Climate Change & Longtermism: new book-length report

Since 2021, as part of the research for What We Owe the Future, I have been working on a report on climate change from a longtermist perspective. The report aims to provide the most complete treatment of that question yet produced. The executive summary is below and the full report is here, available at the What We Owe the Future supplementary materials webpage. I am grateful to the expert reviewers of the report for their comments. Views and mistakes are my own.


Executive Summary

In this report, I will evaluate the scale of climate change from a longtermist point of view. Longtermism is the idea that influencing the long-term future, thousands of years into the future and beyond, is a key moral priority of our time.

In economics, longtermism is embodied by the idea that we should have a zero rate of ‘pure time preference’: we should not discount the welfare of future people merely because it is in the future. Economists who embrace a zero rate of pure time preference will tend to favour more aggressive climate policy than those who discount future benefits.

Climate change is a proof of concept of longtermism. Every time we drive, fly, or flick a light switch, each of us causes CO2 to be released into the atmosphere. This changes the amount of CO2 that is in the atmosphere for a very long time: unless we suck the CO2 out of the atmosphere ourselves, concentrations only fall back to natural levels after hundreds of thousands of years. The chart below shows long-term CO2 concentrations after different amounts of cumulative carbon emissions.

Source: N. S. Lord et al., ‘An Impulse Response Function for the “Long Tail” of Excess Atmospheric CO2 in an Earth System Model’, Global Biogeochemical Cycles 30, no. 1 (2016): 2–17, https://​​​​10.1002/​​2014GB005074.

Some of the ecological effects of climate change get worse over time. The clearest example of this is sea level rise. On current policy, the most likely sea level rise this century is 75cm. However, over 10,000 years, sea levels will rise by 10 metres. Over the long-term, the world will look very different.

From a longtermist point of view, it is especially important to avoid outcomes that could have persistent and significant effects. These include events like human extinction, societal collapse, a permanent negative change in human values, or prolonged economic stagnation. If we go extinct, then that would be the end of the human story, and there would be no future generations at all. If civilisation collapses permanently, then future generations will be left much worse off than they could have been, living lives full of suffering rather than ones of flourishing.

The anatomy of climate risk

The overall size of climate risk depends on the following factors:

  1. Greenhouse gas emissions

  2. The climate change we get from different levels of emissions

  3. The impacts of different levels of climate change

There is uncertainty about all three factors. The main findings of this report are as follows.

Emissions are likely to be lower than once thought

Due to recent progress on clean technology and climate policy, we look likely to avoid the worst-case emissions scenario, known in the literature as ‘RCP8.5’. The most likely scenario on current policy is now the medium-low emissions pathway known as ‘RCP4.5’. Moreover, climate policy is likely to strengthen in the future. For instance, as I was writing this report, the US Senate passed the Inflation Reduction Act, the most significant piece of climate legislation in American history.

Source: Hausfather and Peters, ‘Emissions – the ‘business as usual’ story is misleading’, Nature, 2020.

Climate change is a great illustration of how society can make progress on a problem if enough people are motivated to solve it. This does not mean that climate change is solved, but there is significant momentum, and we are at least now moving in the right direction.

The amount of carbon we could burn in a worst-case scenario is also much lower than once thought. Some of the literature assumes that there are 5 or even 10 trillion tonnes of carbon remaining in fossil fuels, mostly in the form of coal. However, these estimates fail to recognise that not all fossil fuels resources are recoverable. Estimates of recoverable fossil fuels range from 1 to 3 trillion tonnes of carbon.

It is difficult to come up with plausible scenarios on which we burn all of the recoverable fossil fuels. Doing so would require (1) significant improvements in advanced coal extraction technology which is not part of the energy conversation today, but (2) a dramatic slowdown in progress in low carbon technologies that are already getting substantial policy support.

Warming is likely to be lower than once thought

Warming will likely be lower than once feared, in part because of lower emissions and in part because the scientific community has reduced uncertainty about climate sensitivity. Where once current policy seemed likely to imply 4ºC of warming above pre-industrial levels, now the most likely level of warming is around 2.7ºC, and the chance of 4ºC is around 5%. Moreover, where once there seemed to be a >10% chance of 6ºC on current policy, the risk now seems to be well below 1%.

On a worst-case scenario in which we burn all of the fossil fuels, the most likely level of warming is 7ºC, and there is a 1 in 6 chance of more than 9.5ºC.

Climate change will disproportionately harm the worst-off

The climate impacts literature suggests that climate change will impose disproportionate costs on countries at low latitude, which are disproportionately low- and middle-income and have done the least to contribute to climate change. People in Asia will have to deal with increasing flooding due to rising sea levels. Climate change will damage agricultural output, and cause droughts in countries reliant on rainfed agriculture. People in the tropics will face rising levels of heat stress. Fossil fuels also kill millions of people from air pollution in both poor and rich countries.

