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16/1/20. Minor edit to remove linear extrapolation graph, and comment added to acknowledge the uncertainty, starting "We don't know...".

Last year I wrote a forum post asking whether climate change deserved more attention within effective altruism. It's been great to see the recent developments, including Hauke's comparison with global dev, Will's comments, the interesting result that climate change is now #2 most popular cause area in the EA survey, and the answers to this question on what EAs interested in climate change should do. I've really enjoyed this discussion!

At EA Global in London 2019, Niel Bowerman gave a talk and Q&A on the relationship between climate change, longtermism, and effective altruism.

Niel studied for his PhD in Physics on the topic of ‘Emission targets for avoiding dangerous climate change’ from Oxford University. In the US 2008 presidential election Niel was a member of President Obama’s Energy and Environment Policy Team. Niel was previously the Director of the Centre for Effective Altruism, and is now an AI policy specialist at 80,000 Hours.

I thought Niel’s talk was a valuable resource for EAs with a longtermist focus thinking about the implications of climate change. I’ve summarised the key points, including the transcript with images, and provided links to the sources referenced throughout, along with some further resources at the end.

Key points from Niel’s talk

Impacts of climate change

  • Climate change is already impacting a number of areas on Earth, and has already caused hundreds of thousands of premature deaths (Robine et al, 2008), and this is likely to increase to tens or hundreds of millions over future decades (WHO, 2014), and we have a solid case for addressing this problem today
  • We can think about the contribution of climate change to existential risk through three different routes: through potentially making earth uninhabitable for humans; through exacerbating conflict; and through increasing the risk of social collapse
  • The larger contributions to existential risk are probably through the second and third routes, which are strongly deserving of further research incorporating systems theory
  • We can also identify a highly unlikely but not impossible causal chain where extreme climate change could make Earth uninhabitable for humans

Could climate change make Earth uninhabitable for humans?

  • We can think about the zone of habitability to be a function of the social and technological capabilities of future humans, and Earth’s environment.
  • If things continue as they are, it is possible that technological development will mean there is still a zone of habitability, but it is hard to make long-term predictions. These are contested and uncertain areas, deserving of further study.
  • We have far more fossil fuels than we can safely burn. We have burnt around 500 billion tonnes of CO2 so far, and we should aim to keep our total emissions within 1 trillion tonnes to avoid dangerous levels of warming of over 1.5C to 2C of warming. If we burned the total amount of economically viable fossil fuel, a very crude linear model would roughly imply 20-30C of total warming.
  • Several recent papers have looked at tipping points, such as through changes to cloud cover and through increasing moisture in the air, and have put forward claims that there are plausible mechanisms worthy of further investigation that could lead to rapid and drastic runaway climate change

Effective altruism, climate change, and existential risk

  • The biggest contributions of climate change to existential risk probably come through the routes of exacerbating conflict and potentially increasing the risk of social collapse
  • There is sufficient evidence to entertain the hypothesis that climate change could render earth uninhabitable for humans in the future.  It also seems possible that climate change could limit of humanity’s potential in other ways
  • It might now be a good time for some EAs to research climate modelling and uncertainties; future technological development, including food and water; ecosystem and biodiversity importance; planetary systems and boundaries; civilisation collapse and recovery; and possible associations between climate change and other existential risk factors

Selected quotes

This transcript was produced by the CEA team, and the full text, along with other EA Global talks are available through effectivealtruism.org. I personally added the images, links, and references, and the summary above. With thanks to Aaron Gertler, JP Addison, and Niel Bowerman.


Niel: We already know that climate change is causing in the ballpark of hundreds of thousands of premature deaths (Robine et al, 2008). This is likely to increase to as many as hundreds of millions over the coming decades (WHO, 2014). We have a sound economic case for addressing climate change, and it is harming the poorest people the most. And yet these are also the people that have least contributed to climate change, creating a climate justice issue.

