Existential risk

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This post is about how much warming we should expect on current policy and assuming emissions stop at 2100. We argue the risk of extreme warming (>6 degrees) conditional on these assumptions now looks much lower than it once did.

Crucially, the point of this post is about the direction of an update, not an absolute assessment of risk -- indeed, the two of us disagree a fair amount on the absolute risk, but strongly agree on the direction and relative magnitude of the update.

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The damage of climate change depends on three things:

1. How much we emit
2. The warming we get, conditional on emissions
3. The impact of a given level of warming.

The late and truly great economist Martin Weitzman argued for many years that the catastrophic risk from climate change was greater than commonly recognised. In 2015, Weitzman, along with Gernot Wagner, an economist now at New York University, released Climate Shock, which argued that the chance of more than 6 degrees of warming is worryingly high. Using the International Energy Agency’s estimate of the most likely level of emissions on current policy, and the IPCC’s estimate of climate sensitivity, Wagner and Weitzman estimated that the chance of more than 6 degrees is 11%, on current policy.[1]

In recent years, the chance of more than 6 degrees of warming on current policy has fallen quite substantially for two reasons:

1. Emissions now look likely to be lower
2. The right tails of climate sensitivity have become thinner

1. Good news on emissions

For a long time the climate policy and impacts community was focused on one possible ‘business as usual’ emissions scenario known as Representative Concentration Pathway 8.5 (RCP8.5), a worst case against which climate action would be compared. Each representative concentration pathway can be paired with a socioeconomic story of how the world will develop in key areas such as population, income, inequality and education. These are known as ‘shared socioeconomic pathways’ (SSPs).

The latest IPCC report outlines five shared socioeconomic pathways. The only one that is compatible with RCP8.5 is a high economic growth fossil fuel-powered future called Shared Socioeconomic Pathway 5 (SSP5). In combination, SSP5 and RCP8.5 is called ‘SSP5-8.5’. On SSP5-8.5, we would emit a further 2.2 trillion tonnes of carbon by 2100, on top of the 0.65 trillion tonnes we have emitted so far.[2] For reference, we currently put about 10 billion tonnes of carbon into the atmosphere from fossil fuel burning and industry.[3] The other emissions pathways are shown below:

However, for a variety of reasons, SSP5-RCP8.5 now looks increasingly unlikely as a ‘business as usual’ emissions pathway. There are several reasons for this. Firstly, the costs of renewables and batteries have declined extremely quickly. Historically, models have been too pessimistic on cost declines for solar, wind and batteries: out of nearly 3,000 Integrated Assessment Models, none projected that solar investment costs (different to the levelised costs shown below) would decline by more than 6% per year between 2010 and 2020. In fact, they declined by 15% per year.[4]

This means that renewables will play an increasing role in energy supply in the future. In part for this reason, energy systems models now suggest that high fossil fuel futures are much less likely. For example, the chart below shows emissions on current policies and pledged policies, according to the International Energy Agency.

The chart above from Hausfather and Peters (2020) relies on IEA models of future energy systems. These may still be too pessimistic on renewables. You can find the IEA’s cost assumptions here. They show the levelised cost of solar falling by 40% between now and 2030. But if historical trends continue, we should actually expect costs to decline by 89%. Trends may not continue, perhaps because we may be reaching saturation for renewables capacity additions, which drive cost declines. But there seems a decent chance that the cost declines will continue.

Fundamentally, existing mainstream economic models of climate change consistently fail to model exponential cost declines, as shown on the chart below. The left pane below shows historical declines in solar costs compared to Integrated Assessment Model projections of costs. The pane on the right shows the cost of solar compared to Integrated Assessment Model assessments of ‘floor costs’ for solar - the lowest that solar could go. Real world solar prices have consistently smashed through these supposed floors.

Secondly, SSP5-RCP8.5 assumes an enormous expansion in the use of coal, which looks very unlikely in part due to the decline in the cost of renewables and the abundance of natural gas driven by hydraulic fracturing, and in part because countries tend to transition away from coal as they get richer. For example, here is the increase in per capita coal use on different emissions scenarios:

For comparison, China burned what was widely seen to be a prodigious amount of coal from 2000 onwards, but that is dwarfed by the increase in coal use projected on SSP5-8.5. Indeed, total fossil fuel use on SSP5-RCP8.5 by 2100 exceeds some estimates of total recoverable fossil fuel resources.[5] The International Energy Agency claims that coal use peaked in 2014 and is now in structural decline.[6]

