It can NEVER be done, it's not a question of computing power: it's sheer academic and scientific ignorance: it's IMPOSSIBLE to predict such a complex system.
If the Sun eg has a cycle which causes a significant cooling and warming: can they predict that? Obviously not. And yet such an event can change global warming/cooling direction completely.
The naivety of these scientists who model future events of horrible.
And then politicians and the masses belive them.
Go and look eg at their COVID predictions to see how wrong they were.
Funny how they aren't being that loud now when their "great supercomputer models" can be seen to be BS.
The point is that such large scale models CANNOT be made, it's physical impossible.
And no, you can NOT just take an "average" because a complex system can move in such a unpredictable way all of the sudden that the "average" would also completely change in an extreme fasion (if such a system can even be said to have an "average").