The following is an excerpt from Ocean Worlds: The story of seas on Earth and other planets, by Jan Zalasiewicz and Mark Williams

We are getting ever closer to discovering a multiplicity of far-distant ocean worlds, some perhaps life-bearing. Planets, we know, are commonplace in the distant heavens. The next generation of satellites and telescopes will find and examine numbers of Earth-sized planets in the habitable zone, and will glimpse evidence of atmospheres, oceans, and perhaps signs of life itself, via planetary chemistries pushed out of equilibrium.

By then we should also have learned more about the hidden ice covered oceans of such nearer bodies as Europa, Triton, and Callisto (where liquid water has been smuggled far beyond the normal habitable zone by the power of tidal energy), as well as understanding the history of the oceans that seemingly long ago covered, however briefly, the surfaces of our near neighbours Mars and Venus. Each of these planets, present and past (relatively), near and far, will have oceans quite as various and complex as our own. We see them now as simple images, cartoons almost — blank canvases to which we hope to add detail. Those far oceans will be stirred by currents and by differences in temperature and chemistry, each in a specific pattern and combination, and each different — some of them very different — to the patterns and moods we see in our oceans.

There will be different flavours of ocean out there too. We cannot imagine that any of those oceans will be of pure water. Rather, they will be complex chemical cocktails of dissolved salts and minerals — and of organic compounds too — some dilute, others more concentrated than even the dense brines of the Earth’s Dead Sea.

Those far oceans will not be constant, but will evolve through time as their parent planets and stars evolve, either gradually or suddenly and catastrophically. They will, over geological timescales, change in volume, shape, temperature, and chemistry, some waxing larger, others drying out or freezing as water moves between planetary interior, crustal surface, atmosphere, and outer space. The new astronomy will capture oceans young, middle-aged, and old, being born and dying. We are on the verge of not just a new chapter in oceanography — or exo-oceanography, if you like — but of setting up an entirely new library of oceans, for the diversity and complexity of cosmic oceans will be beyond anything that we can dream of. Truth will be stranger than both fiction and scientific hypothesizing alike — and that is even before we think of the kind of life forms that may be evolving in those extra-terrestrial waters.

As these new seascapes open up in front of us, we are here on Earth at another transition: the likely transformation — and biological impoverishment — of our own Earthly oceans that surely still represent a cosmic jewel, even on this widest of universal canvases. For it seems very likely that, over the coming decades, the oceans of Earth will undergo a transformation the like of which has not been seen for many millions of years. The changes wrought by warming, acidification, overfishing, and pollution threaten to kill off not just many species, but also whole ecosystems — not least the extraordinary biological riches of the coral reefs.

It is still — just — not too late to stop or slow this marine holocaust, and there have been many useful initiatives, both national and international.

The setting up of marine reserves helps sharply depleted fish populations to recover to something like their former numbers. Current discussions on how the International Law of the Sea may evolve have as one central theme the effects — and possible control — of harmful human activities.