I guess you could say both, since it seems clear what you mean. But, actually, I don't think there is an "official" antonym, because the phenomenon we call "Global Warming" now, has, in this form, never occured before. So there can be no analog.
In scientific terms, when it comes to phenomena of the distant past, which mean practically the opposite of global warming to the planet, one speaks of the "Ice Ages" or of a "Snowball Earth". Though, put precisely, this describes, in each case, an extreme state, that is, the end result of a development of global climate change. Then, if you want to depict the development (or process) proper, yes, speak of "global cooling".
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