If we just abide by current national commitments under the Paris Agreement to reduce emissions, researchers have estimated we could have a 50% chance of experiencing warming greater than 3.5 ºC, and a 10% chance of experiencing warming greater than 4.7 ºC, by 2100 (relative to 1850-1900 temperatures).1 Especially at the higher end of this range, there will likely be very significant humanitarian harms, including food and water shortages, large scale displacement of vulnerable populations, and decreased global stability.

There is also a non-negligible chance that we could see larger increases in global temperatures, especially if we do not cut emissions in line with current commitments, in which case harms could be much worse. Especially in the more extreme scenarios, a warming climate may increase the risk of human extinction or civilizational collapse.

Promising options for working on this problem include research into the likely outcomes of higher levels of carbon emissions and strategies for mitigating the worst effects. One can also advocate for strategies to reduce emissions (such as carbon taxes or encouraging low emissions technologies) through careers in politics, think-tanks or journalism, or work as an engineer or scientist to develop technologies that can reduce emissions or their impact.

Our overall view

Sometimes recommended

This is a pressing problem to work on, but you may be able to have an even bigger impact by working on something else.


We think work to materially reduce the probability of the worst outcomes of climate change has the potential for a large positive impact. Extreme climate change could have catastrophic and potentially long-lasting consequences for human civilisation, in part by making other issues worse.


This issue is much less neglected than most other issues we prioritize. Current spending is between $100 billion and $1,000 billion per year. However, we think it makes sense to downgrade that to an effective $10–100 billion per year, because much of this spending i) would have happened for other reasons, ii) is not focused on the extreme risks of climate change, or iii) is poorly allocated.


Making progress on reducing the most extreme risks from climate change seems moderately tractable. Coordination is difficult due to the free-rider problem and political tensions. However, there are many known avenues for progress, and some options — such as increasing energy efficiency — are relatively straightforward.

Profile depth


This is one of many profiles we've written to help people find the most pressing problems they can solve with their careers. Learn more about how we compare different problems, see how we try to score them numerically, and see how this problem compares to the others we've considered so far.

What is our analysis based on?

In writing this profile, we drew mainly on Rogelj et al.’s “Paris Agreement climate proposals need a boost to keep warming well below 2°C” (2016), the International Panel on Climate Change (IPCC)’s 5th assessment “Synthesis” (2014) and “The Physical Science Basis” (2013) reports, sections of Climate Shock: The Economic Consequences of a Hotter Planet, by Wagner and Weitzman (2015), John Halstead’s informal report on climate change and existential risk, Chapters 4 and 6 of Toby Ord’s book The Precipice: Existential Risk and the Future of Humanity (2020) (these last two are the only focused treatments of climate change as a possible existential risk we know about), and Open Philanthropy’s 2013 reports on anthropogenic climate change, extreme risks from climate change and geoengineering research.

What is this problem?

The planet is warming over time as a result of increasing carbon dioxide and other greenhouse gas concentrations in the atmosphere due to the burning of fossil fuels. A warming climate has the potential to cause many serious problems, including food and water shortages, large scale displacement of vulnerable populations, and decreased global stability.

How severe these problems will be is very hard to predict. It depends on how sensitive each problem is to different levels of warming, how much warming will occur with different concentrations of carbon in the atmosphere, and how high emissions will be going forward — all of which are uncertain. There is thus a wide range of possible outcomes. This profile is focused on preventing the worst ones, which correspond to the highest degrees of warming.

Two hundred years ago, the level of carbon dioxide in the atmosphere was 280 parts per million (ppm). It’s now around 415 ppm and rising by 2-3 ppm each year.2

If we adhere to the minimum current agreements for reducing carbon emissions under the 2015 Paris Agreement, research from Rogelj et. al suggests we have a 50% chance of experiencing warming greater than 3.5ºC, and a 10% chance of experiencing warming greater than 4.7ºC, by 2100, relative to 1850-1900 temperatures (temperature estimates will be relativized to this period in what follows unless otherwise noted).

If (improbably) we were then to suddenly cut our emissions to zero, IPCC models imply that our chance of eventually exceeding warming of 6.6ºC would still be uncomfortably high, perhaps around 5%3 (though other researchers think the chance would be considerably lower4).

