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Try proving "Not B" in the absence of a reliable "If A then B". That is what he asked me to do (identify the A and B of this discussion). To do that one must accept "If A then B". But how do you prove "Not B" if you cannot say for certain what the relationship is between A and B, or indeed if there is any such relationship at all? It can't be done.

The point is that proving human activity is not responsible for global warming is a in-practice impossible task, as you could not in one lifetime analyze every human action and derive its ultimate effects on the global temperature.

Thus, someone saying global warming is caused by human activity challenging someone else to prove it is not so is asking the impossible. What can be proven is that the information we have to date is not sufficient to prove that global warming is caused by human activity to any degree. And given the total absence of evidence that logically leads to such a conclusion, the positive statement of the insufficiency of global warming theories is proven with the evidence I provided above.

Now that I think about it a little more, it's even more absurd than I thought. "B" in this case is completely undefined. Therefore, "Not B" is a completely meaningless expression.

There IS an exception where you can prove a negative - and that is where the positive is internally contradictory. But we are not arguing over Invisible Pink Unicorns here.

Your first paragraph led me to believe you were on the right track, but then you veered. :-) The problem I had with your argument was not the specific case, but that you used an invalid general claim to argue it. Let the Lefties use the fallacious arguments; let's keep ours tight. That is, certain negatives can be disproven, as you mention later (which yields my point).

You claim: ...proving human activity is not responsible for global warming is a in-practice impossible task, as you could not in one lifetime analyze every human action and derive its ultimate effects on the global temperature.

That ignores other ways that a negative can be disproven...again, via contradiction. Brute force isn't necessary. For example, if we were able to prove (absurdly and hypothetically) that all inputs to a global climate system are dampened out unless they are repeated within 1%, exactly at a frequency greater than a human lifespan, etc., then wouldn't that be proof that humans weren't forcing the system? And we did it without interrogating a single human. We can prove that Tyrannosaurus Rex did not watch TV, even without finding the electric bills of every dinosaur...by proving that the conditions for a "positive" result cannot be met.

...insufficiency of global warming theories...

Insufficiency of theories and status of provability say nothing of the truth, though....they are only measures of validity of an argument. Still, beyond it all, we make decisions with far lower standards than "proof".

...we are not arguing over Invisible Pink Unicorns here.

bbhhh :-)