Different perspectives

I’ve been reflecting a little on some of the recent discussions I’ve had, mostly on Twitter, with those who have more expertise in emission scenarios, and energy systems, than in physical climate science specifically. I’ve found it a somewhat frustrating experience, but it seems that there’s also frustration on the other side too, specifically with the use of RCP8.5, which many seem to think is unrealistic.

One simple issue is that I think many climate scientists simply see the representative concentration pathways (RCPs) as bracketing the range of possible concentration pathways that we could follow, without thinking specifically about how we might do so. One reason for the latter is that there isn’t an easy way to determine how we might follow a specific concentration pathway; it will depend on our future energy pathway, how the carbon sinks respond, and on whether or not some carbon cycle feedbacks start to operate.

My impression is that many emission/energy experts think that we should put more focus on considering the impact of following a pathway that is likely, rather than simply considering a small sample of concentration pathways, at least one of which is seen as very unlikely. Climate models are, however, very computationally expensive, and so there is a limit to how many scenarios can be considered. It’s also important to be able to compare different climate models, so using the same basic scenarios can be important for that reason too. It’s also often possible to extrapolate between scenarios to work out how the system might respond if we followed some scenario that hadn’t explicitly been considered by a climate model.

One might think that if the highest concentration pathway isn’t really all that plausible anymore, that there shouldn’t be as much focus on it. There is certainly some merit to this. However, there are also reasons to still consider this scenario. One is that the impact of climate change depends mostly on how much we are likely to warm, which depends on how sensitive the climate is to these radiative perturbations. If you use RCP8.5 with a climate model that has a climate sensitivity near the middle, or lower half, of the range, then this can also represent what might happen if we follow a lower concentration pathway, but climate sensitivity turns out to be higher.

This is essentially my perspective; until someone can rule out the higher levels of warming (by considering both the plausible range of concentrations pathways and the range of climate sensitivity) then I think it is still important to understand the impact of these higher levels of warming.

I also think that this whole basic debate ignores something that I happen to think is key. We’re moving the Earth’s climate into a regime that we have probably never experienced before. We have some idea of what might happen, but we can’t really know for sure. Maybe we’ll be very lucky and find that it evolves smoothly and in ways that we can manage. Maybe, however, we’ll discover that some natural processes have been masking some of the forced warming, and that we have more warming in the pipeline than we expected, or that we’ve crossed some tipping points that lead to substantial, irreversible, changes on a short timescale.

Clearly we can’t avoid some future warming, since we can’t simply stop using fossil fuels overnight. However, rather than arguing about whether or not climate scientists are using the optimal concentration pathways in their models, maybe we should just recognise that we might not want to face too much more future warming and should spend our time finding ways to limit our future emissions. That’s just my view, of course. Other people probably have a different perspective.

Posted in Climate change, Climate sensitivity, ClimateBall, Philosophy for Bloggers, Policy, Scientists, The philosophy of science | Tagged , , , , , | 9 Comments

Stocks and Flows

Sitting at home waiting for a delivery, so just a quick post. There’s been a new narrative, on social media at least, that we may be heading for a plateau in global emissions. The suggestion, then, is that we are on track to follow one of the lower representative concentration pathways (RCP4.5, for example). This would be a good step, if true, but it seems to be based on a single assessment that suggests that emissions in 2040 will be similar to those today. Not only do there appear to be other assessments that disagree, what some seem to be concluding from this appears to confuse stocks and flows.

What will ultimately determine how much we warm will be how much we emit in total. Emissions in a single year might give a clue as to what this would be, but it’s really going to depend on emissions across the whole time period. In particular, for the mid-range RCPs (RCP4.5 and RCP6) most of the emission reductions happen after 2040. So, emissions in 2040 may be a poor guide to how much we are likely to emit in total.

It would, of course, be great if we are starting to see emissions plateauing and, eventually, starting to drop. One concern I have with the current narrative is the suggestion that somehow it is happening without us really having done very much. It’s possible, I guess, but it would seem rather unfortunate if we start to think that we’ve almost solved this problem and then discover that emissions have continued rising. Maybe a better narrative would be one where we highlight how we might be heading for a plateau in emissions and then suggest that we aim for a fall, rather than simply a plateau.

