Between the Celebes Sea and the Makassar Strait, the last sea nomads live almost permanently on their boats, “balanced on the thread of the horizon”. Their legend has it that a flood swallowed up their land of origin and that only those who were on board survived. The clan chief Baïsaran and his son Kolnirdati take from the sea only what they need, and humour its spirits: no cat on board, it would wake “the guardian spirit of the oceans”. They go where the current goes, for the current commands the fish. A diver-hunter goes down to fifteen metres and walks on the bottom as if on land; a shaman told him that a woman was waiting for him there. But the sky is changing. “These days, the currents and the winds are stronger and stronger. And we don’t know why”, says Baïsaran. The elders knew how to chase away the rain by blowing on the clouds; their formulas are forgotten, like their skills: no one knows any more how to hollow out lepas, the traditional boats, and the engine has replaced the sail.

The Bajau Laut are “like sentinels of the climate”: since the dawn of time, their generations have followed one another, living off the sea.

The Bajau Laut “respect the sea so that it will feed them”; they are “intimately bound to the climate and to the turmoil of the sky”.

“Coral is nothing other than a gelatinous creature of the jellyfish family”: a plankton fixed to the reef, which pumps carbon from the air.

Around them, the settled communities fish with cyanide and dynamite. The Bajau Laut pay the price.

On the shore of the Barents Sea, the Khodovarikha weather station is now nothing but ice, rust and huts. Slava lives there alone, a polarnik, “a cosmonaut of meteorology”, resupplied once a year, in summer, when the sea frees itself of its ice. Every three hours, he records the temperature, the pressure, the precipitation; once a week, he drills through the pack ice to measure its thickness, for this white mantle reflects the sun’s rays and protects the ocean from warming. In the age of satellites, his bulletins, dictated every morning over a temperamental radio, are still used by forecasters all over the world. The Arctic and the Antarctic are, to him, “the furnaces of the weather”. He has seen the pack ice retreat: a few years ago, it stretched for two or three kilometres; last year, it had almost disappeared. Warming does not frighten him; he simply says he has “fallen in love with this corner of the world”. But from the top of the old lighthouse, which he will end up chopping into firewood, he names the price of this love: “It’s one thing to want to save the Arctic in front of your television. It’s another thing to come and work here yourself.”

When he started out, Slava still changed the lamps here; today, “when the wind blows, the whole lighthouse creaks”.

The staircase has rotted and collapsed: the cabin from which the pack ice was studied is waiting for the carpenters.

“Minus 1.4 degrees.” Reading after reading, the same finding: spring comes earlier, autumn later.

“Man is like Slava, able to bow before the sky and embrace the sea.”

“It’s one thing to want to save the Arctic in front of your television. It’s another thing to come and work here yourself.”Slava, polarnik

In Paita, on the coast of Peru, the Humboldt Current brings cold, nutrient-rich waters up from the depths: this is where the sea catches its breath, and the country is the world’s second-largest producer of fish. Captain Victor Silva hunts the pota, the giant squid, aboard the Blanca Eugenia, without a lifeboat, with sailors who cannot swim. “The sea is part of my family”, he says, and when it comes to fishing, he leaves it to God. Yet it is the current that decides, and the plankton it carries. The potas are now moving south: as the planet warms, the plankton spreads into cold waters, and the whole food chain follows it towards the poles. Victor also sees the factory ships coming, Chinese, Korean, Japanese, which have already all but wiped out the sardine and the anchovy. Either the fishermen impose quotas on themselves, or in fifteen years there will be no more pota. In one night, his crew hauls in twelve tonnes of squid. Then he takes his daughter Bianca to where two currents mingle, to show her the sea lions and the penguins, and what we risk losing.

“Half the world’s population lives less than 100 km from the coast.” Paita lives off the pota.

The previous trip went badly: broken engine, raging sea. The next day, it was “as calm as a swimming pool”.

Three in the morning. Something in the air and in the water tells the fishermen that the pota is close by.

“The predator becomes prey”: the Humboldt squid, “the devil of the deep”, as the fishermen call it.

Off Greenland, Tara makes her way through the ice: in less than fifty years, the Arctic has lost half its summer pack ice. The oceanographer Gaby Gorsky trails his net at the surface and hauls it up full of a “soup” of copepods. Plankton, he points out, makes up 90 to 95% of the biomass of the oceans; it absorbs 40% of the CO₂ and releases half the oxygen we breathe. It was these organisms that, billions of years ago, set the climate machine in motion. On the islands, the ornithologists Olivier Gilg and Brigitte Sabard count the nests: with one or two degrees more, new predators are coming up from the south, and rain, once unknown in the north of Greenland, is wiping out entire colonies. A latecomer threatens the balance: plastic, which has existed for only sixty years and is found in every ocean. In the Celebes Sea, on an atoll where nomads have had to settle, Rajamia picks up red debris on the beach to make octopus lures. “Westerners invented plastic. […] We improvise with their rubbish.” The octopuses, for their part, are becoming scarce, like everything else.

A captain, a crew of about ten: the expedition vessel Tara has been sailing the oceans for more than ten years.

In 2015, the pack ice set a new melting record.

Plankton captures carbon dioxide and, as it decomposes on the ocean floor, turns into limestone or oil.

The danger is not the iceberg but the ice at water level, “ten times bigger underneath”, invisible to radar.

Birds are markers of the climate. “A few more degrees and Greenland will turn green.”

Winter in Antarctica, summer in Greenland to nest: the Arctic tern, “a true example of resilience and hope”.

Policies of forced settlement now condemn to immobility families who used to live to the rhythm of the current.

“It’s like us humans: we let ourselves be taken in by whatever glitters.”Rajamia