On the other hand, spending billions of dollars on things carrying questionable benefits for humanity would not be news. At least the reason for this would be noble.

It would seem reducing global air pollution, especially in areas that suffer enormously on a daily basis (China is a prime example), is at least as beneficial to human health as focused efforts in medical research. And that's even without asking how this hypothetical "spending elsewhere" is going to be regulated in your opinion, and that is not a rethorical question. After all, similar complaints are heard every time a robot lands on a planet or satellite in the Solar System.

the nile isn't just a river in Egypt, and it may well stop being a river if runaway global warming continues

Pollution reduction does improve human health.

As r0muald and nitrogen said, reducing pollution definitely is not something that "makes no difference at all". But...

The other problem is that "makes no difference at all" is a good outcome. A bad outcome is that we reduce carbon usage drastically, which reduces economic output drastically, which causes many deaths. (This is not hyperbole. Take China, for instance. They're polluting like crazy, but even with all the pollution, people are better off. They're moving to the cities to get out of rural poverty. That poverty shortens life spans more than the pollution does.)