I believe that the idea that China is obstructing a global carbon tax (among supporters of such a tax) mainly comes from racism and is not supported by the facts. However, people oppose global/national carbon taxes for a number of reasons unrelated to racism. I'm not sure why you are concerned with the authority to levy a carbon tax, since (1) such a tax would almost certainly be beneficial and (2) politicians already have essentially unlimited legal authority to raise taxes of any sort.

We seem to disagree, as I'm convinced that a carbon tax would almost certainly be an unmitigated disaster.

Why? I mean, as a moderate libertarian, I do tend to be suspicious of any proposal to give politicians more power, but given that sooner or later we are going to have to reduce carbon dioxide emissions, surely the best way is to levy a carbon tax and let the market find the most efficient ways to do the reduction?