Thanks for the write-up. I upvoted because I think it lays out the arguments clearly and explains them well but I disagree with most of the arguments. 

I will write most of this in more detail in a future post (some of them can already be seen here) but here are the main disagreements:
1. We can decouple way more than we currently do: more value will be created through less resource-intensive activities, e.g. software, services, etc. Absolute decoupling seems impossible but I don't think the current rate of decoupling is anywhere near the realistically achievable limits. 
2. Renewables are the main bottleneck: The energy per dollar for solar has decreased exponentially over the last 10 years and there is no reason it should not continue; the same is true for lithium-ion batteries. The technology is ready (or will be within the next decade) and it seems to be mostly a question of political will to change. Once renewable energy is abundant most other problems seem to be much easier to solve, e.g. protecting biodiversity if you don't need the space for coal mines. 
3. The global economy is interconnected: It is very hard, if not impossible to stop growth in developed countries but keep growth in developing countries. Degrowth in the West most likely implies decreased growth in the developing world, which I oppose. 
4. More growth is required for a stable future path: Most renewable technology has been developed by rich nations. Most efficiency gains in tech have been downstream effects from R&D in rich nations. If we want to get 1000x more efficient green tech, it will likely come from rich countries that pay their scientists from public taxes. In general, many solutions to problems pointed out by degrowthers require a lot of money. A bigger pie means a bigger public R&D budget and more money to spend, e.g. on better education or national parks. 
5. My vision of the future:  I don't think we can scale to infinite value with finite resources. There clearly is a limit at some point but I don't think we have reached it yet. I want to strive toward a world that could host 100B inhabitants powered by solar, hydrogen and nuclear. People live in dense cities with good public transport. People mostly stopped eating meat and vegetarianism drastically reduced land use and problems of pollution. Many problems that exist in the West today are solved in the future, e.g. the infant death rate is not 0.001 (like it is today in the West) it should be 0! I just can't see why the current level of GDP is optimal for some reason and I think we should aim to grow GDP AND solve other problems (and the two are not mutually exclusive or GDP is even necessary for the other). 
6. GDP growth in the west is not a major goal for  EA anyway: I agree with the fact that GDP growth in already rich countries should not be a major goal for EAs. We should aim to solve global problems, many of which are in less developed countries and we should prevent x- and s-risks. Most of these goals are mostly independent of GDP in rich countries.  However, on the margins, I think more GDP in rich countries probably makes it easier to achieve EA goals, e.g. more GDP means a bigger budget for pandemic prevention. Furthermore, I think it would be bad for EAs to support degrowth both because it seems less relevant than other problems and because I just don't think the arguments are true (as described above).

I will publish a slightly more details version of the above arguments and link it here so that you can engage with them more properly. Thank you, once again, for presenting the arguments for degrowth in this clear and non-judgemental way such that people can engage with them on the object level. 

Regarding your "resource scarcity" section -- I thought that Harrington 2020 was pretty ridiculous for a variety of reasons.  It simply re-runs the original "Limits to Growth" simulations, without engaging with any of the criticism those models have received, or even noting that the real world's performance has been far better than those models predicted.  Furthermore, the Harrington paper blatantly moves the goal-posts, swapping out the environmental problems of the 70s (things like acid rain and industrial pollution and so forth) for climate-related CO2 metrics.  Quoting from my comment about Harrington 2020 on another EA Forum post:
 

This new paper is taking a 1972 computer model of the world economy and seeing how well it matches current trends.  They claim the match is pretty good, but they don't actually just plot the real-world data anywhere, they merely claim that the predicted data is within 20% of the real-world values.  I suspect they avoided plotting the real-world data because this would make it more obvious that the real world is actually doing significantly better on every measure.  Look at the model errors ("∆ value") in their Table 2:

So, compared to every World3-generated scenario (BAU, BAU2, etc), the real world has:
- higher population, higher fertility, lower mortality (no catastrophic die-offs)
- more food and higher industrial output (yay!)
- higher overall human welfare and a lower ecological footprint (woohoo!)

