There is no evidence of climate change scenarios that would render human beings extinct.Michael Mann, cited in “Could climate science make humans go extinct?“
Sometimes, effective altruists give high estimates of the existential risks facing humanity today. In his bestselling book The Precipice, Toby Ord puts the risk of existential catastrophe by 2100 at “one in six: Russian roulette”.
That is a very large number. What does the evidence say? Let’s look at the case of climate risk.
1. Why climate change does not pose a significant existential risk
Ord gives a 1/1,000 chance that climate change will lead by 2100 to irreversible existential catastrophe. That is a frightening number.
But the odd thing about Ord’s discussion is that almost the entirety of the text is devoted to a clear and well-evidenced discussion of why climate change is unlikely to lead to existential catastrophe any time soon.
How could climate change produce an existential catastrophe? Ord argues against six routes to existential catastrophe. Like many of Ord’s writings, this discussion is admirably clear, well-researched, and persuasive.
First, climate change could trigger a runaway greenhouse effect. Emissions would trigger water evaporation, leading to a buildup of water vapor in the atmosphere. That, in turn, would cause more evaporation, until the oceans had boiled off. But, Ord notes, while such an event has occurred on at least one planet, our best scientific models suggest that humanity is incapable of producing even a fraction of the emissions needed to trigger a runaway greenhouse effect on earth.
Second, we could trigger a moist greenhouse effect, a little cousin of a runaway greenhouse effect whose consequences would involve far less evaporation and warming. But again, Ord correctly holds, our best scientific evidence makes a moist greenhouse effect unlikely to happen any time soon, and models predict resulting temperature changes that would be survivable, though catastrophic.
Third, there could be other feedback effects such as melting Arctic permafrost or release of methane from the deep ocean floor. But these are not likely to pose an existential threat this century. Ord notes that even under its “high emissions” scenario, the IPCC expects melting permafrost to contribute 0.3°C of warming by 2100. And although very little is known about the likelihood or scale of ocean methane release, Ord gives no reason to suspect that this would be fatal.
Fourth, we can (and probably will) continue to burn fossil fuels. But, Ord notes, even the implausibly extreme scenario in which we burned literally all accessible fossil fuels, predicted warming would be in the range of 9 to 13 degrees Celsius. That is an astounding number, and certainly poses a catastrophic risk, but there is absolutely no reason to think it would not be survivable: many regions of the earth would remain habitable, above water, and friendly to agriculture.
Fifth, global warming could cause biodiversity loss and ecosystem collapse. But, Ord notes, the evidence linking climate change to biodiversity loss, let alone ecosystem collapse, is mixed. And Ord gives no reason to think that either scenario would be unsurvivable.
Finally, global warming will doubtless cause heat stress, rendering certain areas of the earth uninhabitable during the warmest months without air conditioning. But many areas of the earth will remain temperate, and the rest may be inhabitable with the use of air conditioning and other technologies.
So far, the entirety of Ord’s discussion has given us reasons not to take climate change to pose an existential threat to humanity. We have not been given a single reason to take climate change to pose an existential threat. I don’t mean that we have been given a bad reason. I mean that we have literally not been given a single reason.
On this basis, one might expect Ord to treat existential risk from climate change as negligible. But in fact, Ord gives an estimate of one in a thousand that climate change will produce an existential catastrophe by 2100. That is a shockingly high number. But what, exactly, lies behind it?
2. What might underly Ord’s risk estimate
The only sliver of an argument for existential risk from climate change is contained in a single paragraph. That paragraph begins as follows:
This [discussion] doesn’t rule out unknown mechanisms. We are considering large changes to the Earth that may even be unprecedented in size or speed. It wouldn’t be astonishing if that directly led to our permanent ruin.
And now we are surely going to get an argument for why we should be afraid of these unknown mechanisms? No. In fact, we get an argument against that fear.
The best argument against such unknown mechanisms is probably that the PETM [Paleocene–Eocene Thermal Maximum] did not lead to a mass extinction, despite temperatures rapidly rising about 5 degrees Celsius, to reach a level of 14 degrees Celsius above pre-industrial temperatures.
So far, we have been given nothing to fear. Ord does offer a reason not update too strongly on the PETM:
But this is tempered by the imprecision of paleoclimate data, the sparsity of the fossil record, the smaller size of mammals at the time (making them more heat-tolerant), and a reluctance to rely on a single example.
Fair enough! But what is the positive reason to be afraid of existential risks from climate change? Finally, finally we are given a sliver of an argument:
Most importantly, anthropogenic warming could be over a hundred times faster than warming during the PETM, and rapid warming has been suggested as a contributing factor in the end-Permian mass extinction, in which 96 percent of species went extinct.
Is that the driver behind the view that we have one chance in a thousand of existential catastrophe from climate change by 2100? Surely not. No one would leap to such a strong view on the basis of this single sentence.
But then, what exactly is the argument that climate change poses a near-term existential risk? Could it be that there is no argument? That we have as yet been offered no solid, rational, evidential basis for fear?
“That’s not fair”, you say. “Ord wasn’t claiming to have given us an argument for his existential risk numbers. They were just subjective reports of Ord’s own views, which were never intended to be treated as statements of fact or as reflections of printed arguments.”
Have I been fair to Ord? Are there other reasons to be afraid of existential risk from climate change? Let me know.
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