Thirdly, in order for us to follow SSP5-RCP8.5, there would have to be very fast economic growth and technological progress, but meagre progress on low carbon technologies. This does not seem very plausible. In order to reproduce SSP5-8.5 with newer models, the models had to assume that average global income per person will rise to $140,000 by 2100 and also that we would burn large amounts of coal. It is difficult to imagine that in such a cornucopia, there would not also be a lot of progress on low carbon technology, and that countries would have greatly increasing willingness to pay to protect the environment. Fourthly, climate policy has strengthened substantially over the last few years. Countries representing 66% of global CO2 emissions have committed to achieving net-zero emissions by 2050. Most importantly, China has pledged to get to net zero by 2060. Some of these commitments, such as that of the UK, are enshrined in law. Even if those targets are missed by a decade or more, this is a sharply different trajectory than RCP8.5. For all these reasons, RCP8.5 now looks much less likely. Even the Integrated Assessment Models that were used to generate different versions of RCP8.5, known as MESSAGE and REMIND, now suggest that a moderate emissions trajectory is the most likely outcome, on current policy.[7] Probabilistic assessments of emissions scenarios are thin on the ground, but recent studies suggest that the chance of RCP8.5 is now well below 5%. See for example, this from Liu and Raftery (2021): Various other recent studies suggest that on current policy, we are most likely to follow something in the range of RCP2.6 to RCP6, with RCP4.5 now the most likely scenario.[8] Moreover, this is all assuming that current policy stays as it is. But it now looks like climate policy is going to strengthen in the future. If you would like to read more on emissions scenarios, we would recommend the following papers: 2. Good news on climate sensitivity The main focus of climate science has been to quantify what is known as equilibrium climate sensitivity, which is the warming we get after a doubling of CO2 concentrations, ignoring the effect that the melting of the ice sheets (which occurs over millennia) might have on temperatures.[9] The new IPCC report finds that the uncertainty around equilibrium climate sensitivity has now narrowed, as shown by this chart from Carbon Brief In the 2013-14 IPCC report, the upper 5% bound on equilibrium climate sensitivity was 6 degrees, now it is 5 degrees. In spite of its prominence, equilibrium climate sensitivity is not actually a very useful metric. It measures the warming we get on the assumption that CO2 concentrations reach a certain level and then stay there indefinitely.[10] But, once emissions stop, CO2 concentrations would actually decline due to natural uptake of CO2, initially by the oceans. Atmospheric CO2 would look a bit like this after a given level of emissions: The red line is the result we get after the release of a pulse of 5 trillion tonnes of carbon[11] and the blue line is what happens after the release of 1 trillion tonnes of carbon. In both cases, CO2 concentrations peak and then slowly decline, returning to their natural state after hundreds of thousands of years. CO2 concentrations would only stay at a certain level indefinitely if there were a very precise low level of CO2 emissions sustained over many centuries to precisely compensate for ocean CO2 uptake.[12] This is unlikely to happen in the real world. For this reason, a more useful metric is the Transient Climate Response to Cumulative Emissions, which measures the warming we get for a given amount of cumulative emissions. As it turns out, there is a near-linear relationship between cumulative emissions and global warming, as the chart below shows: Once emissions stop, temperature would stay roughly constant for 100 years[13] before slowly declining over hundreds of thousands of years. In any case, these climate sensitivity metrics are related and the narrowing of uncertainty about the equilibrium climate sensitivity has fed through into the transient climate response to cumulative emissions. In the 2013-14 IPCC report, the 66% confidence range for the transient climate response to cumulative emissions was 0.8°C to 2.5°C per trillion tonnes of carbon.[14] In the latest IPCC report, this has narrowed to 1.0°C to 2.3°C per trillion tonnes of carbon.[15] The IPCC does not give a 5% to 95% range for the transient climate response to cumulative emissions, but we think given the update on equilibrium climate sensitivity, we should expect the narrowing of the 5% to 95% range to be greater still. The main reason that uncertainty about climate sensitivity has narrowed is that formal Bayesian methods that incorporate all lines of evidence have recently been used to estimate it. So, this is a victory for the Reverend Bayes and the Marquis de Laplace, not for expensive climate models. This is discussed at length in the outstanding (and long but readable) Sherwood et al (2020) ‘An Assessment of Earth's Climate Sensitivity Using Multiple Lines of Evidence’. 3. What is the risk of extreme warming? Now, we can bring all of this together. It is not all that easy to estimate how much warming we might get from a given amount of cumulative CO2 emissions. We not only need to predict CO2 emissions from fossil fuel and industry, but also from deforestation and other forms and land use change, and in addition we need to account for non-CO2 greenhouse gases like methane. One thing we can do is to start with projections of which representative emissions pathways we might follow and then use the IPCC’s estimate of how much warming we get on those pathways. Studies that try to project future emissions suggest that the most likely pathway is now something around RCP4.5 (with a range from RCP2.6 to RCP6). According to the IPCC, this suggests that something like 2.5 to 3 degrees of warming relative to pre-industrial levels is now the most likely outcome, on current policy.[16] (The world has already warmed by around 1 degree since the Industrial Revolution). The upper 95% bound for warming is around 2.4 to 4 degrees (The ‘very likely range’ is the 5% to 95% range). So, the chance of more than 6 degrees now seems to be well below 1%, much lower than the 11% estimated by Wagner and Weitzman. 