My political theory includes as a major idea that people dying when they don't have to is typically bad.

Based on this, I judge certain environmental problems to be significant. Someone else, motivated by the belief that environmental problems are overblown, denies that they are. Mind you, this person doesn't deny that we should try to save lives. They just deny the facts about what's happening. [1]

There are (contentious!) philosophical theories on which neither one of us is being objective, but, C'MON, whatever philosophical points you might make, you have to acknowledge a very real sense in which one of us is just reasoning badly.

[1] And to forestall any knee-jerk reactions, yes, it is sometimes the case that people assume a potential environmental problem is a disaster because of the opposite sort of mood affiliation. Which is more common isn't even the point.

If you're against involuntary death, shouldn't your top priority be curing aging? The vast majority of deaths on this planet are due to age-related illnesses. Not accidents, not violence, and certainly not environmental catastrophes.

Possibly! We'd both have to spell out a lot of details to have that argument. I'm not the one to have it either, because I don't know how plausible it is that we can really solve aging anytime soon.

But I'd say it's not super-relevant to the argument at hand, since the person who we're discussing doesn't say "these environmental problems are less important than aging" but just denies the specific harms are happening on the scale they are.

Edit: crap, left out a not. "It's not super-relevant."

Or more generally, people could argue about costs of different ways of mitigating deaths because maybe some non-environment-related issues are "cheaper" or "better" in some way to mitigate. There's not just the question of what causes the most deaths, but also the question of what could be done about it and how.

And people do have lots of ideas and arguments about those questions, like effective altruists' ideas about cheap medical interventions.

That's true in the sense that politics is war by other means. But the materials of politics, like war, can to some extent be studied objectively. The forces that make this hard—e.g. tribal identification—impede other forms of objective study too, though to a lesser extent.