Epistemic status: crank reportage.

It suits me that climate change isn't an x-risk. (The movement has trillions of dollars already, and persistently drains talent, attention, and political capital away from actual x-risks.)

... But is it one?

One palaeontologist, Peter Ward, is semi-famous in the field for suggesting a mechanism by which runaway climate change could kill everything: by turning the ocean into a toxic gas factory.

an increase in carbon dioxide... warms the oceans enough to change circulation patterns. When this happens, sulfur-eating microbes sometimes thrive. These bacteria produce hydrogen sulfide, which, in sufficient quantities and under certain conditions, outgasses into the air, shreds the ozone layer, and poisons other living things. The warming also causes methane ice under the seas to melt and, well, burp, adding to the nasty mix.

As usual in Zoomed Out Sciences like epidemiology and climatology, the model stops short at the inevitable massive effort to reverse this process. The modellers prefer to think of humans as the pink fella from these comics: inertly lamenting.

Ward argues that this (and similar mechanisms) is responsible for all past mass extinctions except the dinosaurs one everyone fixates on.

This is correct as chemistry (I think), and apriori could happen, and for all I know he's right and it actually has happened before. So I have to give it nonzero probability - and one of the real probabilities, with only a few zeroes in it.

I can't really evaluate this. There are some hallmarks of crankery in the book - most of the citations of the book are unscientific or pseudoscientific - and (as Halstead and others have long noted) climate is one of the slower and more detectable ways to kill a biosphere. 

But in concert with the usual vague definition of an x-risk (where there's no threshold on the probability), I've been thinking of climate change as a (lesser) xrisk for a while and thought I'd come clean, even though I don't think this changes anyone's decisions.