We know remarkably little about the oceans that cover most of the Earth, provide half of our oxygen and help to regulate the climate. Maps of the ocean floor are less detailed than those of Mars or the moon. Marine biologists have discovered deeply weird and genuinely wonderful species: boxer crabs wielding anemones like weapons; the rope-like Praya dubia, up to 50 metres long; immortal jellyfish, which unlike any other known creature can revert from maturity to an earlier stage of development, akin to a butterfly becoming a caterpillar. But on one estimate we have identified less than a tenth of ocean-dwelling creatures.

What we can be certain about is that the extraordinary diversity of life in the oceans is under immense and growing threat. This week we learned that the last five years were the hottest on record. Global warming has heated the oceans by the equivalent of one atomic explosion per second for the last century and a half; in recent years the pace has accelerated to between three and six atomic bombs per second.

More than 90% of the heat trapped by greenhouse gas emissions has been absorbed by the seas. Billions of people living in coastal areas are under threat from rising sea levels, due both to the melting of ice – happening at a frightening and increasing rate – and the physical expansion of water as it warms. Climate change is making storms more powerful and disrupting the patterns of marine life on which communities depend.

But it is only the most comprehensive of the dangers. Carbon emissions are also causing ocean acidification. Intensive fishing, pollution and the exploitation of mineral resources are all taking their toll. Between a fifth and a quarter of marine species are already threatened with extinction; global marine populations have halved since 1970. Though only three humans have reached the deepest known point, the Mariana trench, our collective impact is felt there in the form of pollutants and plastics.

Recently, an expert warned that today’s children may be the last generation to see the glory of coral reef systems which have survived for tens of millions of years. Bleaching, first observed in 1983, has already affected up to a third of warm-water reefs. Scientists around the world are “farming” corals in an ingenious attempt to sustain them, but the only real solution is to stave off the worst climate change scenario.

The New York summit overseen by the UN secretary general this September will be a key test of whether nation states are serious about cutting emissions. The new reports provide yet more evidence of how urgent this is. As one expert on ocean warming notes in the Guardian today, it can still be tackled if we act immediately; this is a test of will, not ability. We know that the human consequences if we fail will be catastrophic. And yet, because we have still to explore the oceans more fully, we may never realise quite how much we have lost.