So how could this new world authority come to an economic judgment (your country must tax more or less than it does) and enforce it from “within our country,” then?
I don’t think the Pope suggests the the reformed UN regulate domestic taxation. It might regulate economic activity that is transnational in scope, but not the internal affairs of countries.
Let me add a general observation. The US politics are shaped by the economic realities in the US, which is still a very prosperous country. Our unemployed have a living standard most of the world would consider a windfall. So, American conservatism correctly views most taxation, especially for social programs, as waste or, worse, theft. When an American conservative reads about redistribution of wealth, he thinks of the government taking from an honest productive man to give to a couch potato who won’t work and is not in danger of starving to death.
However, if you take a larger view, you would see people literally starving, or armed thugs engaged in genocide. That is serious poverty. The Catholic teaching always was that rules of market exchange do not apply in extreme poverty. That is, of course, what the gospel teaches. Consider for example, the story of Dives (the name itself is ascribed to the rich man by tradition) and Lazarus (Luke 16). There, Dives is condemned for lack of compassion; no suggestion is made that Dives obtained his wealth dishonestly. The market made Dives rich and Lazarus poor, yet dives owed Lazarus food. This is a situation we are not accustomed to in America, but as we contemplate the global economy, we should reflect on that need for “gratuitousness” or “distributive justice” as the Pope calls it.