If we don’t fulfill our promises under the Paris Agreement and we repeal current policies, or if feedback loops in emissions (such as from melting permafrost) are common or severe enough, we could end up in what the IPCC calls a “high emissions” scenario, for which they give a mean estimate of 4.3ºC warming by 2100, and a chance of exceeding 5.4ºC by 2100 between 10% and 34%.5

If we stayed on such a track, the mean estimate for warming in 2200 is close to 6.6ºC (again relative to 1850-1900), and the 95% confidence interval comfortably includes 9.6ºC. (Though note that the change would happen gradually, making it easier to adapt.)

Screen Shot 2020-05-08 at 12.48.24 PM
Modeled temperature change for the IPCC’s “high emissions scenario” (red), and a “stringent mitigation scenario” (blue). Solid line represent the multi-model mean. Shading represents the 5% – 95% range across models. From the IPCC Synthesis report, p. 59.

If we eventually opted to burn all the fossil fuels on the planet (and didn’t use negative emissions or geoengineering technologies), average temperatures would rise much more than any of these estimates — perhaps eventually yielding warming in the mid teens.6

It’s worth emphasizing that we have little understanding of the likely effects of these higher temperature increases, as most research focuses on the effects of warming at 4ºC and below. (Find some exceptions in this footnote7.)

We are also far from sure that our climate models are correct, especially when it comes to the higher end of possible emissions. So there may be a larger chance than the figures above suggest that, even if everyone fulfills their promises under the Paris Agreement, we will eventually see a high degree of warming.

More extreme scenarios (say, warming of 6ºC or higher) would likely have very serious negative consequences. Sea levels would rise, crop yields could fall significantly, and there would likely be large water shortages. If we fail to adapt, hundreds of millions of people could die from shortages, conflict, or increased vulnerability to diseases, and billions of people could be displaced.

This profile is focused on these ‘tail’ risks — the chance that the planet will experience extreme warming. Though unlikely, the chance of warming over 6ºC still seems uncomfortably high.8 We focus on this possibility, rather than the issue of climate change in general, because preventing the most extreme levels of warming helps prevent the worst possible outcomes. Expected harms from an extra degree increase in temperature are generally assumed to be worse at higher temperatures — going from 3ºC to 4ºC hotter is likely worse than going from 2ºC to 3ºC — so it seems even more important to prevent each additional increase.9

That said, the more likely scenarios — including warming of 2ºC, 3ºC, or 4 ºC, are still a big problem, and the tail risks and the more likely scenarios are also intimately related. In particular: most approaches for reducing the chance of extreme warming will also reduce the severity of more moderate warming, and vice versa.

Is the possibility of extreme climate change an existential risk?

Although there are many important global issues that deserve more attention in society, we’ve argued that when choosing what problem to work on, it often makes sense to prioritize problems that may give rise to existential risks — meaning they could lead to the extinction of humanity or could permanently and drastically curtail its potential.

Why? In short: the reason is longtermism — the view that the very long-term consequences (e.g. in the next thousand or hundred thousand years) of actions often determine what we should do. We think the easiest-to-prevent bad long-term outcome is human extinction, so we think it makes sense to prioritize spending marginal resources on fixing problems that pose existential risks. (Read more.)

Does extreme climate change pose an existential risk?

Extreme climate change could increase existential risk if

  • It could make Earth so hot that it becomes uninhabitable for human life;10
  • It acts as a ‘risk factor’, increasing other existential risks.

Climate change’s role as a risk factor seems to us to be the main way it contributes to total existential risk.

Could extreme climate change make Earth uninhabitable?

Toby Ord puts the chance of climate change making Earth uninhabitable for the human species around 0.1%.11 We roughly agree — it seems very unlikely, but not impossible.

Large temperature increases could make large swaths of the Earth too hot for people to live. Humans (along with many other mammals) can suffer hyperthermia above 35ºC, a temperature that would be common in some parts of the world if we experienced warming over 7ºC. An average temperature rise of 12ºC might make it impossible for people to survive outdoors for extended periods in the majority of the regions humans currently occupy. 12

However, such an increase in temperature would most likely take place over hundreds of years, giving people time to migrate to cooler climates. Even if people would be unable to migrate, many regions would likely remain habitable for human life.

