NOT much is known about the sea, it is said; the surface of Mars is better mapped. But 2,000 holes have now been drilled in the bottom, 100,000 photographs have been taken, satellites monitor the five oceans and everywhere floats fitted with instruments rise and fall like perpetual yo-yos. Quite a lot is known, and very little is reassuring.

The worries begin at the surface, where an atmosphere newly laden with man-made carbon dioxide interacts with the briny. The sea has thus become more acidic, making life difficult, if not impossible, for marine organisms with calcium-carbonate shells or skeletons. These are not all as familiar as shrimps and lobsters, yet species like krill, tiny shrimp-like creatures, play a crucial part in the food chain: kill them off, and you may kill off their predators, whose predators may be the ones you enjoy served fried, grilled or with sauce tartare. Worse, you may destabilise an entire ecosystem.

That is also what acidification does to coral reefs, especially if they are already suffering from overfishing, overheating or pollution. Many are, and most are therefore gravely damaged. Some scientists believe that coral reefs, home to a quarter of all marine species, may virtually disappear within a few decades. That would be the end of the rainforests of the seas.

Carbon dioxide affects the sea in other ways, too, notably through global warming. The oceans expand as they warm up. They are also swollen by melting glaciers, ice caps and ice sheets: Greenland's ice is on track to melt completely, which will eventually raise the sea level by about seven metres (23ft). Even by the end of this century, the level may well have risen by 80cm, perhaps by much more. For the 630m people who live within 10km (six miles) of the sea, this is serious. Countries like Bangladesh, with 150m inhabitants, will be inundated. Even people living far inland may be affected by the warming: droughts in the western United States seem to be caused by changing surface temperatures in the tropical Pacific.

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.

Full story: http://www.economist.com/opinion/displaystory.cfm?story_id=12853926