Relationship between climate change and existential risk

Niel: Now, if I had to guess, I would guess that the second and third risks here — the contribution to other existential risks (Mach et al, 2019) and to societal collapse — are probably larger than the first, but they're also less tractable [for me] to study. And so I'm going to do the thing that academics often do: ignore the big important things and address the thing that I know about as a climate physicist.

Social collapse

Niel: [This would be] something in the vicinity of a complete breakdown of political institutions, a collapse of international trade, and really a breakdown of the fabric of society. To make this really concrete, I'm talking about a reduction in global GDP of more than 80% in less than a decade. This would be drastic and very dramatic, but I have no idea how likely it is to occur. I also have no idea how likely it is that societal collapse would contribute to existential risk.

Historical evidence

Niel presents three examples, which provide some evidence about human adaptability within the context of temperature change, but which also all have limitations that should make us cautious about making comparisons.

First, the Paleocene-Eocene Thermal Maximum, which happened about 55 and a half million years ago.

Niel: [This] saw warming of about 12 degrees above what we currently see today, and yet we didn't see a mass extinction... this may be some evidence that dramatic warming could be habitable for humans, but the rate of warming was really a lot slower than it is today. So it's not a perfect analogy.

Next, the example of the Younger Dryas period, about 11,000 years ago, gives us some evidence.

Niel: Snow and dust accumulation samples tell us that the earth warmed by about seven degrees in certain parts in less than a decade. And humanity also survived that, though it was moving from a much colder climate to the climate we enjoy today. So again, not a perfect analogy, but it's maybe some evidence that humanity is able to survive rapid rates of change.

Finally, there are the range of conditions that humans are able to live in today.

Niel: Humans thrive in climates as diverse as Bahrain and St. Petersburg — climates that vary in average over 16 degrees centigrade. So this is maybe some evidence that even [on a warmer planet], provided that everything else didn't fall apart around us, we would be able to survive in a range of different climates provided that the environment and ecosystems adapted. But again, really not a perfect analogy because these are static conditions as opposed to moving ones.

Is there a causal chain leading to potential human extinction?

Niel: During my PhD, I would talk about this question a fair bit with my old supervisor. He would always say that in order to address climate change as a contribution to X-risk, you need a story you can tell about what events occur that get us from now to a point where earth is uninhabitable or where humans have gone extinct. And so what I want to do for the rest of this talk is to try and sketch out a story that I find [vaguely] plausible for how we could get there. I'm not saying this is likely. I'm not saying this is going to happen. I think this is incredibly unlikely. But the challenge I gave myself was: could I tell a story where we get to a world like that? So that's what I'm going to do here.

Long-range potential of global carbon emissions

In the talk, Niel referred to this graph (Figure SPM10 from the IPCC AR5). The chart shows cumulative total anthropogenic CO2 emissions (i.e. the total carbon released into the atmosphere by human activities) against surface mean average global temperature change. Note that this excludes potential tipping points, is average temperature change, and the impacts of climate change on weather variability increase non-linearly.

Niel: For context, international climate negotiations are aimed at keeping us below two degrees. Five degrees would be a dramatically and drastically different world from the one that we enjoy today, with mass loss of species, [many risky] tipping points...

The world was warmed by around 1.1C so far, and there are discussions in the IPCC on the comparison between 1.5C and 2C of warming. The Climate Action Tracker gives a baseline of 4.1 to 4.8C by 2100, and 2.8 to 3.2C, assuming that current policies are implemented.

Niel: Today, we have burnt about half a trillion tons of carbon dioxide; by the year 2100, if we carry on burning CO2 under business as usual scenarios, we'll get up to about 2 trillion tons of carbon burnt. And that's burning the amount of carbon that we would under business as usual. What if we somehow got really carbon-hungry and [eventually] ended up burning all the potentially economically viable carbon [in fossil fuel reserves]?

Niel then extends the range of the chart above, from 0-2500 TtC, to 0-9,500 TtC, to show the total potential economically viable fossil fuel resources that humanity could burn.