4. Conclusions Several conclusions, clarifications and talking points follow from this. Firstly, on the assumption that the direct or indirect global catastrophic risk (defined as killing >10% of the global population or doing equivalent damage) of climate change depends on warming of more than 6 degrees, the global catastrophic risk from climate change is at least an order of magnitude lower than previously thought. If you think 4 degrees of warming would be a global catastrophic risk, then that risk is also considerably lower than previously thought: where once it was the most likely outcome, the chance is now arguably lower than 5%. None of this is to say that climate change is solved. But we need to acknowledge progress when it occurs and those of us trying to find the best ways to make a difference on the margin should adjust priorities accordingly. This is also not to say that warming of 2-4 degrees would not be bad. We have ample reason to transition to a zero carbon economy from the effects of climate change and also of air pollution. Secondly, this illustrates the importance of the neglectedness of a problem. Due to environmental activism, governments and the private sector have spent trillions of dollars on climate change over the years. This is now starting to yield fruit. If the same amount of effort went into dealing with pandemics, biorisk would also fall a lot. Thirdly, recent progress illustrates the value of an innovation-led approach to climate change, which Johannes first introduced to the EA community via John and Hauke Hillebrandt. Solar, wind and batteries have declined dramatically in cost. The lifetime cost of electric cars is set to be lower than petrol or diesel cars within five years. In many places, countries will start to choose these technologies simply because they are cheaper and better, not because they care about the climate. Because most (>85%) emissions in the future will come from outside the West, innovation is a uniquely tractable strategy because it provides leverage on global emissions without coordination. Fourthly, it is not a foregone conclusion that renewables will take over the entire energy supply. Way et al (2021), projects that renewables, batteries and hydrogen (as the long-term storage option) will take over the global energy supply and save us money, within 20 years if cost declines continue. It is a brilliant paper on cost reductions for modular technologies, but it only examines cost barriers to the scale-up of renewables. But the main barriers to the scale-up of renewables so far have been social and political opposition surrounding land use, value deflation and grid integration. Renewables and the associated transmission infrastructure take-up a lot of land, and so may run into one of the few iron laws of politics: NIMBYs always win As this shows, even in renewables-loving Germany, solar capacity additions have stalled for several years despite radically falling prices. Germany built solar when it was expensive and is building less as it has become cheap, showing the severity of value deflation, grid integration and related issues and the relative non-informativeness of PV module prices alone. One way to reduce the risk that decarbonisation efforts fail is by encouraging a wider range of low carbon energy technologies, such as nuclear fission, nuclear fusion and enhanced geothermal. Finally, this analysis does not give a full picture of climate risk because it only explores the most likely emissions scenarios conditional on current policy up to 2100. But technological progress might be slower than we expect and there might be backsliding on climate policy perhaps due to tensions between the Great Powers. Emissions might also continue past 2100. It is important to think about the probability of very high emissions over long time periods. That, however, is the subject for another post. Thanks to Luca Righetti, Matthew Ives of the Oxford Martin School, Zeke Hausfather and Leslie Abrahams of Clean Air Task Force for discussion. Endnotes 1. Wagner and Weitzman, Climate Shock, Table 3.1. 2. IPCC, Climate Change 2021: The Physical Science Basis, Summary for Policymakers, Assessment Review 6, Figure SPM.7. 3. Global CO2 emissions are around 36 billion tonnes, which equals around 10 billion tonnes of carbon. (3.667 tonnes of CO2 = 1 tonne of carbon). https://ourworldindata.org/co2-emissions 4. “Sound energy investments require reliable forecasts. As illustrated in Figure 2(a), past projections of present renewable energy costs by influential energy-economy models have consistently been much too high. (“Projections” are forecasts conditional on scenarios, so we use the terms interchangeably.) The inset of the figure gives a histogram of 2,905 projections by integrated assessment models, which are perhaps the most widely used type of global energy-economy models19,20,21,22, for the annual rate at which solar PV system investment costs would fall between 2010 and 202019. The mean value of these projected cost reductions was 2.6%, and all were less than 6%. In stark contrast, during this period solar PV costs actually fell by 15% per year. Such models have consistently failed to produce results in line with past trends3,23” Way et al, ‘Empirically grounded technology forecasts and the energy transition’, Oxford Martin School, 2021: p.3. 5. On SSP5-8.5, we would burn a further 2.2 trillion tonnes of carbon. IPCC, Climate Change 2021: The Physical Science Basis, Assessment Review 6, Summary for Policymakers: Figure SPM.7. Mohr et al (2015) estimate that ultimately recoverable fossil fuel resources are less than 1.6 trillion tonnes of carbon. S. H. Mohr et al., ‘Projection of World Fossil Fuels by Country’, Fuel 141 (1 February 2015): Table 2, https://doi.org/10.1016/j.fuel.2014.10.030. 6. “Covid-19 has catalysed a structural fall in global coal demand” IEA, World Energy Outlook 2020 7. Zeke Hausfather, ‘Flattening the Curve of Emissions’, Breakthrough Institute, 2021. https://thebreakthrough.org/issues/energy/flattening-the-curve-of-future-emissions 8. For an overview, see Hausfather, ‘Flattening the Curve of Future Emissions’ Breakthrough Institute, 2021. 9. “In order to calculate an ECS, which is defined here to include all feedback processes except ice sheets, the approach of Rohling et al. (2012) can be used.” IPCC, Climate Change 2021: The Physical Science Basis, Assessment Review 6, Chapter 7 p. 102. 10. It also ignores the feedback effects from melting ice sheets. 11. As mentioned, it is not clear whether this is even technologically possible. 12. “Physically this can be understood by realizing that the ECS is a theoretical quantity representing the warming that would occur only if atmospheric concentrations of greenhouse gases were held constant indefinitely while the climate system was allowed to come into equilibrium. Such a ‘constant radiative forcing’ scenario would require a very precise low level of emission of CO2 sustained over many centuries to precisely compensate for ocean CO2 uptake. This is clearly not a particularly policy-relevant scenario.” Richard Millar et al., ‘The Cumulative Carbon Budget and Its Implications’, Oxford Review of Economic Policy 32, no. 2 (2016): 323–42. 13. “The Zero Emissions Commitment (ZEC) is the change in global mean temperature expected to occur following the cessation of net CO2 emissions and as such is a critical parameter for calculating the remaining carbon budget… Overall, the most likely value of ZEC on multi-decadal timescales is close to zero, consistent with previous model experiments and simple theory.” Andrew H. MacDougall et al., ‘Is There Warming in the Pipeline? A Multi-Model Analysis of the Zero Emissions Commitment from CO2’, Biogeosciences 17, no. 11 (15 June 2020): Figure 3b. https://doi.org/10.5194/bg-17-2987-2020. 14. “The transient climate response to cumulative carbon emission (TCRE) is likely between 0.8°C to 2.5°C per 1000 PgC (high confidence), for cumulative carbon emissions less than about 2000 PgC until the time at which temperatures peak” IPCC AR5 15. “In the literature, units of °C per 1000 PgC are used, and the AR6 reports the TCRE likely range as 1.0°C to 2.3°C per 1000 PgC in the underlying report, with a best estimate of 1.65°C.” IPCC AR6. 16. Hausfather, ‘Flattening the Curve of Future Emissions’ Breakthrough Institute, August 2021; Hausfather ‘Blog: The New IEA Report Shows How We Are Flattening the Curve of Future Emissions’, October 2021; Climate Action Tracker. 