Is it possible for the world to warm much more than 12ºC? It does seem possible. The highest prediction we’ve seen made is an expected rise of ~13.6ºC by 2300 in the scenario in which we burn 5,000 Gt. C of fossil fuels — a conservative estimate of the total amount of fossil fuels on the planet. But this estimate is very uncertain, and there may be up to 13,600 Gt. C in fossil fuel stores13, in which case if we burn it all we would presumably see much higher levels of warming.

That said, it seems very unlikely (though not impossible) that we would continue extracting and burning fossil fuels over the centuries required to reach such temperatures in a situation in which a progressively greater majority of the population is dying due to increasing temperatures.

But this is just if emissions are higher than expected. What if our models are wrong, or there are feedback loops that lead to more rapid or much more extreme and irreversible temperature increases?

This is possible. However, most known feedback effects seem to be relatively minor compared to direct temperature increases due to anthropogenic emissions.14

The most likely known candidates for really extreme feedback loops seem to be “cloud-based tipping points”, where reaching an estimated 1,200 ppm CO2 could cause clouds to ‘burn off’ and expose the Earth to more sunlight — resulting in a projected comparatively rapid temperature increase of 8ºC on top of warming due to the greenhouse effect. So although the feasibility of these scenarios is extremely uncertain, seeing warming up to 20ºC or even more, some of which could happen relatively quickly, is not completely out of the question. Because these scenarios would be so damaging, they are worth thinking about despite being so unlikely.

Even in extreme feedback scenarios, however, we would still probably survive as a species. The highest levels of warming would still leave parts of the Earth habitable, and (assuming society does not collapse – a possibility we will return to in a moment) technological advances would help us adapt to our more hostile environment.

That said, there may be “unknown unknowns” here. How likely extreme feedback loops are, what might happen if they were triggered, and how fast this could all happen are questions on which we would like to see much more research.15

Does extreme climate change increase other existential risks?

The short answer is: probably.

In general, because humanity is currently adapted to certain climatic conditions, changing those conditions comparatively rapidly can be expected to act as a ‘stressor,’ causing tensions, reducing economic growth, and making it harder to handle other issues.

There seem to be many different, complex ways that climate change could contribute to our overall risk of extinction in the coming centuries, though the mechanisms are speculative.16 We will briefly mention a couple of possibilities.

It seems like climate change could contribute to effects like food or water shortages, displacement, or dissatisfaction with governments — which might in turn increase the risk of national or international conflict. The evidence for climate change increasing conflict is somewhat mixed, and often draws from studies of more agrarian societies, which might be more easily affected by climate.17 Still, especially for higher temperature increases (which are also understudied) some such effect seems plausible, since scarcity and stress tends to breed conflict.

In general, conflict seems to increase existential risk, especially when it is between great powers. This is in part because governments in conflict are less cooperative and more often willing to sacrifice safety measures to grow their militaries, capabilities, and economies as fast as possible. For example, the biggest leap forward in our ability to cause destruction — the development of the atom bomb — happened in a war context.

Especially in an economically and ecologically stressed context, great power conflict could also lead to societal collapse, for instance if enough economic, legal, and technological infrastructure is destroyed. At this point, world population might dwindle, and the world’s technological and cooperative capacities might be arrested for a long time. If this goes on long enough, especially if extreme climate change means we are living in a more hostile environment, we could perhaps be wiped out by petty warfare, famine, or disease.

Another possibility is that extreme climate change could threaten so much damage that people are tempted to employ a risky and untested technology to address it — such as some forms of geoengineering. Such technological solutions might conceivably themselves pose existential risks.18

Finally, there is an intuitive argument that a world in which the threat of climate change is greatly reduced just seems like it would be a safer world — perhaps in virtue of greater economic growth, more focus on managing other risks, greater stability, or all of the above. This suggests that climate change is somehow contributing to existential risk, perhaps through various indirect means.

Overall it does seem like climate change probably increases other existential risks and contributes to total risk, though it’s unclear how (or by how much). Importantly, it seems like the risks are again much greater the more extreme the warming is, since smaller increases in temperature will be easier to adapt to.

The analysis here is highly speculative, as not much research has been done on the subject. A few resources that do discuss climate change and existential risk — some of which present reasons to be more pessimistic — can be found below.

Arguments for working on this problem

Climate change is an important problem

Climate change is likely to have substantial negative effects, and, as discussed above, has a small chance of causing extremely bad effects (perhaps even extinction) if we continue to leave it mostly unchecked. It’s clear that much more work and funding should be going towards fighting climate change.

