Burning a tree however should incur some penalty, after all - wildfires are a positive feedback loop in reality and it'd be trivial to detect.
Might have to deviate from the idea that the global carbon score is the sum of everyones individual carbon score, since there will certainly be lots of events which contribute CO2 without player intervention.
Thanks!
In terms of minimizing exploits, perhaps the easiest approach is to pre-compute a carbon score for all blocks and items, and then focus on catching all the situations where those blocks/items can be destroyed. By default, assume 100% of the carbon goes back into the biosphere, and then write special-case logic for stuff like boats breaking into wood.
A few more what-ifs:
* If players toss wood-bits into the deep sea to disappear (as opposed to letting them disappear on land) should that count as sequestering it? Or would that almost be too easy?
* I forgot about trades -- if a villager provides/accepts wood, perhaps it is best to assume it was just freshly grown or is about to be burnt.
* Other dimensions. What should happen if players make a gate into a room in the Nether and do all their smelting there? Alas, we don't have that option in real life :P