228 New Comment 34 comments, sorted by Click to highlight new comments since: Firstly, on the assumption that the direct or indirect global catastrophic risk (defined as killing >10% of the global population or doing equivalent damage) of climate change depends on warming of more than 6 degrees, the global catastrophic risk from climate change is at least an order of magnitude lower than previously thought. If you think 4 degrees of warming would be a global catastrophic risk, then that risk is also considerably lower than previously thought: where once it was the most likely outcome, the chance is now arguably lower than 5%. I think that the crux between climate pessimists and optimists is, at the moment, mostly about how much damage the effects of 2-4 degrees of warming would cause. This has been a recent development - I feel like I saw a lot more arguments that 6+ degrees of warming would make earth uninhabitable in the past when that seemed more likely, and now I see more arguments that 2-4 degrees of warming could cause way more damage than we think. Mark Lynas in a recent 80k podcast puts it this way when asked about civilisational collapse: Mark Lynas: Oh, I think… You want to put me on the spot. I would say it has a 30 to 40% chance of happening at three degrees, and a 60% chance of happening at four degrees, and 90% at five degrees, and 97% at six degrees. Arden Koehler: Okay. Okay. No, I appreciate you being willing to put numbers on this because I feel that’s always really hard, but it’s really helpful. Mark Lynas: Maybe 10% at two degrees. These new environmentalist arguments for climate posing a GCR aren't that we expect to get a lot of warming, but that even really modest amounts of warming, like 2-4 degrees, could be enough to cause terrible famines by reducing global food output suddenly or else knock out key industries in a way that cascades to cause mass deaths and civilisational collapse. They don't dispute the basic physical effects of 2-4 degrees of warming, but they think that human civilisation is way more fragile than it appears, such that a modest loss of agricultural productivity and/or a couple of key industries being badly damaged by extreme weather could knock out other industries and so on leading to massive economic damage. Now, I've always been very sceptical of these arguments because they seem to rely on nothing but intuition and go against historical precendent, but also because I thought we had reliable evidence against them - the IPCCs economic models of climate change say that 2 degrees of warming, for example, represents only a few percent in lost economic output. E.g. this: https://marginalrevolution.com/marginalrevolution/2021/02/the-economic-geography-of-global-warming.html So the damage is bounded and not that high. However, I found out recently that these models are so oversimplified as to be close to useless - at least according to Noah Smith: https://noahpinion.substack.com/p/why-has-climate-economics-failed For example, in 2011, Michael Greenstone and Olivier Deschenes published a paper about climate change and mortality (I studied an earlier version of this paper in a grad school class). Their approach is to measure the effect of temperature on mortality rates in normal times, and use that estimate to predict how a warmer world would affect mortality. The authors make the obvious and grievous mistake of assuming that climate change affects human mortality only through the direct effects of air temperature — heatstroke, heart attack, freezing, and so on. The word “storm” does not appear in the paper. The word “fire” does not appear in the paper. The word “flood” does not appear in the paper. The authors do mention that climate change might increase disease vectors, but wave this away. Near the end of the paper they write that “it is possible that the incidence of extreme events would increase, and these could affect human health…This study is not equipped to shed light on these issues.” You don’t say. The big conceptual mistake here is to assume that whatever economists can easily measure is the sum total of what’s important for the world — that events for which a reliable cost or benefit cannot be easily guessed should simply be ignored in cost-benefit calculations. That is bad science and bad policy advice. His source for a lot of these criticisms appears to be this (admittedly very clearly biased) paper: https://www.tandfonline.com/doi/full/10.1080/14747731.2020.1807856 by Steve Keen, who seems to be some sort of fringe economist. But I see them repeated by environmentalists a lot. The claim is that the economic models are really wrong and therefore we should expect lots more damage from relatively minor amounts of global warming. So, if we accept these criticisms of the IPCCs climate economic forecasts (and please let me know if there are good responses to them), then where does that leave us epistemically? It means that the total economic damage caused by e.g. 3 degrees of warming doesn't have a clear, low, upper bound and that the 'extreme fragility' argument doesn't have strong evidence against it. However there still isn't any positive evidence for it either! And it still strikes me as implausible, and against historical precedent for how famines work (plus resource shorages are the sort of problem markets are good at solving). As far as I can tell, this really is the epistemic situation we're in with regard to the economic side of climate change forecasting - in the podcast episode with Rob Wiblin and Mark Lynas, they discuss this extreme fragility idea and neither cite climate forecasts to try and assess if modest losses to agricultural productivity would cause massive famines or not - it's just intuition Vs intuiton https://80000hours.org/podcast/episodes/mark-lynas-climate-change-nuclear-energy/?startTime=2614&btp=476595d6 Mark Lynas: So that’s, for me, the main question. And one of the most important studies I think that’s ever been performed on this was a study in the PNAS Journal, which looked at what they called synchronous collapse in breadbaskets around the world. So at the moment, the world still produces enough food every single year very reliably. We’ve never had a major food shortage which has been as a result of harvest failure. Mark Lynas: So I mean, if the U.S. Corn Belt was knocked out one year, that would have a huge impact on food prices, and have a huge impact on food security, in fact, as a direct result of that. But imagine if it really wasn’t just the U.S. Corn Belt. It was Australia, it was Brazil, and Argentina, it was breadbaskets of Eastern Europe, and the former USSR, all of that added together, then you enter a situation which humanity has never experienced before, and which looks very much like famine. https://80000hours.org/podcast/episodes/mark-lynas-climate-change-nuclear-energy/ Robert Wiblin: So when I envisage a situation where there’s a huge food shortfall like that, firstly, I think we’ll probably have some heads up that this is coming ahead of time. You start to notice the warning signs earlier, like food prices going up, and food futures going up. And then I imagine that people would start… Because it’ll be a global emergency much worse than the coronavirus, say. You just start seeing everyone starts paying attention to how the hell can we get more calories produced? And fortunately, unlike 500 years ago, we are in the fortunate situation where most people today aren’t already