I'd apply the tax globally, but make it proportional to GDP/capita.

A way to enforce this is tariffs.

Both economically inefficient and unlikely to be agreed upon across countries. Perfect.

Problem is, some people barely afford important things, while others easily affords unimportant things. Taxing everything at the same level will either: 1. Push millions of people into extreme poverty. 2. Or, be so low impact that it is ineffective for reducing carbon emissions.

What's the solution? We should not tax all things equal.

In a perfect market, a global carbon tax would be excellent. Unfortunately we do not live in (anything resembling) a perfect market.

Ideally and eventually, but I have no qualms with the US implementing a carbon tax first to demonstrate leadership in this space.