We don't know exactly how large global fossil fuel reserves are, and we don't know how much burning all that fossil fuel would warm the world.  But one paper highlighted by Linch in the comments of this post suggests 10 or even 12 degrees of warming, which would be a very different world indeed from the world that we live in.  If climate and carbon cycle feedbacks are worse than expected, then it could warm even further.

Niel: There's a bunch of factors that are going to go into this. But this is more to prime your intuitions that our burning that much fuel would probably lead to a very bad world.

Cloud-based tipping points

Runaway greenhouse effects work as follows: increased thermal forcing heats up the ocean, which leads to leads to more evaporation and water vapor in the atmosphere, which traps thermal radiation, and in turn heats up the ocean. Is this feasible on Earth?

Niel: On Earth, if this were to occur in a very impossible worst-case scenario, you could imagine boiling all the oceans or something like this if the effect didn't stop. People have looked into this and they've said, "Actually, this isn't really very plausible. You'd need a lot more incoming solar radiation than is ever really going to occur, because we know how the Earth orbits the sun and we know the range of solar radiation that tends to reach Earth and we know this is really not going to happen anytime soon." I think we can rule out this runaway greenhouse effect. But is there something like this that could happen?

Recent developments in climate modelling

Niel refers to a paper by Popp et al (2016), Transition to a Moist Greenhouse with CO2 and solar forcing, looking at cloud feedback loops.

Niel: A very simple climate model with a strange effect: cloud feedback kicked the temperatures up to stabilizing at about 20 degrees [celsius above current temperatures]. So you basically saw this runaway affect that I've been talking about. Popp et al.’s model wasn't very plausible; it was incredibly simplistic and it made enough assumptions that no one really believed it. But it started to get people scratching their heads and wondering: is this a thing that could happen? Is there a feasible mechanism here?

Niel also refers to a paper by Schneider et al (2019), Possible climate transitions from breakup of stratocumulus decks under greenhouse warming, also looking at cloud-based feedback loops.

Niel: It uses another simplistic model of the atmosphere. And what happens in this model is they triple carbon dioxide concentration (the amount of CO2 in the atmosphere). And what they find is that the stratocumulus cloud layer in the tropics would burn away. Clouds have this lovely white fluffy property that makes them really reflective to incoming sunlight. And so that stops the Earth from warming too much. But when the clouds disappear, the dark ocean underneath of them absorbs a lot of sunlight. And that in turn heats up the ocean.... you see eight extra degrees of warming on top of the tripling of CO2 concentrations that happened simply from what they did in the model.

However, this is has had a range of reactions from climate scientists.

Niel: This model, [like Popp et al (2016)], had a mixed reception from scientists. Some people said, "Hmm, that's really interesting." Others said, "No, it's a simple column model. It's not going to apply globally. We shouldn't kid ourselves enough about this." But I think it is fair to say that we now at least have a physically plausible mechanism here. Something worth investigating further.

Finally, there is an open question on the link between drastic temperature change and uninhabitability.

Niel: We don't know if this is going to be a big deal or a small deal, but at least there's some evidence that maybe this is the thing we should be worrying about. And so what I've been doing here is scratching my head and asking myself: can I tell a plausible story about how earth becomes uninhabitable due to climate change? These are the two main pillars that I'm looking at. How does that lead to uninhabitability? Well, mainly I'm going to ask you to use your imagination regarding a world with 20 to 30 degrees [centigrade] of warming.


Niel concludes by summarising the points raised, that climate change contributes to existential risk through the three routes described above, and that the second and third are probably the larger terms.

Niel: We then saw what it would look like to burn a lot of fossil fuels, and how that could lead us to a really warm climate. And then we hypothesized and spitballed about cloud-based tipping points, and how maybe that's a thing that we should pay attention to. Maybe, I don't know. And together, these things gesture in the direction of a story of how Earth might become uninhabitable.

