Usually agriculture is very subsidied everywhere yes because countries want cheap food for stability which is totally understandable.
There are nasty side effects to that though, the first one being that these subsidies usually are based directly or indirectly on the land size which made less efficient food products like meat much more subsidized than anything else.
We'll have to bring some more rational approach there if we want to improve the land use for climate change.
And then there's also the utility of the fuel used as you mentioned, there should be a difference of treatment between tractors and private jets indeed. But I don't think it needs to be too precise, cutting the worst offenders and taxing otherwise should be good enough.
That's only one dimension - Another major dimension is country.
The same tax applied globally will have very different effects in a third world country to a first world country. Farmers in the USA will react very differently to a 5p per litre tax to farmers in Burundi.
Well China still sees itself as a developing country, and would make up about 30% of CO2, so it all depends on definitions and international agreement.
I'd argue that China is pretty bad at this, it's a developed country indeed but has the same CO2 per capita as France when excluding the exports, that's a very bad result and that's why it should be included since it's a big polluter.
Usually developed countries are emitting less, the cause of that abnormally bad result is the high number of coal plants in China.
There are nasty side effects to that though, the first one being that these subsidies usually are based directly or indirectly on the land size which made less efficient food products like meat much more subsidized than anything else.
We'll have to bring some more rational approach there if we want to improve the land use for climate change.
And then there's also the utility of the fuel used as you mentioned, there should be a difference of treatment between tractors and private jets indeed. But I don't think it needs to be too precise, cutting the worst offenders and taxing otherwise should be good enough.