Some questions* already use the tag earth.
(*) edit: ignore the previous link, see the first comment by plannapus following naught101 answer below.
I'm wondering if it is meaningful on Earth Science SE to use earth as a tag... I guess the idea in those questions is to refer to the Earth as a whole, but I don't know what would be the best alternative. I have some ideas, such as global geodynamic or something similar but it's probably uncommon for most users.