Climate Feedback

Climate Feedback is a site on which scientists grade articles about climate science. Not only do they provide comments, they also give a final grade, indicating the scientific credibility. For example, Chris Mooney’s article on what we’re doing to the Earth, was – unsurprisingly – graded as having a high/very-high scientific credibility. On the other hand, Ben Webster’s article on scientists are exaggerating threats to reefs and marine life, was – also unsurprisingly – graded as having a very low scientific credibility.

The reason I’m writing this is because they’re doing a campaign to try and raise some money. You can read more on Victor’s site. The same article was also posted on Dana Nuccitelli and John Abraham’s Guardian site. I think Climate Feedback does a very good job of commenting on, and grading, articles about climate science. If you feel like helping, the link to the funding campaign is here. It’s already doing pretty well.

Posted in Climate change | | 16 Comments

Airborne fraction

Given the recent news that the rise in CO2 has greened the planet, I can see a new “skeptic” meme appearing. Essentially, I can see them arguing that both climate sensitivity is too high, and that nature will take up more of our emissions than we currently think. Consequently, they will argue that we will not warm much because climate sensitivity is low and because projections of atmospheric CO2 rises are too high. For example Judith Curry ended her post by saying

As Nic Lewis points out, this paper alters the RCP scenarios in terms of resulting atmospheric CO2 content; i.e. the RCP scenarios are significantly too high.

I actually don’t understand this particular issue all that well, so thought I’d write this in the hope that some of my commenters might know more than I do (not hard, I hear you say). I should start, however, by pointing out that I don’t think that this greening of the planet is such a surprise and I think it has been included in climate studies for quite some time already. The rise in atmospheric CO2 is about 46% of what we’ve emitted in total; this is called the airborne fraction. About one-quarter of our emissions have been absorbed by the biosphere, and greening of the planet is probably an expected outcome of this.

Credit: Pierre Friedlingstein

The key issue is really how the airborne fraction will change as we continue to emit CO2. One of the authors of the paper that Nic Lewis mentions above is Pierre Friedlingstein. I found this presentation of his, which includes the figure on the right. It looks rather uncertain, but it seems that the airborne fraction only really starts to increase if we follow an emission pathway above RCP4.5.

So, maybe some of the higher emission pathways will lead to slower increases in atmospheric CO2 than we currently think, but then again, maybe not. If it is only likely to impact the higher emission pathways, this would be good, but not necessarily a reason to pay less attention to the potential risks associated with continuing to emit CO2. Of course, the uncertainty means that we can’t even be confident that this will indeed be the case, and there are other also factors, like ocean acidification, to take into account.

Okay, this post has got somewhat convoluted and isn’t really going anywhere, which is partly because I’m a bit tired, and partly because this is a topic about which I’m not that familiar. I’ll make one point as to why this may be less relevant than it might at first seem. At the moment, the airborne fraction is about 46%. If we were to halt emissions, we’d expect about 20-30% of our emissions to remain in the atmosphere for thousands of years – technically, the long-term enhancement in atmospheric CO2 will be equivalent to 20-30% of our emissions.

One interesting result from climate models is that if we halt emissions, atmospheric CO2 will reduce in such a way that global temperatures will – on average – stabilise (i.e., they will essentially stop rising). If we’re over-estimating the airborne fraction along some of the higher emission pathways, but not the long-term airborne fraction, this would seem to suggest that if we were to halt emissions, global temperatures would – rather than stabilising – continue to rise to the same equilibrium. In other words, if our estimates of the long-term airborne fraction is about right, then even if we’re over-estimating the airborne fraction along some of the higher emission pathways, our long-term warming projections are – I think – unchanged. At least, this would seem to be the case.

So, if this whole greening issue has long-term significance, then it would seem to require that we’re both over-estimating the airborne faction along some of the emission pathways, and over-estimating how much CO2 will remain in the atmosphere once we stop emitting. I’m not even sure if the former is a reasonable inference, but the latter seems a bit of a stretch; at the moment, at least. Anyway, I’ll stop there. If anyone has a better understanding of this than I do, or know where to find out more, comments would be appreciated.

Matt Ridley doesn’t understand free speech

Matt Ridley is, once again, complaining that the climate change lobby wants to kill free speech (you can read it here). What it mainly seems to illustrate is that Matt Ridley doesn’t really understand the concept of free speech.

One example that Matt Ridley provides is a letter from some scientists in the House of Lords, to the editor of the Times. Matt Ridley says that this letter

denounced the two articles about studies by mainstream academics in the scientific literature, which provided less than alarming assessments of climate change.

