The End of Earthly Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 206

though, then the dying Earth would be undoubtedly dramatic. The
stifling heat, sky-high humidity, and roiling clouds would generate
extraordinary firework displays of lightning strikes 118 and rolling
thunder—truly a spectacle worthy of the Götterdammerung , a twilight
of earthly gods and earthly powers.
Water vapour is a powerful greenhouse gas, and its build-up in the
ever-hotter atmosphere will raise temperatures yet higher. The warm-
ing will affect upper levels of the atmosphere, including, crucially, the
stratosphere. It is the stratosphere today that acts as a lid on the Earth's
water (see Chapter 3). A highly effective cold trap, it prevents anything
but minuscule amounts of water molecules from reaching the top of
the stratosphere. If the stratosphere can be kept sufficiently cold, then
the Earth could preserve its water supply for many billions of years.
However, in a warming stratosphere water vapour molecules can
drift into its upper levels, where they become vulnerable to being split
into separate hydrogen and oxygen ions by solar radiation. Once in
that state they—in particular the small, light hydrogen ions—can be
stripped out into space by the solar wind. This is the moist green-
house state, as elaborated particularly by the planetary scientist James
Kasting of Pennsylvania State University. 119 On such a pervasively
warm, humid world, an Earth's supply of ocean water can be siphoned
off into space in something of the order of a billion years.
Another factor, over that time, will be the weakening and subse-
quent loss of the Earth's magnetic field. The Earth's iron-nickel core
was originally completely molten, before beginning to freeze into the
solid inner core and the liquid outer core, currents within which gen-
erate the Earth's magnetic field. The solid core, though, is growing as
the liquid iron solidifies (currently at something like 5,000 tonnes a
second). A billion years from now the core will be entirely solid, and
so unable to generate an effective protective magnetosphere. It will be
all the easier, then, for the solar wind to remove the Earth's water.