The demise of an entire ocean is almost too enormous to grasp, but as the expedition sails deeper into the Arctic, the colossal processes of breakdown are increasingly evident.

The first fragment of ice appears off the starboard bow a few miles before the 79th parallel in the Fram strait, which lies between Greenland and the Norwegian archipelago of Svalbard. The solitary floe is soon followed by another, then another, then clusters, then swarms, then entire fields of white crazy paving that stretch to the horizon.

From deck level it is a stunning sight. But from high above, drones and helicopters capture the bigger, more alarming picture: a slow-motion blast pattern of frozen shrapnel radiating from the high Arctic southwards through this strait, which is the interchange of 80% of the water between the ice cap and the world’s oceans.

Play Video 0:40 Stunning drone footage captures scale of Arctic ice melting - video

This is where ice floes come to die, and the cemetery is filling faster each year, according to the leader of this scientific expedition, Till Wagner, of the University of North Carolina Wilmington (UNCW). One of the objectives of the expedition is to investigate why the collapse of Arctic ice is happening faster than climate computer models predict and to understand what this augurs for the rest of the planet.

Greenpeace vessels begin pole-to-pole ocean journey - in pictures Read more

The melt is not simply a seasonal process. The natural thaw that starts with spring’s warm weather is being amplified by manmade global heating. The Arctic has heated up by 2C above pre-industrial levels, twice the global average. Some hotspots, including parts of the Fram strait, have warmed by 4C. There are variations from year to year, but the trend is clear and accelerating. Sea ice is melting earlier in the spring and freezing later in the autumn. Each summer it thins more and recedes further, leaving greater expanses of the ocean exposed to 24-hour sunlight. This is driving back the frontiers of ice and fragmenting one of the planet’s most important climate regulators. It is also creating a series of feedbacks that are accelerating the Arctic melt. Several are only partially understood.

Facebook Twitter Pinterest Greenpeace’s MV Arctic Sunrise anchored next to Dahlbreen glacier in Svalbard. Photograph: Denis Sinyakov/Greenpeace

The team and crew set out on the Greenpeace ships Arctic Sunrise and Esperanza from Svalbard’s port at Longyearbyen in May, less than a month after the sea ice reached yet another record low. At the start of the melt season, this is a fast-changing body of water. The Greenpeace crew say high winds and strong currents have pushed the ice front southwards by 50 miles in the last four days. The floes appear on the radar screen as a thickening swarm of yellow flies. When the bands of ice cannot be evaded, they are nosed aside by the prow, scraping the hull as we pass. Occasionally, they are smashed through with a clanging jolt.

The combinations of shade and shape are endlessly varied: here an opal gorge, there an emerald grotto, some floes so perfectly rectangular they could have been cut with a guillotine, others as jagged as a Cornish coastline. Many surfaces are topped with snowy peaks or crenellations; others look as flat as a tennis court. Depending on the sea motion when it was formed, the ice can be frazilled into loose white ice crystals, pancaked into bluish layers, or dense, hard nilas so transparent that they appear to be black.

But all the floes have one thing in common: they are steadily diminishing as they head south. On the surface you can see the outliers, tiny translucent chunks that are destined to shrink to the size of an ice cube before they finally melt into the Atlantic.

Facebook Twitter Pinterest Floating ice floes along one edge of Fram Strait. Photograph: Denis Sinyakov/Greenpeace

Since the start of the satellite era in 1979, the summer Arctic has lost 40% of its extent and up to 70% of its volume, says Wagner. Other scientists calculate the rate of decline at 10,000 tonnes a second. Much of the multiyear ice is now gone. Most of what is left is the younger, thinner layer from the previous winter, which is easier for the sun to melt and the wind to push around. Wagner expects ice-free summers in 20 to 40 years, which would allow ships to cruise all the way to the north pole.

Ice-free summers are 10 times more likely if the world warms by 2C rather than 1.5C, according to the United Nations Intergovernmental Panel on Climate Change. The body of top international scientists said last year that the Arctic and coral reef systems were the ecosystems at greatest risk.

The Fram strait is one of the few places in the Arctic where the ice extent has not declined, but that is probably not good news. One of the theories being tested on this trip is whether this is because more ice is being flushed through this channel. The pace of change is mindblowing, Wagner says. “What we are looking at is whether this exit is accelerating.”

