Why Is Mortality Higher in the Winter Than in the Summer?

https://astralcodexten.substack.com/p/chilling-effects

[Epistemic status: Extremely confused! Low confidence in all of this]

I.

On the recent global warming post, a commenter argued that at least fewer people would die of cold. I was prepared to dismiss this on the grounds that it couldn’t possibly be enough people to matter, but, um:

There are only about sixty million deaths per year total, so if this is true then almost 10% of all deaths are due to cold. That sounds…extremely untrue, right?

You can find the source here (study, popular article). The study confirms that it is claiming that 8.52% of all deaths are cold-related (plus an additional ~1% heat-related). It separates the world into a grid of 0.5 degree x 0.5 degree squares. It uses a bunch of assumptions and interpolations to get a dataset of daily average temperatures and mortality rates for each square over ten years. Then it calculates a function of how mortality varies with respect