That is true, but in a situation where I don't know the percentages, I think it's safer to err on the side that will end up with this planet being more inhabitable.
There are several unknown situations like this though, not just global warming. In many ways, by taking "global warming" seriously, you are doing so at the expense of other extremely good causes, where there aren't so many unknowns. In that case you are likely doing harm, but with good intentions. The problem with your mindset is that it assumes that there's no harm done, and possibly a lot of good done.
I think that reducing pollution is a good thing to do, whether or not it's going to have any influence on climate change.
It's another cost benefit situation. If the cost to reducing pollution, which can be
extremely significant, outweights the benefit, then it's not worth doing. Yes, pollution continues to cause harm, but the resources in combatting it may be more effectively allocated. Especially when we may not know what pollution we're looking for, and what it really does.