Posted in advocacy, Carbon tax, Climate change, Global warming, Policy | Tagged , , , , , | 110 Comments

Tipping points/elements

There has been quite lot of discussion recently about climate tipping points, or tipping elements. It’s mostly motivated by a recent Nature comment suggesting that Climate tipping points [are] too risky to bet against. The suggestion is that some the tipping points (West Antarctic Ice Sheet, Greenland Ice Sheet) may triggered sooner than we have anticipated. Hence, we should take this seriously and that international climate action should reflect this. One particular issue would be if there were a cascade of tipping points that, when combined, lead to some new climate state.

However, there do seem to be some who don’t entirely accept this argument, and I can see why. A lot of what is suggested about tipping points is quite speculative; it’s very difficult to quantify the actual likelihood of them being triggered. Also some (like ice sheet retreat) might still be quite slow and may not even be truly irreversible. Hence, I understand why there is some reluctance to make this a major aspect of the narrative.

However, I am starting to think that there is a problem with how we typically discuss this topic; we tend to focus more on what we think will probably happen and not enough on what might happen. Even though what will probably happen could be pretty severe, the low-probability, high-impact outcomes carry the greatest risk. So, I do think we should be talking more about the potential worst-case scenarios, but I’m not entirely sure of how best to do so.

One problem is that the outcome is conditional on what we do in future, and there are a large number of uncertainties associated with that future – what do we do in terms of future emissions, how will our emissions be taken up by the carbon sinks, how will the climate respond to the resulting atmospheric concentration, what will be the impact of this climate response, and how will we then respond to these impacts? In some sense, it’s a continually moving target.

If we fail to limit our emissions, then it becomes more likely that some of the more serious outcomes will materialise. If we start to limit our emissions, then they become less likely. If we have to consider what future pathway we might follow, what will probably happen if we follow that pathway, and also what could happen if we’re unlucky, it all gets rather convoluted. Also, if we do start to limit our emissions, do we stop talking about some of the worst case scenarios that would now be less likely than they had been before, or do we continue to highlight them in case we then start to take emission reductions less seriously?

Also, some of the tipping points/elements are so uncertain that we may have already almost triggered them, or it could still take a fairly substantial amount of additional warming. How should this influence our thinking? Clearly, if we want to avoid them we should aim to limit our emissions, but if we’re not really sure when they’d be triggered, how do we balance this with all the other factors that should influence how we go about reducing our emissions?

As you can probably tell, even though I think it is important to highlight some of the more extreme, worst-case outcomes, I’m still not sure how to do this in a way that accounts for all of the uncertainties, without it becoming so convoluted that it’s difficult to explain clearly. Similarly, how do you avoid simplifying it to the point where it is open to valid criticisms? Maybe other people have some ideas of how to do this. If so, I’d be interested to hear them.

Posted in Climate change, Global warming, Philosophy for Bloggers, Policy, Severe Events | Tagged , , , , , , | 182 Comments


I’ve always been a little confused as to why so much attention is paid to methane emissions. It’s short-lived, so isn’t it maybe somewhat less important that CO2 emissions, which are long-lived? One reason it is quite prominent is because it is regarded as having a large global warming potential; one tonne of methane is regarded as having about 28 times the impact of one tonne of CO2. The global warming potential (GWP) is defined as the time-intergrated climate forcing due to a one-off pulse of methane when compared to a one-off pulse of CO2 of the same mass.

Credit: Allen et al. (2016)

However, as this paper by Myles Allen, and colleagues, highlights, the actual impact of a pulse of methane is very different to that of a long-lived greenhouse gas like CO2. The figure on the right shows warming due to equivalent pulses of short-lived, and long-lived greenhouse gases. A pulse of a long-lived greenhouse gas like CO2 will lead to sustained warming that will persist for a long time. An equivalent pulse of a short-lived greenhouse gas, like methane, will have a large impact initially and then decay away. After a few decades, the impact will be largely negligible.

Sustained emission of short-lived greenhouse gases (solid lines) compared to a single pulse of CO2, a long-lived greenhouse gas [Credit: Allen et al. (2016)]

Hence, simply comparing the global warming potentials of short- and long-lived greenhouse gases can be somewhat misleading. However, a sustained emission of a short-lived greenhouse gas can be equivalent to a single pulse of a long-lived greenhouse gas. The figure on the left shows the sustained emission of short-lived greenhouse gases compared a single pulse of long-lived greenhouse gases, with the total emission of the short-lived greenhouse gases over a century being equivalent to the single pulse of the long-lived greenhouse gas.