The only areas where humanity ends up looking bad are in pollution and "services per capita", where the real world has more pollution and fewer services than the World3 model.  But on pollution, the goal-posts have been moved: instead of tracking the kinds of pollution people were worried about in the 1970s (since those problems have mostly been fixed), this measure has been changed to be about carbon dioxide driving climate change.  Is climate change (which is predicted by other economists and scientists to cut a mere 10% of GDP by 2100) really going to cause a total population collapse in the next couple decades, just because some ad-hoc 1970s dynamical model says so?  I doubt it.  Meanwhile, the "services per capita" metric represents the fraction of global GDP spent on education and health -- perhaps it's bad that we're not spending more on education or health, or perhaps it's good that we're saving money on those things, but either way this doesn't seem like a harbinger of imminent collapse.
 
Furthermore, the World3 model predicted that things like industrial output would rise steadily until they one day experienced a sudden unexpected collapse.  This paper is trying to say "see, industrial output has risen steadily just as predicted... this confirms the model, so the collapse must be just around the corner!"  This strikes me as ridiculous: so far the model has probably underperformed simple trend-extrapolation, which in my view means its predictions about dramatic unprompted changes in the near future should be treated as close to worthless.

There are a couple of bait-and-switch  moves in this post that I don't understand:

1. Paris-targets urgency vs big-picture eco-philosophy
"We're not on track to meet our Paris climate agreements; if we want to meet these targets we'll need a big transition right away" creates a reasonable sense of urgency attached to the issue of climate change.  But then this urgency inexplicably carries over to big-picture philosophical ideas like "we are on a finite Planet Earth... at some point [maybe centuries or millennia from now], we will run out of crucial resources and we'll have to transition to zero resource-use growth", when it seems like we might have plenty of time to solve problems like the eventual scarcity of certain metals (including by doing far-future stuff like space settlement & asteroid mining).

2. Exactly how much degrowth are we talking here?
You don't clarify exactly what "post-growth" or "degrowth" means.  Sometimes it sounds like you are advocating a massive worldwide economic depression of similar impact to the Covid-19 lockdowns, but lasting much longer.  (Yes, you say that it wouldn't be as bad because only certain industries would be shut down, but the recession would also have to be more severe than 2020 in many ways, since the 2020 lockdown-recession didn't actually reduce emissions by much.  So, figure the impact might feel about the same overall.)

But other times, you say that actually the ultimate goal is "avoiding an ecological collapse and its associated economic collapse", giving the impression that you favor approximately whatever mix policies lead to the best long-run outcome for human civilization -- striking the right balance between economic harms from global warming and economic harms from global warming prevention.   Estimates from the UN IPPC say that even 3-4 degrees of warming (aka, blowing past the Paris Agreement targets) would only penalize the economy by a few percent by 2100.  So (unless you think that all these climate studies are wildly wrong), it seems like it is only worth paying a small cost to prevent global warming: stuff like subsidizing green power (as in the bill just passed by the United States -- we closed half the gap between the satus quo and our Paris goals for only $300 billion!), approving more nuclear plants, implementing a carbon tax, and so forth.  The couple-percent-of-GDP damages that mainstream climate science expects, don't seem like they are worth embarking on a many-percent-of-GDP, society-wide sacrifice of human wellbeing and development.  So maybe "degrowth" is just a sexy, radical-sounding word for these sensible global-warming mitigation policies like carbon taxation and the like?  In that case, I don't understand your choice of vocabulary but I am otherwise totally with you.

The post is really nicely structured and written. 

However, to me the key debate is not whether it's possible to have growth while (1) protecting the planet  and (2) eradicating poverty. The question is how probable it is. I have generally found the arguments by the degrowth people quite convincing that it is in many ways improbable. 

