Of Hume, zombies, and tails
My new paper in Climatic Change is on human survival. There is a substantial part of the public who believe that climate change is an existential threat. Extinction Rebellion rebels against human extinction. The Letzte Generation is motivated by the fear of being the last generation.
Some academics feed this fear. Tim Lenton is one. My paper responds to Xu et al. (2020), repackaged as Lenton et al. (2023). In these papers, the authors place many future people outside the range of past climates, and argue that this is dangerous.
I have three objections. The first is philosophical. Unprecedented does not mean bad, as Hume and Moore noted. We experience many things that no human has ever experienced before — some are good, some are bad — and there are unprecedented things I long for — a world without poverty and bigotry is high on my list.
The second objection is statistical. The papers by Lenton and colleagues are about what happens if people would move beyond the current extremes. This is a question about the tails of the distribution. Lenton, however, assumes a mixture of normal distributions, a method well-suited to study the central tendency of a population. Univariate extreme value statistics is well-established, but climate is multivariate (although only temperature and rainfall are considered). I therefore had to cobble together a new estimator: Dominicy’s multivariate generalization of Hill’s estimator of the tail-index, applied to Worton’s proposed multivariate order statistics. For good measure, I also throw in a QQ-estimator of the tail-index because it can be readily corrected for top-censoring (dubbed Tobin’s QQ). This satisfies the statistician in me, but the results indicated that the tails of the distribution of the human population in climate space are thin. Lenton assumed this. I show that this is a fine assumption.
My third objection is substantial. Lenton assumes that 10% of the world population are located in unliveable climates. Yet they are alive. Climate change will move some 20% of the world population into Lenton’s areas of the undead. This drives the difference in results: Lenton puts billions of future people in unprecedented climates, whereas I find that it is hundreds of millions.