Many low- and middle-income countries have essentially never experienced sustained improvements in living standards, and a significant fraction may be left worse-off than today due to climate change. This undermines one common argument for discounting the future costs of climate change—that future generations will be richer and so better able to adapt to the effects of climate change.

We have a clear moral responsibility not to impose this harm, to reduce emissions, and to encourage economic development in poorer countries.

Average living standards will probably continue to rise

Climate-economy models confirm that the costs of climate change will fall disproportionately on poorer people, but almost all models also suggest that global average living standards in the future will be higher than today, on plausible levels of warming. Income per person looks set to increase by several hundred percent by the end of the century, notwithstanding the effects of climate change.

‘Bottom-up’ climate-economy models included in the IPCC’s Sixth Assessment Report that add up the effects of climate impacts in different sectors and plug them into modern economic models suggest that warming of 4ºC would do damage equivalent to reducing global GDP by around 5%. One recent model, Takakura et al (2019), includes the following impacts:

  • Fluvial flooding

  • Coastal inundation

  • Agriculture

  • Undernourishment

  • Heat-related excess mortality

  • Cooling/​heating demand

  • Occupational-health costs

  • Hydroelectric generation capacity

  • Thermal power generation capacity

For instance, in agriculture, the message from the climate impacts literature is that although climate change will damage food production, average food consumption per person will be higher than today, even for 4ºC of warming, due to progress in agricultural productivity and technology. This is illustrated on the chart below from van Dijk et al (2021), which shows per capita food consumption on different socioeconomic pathways.

Source: Michiel van Dijk et al., ‘A Meta-Analysis of Projected Global Food Demand and Population at Risk of Hunger for the Period 2010–2050’, Nature Food 2, no. 7 (July 2021): 494–501, https://​​​​10.1038/​​s43016-021-00322-9.

I have previously been critical of climate-economy models, but now believe they are more reliable than they once were. Until recently, a key determinant of aggregate impact assessments was how to model the effects of >4.4ºC because the chance of that level of warming was so high. Estimates that models arrived at were unmotivated and arbitrary in part because the literature on the impacts of >4.4ºC was sparse. However, warming of >4.4ºC now seems increasingly unlikely (<1% given likely trends in policy), and there is a rich and voluminous literature on the impact of warming up to 4.4.ºC. This makes recent bottom-up models more reliable.

However, even the best bottom-up climate-economy models underestimate the costs of climate change because they do not account for some important direct costs:

  • They do not include tipping points

  • They do not explicitly model the potential effects of climate change on economic growth and technological progress

It is unclear how much these factors would increase the overall direct costs of climate change; that is an important area of future research for climate economics. However, for levels of warming that now seem plausible, these effects seem unlikely to be large enough to outweigh countervailing improvements in average living standards.

Bottom-up climate-economy models also do not account for indirect effects, such as conflict, which I discuss below.

‘Top-down’ climate-economy models try to directly measure the effects of climate change on aggregate economic output, and some of these find much higher impacts from climate change, on the order of a 25% reduction in GDP for 4ºC warming. However, these results are highly model-dependent, rely on questionable econometric assumptions, and exclude several important climate impacts. In my view, the best bottom-up studies are a more reliable guide, notwithstanding their flaws.

Although average living standards are likely to continue to rise, we also need to consider the possibility of societal collapse for other reasons, such as a pandemic or nuclear war. If there were to be a major global catastrophe, then future living standards may not actually be higher than today. Future generations trying to rebuild society would have to do so in a less hospitable climate.

Some tipping points could have very bad effects

In my view, the most concerning tipping points highlighted in the literature are rapid cloud feedbacks, collapse of the Atlantic Meridional Overturning Circulation and collapse of the West Antarctic Ice Sheet.

Some models suggest that if CO2 concentrations pass 1,200ppm (compared to 415ppm today), cloud feedbacks could cause 8ºC of additional warming over the course of years to decades, on top of the 5ºC we would already have experienced. The impacts of this sort of extreme warming have not been studied, but it seems plausible that hundreds of millions of people would die. Moreover, people would be stuck with an extreme greenhouse world for millennia. This would extend the ‘time of perils’: the period in which we have the technology to destroy ourselves, but lack the political institutions necessary to manage that technology. It would also make it much harder to recover from a civilisational collapse caused by something else (such as a pandemic or nuclear war). However, given progress on emissions, it is now difficult to come up with plausible scenarios on which CO2 concentrations rise to 1,200ppm.

Collapse of the Atlantic Meridional Overturning Circulation would cause cooling and drying around the North Atlantic, and more importantly would probably weaken the Indian monsoons and the West African monsoons, with potentially dire humanitarian implications. For 4ºC, models suggest that the chance of collapse is 1-5%, though they probably understate the risk.