We want to be a bit cautious about this consideration, as we think many issues are important and need more attention. The question we want to help you answer is whether this is the best problem for you to focus on in your career. That means you’ll want to compare this issue (and your ability to make progress on it) with others, as discussed to some extent below.

You can be confident that climate change will affect a large number of people.

If you’re able to contribute to mitigating climate change, your chance of making a positive difference is therefore high. The same argument goes for mitigating risks of extreme climate change, since many of the interventions are similar. And in some scenarios — especially if you can get into a position of influence over government policy or another high leverage position — you might be able to make a large difference. For instance, a climate scientist who discovered a potential extreme feedback loop and allowed us to avoid triggering it would be making a large difference.

There’s a lot of opportunity.

Public consensus is that climate change is one of the most important issues of the 21st century, and there are therefore many opportunities to work on the issue in government, business, and academia. This also means that if you are able to get into a position of power, you can leverage a lot of resources. (Although this also means the issue is less neglected, as we discuss below, so it’s not an unalloyed argument for working on it.)

Some important work is relatively neglected.

Although climate change as a whole gets a lot of attention, not very much of this is focussed on research into the likelihood of extreme climate change scenarios or the feasibility and risks of geoengineering.19 Moreover, some potentially promising solutions — like scaling up nuclear energy — currently seem relatively neglected.

Your work might have positive side effects.

Because climate change threatens to act as a general “stressor” on society, mitigating it might have positive effects on other areas, such as reducing the chance of great power conflict. Reducing our reliance on burning fuels might also reduce air pollution, which causes millions of deaths per year. Moreover, working on extreme climate change could indirectly help promote positive values, such as caring about future generations, and it’s possible that finding effective ways of mitigating climate change could serve as a blueprint for future efforts to tackle global threats.

However, this consideration is not as strong as it might at first appear. This is because working on other problems — including reducing the chance of great power conflict, improving institutional decisionmaking, and voting reform — might also indirectly mitigate climate change by helping people cooperate to address it (as well as help with other issues).

Arguments against working on this problem

Other existential risks seem considerably greater.

The main argument against working on risks of extreme climate change is that there are other problems that seem like they pose considerably larger threats to humanity. People who study risks of human extinction usually think nuclear war, great power conflict in general, and certain dangerous advances in machine learning or biotechnology all have a higher likelihood of causing human extinction than climate change.

This seems roughly right to us. While climate change is likely to be very harmful, for reasons described above it is hard to see how it can make the whole Earth uninhabitable. Moreover, although we would guess that climate change increases other existential risks to some extent (such as those from great power conflicts), these might be best addressed by working to reduce those other risks more directly rather than working on climate change. So all else equal we’d usually advise people to work on those issues instead.

That said, this consideration only applies if you’re a good fit for working on one of these other threats or another more promising area. We think there are many promising opportunities to work on these issues (see our ‘priority paths’). But if you are particularly well-suited to working on climate change (for instance, you’re a climate scientist) or your alternatives would be to work on a less pressing issue, it could well be your highest-impact path.

The area is somewhat crowded.

Climate change as a whole gets a lot of attention and funding. In particular, it gets much more attention than many other pressing global issues. In 2015, the US government spent about $8 billion per year on direct climate change efforts20 and more on regulations designed to limit carbon emissions (although some of those regulations are now being rolled back.) The UK spends about £1 billion per year on climate change projects in developing countries21 and several hundred million dollars are spent each year by foundations.22 On top of this many businesses and universities around the world work on general climate change research or technologies designed to reduce emissions. The Climate Policy Initiative counted almost $400b in climate change related spending in 2015.

Although as we said only a small amount of this effort focuses on the extreme risks from climate change, reductions in greenhouse emissions disproportionately reduce the risk of extreme temperature increases. Therefore most of the existing funding directed at climate change is going to a quite reasonable strategy for working on the extreme risks from climate change, and so you might think that this problem is already receiving quite a lot of resources — at least relative to many other, less well-known issues. In general, a problem being less neglected suggests it will be harder for an additional person to make as much of a difference working on it.

