Question

I was wondering, is there more or less water on the Earth than there used to be a million years ago?

Answer 1

In that sort of time frame I reckon the answer is roughly the same. It does increase by a small amount. I think the stated geological figure is about 1 inch every 20,000 years or so. That's extrapolated over the lifetime of the Earth. Most of the water we have comes in the form of comets hitting the Earth. When the Earth first formed, of course, there was a disc of stellar debris which was basically the material left over from when the sun formed. Out of that we condensed planets as planetesimals. They slowly aggregated more material and formed bigger planets. The material left over was comets and other asteroids and other bodies which were out there in orbit.

Comets are viewed as what are called, "dirty ice balls," basically lots of water with some other stuff chucked into them. Occasionally one of them's going to cross the orbit of another planet, get drawn in by gravity and crash land. Most of the water on Earth, we think, comes from comets, originally. Given that they're not actually that common these days but over the millions of years time scale I'd say the amount of water on Earth hasn't changed a huge amount. I would add that it probably is increasing very, very slightly.