There are some suggestions for further research.

Niel: I'd love for people to go away and think about each of these different parts — uninhabitability, contribution to other X-risks, and societal collapse — and maybe investigate some of these uncertainties, with a particular focus on thinking about the chance of climate change really contributing to humanity going extinct altogether.


Epistemic status of climate change science

Niel Bowerman: We know for certain that the world is warming, we know for certain that carbon dioxide concentrations are going up, and there's a lot of consensus on the physical mechanisms whereby those carbon dioxide concentrations would lead to warmer temperatures. I think that 97% of scientists agree that climate change is most likely caused by humans.
When we zoom out and talk about climate change as a contribution to existential risk, now we're just conducting pure speculation.... This is more of a prompt to encourage people to go and work on this question a bit more and try and pin down some of these answers. Because I think that other than maybe my first slide, everything I've said today is largely speculation — but it's speculation trying to get at the question of where this chance of extinction might lie, which is very important from a longtermist perspective. If we come at the problem from this perspective, then I think there are a bunch of really interesting questions to ask that I'd love to see the broader community addressing.

Severity of climate change

Nathan: Sticking with more mainline questions just for a second: Some of your earlier comments toward the beginning of the talk suggest that your outlook would be: “Humanity will be pretty adaptable. Yes, there will be costs to climate change, but in the absence of a story where other things happen as knock-on effects, the mainline expectation is not so bad.” Is that a fair reading of your view?
Niel: Yeah. It depends on what you mean by “not so bad”. I expect tens of millions, maybe even hundreds of millions of people to die prematurely because of climate change. And that's really bad. It's similar on a vague scale of badness to things like the number of people that die in traffic accidents. So when you look at climate change [having this level of impact] over many decades, it's very much a big problem.
And then, from a longtermist perspective, my question is: “Where does it rank relative to the other existential risks?” If I had to guess, I would guess that it's maybe lower down on the list of things likely to contribute the most to existential risks than, say, AI or biorisk. I'd put it more among the cluster of things like nuclear weapons and geoengineering.
For me, that's still very much a thing deserving of attention and worth working on, and whether it's really bad depends a lot on your definition of “really bad”.

Urgency of tackling climate change

Niel: I think we should be cutting carbon emissions much, much more dramatically than we are right now. I think we should be going to net zero emissions. I think there's a relatively solid economic case. So this is a good idea and [will give us] a positive return on our investment. My guess is that we should be keeping the total amount of carbon that humanity admits to less than a trillion tons, which would hopefully keep us at less than two degrees of warming... [we need to make] pretty dramatic and quick reductions.

Methane clathrates

Nathan: Is there a story about methane deposit release that would crack your set of possible narratives here? I don't know a ton about it, but it's supposed to be the most powerful greenhouse gas. There's a lot of it sitting at the ocean floor and in the Tundra.
Niel: Yeah, methane clathrates. This is one of the things that I've always been a little worried about. The thing about methane clathrates is that they are very slow to bubble up. So in most of the plausible scenarios that we see, unless you go into the sorts of crazy scenarios that I was talking about, you end up seeing methane clathrates not emerging for something like hundreds or maybe even a thousand years after you see your initial warming.
If we got the climate science wrong, or if the bottom of the ocean heated up way faster than we expected, then you could see [clathrate emergence] happening faster. But the ocean doesn't really turn much in the vertical direction. Almost all the movement in the ocean is horizontal. So this just makes it very hard for heat to propagate downwards. And that means it's going to take a little while for those methane clathrates to heat up.