producing food, and most capital today isn’t already allocated towards producing more food. So there’s potentially a bunch of elasticity there where, if food prices go up tenfold, that a lot more people can go out and try to grow food one way or another. And a lot more capital can be reallocated towards agriculture in order to try to ameliorate the effects. Robert Wiblin: And you can also imagine, just as everyone in March was trying to figure out how the hell do we solve this COVID problem, everyone’s going to be thinking “How can I store food? How can I avoid consuming food? How can we avoid wasting food? Because every calorie looks precious”. And maybe that sense of our adaptability, or our ability to set our mind to something when there’s a huge disaster and just throw everything at it, perhaps makes me more optimistic that we’ll be able to muddle through, perhaps more than you’re envisaging. Do you have a reaction to that? Mark Lynas: My reaction is: imagine if Donald Trump is in charge of the response. It’s all very well to have optimistic notions of technological progress and adaptive capacity and things. And yeah, if smart people were running the show, that would no doubt be the most likely outcome. But smart people don’t run the show most of the time, in most places, and people are amenable to hate and fear, and denial and conspiracies, and all of those kinds of things as you’ve seen, even in the very short term challenges of COVID. My point is that, unlike temperature forecasts, there aren't any concrete models to support either Rob or Mark's position. And elsewhere in the article Mark claims this scenario is 10% likely with 2 degrees of warming. If he's right, butterfly effects of 2 degrees of warming causing civilisational collapse is twice as likely as the 5% chance of 4 degrees of warming cited in this post, and it's therefore where the majority of the subjective risk comes from. Regardless, as the physics side of climate change modelling has started to rule out enough warming to directly end civilisation by clear obvious mechanisms, this 'other climate tail risk' (i.e. what if the fragility argument is right) seems worth investigating if only to exclude the possibility. I still place a very low weight on these arguments being right, but it's probably higher than the chance we get 6+ degrees of warming. Again, this isn't my area so please let me know if this has all been heavily debunked by climate economists. But currently it seems to me that the main arguments of climate pessimists aren't addressed by ruling out extreme warming scenarios. Speaking for me personally and not Johannes. I strongly disagree with the claim that 3,4, 5 or 6 degrees of warming would do anything even remotely close to ending human civilisation or causing civilisational collapse. However, I don't think this post is the best place to discuss the question of climate impacts. I am working on a large report on that question which will be out next year. I see - that seems really valuable and also exactly the sort of work I was suggesting (I.e. addressing impact uncertainty as well as temperature uncertainty). In the meantime, are there any sources you could point me to in support of this position, or which respond to objections to current economic climate models? Also, is your view that the current Econ models are fundamentally flawed but that the economic damage is still nowhere near catastrophic, or that those models are actually reasonable? For sources, I would recommend just reading the technical summary of the 2014 IPCC Impacts report. There is no indication there that civilisation will end at 4 degrees. I think a lot of the economic models are very flawed yes. I think it is more useful to look at the impacts literature and try and make your own mind up from there. But I also think it is instructive that the most pessimistic models suggest that 4 degrees of climate change would leave us with something like a 400% increase in GDP compared to a counterfactual 900% increase without climate change by 2100. There is absolutely no indication from any of the economic literature that civilisational collapse is on the cards, and that is an update, even if the models are bad, which they likely are. There are a couple of sources which I'd recommend taking a look at. • 2015 Climate Change Risk Assessment - particularly worth reading the section which includes the quote "A simple conclusion is that we need to know more about the impacts associated with higher degrees of temperature increase. But in many cases this is difficult. For example, it may be close to impossible to say anything about the changes that could take place in complex dynamic systems, such as ecosystems or atmospheric circulation patterns, as a result of very large changes very far into the future." • Climate change risk assessment 2021 - conclusion starts out by saying "Unless NDCs are dramatically increased, and policy and delivery mechanisms commensurately revised, many of the climate change impacts described in this paper are likely to be locked in by 2040, and become so severe they go beyond the limits of what nations can adapt to." Section 4 of this paper considers various cascading systemic risks. These are a big part of what makes predicting the future impacts of climate change so hard. agree on the first one - it is very good. I hadn't seen the second one thanks for sharing! Agree that these seem like useful links. The drought/food insecurity/instability route to mass death that my original comment discusses is addressed by both reports. The first says there's a "10% probability that by 2050 the incidence of drought would have increased by 150%, and the plausible worst case would be an increase of 300% by the latter half of the century", and notes "the estimated future impacts on agriculture and society depend on changes in exposure to droughts and vulnerability to their effects. This will depend not only on population change, economic growth and the extent of croplands, but also on the degree to which drought mitigation measures (such as forecasting and warning, provision of supplementary water supplies or market interventions) are developed." The second seems most concerned about brief, year-long crop failures, as discussed in my original post: "probability of a synchronous, greater than 10 per cent crop failure across all the top four maize producing countries is currently near zero, but this rises to around 6.1 per cent each year in the 2040s. The probability of a synchronous crop failure of this order during the decade of the 2040s is just less than 50 per cent". On its own, this wouldn't get anywhere near a GCR even if it happened. A ~10% drop in the yield of all agriculture, not just Maize, wouldn't kill a remotely proportionate fraction of humanity, of course. Quick googling leads to a mention of a 40% drop in the availability of wheat in the UK in 1799/1800 (including imports), which led to riots and protests but didn't cause Black Death levels of mass casualties. (Also, following the paper's source, a loss of >20% is rated at 0.1% probability per year) What would its effects be in that case (my original question)? This is where the report uses a combination of expert elicitation and graphical modelling, but can't assign conditional probabilities to any specific events occurring, just point out possible pathways from non-catastrophic direct impacts to catastrophic consequences such as