| MadSci Network: Astronomy |
Great question!
I'll answer the easy part first: No. If the amount of water that
"cushions" the Earth (and really supplies the tides that are created
by the pull of the Moon on the Earth) changes, nothing would happen.
This is because the amount of water on Earth only amounts to about
0.02% of Earth's entire mass (1).
As to a change in lunar orbit: Yes, this could severely affect the Earth, provided the orbit became smaller. Newton's Universal Law of Gravitation tells us that the force between 2 objects is inversely proportional to the square of the distance between them (2). So, the smaller the distance, the greater the force. The Roche Limit is the smallest distance that a satellite can orbit from the center of a planet without being torn apart by tidal forces (3). So, as long as the object (Moon) does not get closer than the Roche Limit, tidal forces will not break the object apart. Currently, the Moon is about 60 Earth radii away, and the Roche Limit for the Earth-Moon system is 2.9 Earth radii (4).
If the Moon did find itself in such a small orbit, things would be disasterous for Earth and its inhabitants. Pieces of the Moon as large as 1000 km (or more) could crash onto the surface, obliterating continents and evaporating oceans in intense impact heating events. No worries, though, as a consequences of their formation, the Moon is actually drifting away from the Earth at a rate of about 3.8 centimeters per year (5).
To conserve angular momentum, then, as the Moon is drifting away (its orbit is getting bigger), the Earth is actually slowing down, at a rate of about 2 milliseconds per century (6), so our days are getting longer.
The Moon won't move away forever, though, and the whole process will stop around 15 billion years from now, when the Earth's rate of rotation, the Moon's rate of rotation and the Moon's orbital period are all around 55 days long---at this point, the Moon will be about 1.6 times farther from the Earth than it is now (7). However, our Sun is expected to "die" as a planetary nebula in about 4.5 billion years (6), puffing the outer layers of its atmosphere clear out to Mars' orbit, frying everything in its path, so Earth's slowdown really won't be an issue!
[Actually, because of mass loss during the red giant phase, the Sun's mass will decrease, which means that the Earth's orbit will increase in size. Although somewhat uncertain, it is thought currently that the Earth will avoid getting engulfed by the Sun. However, as the Sun becomes much larger, the solar tides on the Earth-Moon system will change, which will affect its evolution. Moderator]
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