Actually, it's not a hoax, nor is it an established scientific fact that should be the basis of public policy.
First, "climate change" as the phrase is used is real-world Newspeak, which, unlike Orwell's fictional corruption of language for political ends, does not narrow the meaning of words in the service of Party ideology, rather the real version overloads words with meanings, which can be shifted at need to serve the interests of "the Party" (in the US context that could be the Democrats, or it could be just the professional managerial class): "climate change" can mean (when needed) the indisputable fact that the earth's climate changes over time, or it could mean the same thing as the more precise standard English meanings of any of the phrases "anthropogenic global warming", "greenhouse gas induced warming", "anthropogenic climate change", and probably a few others as needed for political ends, and what it means can be changed from when the phrase is used with one meaning to when that use is criticized for being false.
Second, even the narrowest meaning -- anthropogenic greenhouse gas induced global warming -- is not a hoax, but a plausible scientific hypothesis, which has not been falsified, even though essentially all of the computer models that have been used to suggest that a run-away version of it imperils humanity and/or the natural world as we know it, have all been falsified by actual measurements. To be a hoax, those pushing this theory would need to know it is false, which they plainly do not, because it might be true. (The runaway scenarios always involve something besides just greenhouse gas warming, which is logarithmic in effect -- each doubling of CO2 concentration raises the mean temperature by the same number of degrees as the previous doubling.)
That said, the political uses of the notion involve a lot of fakery: pretending science works by consensus, ignoring countervailing evidence (for the alarmist scenarios), ignoring cost-benefit analyses in pushing favored policies, and so forth. In this regard, it is worse than a hoax, as a hoax is merely a lie, while the political use of "climate change" is in the definition of Harry G. Frankfurt, "bullsh*t" -- saying whatever is expedient without regard to whether it is true or false -- bullsh*t being a worse enemy of the truth than lying (or hoaxing) since the liar (or hoaxer) has enough regard for the truth to know it and say the opposite for some purpose, while the bullsh*tter has no regard for the truth at all.