If you believe that global warning is largely a result of human activity, and that we should spend money to try to prevent it, it make perfect sense to spend that money wherever on the planet you think will have the biggest impact. It's a global problem, so it need a global solution. We could be everything possible in the USA to try to fix it, but it wouldn't do any good if the rest of the people on the planet keep doing the things that cause it. I doesn't matter where on the globe you fight it when were all on the same globe.
Now if you on't believe that it's a man made problem, then you're right, it makes no sense to fight it, here or anywhere.