There is deep uncertainty about potential sea level rise once warming passes 3ºC. For higher levels of warming, there is a risk of non-linear tipping points, such as collapse of the West Antarctic Ice Sheet, which would cause sea levels to rise by around 5 metres over 100 years, which would probably cause flooding of numerous highly populated cities, especially in Asia.

Due to progress on emissions, these tipping points now look less likely than they did ten years ago, but their expected costs (impact weighted by probability) may still be large. Furthermore, our understanding of the climate system is imperfect, and there may be other damaging tipping points that we do not yet know about.

All this being said, contra some prominent research, the evidence from models and the paleoclimate (the deep climate history of the Earth) suggests that it is not the case that, once warming passes 2ºC-4ºC, runaway feedback loops will kick in that make the world uninhabitable.

Direct impacts fall well short of human extinction

Given progress in emissions, the risk of human extinction from the direct effects of climate change now seems extremely small. The most plausible route to human extinction is via runaway feedback loops. However, models and evidence from the paleoclimate suggest that it is impossible to trigger such runaway effects with fossil fuel burning. Models suggest that we could only trigger a runaway greenhouse if CO2 concentrations pass 3,000ppm (at the very least), which is out of reach on revised estimates of recoverable fossil fuels.

Moreover, global average temperatures have been upwards of 17ºC higher several times in the past without triggering runaway feedback loops that killed all life on Earth. Indeed, since the Cretaceous, 145 million years ago, periods of high temperatures and/​or rapid warming have not been associated with ecological disaster. However, prior to the Cretaceous, climate change was linked to ecological disaster. In the report, I discuss the theory that this was because of ecological and geographical factors unique to the pre-Cretaceous period.

I construct several models of the direct extinction risk from climate change but struggle to get the risk above 1 in 100,000 over all time.

One argument that climate change could directly cause civilisational collapse is that it could be a contributing factor (along with deforestation, human predation, and pollution) to ecosystem collapse, which could in turn cause the collapse of global agriculture. I argue in the main report that this risk is minimal.

Indirect risks are under-researched but now seem fairly low

Because interstate war has become increasingly rare since the end of World War II, most of the literature on climate change and conflict has focused on the connection between climate and civil conflict: conflicts between a government and its citizens in which more than 25 people are killed.

Scholars in the field agree that, so far, climate-related factors have been a much weaker driver of civil conflict than other factors such as socioeconomic development and state capacity. However, there is strong disagreement in the field about how important climate change will be in the future. It is widely agreed that the risk of climate-induced conflict is greatest in low- and middle-income countries, and that the most important mechanism is damage to agriculture.

The potential impact of climate change on the risk of interstate, rather than civil, war is potentially much more important but also much less studied. Among interstate conflicts, conflicts between the major powers pose by far the largest risk to humanity. This is because the major powers have far more destructive weaponry and have the capacity to alter the trajectory of humanity in other ways.

The most plausible way that climate change could affect the risk of interstate war is by causing agricultural disruption, which causes civil conflict, which in turn causes interstate conflict. Indeed, there is some evidence that countries embroiled in civil conflict are more likely to engage in military disputes with other countries.

It is difficult to see how climate change could be an important driver of some of the most potentially consequential conflicts this century—between the US and Russia, and the US and China. It is more plausible that climate change could play a larger role in driving conflict between India and Pakistan and also India and China. However, for plausible levels of warming, other drivers of this conflict seem much more important.

It is extremely difficult to provide reliable quantitative estimates of the risk of Great Power War caused by climate change. Nonetheless, I have built a model that attempts to put some numbers on the key considerations. I think this is valuable for several reasons. Firstly, it clarifies the cruxes of disagreements and allows focused discussion on those cruxes. Secondly, it allows us to prioritise different problems. If we do not quantify, we will still have judgments about how important different considerations are. Models make these considerations precise.

The downside of quantitative models is that they can cause false precision and anchor readers, even if the model is not good and has not been subject to scrutiny. Many of the considerations I have discussed are very difficult to quantify because there is essentially no literature on them.

With those caveats in my mind, my best guess estimate is that the indirect risk of existential catastrophe due to climate change is on the order of 1 in 100,000, and I struggle to get the risk above 1 in 1,000. Working directly on US-China, US-Russia, India-China, or India-Pakistan relations seems like a better way to reduce the risk of Great Power War than working on climate change.

My personal thoughts on prioritising climate change relative to other problems

My primary goal in this report is to help people to answer the following question:

If your goal is to make the greatest possible positive impact on the world, what should you do with your time and money right now, given how the rest of society is spending its resources?

