Thanks for the recommendations! Glad to see that you've compiled an IR reading list related to EA causes.

I am very uncertain about these predictions but would love to hear your thoughts!

Thanks, Jack!

Here are a couple of thoughts from an IR perspective: 

Case 1: 50% population loss, no infrastructure damage, no climate change (e.g. a limited pandemic)

• The nation-state system is likely to remain intact, aside from a few states that have been totally depopulated. 
• A fundamental reorganization of the global balance-of-power (might lead to the total demise of current superpowers like the US and China), which would cause a collapse of the international order (no more UN, no more meaningful alliance arrangements).
• If resources are damaged, we will likely see intense international competition over scarce resources (e.g., food, oil, raw materials). This entails hoarding, export bans, using coercive force to acquire resources, etc. 
• Internal turmoil might cause the economic collapse of certain countries and regime change (think interwar Weimar Germany, late 1980s Soviet Union, Venezuela). This would cause massive refugee flows.
• In other words, domestic and international politics will likely reinforce existing issues caused by the catastrophic risk.

Case 2: 90% population loss, infrastructure damage, and extreme climate change (e.g. nuclear war that caused nuclear winter)

• The nation-state system is severely damaged. Aside from a few countries, most have lost a meaningful form of government.
• IR theory does not really apply in this scenario, as there are few states.
• Still, humans will struggle to see themselves as "one" because they have sufficient ethnic/national ingroups to rely on. This means that there will still be identity-based competition over resources (e.g., hoarding food for one's ingroup).

Case 3:  a catastrophe causes the deaths of 99.99% of people (leaving 800 thousand survivors), extensive infrastructure damage, and temporary climate change (e.g. a more severe nuclear winter/asteroid impact, plus the use of biological weapons).

• The nation-state system totally collapses. 
• The 800,000 survivors regroup as a nation/country and organize a government to distribute resources. However, the effectiveness of such an organization is questionable due to linguistic/cultural differences.

As you can see, the higher the percentage of deaths, the less IR theory has to say about the situation. I think the central insight is that the number of survivors determines people's identities (whether they think of themselves as part of a nation or as part of humanity as a whole). This, in turn, affects whether there will be internal competition over resources among different groups. In other words, we cannot simply assume that the survivors can effectively allocate resources among themselves. Internal resource competition can lead to excess/unnecessary deaths. Paradoxically, the percentage of such excess deaths might be Case 1 > Case 2 > Case 3.