state collapse. Note that this isn't a criticism - I've worked on a project with the same methodology (graphical modelling based on expert elicitation) assessing the causal pathways towards another potential X-risk that involves many interacting factors. These questions are just really hard, and the Chatham house report is at least explicit about how difficult modelling such interactions is. Yeah the IPCC seems to think that there is a lot of scope to use irrigation to adapt to increasing droughts. in Bangladesh >50% of agricultural land is irrigated and people in the fertile crescent at the dawn of agriculture made extensive use of irrigation. I'm still pretty worried about the potential effects on very poor agrarian countries. But this still falls well short of a GCR on any reasonable projection of future changes in agricultural technology. The effect outlined is more like - maybe the price of one staple crop increases by at most 50%. For example, the right hand pane shows the effect on food prices of climate change - up by on the order of 25% than the counterfactual, depending on the socioeconomic scenario. Yeah, between the two papers, the Chatham house paper (and the PNAS paper it linked to, which Lynas also referred to in his interview) seemed like it provided a more plausible route to large scale disaster because it described the potential for sudden supply shocks (most plausibly 10-20% losses to the supply of staple crops, if we stay under 4 degrees of warming) that might only last a year or so but also arrive with under a year of warning. The pessimist argument would be something like: due to the interacting risks and knock-on effects, even though there are mitigations that would deal easily with a supply shock on that scale, like just rapidly increasing irrigation, people won't adopt them in time if the shock is sudden enough, so lots of regions will have to deal with shortfalls way bigger than 10-20% and have large scale hunger. This particular paper has been cited several times by different climate pessimists (particularly ones who are most concerned about knock-on effects of small amounts of warming), so I figured it was worth a closer look. To try and get a sense of what a sudden 10-20% yield loss actually looks like, the paper notes 'climate-induced yield losses of >10% only occur every 15 to 100 y (Table 1). Climate-induced yield losses of >20% are virtually unseen'. The argument would then have to be 'Yes the sudden food supply shocks of 10-20% that happened in the 20th century didn't cause anything close to a GCR, but maybe if we have to deal with one or two each decade, or we hit one at the unprecedented >20% level the systemic shock becomes too big'. Which, again, is basically impossible to judge as an argument. Also, the report finishes by seemingly agreeing with your perspective on what these risks actually consist of (i.e. just price rises and concerning effects on poorer countries): Our results portend rising instability in global grain trade and international grain prices, affecting especially the ∼800 million people living in extreme poverty who are most vulnerable to food price spikes. They also underscore the urgency of investments in breeding for heat tolerance. Update: looks like we are getting a test run of sudden loss of supply of a single crop. The Russia-Ukraine war has led to a 33% drop in the global supply of wheat: https://www.economist.com/finance-and-economics/2022/03/12/war-in-ukraine-will-cripple-global-food-markets Now, I've always been very sceptical of these arguments because they seem to rely on nothing but intuition and go against historical precendent What historical precedent do you have in mind here? The reason my intuitions initially would go in the opposite direction is a case study like invasive species in Australia tl;dr is when an ecosystem has evolved holding certain conditions constant (in this case geographical isolation), and that changes fairly rapidly, even a tiny change like a European rabbit can have negative consequences well beyond what was foreseen by the folks who made the change. I won't pretend to be an expert on how analogous climate is to this example, but if someone wanted to shift my intuitions, a good way to start would be to convince me that, for some given optimistic economic forecast, the likelihood it has missed significant knock-on negative consequences of an X degree average rise in temperature is <50%. As it turns out, there is a near-linear relationship between cumulative emissions and global warming Very interesting. Thanks! What role would potential tipping points like permafrost loss play on the "near-linear relationship between cumulative emissions and global warming"? As far as I understood, there was a lot of uncertainty on that. I had a similar question. I've been reading some sources arguing for strong action on climate change recently, and they tend to emphasise tipping points. My understanding is that the probability of tipping points is also accounted for in the estimates of eq climate sensitivity, and is one of the bigger reasons why the 95% confidence interval is wide. It also seems like if ultimately the best guess relationship is linear, then the expectation is that tipping points aren't decisive (or that negative feedbacks are just as likely as positive feedbacks). Does that seem right? The new models account for potential feedbacks from permafrost carbon. I'm also not especially worried about that feedback or the one from methane clathrates. The world was about 4 degrees warmer a few million years ago, and we didn't get a rapid carbon input from these sources. And the models and basic physics suggest that these would be slow acting multi-centennial scale feedbacks. The Sherwood et al (2020) paper accounts for evidence from the paleoclimate which should in principle pick up some tipping points from the past, though what we are doing now is not a perfect analogue for past climate change in various ways, and paleoclimate proxies are imperfect. Our confidence in the linear relationship between cumulative emissions and warming is lower the higher emissions get. The IPCC is less sure it holds once we get past 1,000 billion tonnes of carbon (on top of the 650 billion tonnes we have already emitted). The Sherwood et al (2020) paper only estimates ECS for up to two doublings of CO2 concentrations, so 1,100ppm. Beyond that, we have less of a clue, especially as CO2 concentrations wouldn't have been that high for tens of millions of years. I am worried about feedbacks if emissions do get that high. Imo, the most worrying thing about climate change is the potential for unexpected surprises, especially from cloud feedbacks, eg here. That is the first time a fast feedback has shown up in the models. But that is something we reach when we get to 1,300ppm, which is probably several centuries away. There is some stuff from the planetary boundaries people arguing that we are on the brink of massive and disastrous tipping points even at 2 degrees, eg this widely cited paper from 2018. That paper fits the planetary boundaries pattern of arguing that there is a potentially significant environmental tipping point close by, on the basis of limited or non-existent evidence and argument. Some more good news: it looks like the US is going to be spending$555B over the next 10 years to combat climate change. Hopefully a decent chunk of this will be spent somewhat effectively.

