If global warming were a hoax, you would need almost every climate science expert to be in on it (because they're the ones collecting and analyzing the data and concluding that global warming is primarily caused by humans and will have very bad consequences if we don't do something about it soon).
So then global warming deniers think 'okay, what could scientists have to gain by creating this hoax?'. Actually many will just say "It's all Al Gore!" or "Global warming alarmists!" because they don't even bother to think about who discovered that global warming was occuring and what was causing it in the first place (hint - it wasn't Al Gore!).
So if they get to that point, they think "well, the scientists want to get rich!". But if they know anything about how science works, they know that scientists don't get rich (unless they invent something good and patent it, and manage to keep the rights away from the university or company they're working for, which they usually don't).
So then they think "well, they wouldn't be getting funding if it weren't for global warming, so they're just inventing this hoax to save their jobs!". Except that climate scientists had jobs and funding before this supposed "global warming hoax" began, so that's not really true either. Even if global warming weren't a problem, they'd still get funding to study global climate. It wouldn't be as easy to get the funding, but they would manage like they have since the start of climate science.
And the final point is that it's impossible for it to be a hoax, because that's not how science works. If a scientist is found to be falsifying data, his reputation is ruined and his career is over. Science is all about gathering and analyzing data in an unbiased manner.
I'm with you - I wish global warming were a hoax. Unfortunately that's simply wishful thinking, and not the case.