One of the articles discussed this report which was so ridiculous, it was mocked on Twitter. Describing it as providing a less than alarming assesment of climate change is either wilfully disingenuous, or illustrates how clueless Matt Ridley really is (one should note that the report in question was funded by the Global Warming Policy Foundation, of which Matt Ridley is an Academic Advisor). It has also never appeared in the scientific literature; well, not what I would regard as the scientific literature.

He then goes on to promote the, apparently poorly funded, Global Warming Policy Foundation, saying

The GWPF often draws attention to the many studies ignored by greens that suggest climate change is not so dangerous, and to the economic and environmental harm done by climate policies.

When I cover this topic I am vilified as on no other subject,

Well, maybe what he said earlier illustrates why. If he wants to argue that reducing poverty and dealing with other issues in the developing world should be done by using cheap fossil fuels (as he does) he really should do so in light of all the available evidence. Cherry-picking evidence that appears to support his policy preference is something that – IMO – should be criticised. That the changes that could occur due to our continued emission of CO2 might produce less severe impacts than we currently think, is not really an argument for ignoring that the impacts may well be severe and that we should really consider doing something to address this.

You might argue that those who support climate action are essentially doing the same by focusing on the evidence that suggests that the impacts could be severe and damaging. Well, this seems pretty understandable; if there is a chance of bad things happening, the tendency will be to consider the probability of these outcomes, and whether or not we should do anything to minimise the chances of them happening. It’s not typical to argue that these bad things might not happen and that we should therefore base our actions on this possibility. There is only one actual future and the evidence suggests that the changes that do occur will likely be irreversible on human timescales; we don’t easily get to reverse them if the decisions we do make turn out to be less than optimal.

The rest of his article is just a repeat of pretty standard “skeptic” themes. People who criticise Ridley, and other “skeptics”, are apparently bullies who want to close down the debate. Considering that maybe what he’s promoting is, at best, a cherry-pick and, at worst, wrong, doesn’t seem to cross his mind. You really get the impression that he thinks that free speech means that he should get to promote his views without criticism; don’t those who disagree with him have the right to speak too? This is maybe the most irritating thing about his whole article. By arguing that those critical of what “skeptics” promote in public are trying to kill free speech, he’s essentially trying to deligitimise their arguments on the basis of them violating something that we regard as a fundamental part of our democracies. This seems to be much more of an attempt to kill free speech than anything those he’s criticising have done.

The TCR-to-ECS ratio

Something I’ve been interested in is whether or not the ratio between the Transient Climate Response (TCR) and the Equilibrium Climate Sensitivity (ECS) best estimates from most of Nic Lewis’s work makes sense. Typically his TCR best estimate is around 1.35oC, while his ECS best estimate is around 1.65oC; giving a TCR-to-ECS ratio bigger than 0.8. For climate models, the ratio is typically below 0.8 and, most likely, below 0.7. I did ask Nic Lewis if he could explain his rather high TCR-to-ECS ratio, and he responded in his normal charming fashion. His argument, I think, is simply that you take the energy balance forms of the equations that describe TCR and ECS

$TCR = \dfrac{F_{2\times} \Delta T}{\Delta F},$

and

$ECS = \dfrac{F_{2\times} \Delta T}{\Delta F - \Delta N},$

and you simply plug in the values for the change in temperature ($\Delta T$), change in forcing ($\Delta F$), and change in system heat uptake rate ($\Delta N$) and out pops an answer. My question, however, was trying to delve slightly deeper into this issue, which I’ll try to explain below.

As discussed in Section 4 of this paper (Earth’s energy imbalance and implications by Hansen et al.) for a given climate sensitivity, the climate response function (defined as the fraction of the fast-feedback equilibrium response to a climate forcing) depends on the rate at which energy is mixed into the deep ocean. If energy is mixed into the deep ocean slowly, then most of the energy will heat the upper ocean, land and atmosphere, and the climate response will be fast. If, however, it is rapidly mixed into the deep ocean, then the climate response will be slow (the upper ocean, land and atmosphere will warm more slowly). This will manifest itself as a difference in the magnitude of the planetary energy imbalance that can be sustained; if the climate response is fast, we’d expect the typical planetary energy imbalance to be small; if it is slow, we’d expect it to be larger.

This is illustrated in the figure below, which shows examples of 3 different climate response functions, all of which can match the historical surface temperature record, but which produce different planetary energy imbalances. If the climate response is slow, we might expect a planetary energy imbalance today of around 1Wm-2. If it is fast, we might expect it to be less than 0.5Wm-2. Given that, today, it is probably somewhere between 0.6 and 0.8Wm-2, Hansen et al. (2011) suggest that an intermediate response is more realistic and that most climate models possibly mix energy into the deep ocean too rapidly (they tend to have slow climate response functions). However, is this sufficient to explain the discrepancy between a TCR-to-ECS ratio of 0.6 to 0.7 – suggested by most climate models – and a value in excess of 0.8, as suggested by Nic Lewis’s work?

