And then there are the red tides of algal blooms, the plagues of jellyfish and the dead zones where only simple organisms thrive. All of these are increasing in intensity, frequency and extent. All of these, too, seem to be associated with various stresses man inflicts on marine ecosystems: overfishing, global warming, fertilisers running from land into rivers and estuaries, often the whole lot in concatenation.

Some of the worrying changes may not be entirely the work of man. But one that surely has no other cause is the dearth of fish in the sea: most of the big ones have now been hauled out, and the rest will be gone within decades if the pillage continues at current rates. Indeed, over three-quarters of all marine fish species are below, or on the brink of falling below, sustainable levels. Another change is the appearance of a mass of discarded plastic that swirls round in two clots in the Pacific, each as large as the United States. And the sea has plenty of other ills, as our special report this week explains.

Neptune would weep

Each of these changes is a catastrophe. Together they make for something much worse. Moreover, they are happening alarmingly fast—in decades, rather than the aeons needed for fish and plants to adapt. Many are irreversible. It will take tens of thousands of years for ocean chemistry to return to a condition similar to its pre-industrial state of 200 years ago, says Britain’s most eminent body of scientists, the Royal Society. Many also fear that some changes are reaching thresholds after which further changes may accelerate uncontrollably. No one fully understands why the cod have not returned to the Grand Banks off Canada, even after 16 years of no fishing. No one quite knows why glaciers and ice shelves are melting so fast, or how a meltwater lake on the Greenland ice sheet covering six square kilometres could drain away in 24 hours, as it did in 2006. Such unexpected events make scientists nervous.

... contd.