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"Population to 2100" is the tab to look at.

The end goal is to calculate the number of lives lost if we faced an existential catastrophe tomorrow. Not just for the people who died during the catastrophe itself, but the unrealised future humans who never come into being.

Future lives are discounted: this means that we don't attach quite as much value to a life in 2099 as we do to a life in 2020. This isn't a universally accepted practice but arguments can be made for it.

To complicate the calcuations, however, I use variable rate discounting – rather than normal fixed-rate exponential discounting. This is due to the work of Martin Weitzman who showed that for non-financial utility discounting we should start with a rate equal to the average of sensible estimates for catastrophic risk, reducing over time towards the smallest sensible risk estimation we can imagine.

To put it another way, let's assume that there are a range of estimates for our annual existential risk, from 50% for the hyper-pessimists right down to 0.001% for climate change denier-types.

If the average of all those estimates is 10%, we would start with a discount rate of 10%, and then reduce that rate year on year towards 0.001%.

In the actual calculations in the "Population to 2100" tab, we start with an annual x-risk of 0.18% – this is derived from the assumption that cumulative risk over a century is 16.67%.

That 0.18% gives us our starting point for the discount rate. The rate itself decays at a 1% rate towards a minimum of 0.001% (the backgrond natural exinction risk).
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