All of this implies that one should be careful when comparing short-lived and long-lived greenhouse gases. When considering long-lived greenhouse gases, stabilising temperatures requires getting emissions to zero. When considering short-lived greenhouse gases, it requires getting emissions to stabilise. Constant emission of a long-lived greenhouse gas is equivalent to increasing emissions of a short-lived greenhouse gas. Finally, reducing emissions of a long-lived greenhouse gas leads to continued warming until emissions get to zero, while reducing emissions of a short-lived greenhouse gas can lead to cooling. Also, if emissions of short-lived greenhouse gases are reduced to zero, their overall warming impact will eventually become negligible.

As a consequence of this, there are a couple of papers that have suggested a modification to the global warming potential metric, called GWP*. It still uses the original GWP metric, but depends on how the emission rate of the short-lived GHGs changes (essentially because one can relate a single pulse of a long-lived greenhouse gas to an increase in the emission rate of a short-lived greenhouse gas). This allows one to more properly compare changes in the emission of short-lived and long-lived greenhouse gases.

The reason this is important is because if we simply use the original GWP metric, we could end up committing a lot effort to reducing emissions of a short-lived greenhouse gas (that only has a warming impact for decades) at the expense of emission reductions of a long-lived greenhouse gas, that can have a warming impact that lasts for centuries, or longer. Similarly, the original GWP metric would suggest that reducing the emissions of short-lived greenhouse gases should still lead to warming, as it does for long-lived greenhouse gases. This, however, is wrong. Reducing the emissions of short-lived greenhouse gases should lead to cooling. Hence, we could end up over-estimating what we need to do to achieve some target.

On the other hand, because short-lived greenhouse gases have a large impact on decadal timescales, they are still important if we are to achieve some of our near-term targets. So, they certainly can’t be ignored. There are also other factors associated with them that are also important (for example, land use change and excess water use associated with livestock farming).

There are also some potential issues with the new global warming potential metric, as pointed out in this paper. Under the new metric, a region with a high, but constant, level of methane emission, could argue that they’re no longer contributing to global warming and, hence, don’t need to do anything. However, reducing their methane emissions could still account for some of the impacts that they’ve already had, and would clearly make it easier to achieve some of our targets.

It’s clear that this isn’t a simple issue, but it does seem that there is still some confusion about how short-lived greenhouse gases, like methane, compare to long-lived greenhouse gases, like CO2. I hope this post clarifies some of the issues. I hope I have indeed explained things properly. Feel free to correct me if I have got something wrong. I’ve also provided links, below, to the papers I’ve mentioned, plus links to a couple of other articles about this topic, including a very nice Carbon Brief guest post by Michelle Cain (one of the authors of the papers I mentioned). If you want to learn more, I can also recommend this podcast with Michelle Cain.

New use of global warming potentials to compare cumulative and short-lived climate pollutants – Allen et al., Nature Climate Change, 2016.
A solution to the misrepresentations of CO2-equivalent emissions of short-lived climate pollutants under ambitious mitigation – Allen et al., NPJ climate and atmospheric science, 2018.
Improved calculation of warming-equivalent emissions for short-lived climate pollutants – Cain et al., NPJ climate and atmospheric science, 2018.
Unintentional unfairness when applying new greenhouse gas emissions metrics at country level – paper by Joeri Rogelj about an unintentional issue with the new GWP* metric.
Guest post: A new way to assess ‘global warming potential’ of short-lived pollutants – Carbon Brief guest post by Michelle Cain.
Why we’re still so incredibly confused about methane’s role in global warming – Chris Mooney article about this topic.

Posted in Environmental change, Global warming, Policy, Research, Scientists | Tagged , , , , , | 229 Comments

Science and Technology Studies Podcast

I’ve been trying to listen to more podcasts, and came across a new one that might be of interest to my readers. It’s called The Received Wisdom with Shobita Parthasarathy and Jack Stilgoe. Both are researchers in Science and Technology studies, and my regulars will know that this is an area I’ve discussed on a number of occasions. The first episode was about climate activism – a topic I also find interesting – and featured Dan Sarewitz.

The initial part of the podcast was the two presenters just discussing some aspects of science/technology in society. There was an interesting discussion about how science and technology is used in society and how it can often be used in a way that benefits many people, but can also be used in ways that do a lot of harm. However, even here there was a suggestion that scientists don’t understand this and that they might do better if they were lucky enough to stumble into a Science and Technology Studies class.