Key judgement calls you need to make

You are more likely to think this is one of the most pressing problems for you to work on if:

  • You think that we have obligations to people who do not yet exist, in addition to people who currently exist.
  • You think that there is great value in ensuring that human civilization continues in the long term.
  • You think action on climate change has a substantial chance of having a large impact on the long-term future, either directly or indirectly (e.g. through its effects on other issues, such as great power wars).
  • You think climate change is one of the larger contributors to existential risk.
  • You have relevant experience that makes you much better equipped to work on climate change than other issues.
  • You want to work on a problem that most people agree is pressing.
  • You think there is great value to preserving the Earth’s ecosystems and biodiversity.
  • You are comfortable with pursuing high-risk, high-reward methods for improving the world, such as social advocacy, lobbying governments, and speculative research.
  • Relatedly, you are comfortable with working on highly uncertain problems – where you are reducing the chance of low-probability, very bad outcomes.

What approaches exist for solving this problem?

In general, we most want to see people working on currently neglected strategies to mitigate climate change, especially if these strategies are targeted toward reducing the chances that the most extreme negative potential consequences of climate change will come to fruition.

Reducing net greenhouse gas emissions

Reducing greenhouse gas emissions disproportionately reduces the risk of extreme temperature increases relative to median temperature increases — so the best ways of reducing emissions may also be among the most effective ways to mitigate extreme risks.23

Promising approaches to reducing emissions include:

Read more about high leverage strategies for reducing emissions.

Research into geoengineering

‘Geoengineering’ refers to large-scale interventions in the Earth’s climatic system with the aim of limiting climate change. Geoengineering may become more attractive to governments in the future if large temperature increases occur. Research we do now about its feasibility, likely side-effects, risks and optimal governance could help future policymakers make more informed decisions about whether to use geoengineering if they face extreme climate change. However, the concern has also been raised that continued investment in geoengineering research may also cause less investment in other mitigation and adaptation strategies.

Listen to our podcast episode with Kelly Wanser on whether to deliberately intervene in the climate and check out the resources in the show notes to learn more.

Research on extreme risks from climate change

More climate studies research could better inform policymakers about the likelihood of the extreme risks of climate change as well as strategies to reduce those risks. There are large gaps in our knowledge about the range of possible outcomes of higher levels of warming, as well as the likelihood and magnitude of potential feedback effects. It may also be high-impact to research and develop adaptation strategies for extreme climate change scenarios.

What skill sets and resources are most needed?

  • Researchers with expertise in climate science or how to coordinate countries to reduce emissions.
  • Engineers who can develop new clean technologies, negative emissions technologies, or infrastructure around alternative energy sources.
  • Policymakers, activists, and lobbyists who can push through policies to reduce greenhouse gas emissions.

Who is already working on this problem?

What can you concretely do to help?

  • Do a PhD in climate science and investigate the likelihood of extreme climate change, or its possible physical effects, in academia. (See our profile on academic research.)
  • Do graduate study in economics, public policy, or a related field, and try to determine the best policy-related solutions to extreme climate change, or investigate the likelihood and nature of the worst possible economic and political effects of climate change.
  • Work in policy, academia, or nonprofits to develop strategies for mitigating the worst economic and political impacts of climate change, or to help society develop resilience in case extreme climate change does happen.
  • Work at ClimateWorks, or other foundations focused on climate change to direct funding and other resources where they can do the most good.
  • Get into positions where you can advocate for climate change mitigation policies and legislation, for example by going into national politics, journalism or think-tanks.
  • If you’re an engineer or scientist, work in R&D for developing lower emissions technologies. See some suggestions for how to do that here.
  • If you want to contribute but none of these paths are for you, you can still help by donating to (or trying to work) for effective organizations working on climate change. Three organizations we’ve seen highlighted as particularly effective are the Clean Air Task Force, Carbon 180, and TerraPraxis.

Learn more

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Notes and references

  1. ‘Current national commitments’ here refers to Nationally Determined Contributions (NDCs) promised as part of the 2015 Paris Agreement. Rogeli et. al. (2016) estimate that if all countries fulfill their NDCs, but they do not grow more aggressive over time, we have a 90% chance of not exceeding a rise of 4.7 ºC, and a 50% chance of not exceeding a rise of 3.5 ºC (relative to 1850 – 1900) by 2100. These estimates are based on pessimistic assumptions about the scenarios involved, including that the conditions on states’ conditional NDCs are not met. See table 1 on p. 4 for the full range of projections.