The team is also examining other processes to help gauge the health of the ocean and how it is changing. To get a baseline, they conduct tests at intervals along the 79th parallel. First, they identify a suitably sized floe, then an armed polar bear guard ensures there are no predators and checks the solidity of the ice. After the all-clear, a dozen or so scientists and volunteers pull up on motor dinghies to drill, measure, take water samples and extract cores. Every five metres a square trench is shovelled out of the surface snow down to the hard surface of the ice below. All the while, the floe drifts with the current and bobs gently in the swell. On the ship, crew members test the water at different depths for acidity, temperature, turbidity, dissolved oxygen and acidification.

Facebook Twitter Pinterest Dr Till Wagner (right) drills an ice floe in the Fram Strait. Photograph: Denis Sinyakov/Greenpeace

In the past, the data would have been related only to physics and chemistry, but there is a growing awareness of the crucial climate role played by marine organisms, so biologists are also part of this interdisciplinary team. They scan the horizon for whales, seals and walruses and use tightly meshed zooplankton nets to capture teeming masses of minute shrimp- and wormlike creatures.

Far more than polar bears, these tiny creatures and the algae on which they graze play an essential role in not just the local ecosystem, but the global climate and food chain, according to a growing body of research. The largest synchronised movement of biomass on the planet is the vertical migration of zooplankton, which happens daily as the tiny creatures move from the depths to surface feeding waters. The area under the ice floes is such a rich source of nutrients that it has been described as an all-you-can-eat buffet for zooplankton.

Researchers are discovering that the floes are home to an extraordinary variety and abundance of life, even during the four-month darkness of Arctic winter. Depending on how the ice is formed, it can be permeable and elastic, with space inside that can be colonised by bacteria, fungal spores and the tiny creatures that feed on them such as the transparent jellyfish Sympagohydra tuuli, which squeezes inside the cracks in the ice to hunt for food. The green and yellow shades at the base of floes indicate the presence of phytoplankton – algae that use sunlight to convert carbon dioxide and water into oxygen and energy. These are the pastures on which the zooplankton graze. Most important among them are copepods, a fat-filled staple in the diet of whales and fish.

Facebook Twitter Pinterest A microscope image of zooplankton collected in the Fram Strait. Photograph: Denis Sinyakov/Greenpeace

Together, millions of these species form an oceanic pump, says Mattias Cape, a biological oceanographer with the University of Washington. Phytoplankton help the oceans produce more oxygen than all the world’s forests. They also sequester carbon dioxide more effectively because copepods and the bigger creatures that eat them take the gas down to the depths, where it can be stored for hundreds of years. Nowhere is this pump more effective than near the poles – the zooplankton here are bigger, so they sink deeper.

But this is changing. When Cape observes the zooplankton through a microscope in the hold of the ship, he can see that the chubby Arctic copepods have competition from their slimmer and shorter Atlantic counterparts. This invasion has been recorded in other parts of the ocean. “We see a shift from big to small, which is a concern, because it will make this pump action weaker,” he says. The study may help to explain why the Arctic is losing oxygen faster than almost anywhere on Earth. Another factor is that cold water absorbs more carbon dioxide, which gives it high levels of acidity. “We talk about the ocean being hot, sour and out of breath,” Cape says.

If the Arctic were a patient, doctors would be alarmed by its vital signs. As well as hot flushes, asthma and contamination (the researchers are following up on studies that suggest the Fram strait has one of the highest levels of microplastics in the world), the ocean has also been diagnosed with a weakening of its immune system. For centuries, the Arctic’s distinctive character has been shaped by a layer of cold, relatively fresh water just below the surface, produced by melting ice and glaciers. This has insulated the sea ice from the warmer, denser, saltier waters of the Atlantic currents that flow in the depth. But this stratification is collapsing as temperatures rise.

Facebook Twitter Pinterest The Arctic is losing oxygen faster than almost anywhere on Earth. Photograph: Denis Sinyakov/Greenpeace

The oceanic shift was outlined in a landmark study published last year in Science, which found that the water density and temperature of the Fram strait and Barents Sea were increasingly like those of the Atlantic, while further east, Russia’s Laptev sea was starting to resemble what the Barents used to be. “The polar front is shifting,” the lead author, Dr Sigrid Lind, of the Institute of Marine Science and the University of Bergen, told the Guardian this year. “The Arctic as we know it is about to become history. It will go when stratification breaks down completely and the Atlantic takes over the whole region.”