However, strictly speaking, the question is not even how improbable it is but the comparative question whether it is more improbable than having degrowth while (1) protecting the planet and (2) eradicating poverty. And this, I find even more improbable.

In other words: we should not just examine how growth makes poverty-eradication-cum-protecting-the-planet hard, but we should equally carefully examine how degrowth makes it hard -- and probably even harder.  
 

"""Poor nations would suffer unintended consequences because they rely heavily on exports to rich countries → In fact, poor nations will benefit from it because as we internalize costs, they’ll get a fair salary and get compensated for environmental costs."""

This is, to put it mildly, implausible, and requires strong evidence IMO.

That you dismissed the most important issue with your claim so tersely without really engaging with it suggests you simply do not care very much about the effects on the global poor, which in a scenario without economic growth would I expect be much, much worse than the worst plausible effects of 3-4° warming (which I take to be the likely "business as usual" outcome). Citing Hickel here, a known bad faith/disingenuous actor in this area (see here) doesn't serve to provide much evidence either.

How realistic are the optimistic views that technology will be able to deliver, fast enough, green energy, mitigate climate change, and bring us back under all the remaining planetary boundaries?
Predictions by International Energy Agency’s World Energy Outlook versus reality:

How likely is it that we won’t run out of any crucial resource in the next century? 
 

On a planet of finite resources, it doesn’t seem like we can continue growing indefinitely – Under business-as-usual, when would we reach plateau/collapse?
Think of how foolish it would have been to throttle energy/resource use 100 years ago to protect ourselves from running out at 100-years-ago levels of use. When we start to run out, we have many options: alternative resources, efficient use, dilution, recycling, etc. This will extend the clock many fold. Resources may be finite, but need for them is not constant. I am confident they will become outdated in time, as resources historically have thusfar.

There are lots of  very concerning problems arising from overapplying GDP, capitalism, gerontocracy, and neglecting environmental issues. But the above particular points do not seem substantive to me.

Thanks for the post ! I liked it. 

I also think that EAs are way too optimistic when it comes to reach green growth.  Most large scale environmental problems have gone worse globally, and continue to do so today, despite way better technology. 

In my view, thinking that we'll somehow solve something we failed to adress for the last 50 years, despite ample alerts, and despite decoupling being the main environmental policy... well, this strikes me as rather risky.

If you're interested, I made a post on energy depletion that also adresses the issue of limits to growth. The second post adresses decoupling of energy and GDP. However, I'm more skeptical of the possibility that we'll do a voluntary degrowth: the third post adresses that.

Regarding your "Do we need growth in rich countries?" section -- this strikes me as a failure of imagination.  You are willing to look back across history and say that life has gotten vastly better in many ways with advancing science and industry- -- modern medicine, the conveniences of travel and telecommunications, etc.  But you don't seem willing to make the obvious extrapolation into the future -- in a more prosperous and more energy-abundant world, don't you expect that society could become even better?  People could afford better medical care, society could more easily afford to redistribute and create equality, society might even become more participatory and democratic?

People in 1960 would have made your same argument -- "The USA and Europe don't need growth; we're already so prosperous, we've basically achieved everything you could possibly want."  But looking back, we can tell that they were wrong -- life has improved in important ways since 1960, and even since 1990!  What makes your argument any more likely to stand the test of time?

In other words -- sure, the USA doesn't do quite as well on quality-of-life metrics as some countries in northern Europe which have a slightly lower GDP.  It would be great to learn from those countries.  But also, neither the USA or Europe represent the highest potential of human civilization -- so much more is possible!  For a vision of what this better future might look like, here is an optimistic, utopian story I put together with some friends of mine that tries to illustrate what a fairer, more democratic, and more abundant world could be constructed.  Here are some relevant quotes from that project:

Q. What is a new non-AI technology that has played an important role in the development of your world?

A. Improved governance technology has helped societies to better navigate the “bulldozer vs vetocracy” axis of community decision-making processes. Using advanced coordination mechanisms like assurance contracts, and clever systems (like Glen Weyl’s “SALSA” proposal) for pricing externalities and public goods, it’s become easier for societies to flexibly make net-positive changes and fairly compensate anyone affected by downsides. This improved governance tech has made it easier to build lots of new infrastructure while minimizing disruption. Included in that new infrastructure is a LOT of new clean power.