Potential future research areas

Nathan: Okay, cool. That's very interesting. So you've covered this one a little bit, but: how important is it for people to work on this? I think your suggestion would be that exploring the corner cases or the far-out cases is really where the highest value work is to be done.
Niel: That'd be my guess. Yeah. I'm excited to see people exploring societal collapse like the people at Cambridge are doing. [More people should be] looking at some of these worst-case scenarios and [how feasible they are]. And also, geoengineering is a giant question mark in my head. I don't really understand how that interacts with existential risk [from societal collapse]. But my guess is that people should be figuring that out, too.
Nathan: You may want to pass on this because we don't have a ton of time, but how does this 20-degree warming translate to true uninhabitability?
Niel: This part I didn't really know, I would again just be spitballing here, but when you look at agricultural yields, they start dropping off a cliff at some point. And then if we move agricultural zones up into the poles, you end up with very different growing seasons than the ones that they were built for. We don't have a great sense of how that would work. But the most likely story you could tell is something involving societal collapse, and the breakdown of society having a bunch of other bad knock-on effects.

Relative importance of climate change for effective altruism

Nathan: How have your views changed about all of this over time?
Niel: My guess is that my views on climate change have stayed relatively stable over time, and my views on the importance of working on AI have gone up. For me, climate change still seems just as important as when I worked on it as my full-time job. And now that there's a ton of folks flooding into AI policy and AI safety and things like that, my guess is that it's time for EAs to revisit working on climate change tail risks and nuclear weapons and geoengineering and some of the other problems in this space.

Carbon tax and other interventions

Nathan: How do you think about a more conventional carbon tax, versus a carbon capture (which might fall under geoengineering, depending on the strategy). What do you think we should be pursuing first to reduce carbon?
Niel: On the immediate question of carbon taxation versus carbon capture: carbon capture is an unproven, very expensive technology, and carbon taxation is a very proven but somewhat politically infeasible mechanism. I'd be way more excited about a carbon tax. A carbon tax of the right size, if implemented across the world, could go a really long way to helping solve climate change. I don't know about the political feasibility of this, but if it were implemented, it would do a lot of good. Carbon capture, I think, is going to be very expensive and is a long way off, but it's hopefully part of the solution in the future.

Further reading

Avin, Shahar, Bonnie C. Wintle, Julius Weitzdörfer, Seán S. Ó hÉigeartaigh, William J. Sutherland, and Martin J. Rees. 'Classifying global catastrophic risks'; Futures 102 (2018): 20-26.

Robine, J., et al. 'Death toll exceeded 70,000 in Europe during the summer of 2003', Compes Rendus Biologies (2008)

WHO, 'Quantitative risk assessment of the effects of climate change on selected causes of death', 2030s and 2050s' (2014)

Diamond, J. 'Why societies collapse', TED talk (2008), see Collapse and Upheaval.

Bostrom, N., 'Existential Risks', Journal of Evolution and Technology (2001)

Mach, et al. "Climate change as a risk factor for armed conflict" Nature (2019)

Beard, S., et al. "Assess climate change’s contribution to existential risk” in draft(2019), see https://www.cser.ac.uk/team/simon-beard/ and 'The Climate Crisis as an existential threat', Future of Life Institute Podcast, 2019.

Kemp, L., et al. “Climate end game” in draft, see https://www.cser.ac.uk/team/luke-kemp/ and see also

Steffen et al. (2018) 'Trajectories of the Earth System in the Anthropocene'

King et al. (2015) 'Climate Change: A Risk Assessment'Sluijs, A., et al. 'Subtropical Arctic Ocean temperatures during the PETM' Nature (2006)

Raup, D. & Sepkoski, J. 'Mass extinctions in the marine fossil record', Science (1982)

Alley, R. et al. 'Abrupt increase in the Greenland snow accumulation at the end of the Younger Dryas event', Nature (1993)

Stocker, et al. Summary for Policymakers of IPCC Working Group 1 Fifth Assessment Report. Figure SPM.10, doc here

Matthews, D. et al. 'Focus on cumulative emissions, global carbon budgets and the implications for climate mitigation targets' Environmental Research Letters (2018)

Bruckner, et al. Ch 7 of IPCC Working Group 3 Fifth Assessment Report. See table 7-2, shown below. (Fossil fuel reserves + resource (potentially economically viable to be extracted in the future) are estimated at 9.5-14.5 TtC.)