  2. According to the US’s National Oceanic and Atmospheric Administration, atmospheric CO2 levels in May 2019 were just under 415 ppm, representing a 3.5 ppm increase from the year before. A number of sources on the future of climate change, like Wagner and Weitzman’s Climate Shock use a 2 ppm/year figure.

  3. On p. 54 of Wagner and Weitzman’s Climate Shock they give a range of probabilities for eventually exceeding an increase of 6 ºC relative to 1981-2005 on the basis of various CO2-equivalent concentrations of greenhouse gases, based on IPCC models. According to NASA’s climate website, in the period of 1981-2005 the atmospheric temperature was about 0.6 ºC warmer than 1850-1900, so making all temperature rises relative to 1850-1900 we get a range of probabilities for a 6.6 ºC eventual increase. Unfortunately, Wagner and Weitzman do not give probabilities of this increase for the scenario we are discussing, in which nations fulfill their minimal NDCs; and Rogeli et. al. (2016) do not give estimates of CO2-equivalent concentrations for the scenarios they study. Therefore it’s not straightforward to line the predictions up with one another. However, Rogeli et. al. do give estimates of cumulative CO2 emissions for their scenarios. They give a mean estimate of 2,800 gigatons of C02 cumulative emissions for 2011-2100 for the scenario we are discussing. This is pretty close to the IPCC’s RCP4.5, for which the mean estimate is 2860 Gt CO2. The IPCC gives a mean estimate of 625 ppm CO2 eq by 2100 for RCP4.5. Wagner and Weitzman’s analysis yields a 5% chance of an eventual 6.6 ºC rise for 600 ppm CO2-equivalent, and an 8% chance for 650 ppm.

  4. In particular we have in mind John Halstead’s blog post “How Hot Will it Get?”, which argues that the models underlying Wagner and Weitzman’s conclusions (which are from the IPCC’s report) result in too high a prediction of large temperature increases due to incorrectly estimating climate sensitivity. Halstead’s post relies on Annan and Hargreaves’ “On the generation and interpretation of probabilistic estimates of climate sensitivity” (2009).

  5. According to table SMP.2 on p. 23 of the IPCC’s AR5 report The Physical Science Basis, the mean estimate for warming by 2100 for the IPCC’s “high emissions” scenario (RCP 8.5) is 3.7 relative to 1986-2005. The “likely” range is 2.6 – 4.8 relative to 1985-2005. The way the IPCC uses the word “likely” it means 66-90%. (They say it means >66% but they use “very likely” for >90% — see fn. 2 for their explanation of this terminology.) This suggests they think there’s a 10-34% chance of warming greater than 4.8 degrees. Relativized to 1850-1900 (again using NASA’s historical temperature data) this yields a 10-34% chance of exceeding 5.4 ºC by 2100, and a mean estimate of 4.3.

  6. See Tokarska et al. (2016) for a discussion of what would happen if we burned 5 trillion tonnes of carbon. Note that 5 trillion tons is a conservative estimate of the amount of carbon left on Earth.

  7. Exceptions include “Climate Change: a Risk Assessment” from the Centre for Science and Policy at Cambridge, as well as research from helix and Impressions which look at warming scenarios up to 6 ºC. The Cambridge report suggests a lack of mainstream work on scenarios of warming over 4 ºC in their section “Knowing the least about what matters most”, pp. 45-46. (Note 4 ºC can mean somewhat different things depending on when the temperature change is relative to, as mean global temperatures have risen about 1.15 4 ºC since 1850.) Find more sources in footnote 15 below.

  8. It’s very hard to say how high, but my (Arden’s) rough guess would be somewhere between 1% and 15%. Following the reasoning in footnote 3 above, Wagner and Weitzman’s analysis seems to imply a 5%-8% chance of 6 ºC eventual warming (relative to 1981-2005, so ~6.6 ºC relative to 1950-1900) if we do the minimum we’ve promised to do and then suddenly stop emitting at 2100. Putting aside the time after 2100 for a moment: we might not meet our promises of today, but we might do more — I’d guess the latter is actually more likely over the next 80 years as damages from climate change build political pressure for more mitigation. But then there’s the period after 2100: though I’m optimistic about negative emissions technologies I’d expect us to net positive emissions next century and perhaps the century after that (though it’s obviously hard to say what will happen in such a long time). So I might roughly double the chance of greater than 6 degrees eventual warming to ~10-15%. However, the critique of Wagner and Watzman John Halstead makes in How hot will it get? makes me think a lower bound of 10% might be too high. Halstead concludes that under plausible assumptions if we do nothing to mitigate climate change we have <1% chance of >6 degrees warming eventually. I don’t want to put too much weight on Halstead’s analysis given that it hasn’t received the scrutiny of the broader climate science community, but it does make me think the chance could be quite low. So 1-15% seems like a reasonable range to me.

