Medium article by Aaron Huertas.