Not sure what you're trying to say. Global warming is a human activity, global warming is unproductive, global warming is caused by one human activity, that activity is unproductive?
Not sure if you are writing this in good faith or not, but let me assume you are:
The parent said "War is the single most unproductive activity humans can do", without giving much details about a metric (it is very productive if your business is to build weapons, but counter-productive if your business is to save lives).
But assuming that the metric was something along the lines of "doing good for society", then global warming is a lot more counter-productive than wars. Global warming and wars are the result of human activities (in case that was not clear).
So yeah, we would certainly save more human lives by keeping our wars (I mean, without nuking the whole planet) but working all together to reduce the impact of global warming. Meaning that IMHO, "war is NOT the single most unproductive activity humans can do". Not that it is good, quite obviously.
