I think HBD is a fantastic test for true rationality. It's a rare case where a scientific fact conflicts with deeply ingrained political and cultural values.

Unfortunately it might also be an area where epistemic and instrumental rationality clash. In fact, most of the world does not have freedom of speech in the same way the US does - if one advocated HBD in, say, Germany, could one be thrown in prison in the same way people are imprisoned for saying 'seig heil'? On reflection, that might be a bad example, because there is a good reason why laws prohibiting Nazism exist in specifically Germany, but laws against racial hatred exist in many other countries too, and I wonder if "I am just pointing out a scientific fact, it doesn't mean I hate anyone" would be a successful legal defence when the judge and jury are not rationalists.

The people directly killed by bombs are only a tiny fraction of the casualties. Afterwards you have the possibility of nuclear winter, which could radically change the climate for the worse. You have contamination of vast amounts of farmland and food supplies, and the destruction of the global economy. Even countries that weren't hit directly would be have tons of issues and would probably collapse.

The study I am thinking of did account for this. It is thought that nuclear winter would last 6 months (based on global cooling from very large volcanic eruptions, 3 km^3 of ash cools the planet by 0.5 degrees for 6 months) and in the worst-case scenario this happens at the right time of the year to make the harvests fail. Africa starves and takes high casualties even without being nuked, while Japan raids nearby countries for food. I'm not sure why they singled out Japan for stealing food, maybe they were still angry over WWII or had some other reason to believe that Japan would behave like that.

There have been quite a few airburst nuclear tests, and the background radiation on earth has barely changed. While the half life of uranium is millions or billions of years (depending upon isotope) the products of a nuclear explosion have very short half-lives - the worst would be over within an hour. Not only do we not have enough bombs to contaminate the world, but ground zero would be habitable again after a few months. Hiroshima was rebuilt, not abandoned, after WWII.

Yes, I agree many countries would collaspe, and moreover that the primary combatants quite possibly would take over 90% casualties, the low global rate being largely because many countries emerge unscathed. If there were some reason for most countries to be nuked, and enough bombs to nuke them all, then 90% global casualties is a possiblity, and while I think a nuclear war between allmost all countries is unlikly, its still a lot more likly then 90% of humanity killed by environmental or political collapse.

Sudden changes to the climate are bad. In the past the climate changed very very slowly, so that life had a chance to adapt.

Life is pretty good at surviving. Humans can live in the arctic circle and in tropical rainforest. Hopefully, by this time we would also have better technology to help us.

CO2 is acidifying the oceans and could destroy most of the oceans ecosystems.

This is a little worrying. I thought it was mostly 'just' the corral reefs, which, while very pretty, are not vital to everything.