Credit : Hansen et al. (2011)

Credit: Miller et al. (2015)

In trying to understand this better, I came across a paper called Model structure in observational constraints on transient climate response, by Miller et al. It includes the figure on the right that shows the TCR-to-ECS ratios for Perturbed Physics Ensembles (PPE), CMIP5 models, and also includes the AR4 and AR5 ranges for TCR and ECS. Judging by this figure, it would seem that a TCR-to-ECS ratio between 0.8 and 0.9 is possible, but would seem to require that the TCR be quite low; less than 1.5oC, which is indeed consistent with Nic Lewis’s best estimate. However, this still doesn’t really indicate that a TCR-to-ECS ratio greater than 0.8 is consistent with the planetary energy imbalance being greater than 0.6Wm-2.

I’m not quite sure where to go with this. This post is really just some thoughts on this. It seems that a TCR-to-ECS ratio above 0.8 is possible if the TCR is sufficiently low. Given how much we’ve now warmed, however, a low TCR is becoming less likely. Also, if the system can sustain a planetary energy imbalance of between 0.6Wm-2 and 0.8Wm-2 might also suggest that a TCR-to-ECS ratio above 0.8 is unlikely, because the potential amount of committed warming could then be larger than 20% of a low TCR.

I should say that I think Nic Lewis’ work is very interesting and makes a valuable contribution to our understanding of climate sensitivity. However, it does suggest some things that are at odds with other estimates; such as a much larger probability of the ECS being below 2oC, and a higher TCR-to-ECS ratio. Also, there is the possibility that feedbacks are time-dependent in such a way that observationally-based estimates of climate sensitivity may suggest that climate sensitivity is lower than it actually is.

I should also add that focusing only on climate sensitivity can be a bit misleading, as how much we will actually warm will depend on climate sensitivity and on carbon cycle feedbacks. These can be combined into a single quantity called the transient response to cumulative emissions, which is thought to be between 0.8oC and 2.5oC per 1000GtC. Since we’ve warmed by almost a degree after emitting just under 600GtC might suggest that the lower end of this range is rather unlikely. I’ll stop there. Comments welcome.

We can never “know” anything

Judith Curry has waded into the consensus discussion by highlighting a post written by two people I’ve never heard of and – from what I can find – have no actual expertise in this topic whatsoever. Apparently the key point is that

….. the fact that so many scientists agree so closely about the earth’s warming is, itself, evidence of a lack of evidence for global warming.

Huh? Yup, you read that right. Apparently the idea is that as the level of agreement increases, our willingness to consider – or fund – alternative ideas decreases, and we actually start encouraging others to avoid considering these alternatives. Therefore we restrict our ability to increase our knowledge. Drawing this conclusion doesn’t require any actual evidence that we are discouraging the investigation of alternatives. Also, that we might have already elminated most alternatives does not seem to be an option.

I keep writing, and then deleting, a discussion about this. This is because it is such nonsense, actually discussing it further seems to be giving it way too much credence. If you’ve wondered why I haven’t mentioned Judith Curry’s blog recently, this is a good illustration of why. If Judith is going to promote this crap, there’s not really much point in even reading her blog, let alone wasting any time writing about what she promotes. I think I shall endeavour to continue in that vein.

Posted in Judith Curry, Science | | 373 Comments

Time dependent feedbacks

There’s a very interesting new paper by Gregory and Andrews called Variation in climate sensitivity and feedback parameters during the historical period. The motivation seems to be to try and reconcile why estimates of the effective climate sensitivity using recent observations seems to suggest climate sensitivities that are somewhat lower than climate model estimates of the Equilibrium Climate Sensitivity (ECS). For simplicity, I’ll use ECS for both effective climate sensitivity and Equilibrium Climate Sensitivity, but – technically – they aren’t quite the same thing.