This has not happened for more than 12,000 years, but the shift is well under way. First to succumb, according to Lind, will be the Barents Sea, which will have no fresh water by 2040, then the Kara sea. The consequences will be far-reaching. The food chain is already affected. Atlantic species of cod, herring and mackerel are moving northwards. For the next 20 to 30 years this could boost fishing catches, but forecasts by Norway suggest boom will turn to bust later as the waters grow too warm for fish larvae.

There are signs elsewhere in the Arctic that the arrival of smaller Atlantic copepods may be associated with a decline in the whale populations that rely on them. This is not yet certain. It may be possible for whales simply to eat more. But Heather Koopman, a marine physiological ecologist at UNCW, says she is concerned that the speed of change is outstripping some species’ ability to evolve.

“The bowhead whale, for example, can live for 200 years, so some are having to adapt to a modern climate with faculties developed in the Georgian age,” Koopman says. “Things are moving far too rapidly for them to keep up. Perhaps small invertebrates can cope with year to year because their breeding cycles come round more quickly. But for a 200-year-old whale, how can they change that fast? Things are so accelerated.”

Facebook Twitter Pinterest The Dahlbreen glacier at nighttime. Photograph: Denis Sinyakov/Greenpeace

For humanity, the biggest impact is on the weather. The area between the cold pole and the warm equator is a ramp that propels weather fronts across continents. Its incline has always varied from season to season as the icecap expands in winter and shrinks in summer, providing a global pulse that pumps sea and air currents around the world. But that frozen heartbeat is warming and weakening as the Arctic becomes more like the Atlantic. Lind speculates that ocean destratification is the key driver for ice loss, which in turn affects the jet stream, weakens the polar vortex and can lead to heatwaves in the southern US and cold weather extremes in Asia. “The rules of the game are changing. We seem to be seeing large-scale weather pattern changes connected to the shrinking Arctic. As the Arctic becomes history, we need to understand how it affects the globe.”

Such concerns are part of the motive for a new wave of international research. From September a German research vessel, Polar Stern, will drift across the north pole and be frozen into the ice over the winter so that 600 scientists can conduct tests billed as the biggest ever study of the Arctic. Norway has just launched a new icebreaker, the Kronprins Haakon, which will embark on a series of trips over the next six years in conjunction with Birmingham University to examine ocean acidification and food chain contamination. “This area has been ice-covered and will change once the trawlers move in,” says Marit Reigstad, of the Arctic University of Norway. “We need to know more before we do anything. We need to develop new laws and regulations for the area.”

Greenpeace is calling for the central Arctic to be declared a protected marine area. On World Oceans Day on Saturday, campaigners will rally outside parliaments in several countries to demand a new global treaty to end the overexploitation of the high seas.

Facebook Twitter Pinterest Arctic Sunrise anchored next to the Dahlbreen glacier. Photograph: Denis Sinyakov/Greenpeace

The US, China, Russia, Canada and Korea are more focused on the commercial and strategic opportunities that are emerging as the Arctic melts and opens. Fishing, mining, tourism and cargo shipping could profit, but any gains will be far outweighed by the costs of a diminished Arctic. A recent study found melting permafrost alone would cause $70tn of damage, 10 times the expected revenue from resource extraction and new trade routes.

Adding further to that enormous, existential reckoning could be other feedback loops that are now being investigated. Among them is the possible loss of the Arctic’s soothing influence on the northern seas. Bands of ice buffer the waves. Inside the floes, the ship’s passage is far smoother. Wagner speculates that when this calming barrier melts away, the swells will churn the ocean and bring warm water to the surface, which could further accelerate the fragmentation of the polar cap. It is yet another potentially grim area of study, but he views it – like other signs of the Arctic’s demise – with professional composure.

“Emotionally, I detach myself,” he says. “The Arctic is an object of study. It’s like a doctor observing a patient to see how sick they are.” He is reluctant to offer a prognosis without a longer-term study, but he says the physics make a recovery extremely unlikely.

“I have to hurry up or my science will become archaeology,” the 34-year-old jokes. “There will still be sea ice during the winter, but in the summer it will probably disappear. It won’t be the death of the Arctic, but will be the end of the Arctic as we know it.”

Facebook Twitter Pinterest Walruses on sea ice near the Arctic Sunrise. Photograph: Denis Sinyakov/Greenpeace

• This article was amended on 12 June 2019 because an earlier version referred to the Nansen Legacy as a new icebreaker that Norway had launched. That meant to say the Kronprins Haakon; the Nansen Legacy is an Arctic research project. This has been corrected.