  9. According to “GHG Targets as Insurance Against Catastrophic Climate Damages” (2012) and “Inside the Integrated Assessment Models: Four Issues in Climate Economics” (2008), the majority of models of climate damages assume that damages will be a function of the square of temperature increase. The authors of the latter paper suggest that a cubic function may also be possible.

  10. This section focuses on heat stress because it seems like the most plausible mechanism by which climate change could more or less directly cause human extinction. For discussion of effects on oceans, agriculture, and biodiversity see the sections on climate change in Founders Pledge’s report on existential risk. The report argues persuasively that these are not possible mechanisms of human extinction.

  11. See Table 6.1 on p. 167 of The Precipice.

  12. See “An Adaptability Limit to Climate Change due to Heat Stress” (2010). It’s a bit unclear from the text, but the cited temperature changes appear to be relative to the mid 19th century.

  13. See again Tokarska et al. (2016), p. 852, and The Precipice, ch. 4, p. 108. The warming estimate of 12 ºC is relative to 1986-2005, which is equivalent to an estimate of ~12.6 relative to 1850-1900 by the same method used in footnotes 3 and 5.

  14. According to Section 4 of Halstead’s informal writeup, which interprets Section 12.5.5 of the IPCC’s 5th assessment Working Group 1 Report. See also pp. 106-108 of The Precipice for a summary of our knowledge (or rather, lack of knowledge) about potential feedback mechanisms.

  15. This assessment roughly agrees with David Wallace-Wells’ book on the subject, The Uninhabitable Earth: A Story of the Future. Despite the title, Wallace-Wells concludes that there is most reason to think the Earth will not be made uninhabitable by climate change, though parts of the Earth could be.

  16. Find a list of possible mechanisms in the appendix (Section D) of Climate, conflict, and social stability: what does the evidence say? (2014).

  17. The IPCC’s 2014 “Impact Report” (ch. 12) finds the impact of climate change on international security to be highly uncertain, but suggests it’s likely to be a stressor. A study of expert opinion on the subject discusses a number of factors and reports a perceived weak positive effect of warming on the risk of armed conflict.

  18. Read about the risks and benefits of one such technology — solar radiation management — in Sections 1.1.2 and 2.1 of the Future of Humanity Institute’s 2017 report “Existential Risk: Diplomacy and Governance”. See also John Halstead’s –“Stratospheric Aerosol Injection Research and Existential Risk” (2018).

  19. About $11 million is spent per year on Geoengineering research.
    “Our total tally of funding for such projects (for which we have funding information) amounts to about $11 million/year.” Source

  20. “…U.S. federal government reports seem to indicate that they spend roughly $8 billion a year on climate change efforts, mostly (~$6 billion/year) on developing technology to reduce emissions.” Open Philanthropy: Anthropogenic Climate Change

  21. “The Government will maintain its effort on urgent action to tackle climate change by supporting low-carbon growth and adaptation in developing countries. The UK’s pool of money for climate change projects in developing countries will be increased to £969 million, funded by DFID, the Department of Energy and Climate Change (DECC) and the Department for Environment, Food and Rural Affairs (DEFRA).” HM Treasury: Spending Round 2013 Archive

  22. “Based on these reports and an off-the-record conversation, we would estimate that typical annual philanthropic spending on climate change mitigation in the U.S. is likely in the range of a few hundred million dollars.” Open Philanthropy: Anthropogenic Climate Change

  23. Givewell: “Reductions in greenhouse gas emissions are expected to disproportionately reduce the risk of extreme temperature increases and extreme impacts (relative to how much they reduce median estimated temperature changes), so mainstream interventions to reduce the negative impacts of climate change in general (discussed on our page on anthropogenic climate change) may be the most effective strategy for addressing extreme risks.”