A standard way to assess ECS using observations is to use a simple energy balance model in which

$F = N + \alpha T,$

where $F$ is the change in forcing over the time interval considered, $N$ is the change in system heat uptake rate, $T$ is the change in temperature, and $\alpha$ is the feedback parameter. The assumption here is that $\alpha$ is constant and – if one wants to equate effective and equilibrium climate sensitivity – that it will remain constant. The ECS is then simply $3.7/\alpha.$

What this paper did was to use Atmospheric General Circulation Models (AGCM) and prescribe[d] time-varying observationally derived fields of [sea surface temperature] SST and sea-ice concentration, but fixed atmospheric concentrations at pre-industrial levels. In such a model, the change in forcing $F$ is 0, but the change in temperature, $T$, will not only be a reasonable representation of the actual change (not quite, because the land is not warming as fast, but close – 85% to 95%), but the spatial distribution of the warming will also be similar to what actually happened. To determine the climate feedback parameter, $\alpha,$ you then use

$N = - \alpha T.$

Credit : Gregory & Andrews (2016)

To estimate $\alpha$, you simply regress $N$ against $T$. As shown in the top left panel of the above figure, the result for the whole period is 1.72 Wm-2K-1 (ECS = 2.2K), where $N$ and $T$ are relative to 1979-2008. The bottom left panel, however, shows what happens if you consider shorter time intervals, and indicates that it can vary quite substantially, depending on the time interval considered. The right-hand panels show $\alpha$ determined by regression in 30-year sliding windows. They also show $\alpha$ during the first 20 and first 100 years of abrupt 4xCO2 simulations and show that it is smaller (and the ECS larger) than is the case when the spatial distribution of the surface warming is chosen to represent what we probably experienced over the last 100 years or so.

As the paper itself says

Our results suggest that the differential climate feedback parameter $\alpha$ varied on multi-decade timescales during the historical period and that it was generally larger than abrupt4xCO2, in particular during the last three decades.

The reason being that essentially

[d]ifferent geographical pattern of SST that produce the same global-mean $T$ can give different $N$.

There seem to be a number of conclusions that one can draw from this study.

• Climate models that have a reasonable representation of the pattern of surface warming do a reasonable job of estimating the resulting feedback response.
• The reason why observationally-based estimates for ECS tend to suggest lower values than other estimates (such as climate models) may well be simply because of the spatial distribution of surface warming that we have actually experienced, rather than because our climate is actually less sensitive than these other estimates suggest.
• Projections for future warming will likely be reasonable as long as the pattern of surface warming in climate models is a reasonable representation of what we will likely experience under increasing anthropogenic forcings.

Three years!

Yesterday was the third anniversary of me starting this blog. I was going to post something yesterday, but it coincided with the release of our consensus on consensus paper, which seemed like it should take precedence. It’s been a very interesting three years, quite stressful, frustrating, challenging, but also – at times – enjoyable. I’ve certainly learned a great deal, and I’ve encountered many interesting and knowledgable people.

The blog has had many more reads and many more comments than I was expecting. Early on it was actually quite hard to manage, and I really do need to thank Rachel who helped with the moderation for the first year or two. It really set the tone and has made subsequent moderation much easier. I also need to thank all those who’ve written guest posts: Rachel, Willard, Michael Tobis, Richard Betts, Richard Erskine, Lawrence Hamilton, An Oil Man, verytallguy, John Russell, and Steven Mosher (I hope I haven’t forgotten anyone). I’ve written lots of posts of my own, but I don’t really know which ones to highlight. I mostly look at old ones and think “oh, that’s not quite right”, or “I wouldn’t have written it that way now”. This is really all just one big learning experience for me.

I was thinking about whether or not to change anything about how I run the blog, but I decided against it. It’s just a blog. I get to write things, and sometimes people kindly write guest posts, but it’s not something I want to take all that seriously, and I don’t really want to feel obliged to do anything specific. I realise that what I’ve focused on has changed with time, but I typically just write whatever happens to be of interest at the time. I started focusing mainly on the actual science, and have more recently become interested in science in society, but I don’t really want to drift too far away from areas where I think I have at least some expertise. Maybe I’ll write a bit more about my own research.

The only other thing I was going to comment on was how things seem to have changed in the last 3 years. It feels as though climate science denial has become much less relevant. It might just be that I’m simply able to ignore it more than I used to be able to do, but it does feel as though it’s being ignored more generally and that it’s getting more difficult to effectively promote climate denial publicly. Maybe it was the fact that virtually all governments reached agreement in Paris at COP21. Maybe the succession of warmest years has made it more difficult to deny climate science without appearing to be denying reality. Maybe illustrating the consensus has some kind of effect. Maybe I’m even wrong, but it certainly feels less relevant than it did when I first started.

Anyway, that’s all I was going to say. It’s been a fascinating three years and I’m amazed it’s gone so quickly. I’m not sure I can quite manage another 3 quite like the last 3, but I certainly plan to keep writing posts when I feel like doing so. We’ll just have to see how it goes.

Posted in Climate change, ClimateBall, Personal, Science | | 52 Comments