Crucially, this question is about what people should do on the margin. It is about what people should do given how society allocates its resources, not about how society as a whole should allocate its resources. Thus, when I say that working on some other problems, such as nuclear war or biosecurity, will have greater impact, this doesn’t mean that society as a whole should spend nothing on climate change and everything on nuclear war and biosecurity. Rather, it is a claim about what we should do with our resources given how other resources are currently spent.

Moreover, the question I am trying to answer in this report is specifically about how to make the greatest possible impact on the world. This is the highest possible bar. In my view, climate change is one of the most important problems in the world, but other problems, including engineered viruses, advanced artificial intelligence and nuclear war, are more pressing on the margin because they are so neglected. One can visualise this in the following way. Green projects are beneficial on the margin, and red projects are harmful on the margin. Deeper green projects are more beneficial whereas deeper red projects are more harmful on the margin.

To emphasise, we should not confuse the claim that other problems are more pressing than climate change with the claim that climate change doesn’t matter at all. I am glad that climate change is a top priority for millions of young people and for many of the world’s smartest scientists, and I would like governments and the private sector to spend more on climate change. I helped to set up the Founders Pledge Climate Change Fund (donate here), which has helped to move millions of dollars to effective climate change charities. The point is that I would like other global catastrophic risks to receive comparable attention, not that I would like climate change to receive less than it does today.

Imagine that only a few hundred people in the world thought that climate change is an important problem (rather than at least tens of millions), that philanthropists worldwide spent a few million dollars a year on climate (rather than $10 billion), that society as a whole spent a million dollars on the problem (rather than $1 trillion), and that the international institutions trying to tackle the problem either don’t exist or have a similar budget to a McDonald’s restaurant. How bad would climate change be? This is how bad things are for the other global catastrophic risks, and then some.

The final important piece of context is as follows: although I am taking a longtermist perspective in this report, my conclusions about the priority of climate change relative to other global catastrophic risks are also true if you think only current generations matter. In my view, the risks from AI, biorisk and nuclear war this century are much higher than commonly recognised.

  • AI: Forecasters on the community forecasting platform Metaculus think that artificial intelligent systems that are better than humans at all relevant tasks will be created in 2042. The most sophisticated attempt to forecast transformative AI is by Ajeya Cotra, a researcher at the Open Philanthropy Project and her model now suggests that it is most likely to be developed in 2040. A 2017 survey of hundreds of leading AI researchers found that the median judgments implied that there is around a 4% chance of human extinction caused by AI before the end of the century.

  • Biorisk: Combined forecasts on Metaculus imply that the chance of synthetic biology killing more than 10% of the world population by 2100 is around 7%. The implied chance of synthetic biology killing more than 95% of the world population before 2100 is around 0.7%.

  • Nuclear war: Forecasters on the community forecasting platform Metaculus think that there is an 8% chance of thermonuclear war by 2070.

These risks are not speculative possibilities, and the case for working on them is not contingent on ignoring the suffering of the current generation for the sake of a tiny probability of techno-catastrophe. I think it highly likely that my daughter will have to live through nuclear war, pandemics created by engineered viruses, and/​or the emergence of transformative AI systems that will radically alter society. It is deeply unfortunate that few people acknowledge these problems, and that many people who are aware of them dismiss them as sci-fi fantasies without attempting to engage with the arguments, or grappling with the fact that many people working in these fields agree that the risks are large.

Although, I contend, my conclusions follow on both neartermist and longtermist perspectives, it is important to reiterate that, in my view, a longtermist ethical point of view is the correct one. I see no compelling arguments for ignoring the welfare of future generations, and an ethical system that does ignore them is obviously difficult to square with concern about climate change.

While many people accept that the direct risks of climate change are lower than these other risks, some argue that the indirect effects of climate change may be large enough to make the total risk of climate change comparable. I do not think this is plausible. As discussed above, my rough models suggest that the total risk of climate change falls well short of the direct risk posed by the other global catastrophic risks. Moreover, the other risks also have indirect effects. As a rule, we should expect greater direct risks to have greater indirect effects. For instance, the indirect effects of trends in biotechnology seem to me much larger than the indirect effects of climate change. If biotechnology does democratise the creation of weapons of mass destruction, the indirect effects for the global economy and geopolitics are hard to fathom but seem enormous.

Overall, because other global catastrophic risks are so much more neglected than climate change, I think they are more pressing to work on, on the margin. Nonetheless, climate change remains one of the most important problems from a longtermist perspective. If progress stalls and emissions are much higher than we expect, then there is a non-negligible chance of highly damaging tipping points. Moreover, climate change is a stressor of political upheaval and conflict, which can in turn increase other global catastrophic risks. Finally, extreme climate change would make recovery from civilisational collapse more difficult.