What are the scores for various countries regarding rendering the planet unfit for human habitation?

Let us imagine that a country can by any of numerous means make the planet unfit for human habitation, maybe one almost instantaneous action or by releasing self-replicating gas that gradually spreads around the atmosphere etc. So, any country that can do that once, which is surely all that is needed, has a score of one?
How many countries have a score of one or more and which are they? Which countries have the highest score? Perhaps some countries feel that they need strategically placed duplicates in case any get rendered ineffective by enemy action. So, maybe there is a need for a score higher than one.
I suspect the following scores :-
USA 3
Russia 2 (would be 3 if the economy was better)
China 2
France 1
UK 1
This is pure speculation by me. Some regimes vaguely boast about such power, others are silent about the matter.