Scientists behind a landmark study of the links between oceans, glaciers, ice caps and the climate delivered a stark warning to the world on Wednesday: slash emissions or watch cities vanish under rising seas, rivers run dry and marine life collapse.

Days after millions of young people demanded an end to the fossil fuel era at protests around the world, a new report by a UN-backed panel of experts found radical action may yet avert some of the worst possible outcomes of global warming.

But the IPCC Special Report on the Ocean and Cryosphere in a Changing Climate was clear that allowing carbon emissions to continue their upward path would upset the balance of the great geophysical systems governing oceans and the frozen regions of the Earth profoundly.

The report from the Intergovernmental Panel on Climate Change says those changes are having direct impacts on human health — including in Canada.

"It doesn't matter where you live in the world or where you live in Canada, the impacts of climate change are going to impact everyone," said Sherilee Harper, an epidemiologist at the University of Alberta who was one of the lead authors.

Finished on Tuesday in a last 27-hour session of talks in Monaco between authors and representatives of governments, the report is the culmination of two years of work by the UN-backed Intergovernmental Panel on Climate Change (IPCC).

Compiled by more than 100 authors who crunched 7,000 academic papers, the study documents the implications of warming oceans, fast-melting ice sheets in Greenland and Antarctica, and shrinking glaciers for more than 1.3 billion people living in low-lying or high mountain regions.

A woman carrying her daughter walks through floods caused by rising sea levels in Pekalongan, Indonesia, in June 2017. Indonesia is known to be one of the largest archipelagic countries in the world, with more than 17,000 islands. But coastal areas have been threatened by rising sea levels, with most of the small islands being only one metre above sea level. (Ulet Ifansasti/Getty Images)

The report found:

Seas are now rising at 3.66 millimetres a year, which is 2.5 times faster than the rate from 1900 to 1990.

The world's oceans have already lost one per cent to three per cent of the oxygen in their upper levels since 1970 and will lose more as warming continues.

From 2006 to 2015, the ice melting from Greenland, Antarctica and the world's mountain glaciers has accelerated. They are now losing 653 billion tonnes of ice a year.

Arctic June snow cover has shrunk by more than half since 1967, down nearly 2.5 million square kilometres.

Arctic sea ice in September, the annual low point, is down almost 13 per cent per decade since 1979. This year's low, reported Monday, was tied for the second-lowest on record.

Marine animals are likely to decrease 15 per cent, and catches by fisheries in general are expected to decline 21 per cent to 24 per cent, by the end of century because of climate change.

The changes are affecting people. For example, there have been outbreaks of vibrio poisoning, causing gastrointestinal illness, from shellfish living in warm water.

"We're starting to see outbreaks of different vibrio species in places we did not see them before, and that's been attributed to ocean warming," Harper told the Canadian Press.

Arctic communities will be most directly affected.

"For both the Arctic and west of B.C., the report talks about how the decreased catch of fish and seafood will impact nutrition for the people who live there," Harper said.

"We'll see anywhere from a 20 to 30 per cent decrease in their nutrient intake because of those climate change impacts on fish distribution."

More than 120,000 Canadians live in northern communities.

The report projects sea levels could rise by one metre by 2100 — 10 times the rate in the 20th century — if emissions keep climbing. Looking further forward, the rise could exceed five metres by 2300.

B.C., Maritime floods will surge

By 2060 — within the lifetime of about half of Canadians now living — coastal floods off British Columbia and the Maritimes that used to occur once a century will be annual events, it says.

Water availability across Western Canada will be disrupted.

Crucial kelp forests and seagrass meadows that nurture sea life off both east and west coasts are threatened. Kelp forests shelter thousands of species, from fish to seals to seabirds.

"The decline of kelp forests is projected to continue in temperate regions due to warming, particularly under the projected intensification of marine heatwaves, with high risk of local extinctions," says the report.

A visitor stands at a spot that in 1963 was under the ice of the Pasterze glacier as the glacier in its current state is seen behind on Aug. 13 near Heiligenblut am Grossglockner, Austria. The Pasterze, Austria's largest glacier, has lost over half its volume since 1850, and its tongue, shrinking in both width and depth, has retreated at least 2.6 kilometres. (Sean Gallup/Getty Images)

In the Himalayas, glaciers feeding 10 rivers, including the Ganges and Yangtze, could shrink dramatically if emissions do not fall, hitting water supplies across a swathe of Asia.

Thawing permafrost in places such as Alaska and Siberia could release vast quantities of greenhouse gases, potentially unleashing feedback loops driving faster warming.

The IPCC galvanized global concern over climate change in October when it published a report that showed the world would need to halve emissions over the next decade to stand a chance of meeting the temperature goals in the 2015 Paris Agreement.

Following a subsequent report published last month on land use and farming, the IPCC Special Report on the Oceans and Cryosphere — or "frozen world" — was the final piece in a scientific jigsaw revealing the global sweep of climate impacts.

Released two days after a one-day UN climate summit in New York closed with scant signs of transformative action by major economies, the latest report underscored the gulf between warnings from science and the policies of most governments.

"If we aren't able to have ambitious action that enables us to reduce greenhouse gas emissions, we will have these high-end impacts," said Nerilie Abram, a paleoclimatologist at the Australian National University and an author of the study. "We're at a point where we have a decision to make."

Triple blow

Carbon emissions, which hit a record high last year, are projected to inflict a devastating toll on oceans, which have so far buffered almost all the man-made warming generated by burning coal, oil and gas.

The report says scientists are now "virtually certain" that the oceans have warmed unabated since 1970. Since 1993, it's likely the rate of warming has more than doubled, with more than 90 per cent of excess heat going straight into salt water.

As the oceans get hotter, what are known as "marine heat waves" are becoming more intense, turning coral reefs boneyard white — including much of Australia's Great Barrier Reef. As more carbon dioxide dissolves in the water, the oceans are also becoming more acidic, damaging ecosystems.

A picture snapped June 2, 2018, shows dead fish on a beach covered with trash, including many plastic items, in the Hann Bay in Dakar, Senegal. (Seyllou/AFP/Getty Images)

'We have a choice'

The rising temperatures are in turn starving the upper layers of the water of oxygen, suffocating marine life, creating growing dead zones and disrupting the circulation of ocean currents, which then unleashes more disruptive weather on land.

While animals in Arctic seas are expected to increase, that comes at the price of dramatic declines everywhere else in the world.

The authors say long lag times at work in oceans mean some of these changes will inevitably intensify over centuries — even if the world stopped emitting all its greenhouses gases tomorrow.

The report ends with a plea for governments to co-operate and calls for "profound economic and institutional transformative change."

"Nations need to act," Harper said.

"But there's also ways that provinces can act and municipalities can act. If we're going to have transformational change in governments, we need to do that from international to national to local.

"We have to act now. We need to start making those decisions today, because the decisions we make today will have impacts centuries from now."

"Everybody in the world will be affected by the changes we are seeing," Michael Meredith, an oceanographer with the British Antarctic Survey and one of the report's authors, told Reuters.

"The key thing that's coming out of the report is that we have a choice. The future isn't set in stone."

But if emissions are allowed to continue rising then the impacts are likely to start accelerating so rapidly that they will overwhelm societies' capacity to cope, with the poorest and most vulnerable communities and countries succumbing first.

"In a high emissions scenario, the chances of having any reasonable foothold to deal with the impacts becomes much smaller," said Matthias Garschagen, chair in human geography at LMU Munich, another author. "Systems are changing in a way in recent history that they've never changed before."