The interview with Dan Sarewitz didn’t really get much better. It was full of generalisations about those who work in science and technology. There were claims that the typical academic scientist doesn’t reflect about how their research interfaces with broader societal issues, that the scientific community doesn’t think deeply about the enterprise in which they’re engaged, and that they’re clueless about the benefits of progress and about truth in the face of fraught politics.

I do think that there are some real issues with how scientific understanding interfaces with society, and with policy making. I do think that there are real issues with how scientists present their research to the public; there is indeed a tendency to over-hype results and to suggest that some new finding will solve all sorts of problems. I do think that the incentive structures in unversities encourage behaviour that may not always be optimal.

However, this is very complex and the issues aren’t the same across all areas of science and technology. People who work in science and technology are not some homogeneous bunch who behave in the same way and who all perceive the role of science in society in the same way. In many respects, the problems are not even unique to those who work in science and technology; over-hyping the significance of what we do is probably a common human failing, rather than something done only by those in science and technology.

I do think that it would be great if there was more interaction between social scientists and physical scientists; I think both could learn a lot from each other. However, I don’t see how this is going to be all that effective if the assumption is that it’s mostly the physical scientists who would benefit. Maybe social scientists who feel comfortable generalising about the failings of those who work in science and technology should reflect on this and should maybe consider that although the knowledge they’ve generated might benefit society, it can also end up doing more harm than good.

Posted in Philosophy for Bloggers, Policy, Scientists, The philosophy of science, The scientific method, We Are Science | Tagged , , , , , | 12 Comments


Climategate is a topic I’ve rarely discussed on this blog. Mostly because it’s clear that it’s not possible to have a constructive discussion with those who have different views about its significance. However, since I watched the BBC show about it last night, I thought I might risk it. It’s a bit of a pity that they decided to cover it, since it really doesn’t have much significance when it comes to climate science specifically. It does illustrate the lengths some will go to in order to undermine our scientific understanding, but it’s not clear how we really benefit from reminding people about this fake scandal.

What struck me was how difficult it was for some of those involved. Phil Jones clearly found it a very difficult time and it was pretty obvious that Tim Osborn found it difficult to talk about some of what he had experienced. One of the main things it highlighted in the emails was Mike’s Nature trick….to hide the decline. This is typically misrepresented by those who promote it. The context was the cover of a report which showed the temperature history for the last 1000 years. Most tree ring series ended in 1980, so Mike’s Nature trick was simply the addition of the instrumental temperature record after 1980.

However, one tree ring series diverged after 1960 and showed cooling. This is clearly wrong, since we know that temperatures kept rising after 1960. Hence the instrumental temperature record was added to this series post 1960, rather than post 1980 as per the other tree ring series. If this was a figure for a scientific paper, you’d expect this to be made clear in the paper itself. However, this was for a cover of a report, the resulting figure was a reasonable representation of our millenial temperature history, and showing the diverging portion of the tree ring series would have been wrong.

The show did illustrate that the stolen emails did indicate that some of what was going on was sub-optimal. In particular, some of the scientists weren’t as open as they could have been. However, we do keep getting told that science is social, and this seems to be an illustration of that. As scientists we have some obligation to engage with critics of our work. However, we do have jobs that require quite a lot of attention, so there is a limit to how much time we can spend responding to people who question what we’ve done. We’re also not obligated to engage with those who do not appear to be engaging in good faith.

Similarly, we should be willing to share what we’ve done with others, but – again – there are limits. A key aspect of scientific research is that others should be able to reproduce what you’ve done so as to check your results. However, this simply requires that it be possible to do so, not that researchers are obligated to share every bit of what they’ve done with those who are trying to reproduce their work. In some cases it may not be straightforward to share everything. I’ve on occasion used codes that you need to ask to use. If someone else wanted to use the same code, they’d need to go to the source, rather than expect me to give it to them. In most cases, this is simply because the developers would like to keep track of who is using their codes, not because they’re aiming to limit who can actually use them.

So, as far as I’m concerned, Climategate is mostly an indication of the lengths some will go to in order to undermine our understanding of an important topic. It may also indicate that some of the conduct by the scientists involved was not ideal, but science is a social endeavour and scientists can get as frustrated as anyone when faced with a barrage of bad-faith requests. There is absolutely no indication that anyone engaged in anything nefarious when it came to the science itself, and attempts to check the results from the groups involved have confirmed their results and further strengthened our scientific understanding.

Posted in ClimateBall, Contrarian Matrix, Gavin Schmidt, Sound Science (tm), Steven McIntyre, The philosophy of science, The scientific method | Tagged , , , , , , , | 138 Comments