Suppose the cause of global warming is not human action but changes in solar activity or some other external factor. Suppose also that the consequences of global warming will be catastrophic. Finally suppose that there is something we can do to prevent global warming, say raising the albedo of the earth with orbital mirrors, high altitude pollution, or whatever. Isn't the argument for doing it precisely the same as if we were causing the warming? Hence isn't "whose fault is it" a wholly irrelevant distraction?
Of course, the questions of causation and prevention are not unrelated. If we are causing global warming that suggests one possible way of preventing it—stop whatever we are doing that causes it. But doing that may be, indeed very likely is, enormously costly, perhaps more costly than letting global warming happen. It might even be impossible, if what we have already done is enough to cause long run catastrophe even if we don't do any more of it. And even if we are causing it and could stop doing so, there might be better solutions.
Concerning Global Warming More Generally
I should add that I am taking no position here on the other usual questions about global warming. I do not know if it is happening, although it seems likely enough. I do not know if, if it is happening, it is due to human action, although that again seems a plausible enough guess. And it is not all clear to me that, if it happens, it will be a bad thing, let alone a catastrophe.
The crucial fact for me is that the more persuasive predictions of bad effects are well into the future; at one point the estimate was a sea level rise of half a meter to a meter over the next century. In my view, the next century is sufficiently uncertain so that it makes little sense to take expensive precautions against risks that far off. By the time the risk arrives, if it arrives, we may have already wiped outselves out in some other way. If we have not wiped ourselves out, our lives may have changed in a way that eliminates or even reverses the problem; communting via virtual reality produces little CO2. If we are still around and the problem is still around, we are likely to have a level of technology and wealth that will make possible a range of solutions well beyond what we are currently considering.
All of these are reasons why I think a persuasive case for doing something about global warming requires evidence, not yet available, of serious negative effects in the fairly near future. But that conclusion does not depend on whether whatever is happening to the climate is or is not our fault.