The End of Earthly Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 203

nearer 2 cubic kilometres), while the amount being erupted each
year appears to be much less—perhaps something like a quarter of
a cubic kilometre. 116 This suggests that something of the order of a
cubic kilometre of water is being siphoned off from the ocean and is
disappearing into the depths of the Earth, to simply dissolve into the
minerals of the mantle.
The Earth's oceans hold some 1.3 billion cubic kilometres of water,
so by projecting presently estimated rates of water loss into the future
the Earth's surface will have lost most or all of its oceans a billion
years from now.
It may not be quite as simple as that. The Earth's interior is changing
through time. It is cooling slowly, and its chemistry is changing as vol-
canoes keep erupting magma, derived from that interior, on to the sur-
face. Its chemistry and physical properties are changing, too, as water is
mixed into it from the oceans. So the balance of water gained and lost
between surface and exterior must change across long periods of geo-
logical time. Attempts to feed this more sophisticated understanding
into a model suggest 117 that the rate of water uptake by the Earth's man-
tle will slow down over time—and that the Earth can never lose all of
its oceans by that means, but will stabilize with over half of its volume
still at the surface. A billion years from now, the oceans might only
have lost a quarter of their volume by leakage into the mantle.
This would still look like a different Earth, and even this relatively
small loss might have big implications for life on Earth. With a quar-
ter of its volume gone, the oceans will not flood over on to the conti-
nental areas as they do today, but they will simply lap up against the
relatively steep slope that separates ocean basin from continental
mass. The continents will therefore loom larger, when viewed from
outer space. That will mean the end of shallow seas, and so the end of
many of the habitats that today we commonly think of as 'the sea',
from the North and Irish Sea around Britain, to the Baltic, to the