EA - Supervolcanoes tail risk has been exaggerated? by Vasco Grilo

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Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: Supervolcanoes tail risk has been exaggerated?, published by Vasco Grilo on March 6, 2024 on The Effective Altruism Forum.This is alinkpost for the peer-reviewed article "Severe Global Cooling After Volcanic Super-Eruptions? The Answer Hinges on Unknown Aerosol Size" (McGraw 2024). Below are its abstract, my notes, my estimation of a nearterm annualextinction risk fromsupervolcanoes of 3.38*10^-14, and a brief discussion of it. At the end, I have a table comparing my extinction risk estimates with Toby Ord's existential risk guesses given inThe Precipice.AbstractHere is the abstract fromMcGraw 2024 (emphasis mine):Volcanic super-eruptions have been theorized to cause severe global cooling, with the 74 kya Toba eruption purported to have driven humanity to near-extinction. However, this eruption left little physical evidence of its severity and models diverge greatly on the magnitude of post-eruption cooling. A key factor controlling the super-eruption climate response is the size of volcanic sulfate aerosol, a quantity that left no physical record and is poorly constrained by models.Here we show that this knowledge gap severely limits confidence in model-based estimates of super-volcanic cooling, and accounts for much of the disagreement among prior studies. By simulating super-eruptions over a range of aerosol sizes, we obtain global mean responses varying from extreme cooling all the way to the previously unexplored scenario of widespread warming. We also use an interactive aerosol model to evaluate the scaling between injected sulfur mass and aerosol size.Combining our model results with the available paleoclimate constraints applicable to large eruptions, we estimate that global volcanic cooling is unlikely to exceed 1.5°C no matter how massive the stratospheric injection. Super-eruptions, we conclude, may be incapable of altering global temperatures substantially more than the largest Common Era eruptions.This lack of exceptional cooling could explain why no single super-eruption event has resulted in firm evidence of widespread catastrophe for humans or ecosystems.My notesI have no expertise involcanology, but I foundMcGraw 2024 to be quite rigorous. In particular, they are able to use their model to replicate the more pessimistic results of past studies tweeking just 2 input parameters (highlighted by me below):"We next evaluate if the assessed aerosol size spread is the likely cause of disagreement among past studies with interactive aerosol models. For this task, we interpolated the peak surface temperature responses from our ModelE simulations to the injected mass and peak global mean aerosol size from several recent interactive aerosol model simulations of large eruptions (Fig. 7, left panel).Accounting for these two values alone (left panel), our model experiments are able to reproduce remarkably similar peak temperature responses as the original studies found". By "reproduce remarkably well", they are referring to acoefficient of determination (R^2) of 0.87 (see Fig. 7)."By comparison, if only the injected masses of the prior studies are used, the peak surface temperature responses cannot be reproduced". By this, they are referring to an R^2 ranging from -1.82 to -0.04[1] (see Fig. 7).They agree with past studies on the injected mass, but not on theaerosol size[2]. Fig. 3a (see below) illustrates the importance of the peak mean aerosol size. The greater the size, the weaker the cooling. I think this is explained as follows:Primarily, smaller particles reflect more sunlight per mass due to having greater cross-sectional area per mass[3].Secondarily, larger particles have less time to reflect sunlight due to falling down faster[4].According to Fig. 2 (see below), aerosol size increases with injected mass, which makes intuitive sen...