Yes though I suppose it is still unclear whether they will get it through or not. China is going to spend a fortune on solar and nuclear over the next few decades, which is good.

Thanks for this report, very interesting.  I think that the question on everyone's mind after reading this is: what does this mean for the EA viewpoint on the importance of climate change as a cause area (which is already somewhat controversial)?  Sounds like the two of you disagree and John is working on a report on this, but I'd say that I am quite interested to see this report and both of your views on the subject.

Thanks Dan!

My personal view is that this is one input to a very complex cause prioritization question.

While it (a) certainly reduces the "naïve"  importance of climate somewhat (though mind the fact that this is only about temperature here, it could be that changed views on the badness of different temperatures went the opposite way), (b) it also shows the incredible tractability and cost-effectiveness of a particular policy (technology-specific support and innovation policy) which underlies most of the change in expected emissions and which we can make more likely for sectors and tech that have not enjoyed widespread popularity but still need solving, and (c) the increase in societal resources dedicated to climate also means that advocacy to improve it is potentially more valuable.

So I would say evaluating this question would require a lot more work than this update on expectable distributions of emissions and warming and I don't have strong and stable views on this.

Thanks Johannes for the reply. I agree with you on (a) and (c), but I'm a bit confused on (b). I understand (and for the most part) agree with your view that "technology-specific support and innovation policy" is a very promising route for philanthropic engagement to fight climate change, but I'm struggling to see how this recent shift in climate badness predictions adds additional support for this route of intervention vis-a-vis other mechanisms (rich-country policy advocacy concentrated on reducing domestic emissions, projects that directly reduce emissions in the short term, etc.)

Thanks Dan! Let me clarify. Whether or not that is additional evidence depends on what informs the prior view.

But if one digs deeper on "what are the causes for the change in emissions predictions?" almost all of those are related to technology-specific support policies, not (i) actions that were meant to maximally reduce emissions in the short term (which, in the early 2000s would not have been massive solar subsidies which were primarily motivated by love of solar and hate of nuclear, in the German case, not climate) nor (ii) rich country policy advocacy to reduce emissions domestically.

It is not that our means to reduce emissions in the short-term have improved dramatically (say, we now have cheap credible offsets and those drive lesser expected warming) nor that increased targets / domestic policy ambition is already reflected in those improved scenarios, what has changed is technology cost and that has been almost entirely driven by innovation policies of various kinds (including early-stage deployment policies),  not target-setting or carbon pricing policies (saying this as someone who has worked in carbon pricing for half a decade).

I think it is also evidence for the importance of targeting policy effort, e.g. a lot of the contents of the infrastructure bill are focused on by now relatively mature technologies (accelerating the adoption of electric vehicles by a bit) which might accelerate some emissions reductions in the US but has relatively less spillover, but the more transformative parts of the bill are those that support technologies at earlier stages of development where the trajectory towards clean is not yet locked in. So, it wouldn't be surprising if the most valuable parts of the infrastructure bill are those that do not reduce emissions in the US in the next 5 years, indeed this is what we should expect.

As I understand it, overestimation of sensitivity tails has been understood for a long time, arguably longer than EA has existed, and sources like Wagner & Weitzman were knowably inaccurate even when they were published. Also, as I understand it, although it has gotten more so over time, RCP8.5 has been considered to be much worse than the expected no-policy outcome since the beginning despite often being presented as the expected no-policy outcome. It seems to me that referring to most of the information presented by this post as "news" fails to adequately blame the EA movement and others for not having looked below the surface earlier.

I think you are right that a lot of these points have been around in the scientific literature for a while. What has changed now is that they are definitely mainstream. The Sherwood et al paper has really helped to formalise the findings of the Annan and Hargreaves paper from years ago, and that has all now been recognised by the IPCC. James Annan told me that he did raise the point about priors with Weitzman a while ago but didn't get anywhere.

One thing that has changed in recent years is that whereas the IEA and others used to estimate that RCP6 was the most likely emissions scenario, it looks like RCP4.5 is the most likely scenario, on current policy. And even that may be too pessimistic

Out of curiosity, what are yours (John's) and Johannes' different views on the absolute climate risk and the main reasons behind these differences?

I think it may be a mistake to look at fast progress in renewables and infer that countries will be able to meet their emissions targets without significant difficulty. Ramez Naam writes:

Our hardest climate problems – the ones that are both large and lack obvious solutions – are agriculture (and deforestation – its major side effect) and industry. Together these are 45% of global carbon emissions. And solutions are scarce.

Agriculture and land use account for 24% of all human emissions. That’s nearly as much as electricity, and twice as much all the world’s passenger cars combined.