Solar, geothermal, and fusion power provide most of humanity’s energy, and they do so at low prices thanks to scientific advances and economies of scale. Abundant energy enables all kinds of transformative conveniences:

• Cheap desalinization changes the map, allowing farming and habitation of previously desolate desert areas. Whole downtown areas of desert cities can be covered with shade canopies and air-conditioned with power from nearby solar farms.
• Carbon dioxide can be captured directly from the air at scale, making climate change a thing of the past.
• Freed from the pressing need to economize on fuel, vehicles like airplanes, container ships, and self-driving cars can simply travel at higher speeds, getting people and goods to their destinations faster.
• Indoor farming using artificial light becomes cheaper; instead of shipping fruit from the opposite hemisphere, people can enjoy local, fresh fruit year-round.

Q. What is a new social institution that has played an important role in the development of your world?

A. New institutions have been as impactful over recent decades as near-human-level AI technology. Together, these trends have had a multiplicative effect — AI-assisted research makes evaluating potential reforms easier, and reforms enable society to more flexibly roll out new technologies and gracefully accommodate changes. Futarchy has been transformative for national governments; on the local scale, “affinity cities” and quadratic funding have been notable trends.
In the 2030s, the increasing fidelity of VR allows productive remote working even across international and language boundaries. Freed from needing to live where they work, young people choose places that cater to unique interests. Small towns seeking growth and investment advertise themselves as open to newcomers; communities (religious beliefs, hobbies like surfing, subcultures like heavy-metal fans, etc) select the most suitable town and use assurance contracts to subsidize a critical mass of early-adopters to move and create the new hub. This has turned previously indistinct towns to a flourishing cultural network.
Meanwhile, Quadratic Funding (like a hybrid of local budget and donation-matching system, usually funded by land value taxes) helps support community institutions like libraries, parks, and small businesses by rewarding small-dollar donations made by citizens.
The most radical expression of institutional experimentation can be found in the constellation of “charter cities” sprinkled across the world, predominantly in Latin America, Africa, and Southeast Asia. While affinity cities experiment with culture and lifestyle, cities like Prospera Honduras have attained partial legal sovereignty, giving them the ability to experiment with innovative regulatory systems much like china’s provinces.

A discussion about GDP growth and climate change (1) doesn't really add anything to the known economic strategies to end emissions (sinking lid carbon caps) (2) nor does it engage with the key pressures preventing us from implementing them. So I agree with you that we shouldn't be too concerned about impact on GDP through carbon mitigation, but also think arguments in favour or against that aren't really engaging with the critical solutions or challenges.

On the solutions side: economically, at the level of economics design, a sinking lid carbon cap makes it very simple. We set a carbon budget from now to 2050 based on the level of temperature increase and other climate impact we deem tolerable. Then, anyone emitting carbon has to buy emissions credit out of the limited budget available. I'm not saying governments should entirely leave market forces to sort it all out--there's a strong argument to helping them along with e.g. tailpipe emissions regulations. But economically that'd be sufficient. Under this approach you don't have to worry about whether you get growth or degrowth or something in between--just set your target and the economy will adjust to achieve it.

I suppose I must concede that some people trying to decide exactly what level of emissions, or temperature increase, we might want to tolerate treat it as a GDP optimisation problem. What level of temperature increase maximized GDP given the cost of avoiding to climate change against the costs of experiencing climate change?

