Yes, we should merge them all because we simply don't have the numbers to launch the sites individually and to infuse them with the numbers necessary to prevent them from getting closed down in the same way that Astronomy.SE and Economics.SE got closed down due to inactivity - bear in mind that there's also significantly more Economics and Astronomy activity on the Internet than there is activity on all the geoscience subjects combined.
In general, there are not many people who discuss these subjects on the Internet (I know, since I'm quite active on the associated topics on Quora, Reddit, and Physics Forums), so something like this would definitely grow if all merged into such a topic. We can follow the model of the main StackOverflow, where there are a huge number of discrete unrelated topics, and people can subscribe to individual tags.
All of these topics would fit under the geoscience journals like Nature Geoscience, and all of them would fit under an American Geophysical Union meeting. Planetary Science topics should go under this too, but it does seem that most of the Planetary Science activity on the Internet ends up on Astronomy forums. I'd be willing to append the word "Planetary Science" (and/or Oceanography) to the above description if people don't consider it too wordy.
It's not just the AGU either - numerous universities combine them all into one area - there's Caltech (Geological and Planetary Sciences + Environmental Science and Engineering), MIT (Earth, Atmospheric, and Planetary Sciences), UChicago (Geophysical Sciences), Columbia (Earth and Environmental Science), Yale (Geology & Geophysics), and Brown (Geological Sciences). And even when they are all combined, they still only reach sizes that are similar to the sizes of other departments found on campus. There are only roughly as many geoscientists (of all subdisciplines combined) as there are physicists or chemists in most universities.
Moreover, in many cases, geologists often value the information contained in fields like oceanography. Geologists who want to study the history of the Earth (or stratigraphers), for example, might often talk more with oceanographers and paleontologists than with people who are more into deep-earth processes, even though both stratigraphers and deep-earth people are technically geologists.
Atmospheric Science and Oceanography are quite related to each other (through fluid dynamics), and we must also remember that there are only a handful of strong oceanography departments in the nation. Most schools (especially strong ones like MIT) put their oceanography in their Geoscience departments.