Industry – steel, cement, and manufacturing – account for 21% of human emissions – one and a half times as much as all the world’s cars, trucks, ships, trains, and planes combined.

Add industry, agriculture, and land use together and you have a very sticky, very difficult-to-improve 45% of carbon emissions.

By contrast, electricity and transportation are 39% of global emissions – nearly as big. The good news is that in electricity and transportation, we have momentum.

We do NOT have momentum in reducing the carbon emissions of industry and agriculture.

Hi, thanks for this comment.  I agree with half of it. We definitely don't have momentum in decarbonising hard to decarbonise sectors like industry and aviation. But the solution there is to make cheap hydrogen, which we can do with super cheap renewables and nuclear hydrogen gigafactories.  Whether we will do this is the great unknown of climate policy. But the IEA estimates account for lack of progress in these sectors, so they don't affect the central point of our piece. Also, some parts of industry are easier to decarbonise. Davis et al (2018) estimate that the hard to decarbonise parts of industry account for 9% of emissions.

On agriculture and land use, the first thing is that I'm not sure I agree on that IPCC assessment of their contribution to emissions. I think that estimate overweights the importance of short-lived pollutants like methane, as discussed here. When we're thinking about tail risks the real problem is CO2 because it is so long-lived. Second, forest area is increasing in temperate regions. Depending on the source, this started in temperate regions in 1990, or perhaps decades earlier. Indeed, some sources argue that once we account for fire management, plantations and replanting, forests hold more carbon now than they did in 1900 - I'm not sure how reliable this study is though. But what all of this shows is that the solution to forest loss is development - things seem to follow a kuznets curve type relationship. As before, this is also accounted for in estimates of likely emissions pathways.

1. "But the IEA estimates account for lack of progress in these sectors, so they don't affect the central point of our piece. "

This is important and possibly a bit confusing in the piece, the 2.5-3 degree world is now the default on fairly pessimistic assumptions about further progress.

2. Also, another thing that the estimates do not reflect are the effects of recent net-zero commitments + uptick in cleantech investment, both public and private. If the current surge in cleantech spending persists, we should expect effects in technologies beyond the usual suspects of wind/solar/electric cars/light bulbs. I think we can probably lock-in a low-carbon trajectory this decade if we are able to (a) maintain/increase momentum on clean energy innovation, (b) make sure resources for energy innovation are better spent, and (c) we manage to avoid massive emissions lock-in of new long-lived infrastructure.

3. I would also say it is not quite correct that we are not making progress on hard-to-decarbonize sectors such as industry, aviation, and agriculture. While these are the sectors to worry about most and those where a trajectory change isn't guaranteed (unlike, say, electrification of light duty transport), Bill Gates + Breakthrough Energy + the tech community more broadly have stepped up their game significantly, also on those technologies (e.g. alternative proteins are arguably booming as a field and Gates & Breakthrough Energy have succeeded in putting those techs a lot more on the mind of the public).

(Sorry for lack of sourcing, this is quick, but I will outline all of those things more publicly and documented fairly soon)

This is a useful post and updated my estimate of the chance of lots of warming (>5 degrees) downwards.

Quick question: Do you have a rough sense of how the different emission scenarios translate into concentration of CO2 in the atmosphere?

The reason I ask is that I had thought there's a pretty good chance that concentrations double compared to preindustrial, which would suggest the long-term temperature rise will be roughly 2 - 5 centigrade with 95% confidence – using the latest estimate of ECS.

However, the estimates in the table are mostly lower than this. Are they lower because:

• Concentrations won't double on these emission scenarios?
• The world will still be warming in 2100, and won't have yet reached equilibrium?
• Something else I'm not understanding?

Hi Ben,

CO2 concentrations on the different shared socioeconomic pathways are shown in Table 5 here. On the most likely scenario - RCP4.5 - CO2 concentrations would double relative to pre-industrial by around 2060.

I think this comes down to the difference between the transient climate response to cumulative emissions and equilibrium climate sensitivity. On the assumption that CO2 concentrations stabilise, ECS tells you the warming you get eventually once the climate system has reached equilibrium (not including ice sheet feedbacks). If CO2 concentrations stabilise, then it would take decades to centuries for the system to reach equilibrium.  Whereas the warming figures in the table is the warming you get at 2100. I have wrestled with trying to convert things to CO2 concentrations and then trying to infer warming from ECS, but it is unnecessary. CO2 concentrations will not stabilise, so the system will never truly be in equilibrium. The TCRE is much more informative.

How cumulative emissions translate into CO2 concentrations is model-dependent. 1 ppm of atmospheric CO2 is equivalent to 2.13 gigatonnes of airborne carbon. However, the amount of carbon that we burn that remains in the atmosphere (the airborne fraction) changes with emissions - the airborne fraction increases the more we emit because land and ocean carbon sinks get exhausted, which you can see in the doughnut charts below.

That all makes sense, thank you!

'RCP2.6 to RCP6'

Sorry if you said this somewhere and I missed it, but is there a well-defined meaning to the numbers here, or are they somewhat arbitrary classifications of increasingly worse scenarios?

hi sasha, RCP is a 'representative concentration pathway'. The number refers to the radiative forcing from GHGs in 2100 measured in watts per square metre. So, on RCP8.5, the extra forcing would be 8.5 watts per square metre.

Do you have thoughts on how potentially rising inflation could affect emission pathways and the relative cost of renweables? I have heard the argument that associated rises in the cost of capital could be pretty bad, because most costs associated with renewables are capital costs, while fuel costs dominate for fossil energy.

Is not the reason that Germany's solar build-out slowed down due to the ramp-down of the subsidy scheme? https://en.wikipedia.org/wiki/Feed-in_tariffs_in_Germany