But there's also the key political pressures that keep us from implementing carbon budgets. Oil prices went up this year by a lot, and this has led to high inflation across the economy, according to some experts, and has been experienced directly by consumers as higher costs to fuel combustion engine vehicles. Political leaders of all stripes including Biden, who ran on a platform of lowering emissions, amongst other priorities, have responded by trying to get oil prices down. That's the exact opposite of what you do to avoid climate change, but it might be the only choice democratically elected leaders have to respond to public concerns in order to be elected again. And voters aren't primarily concerned about maintaining topline growth; they want to know: will I have a job; will I keep my job; can I afford to drive my car? GDP is a contributor to all those things, and SWB measures definitely decline during recessions. but a debate on GDP targets and subjective well-being won't help you understand how to get support from voters for a tractable climate change response.

So I am left wondering the value of framing the question in terms of GDP at all, except perhaps in setting an economically optimal temperature increase limit target for carbon credit schemes. Maybe that kind of puts me in agreement with OP? But I disagree that the inevitable conclusion is that we need to "shrink material throughput". What needs to be shrunk are greenhouse gas emissions, and to target growth in a positive or negative sense from the outset seems misplaced.

Interesting post, here's a few thoughts:

1. what are your arguments against the claim that: 
   "It is too late for organized degrowth because climate change is now driven by intrinsic feedbacks as well as anthropogenic GHG's. Change in both GDP and GHG production will depend primarily on global response to ongoing climate and ecological disasters. Population control will drive itself, through massive suffering and death caused by climate disasters and a change in public sentiment toward conceiving children, or just living, under much worse conditions than now."
   
   You seem to want degrowth, and I agree with you, it's the only honest option, frankly, but you're looking at the climate change aspect through a model of GAST temperature rises driven solely by anthropogenic GHGs. I just want to know why.
    
2. Nanotechnology was popularized by Eric Drexler in his old book "Engines of Creation". His visions of nanotechnology fit the techno-optimists bill for rescue from climate change (in ways that current proposals for BECCS and DACCS technology do not). In particular, nanotechnology manufacturing offers hope of extremely rapid large-scale in-place manufacturing suitable for all sorts of climate and pollution mitigation technologies tested and adopted over very short time frames.
   
   You're looking for some hope and cheer. Seen from this perspective, nanotech offers it. There might be other problems associated with its use, but realizing its full vision would give humanity tools to solve most environmental problems rapidly.
3. Decoupling food supply from farmland seems an important step to ensure the survival of the current human population, depending on the impacts of climate change or the 6th great extinction that you consider too soon,  too fast, or too big to prevent. 
   
   There are many possibilities, I favor nutrition that does not require:
   * refrigeration
   * high-moisture content
   * cooking
   
   In particular, powdered drinks or crisps, vacuum-sealed single-use dissolving oil packs, and liquid extracts of vegies and fruit (when those are available). You add water to the powdered drinks. 
   
   You're probably familiar with fermentation technology used to make milk proteins, do you have an opinion about the technology? I heard a few mentions about it recently, including a potential commercial milk product based on the technology.
    
4. Broadly, the 1.5C ceiling on GAST temperature rise was thought to be low enough to avoid climate tipping points. Obviously, it is not. Given your research on climate change to date, when do you think the arctic will be ice-free in the summer, all other things equal? 
   
   Let me note that governments are making plans for an ice-free arctic rather than looking to prevent the ice loss. There's talk of fishing, drilling, shipping lanes, and military movements in arctic waters once the ice is gone.
    
5. You covered a lot of areas with that doughnut chart. The scenarios involved are more complex when you factor in multiple forcings, and their results are more negative than a first attempt might capture. For example, sticking with climate change, California megaflood frequency is expected to rise this century, driven by global warming. 
   
   A megaflood from an atmospheric river covering California for several weeks would create an inland sea, flood cities built on flood plains( like LA), and cut off critical transit routes. Death by dehydration or starvation could occur. 
   