Schneider, T., Kaul, C., & Pressel, K. 'Possible climate transitions from breakup of stratocumulus decks under greenhouse warming' Nature Geoscience (2019)

Popp, M., Schmidt, H., & Marotzke, J. 'Transition to moist greenhouse with CO2 and solar forcing' Nature Communications (2016)

Goldblatt, C. & Watson, A. 'The runaway greenhouse: implications for future climate change, geoengineering and planetary atmospheres' Phil. Trans. Roy. Soc. A (2012)

Potential relevant organisations

The following organisations, among many others, are researching extreme climate change, civilisation collapse, and existential risk:

Centre for the Study of Existential Risk


Global Priorities Institute

The Forethought Foundation

The Future of Humanity Institute

Sorted by Click to highlight new comments since:

Thanks for this. I think it could be made clearer here how seriously we are meant to take the possibility that climate change could directly make the Earth uninhabitable. Is the claim that this is possible just like the LHC causing a black hole, or is it that there is like a 1 in 100 or 1 in 1,000 chance? Having looked at the evidence, I find it very difficult to see how climate change could directly cause extinction. It is a very slow moving problem, and for the effects we could see over the course of centuries, I just don't see any mechanism by which it could cause extinction.

The only plausible way extinction could happen is from moist greenhouse or runaway greenhouse, but the median view in the literature is that this is not physically possible. e.g. The Popp et al paper mentioned assumes that the Earth is entirely ocean.

Great question. I'm afraid I only have a vague answer: I would guess that the chance of climate change directly making Earth uninhabitable in the next few centuries is much smaller than 1 in 10,000. (That's ignoring the contribution of climate change to other risks.) I don't know how likely the LHC is to cause a black hole, but I would speculate with little knowledge that the climate habitability risk is greater than that.

As I mentioned in the talk, I think there are other emerging tech risks that are more likely and more pressing than this. But I would also encourage more folks with a background in climate science to focus on these tail risks if they were excited by questions in this space.

Is it a mainstream position that we'd get ~20°C warming from burning all the fossil fuels? I thought we'd usually expect a lower (but still very bad!) temperature change from burning all the fossil fuels:

The Earth is not at risk of becoming like Venus. We have done climate model simulations in which all available fossil fuels were burned and the resulting CO2 released into the atmosphere. The planet warmed up about 10 °C in these simulations. This was enough to melt all of the ice sheets and produce 60 meters of sea-level rise, but in no such simulation does the Earth become anything like Venus.



Linked paper here:

https://advances.sciencemag.org/content/1/8/e1500589. (Eyeballing the graphs, it looks like a 12°C change* at the highest).

*The median estimate is 10 °C, with 12 °C being the upper end.

Thanks for flagging this. I think estimating temperature rise after burning all available fossil fuels is mostly educated guesswork. Both estimating the total amount of fossil fuels is hard and estimate the climate response from them is hard.

However, I hadn't seen this Winkelmann, et al. paper, which makes a valuable contribution. It suggests that the climate response is substantially sub-linear at higher levels of warming.

The notes that are currently posted above about how warm it would get if we burned all the fossil fuels were back-of-the-envelope calculations that I did in this slides' notes, and I wouldn't trust them much. They assume a linear model which isn't reliable at these temperatures. I didn't end up including them in the talk as I didn't think they were robust enough. I'll ask Louis about removing them.

Thanks for flagging this Linch!

Thanks for the quick response, and really appreciate your (and Louis's) hard work on getting this type of sophisticated/nuanced information out in a way that other EAs can easily understand!

No worries - edit made.

Humans thrive in climates as diverse as Bahrain and St. Petersburg — climates that vary in average over 16 degrees centigrade.

Or go up to Fairbanks, Alaska where I live, and add another ~8 C to the range!

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