Global warming as existential risk

Why should we be committed to egalitarianism? If you read this blog, it is likely that you are committed to (or reject) egalitarianism because (obviously) of your beliefs about the justice of egalitarianism. However, I think that we must pursue human equality not simply because it is just, but also because it is essential to organizing collective action to prevent the existential risk of catastrophic global warming.

In this post, I want to say why I think global warming is an existential risk. I’ll talk about egalitarianism and collective action in the next post.

Nick Bostrom defines existential risk as

An existential risk is one that threatens the premature extinction of Earth-originating intelligent life or the permanent and drastic destruction of its potential for desirable future development.

There is, of course, uncertainty about whether global warming will cause drastic and possibly permanent destruction of the potential for future human development. However, we do not need to believe that it is certain that global warming will cause catastrophic destruction. To be an existential risk, we just have to be certain that there is an appreciable risk of such destruction.

IPCC projectionsMartin Weitzman considers several sources of uncertainty about the consequences of global warming. The first is uncertainty about future distributions of global temperatures. When the IPCC projects future warming, they present multiple possible paths, as shown in the 2007 Figure to the right. Unfortunately, the recent news about global warming is that evidence is accumulating that warming is likely to follow the most adverse of the IPCC climate projections. The World Bank warns that

On a higher fossil fuel intensive business-as-usual pathway, such as the IPCC SRESA1FI, warming exceeds 4°C earlier in the 21st century.

Right now, business-as-usual is the order of the day. If that continues, it appears that the evolution of the climate may follow one of the more adverse tracks, similar to the highest (red) track in the Figure above.

This does not mean, however, that we can be confident that warming by 2100 will be limited to 4°C, if only because the IPCC did not consider all of the possible positive feedback effects that could amplify global warming. For example, thawing permafrost. Permafrost covers 24% of the Northern Hemisphere. When it thaws,

CO2 and methane emissions from thawing permafrost could amplify warming due to anthropogenic greenhouse gas emissions. This amplification is called the permafrost carbon feedback… If the permafrost thaws, [it will] decay, potentially releasing large amounts of CO2 and methane into the atmosphere. This… release into the atmosphere is irreversible on human time scales.

That is, global warming itself generates greenhouse gas production, amplifying global warming. What you also need to understand is that

None of the climate projections in the IPCC Fourth Assessment Report include the permafrost carbon feedback (IPCC 2007).

So it is possible that the actual temperature pathway may be more adverse than the worst IPCC 2007 pathway.

Weitzman’s second source of uncertainty concerns the environmental damage that these temperatures may cause. The IPCC and others project that 4°C increase could cause coastal flooding, the death of much of the oceanic biome, and widespread desertification; but the extent of these damages is less certain than even the temperature. Again, ‘uncertainty’ includes the possibility that damage could be much greater than we anticipate.

Weitzman notes that we are also uncertain how much a given level of environmental damage will diminish human welfare. No one knows how human societies will cope if, for example, millions of people are driven north and south from an unliveable equator. However, history is replete with examples in which apparently small destabilizing events lead to rapid, prolonged, and unforeseen cascades of warfare and institutional collapse. Because human systems are tightly coupled, Nassim Taleb points out that

real life is not a casino with simple bets. This is the error that helps the banking system go bust with an astonishing regularity… The problem of the illustrative current subprime mortgage mess is not so much that the “quants” and other pseudo-experts in bank risk-management were wrong about the probabilities (they were) but that they were severely wrong about the different layers of depth of potential negative outcomes. For instance, Morgan Stanley has lost about $10 billion (so far), while allegedly having foreseen a subprime crisis and executed hedges against it; they just did not realize how deep it would go and had open exposure to the big tail risks. This is routine. A friend who went bust during the crash of 1987 told me, “I was betting that it would happen, but I did not know it would go that far.”

So: physical models of the climate leave open the possibility that humans could warm the climate to a degree that would radically degrade the ecological networks supporting life. The human consequence of this are uncertain, but experience gives us reason to be concerned that there is a non-zero chance of consequences that are much worse than we currently imagine. These multiple sources of uncertainty mean that we are facing a distribution of future human outcomes with “fat tails,” meaning that on the up-side there is an unknown but appreciable chance that outcomes will be wholly manageable, and on the down side there is an unknown but appreciable chance of deep and lasting catastrophe.

This is an existential risk. In my next post I will make the case that we have to think about egalitarianism in the light of that risk.

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About Bill Gardner

A health care researcher and a child and quantitative psychologist by training. I am an American living in Canada and am Professor of Paediatrics, Obstetrics & Gynaecology, and Community Health & Epidemiology at Dalhousie University; and Professor of Pediatrics, Psychology, and Psychiatry at The Ohio State University. I also blog at The Incidental Economist ( and you can follow me @Bill_Gardner on Twitter.
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3 Responses to Global warming as existential risk

  1. Pingback: Existential Risks and 100 Billion Worlds | Inequalities

  2. Pingback: ‘Existential Risk’ as hysterical rhetoric? | Inequalities

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