    California might lose most of its population to migration or other causes during a megaflood. California megafloods might occur too frequently to make large-scale resettlement of the state feasible or attractive. 
   
   In such a scenario, what are the knock-on effects of that flood? California is a big economy on its own, and a major food grower. It would create a pollution disaster across the state, destroy most California infrastructure, wipe out Silicon Valley businesses, disrupt internet and power delivery in other areas, lead to production losses in some industries, kill some people, and force most people to leave (if they can) as the rains continue, provided the state or the feds can  help in the event of a megaflood, not a sure thing.
   
   Interestingly, California is also vulnerable to mega-tsunami's, but those occur less frequently than megafloods, and afaik, their frequency is not affected by climate change.
   
   You did a lot of work to explore the area of green growth vs degrowth, have you thought about turning your attention to scenarios like large-scale weather disasters and specifics to prepare for them? I would be interested. 
    
6. On the topic of the 6th great extinction, you should have some specific thoughts, as a biologist. Have you seen techno-optimism about how to restore an ecosystem sufficient to support at least a few million people with species that are no longer present?
   
   There's a pathway through which humanity does its best to help ecosystems restore themselves. The scenario is plausible but the devil is in the details. Do you believe that the completion of the 6th great extinction is inevitable? Can it be reversed?
   
   Assuming something happened like a large drop in ocean biomass within a relatively short period, say a decade, what could be done to limit its impact on terrestrial ecosystems? Could we bring back the plankton without removing atmospheric CO2?

I asked a lot here, I don't expect you to address it all, of course, just whatever you'd like. 

Thanks for your excellent post, by the way!

To add to my comments yesterday, while GDP growth doesn't seem to drive subjective well-being (SWB) in Western countries, recessions certainly harm it. You acknowledged unemployment could be an issue. I don't know if shifting to a four day workweek is a good idea to generate extra jobs because (1) it's authoritarian to stop people from working five days a week if they want to (2) people will suffer a real loss of 20% of their income and they may suffer in SWB terms as a result (3) many jobs are not super fungible and 4 workers doing 5 days a week can't simply be replaced by 5 workers doing four days a week without a lot of retraining (4) slowing down parts of the economy unconnected to carbon emissions will lower output unnecessarily (as an example, if you cut teacher hours by 20%, it isn't clear how this meaningfully reduces carbon emissions because teaching isn't a high carbon occupation, but you will suddenly have to higher an extra 25% more teachers if you want to keep total teaching hours the same) and (5) while GDP growth doesn't raise SWB, GDP declines could still lower it. Humans are loss averse, and tend to want to avoid losses more than they want to approach equivalent gains, by a ratio of about 2:1.

It does seem like good public policy partially protects against the SWB impacts of unemployment and recessions (https://link.springer.com/article/10.1007/s42413-019-00022-0). However, (1) that doesn't mean it's entirely protected and (2) we can't snap our fingers to get better public policy --that'll take work.

I don't have strong opinions on the object-level of whether marginal increases in growth in rich countries is net good (I think I have substantially more sympathy than most EAs that it's net bad). But I think the argument is unlikely to primarily route through climate change, for fairly simple reasons.

1. Transformative AI and  engineered pandemics are substantially more important than climate change.

2. GDP growth probably has a substantial impact on AI and pandemics. 

3. Climate change and environmental degradation also has some impact on AI and pandemics.

4. However, I think it will be quite surprising if the third-order impact of GDP growth -> climate change and environmental degradation -> AI and pandemics is higher than the second-order impact of GDP growth -> AI and pandemics.

5. Thus, to investigate whether growth in rich countries on the margin is net good or bad, almost all of the "action" will come from investigating the effects of growth on AI, and maybe bio/pandemics as well. It is unlikely to come from two layers of indirection.

I'm curious where the crux is. My current guess is that the main crux between me and the OP is (1). I agree that this is a nontrivial question (one of the most important cause prioritization questions) and I do not plan to answer it here.