Seas are heating and rising faster and faster, a definitive United Nations report warns, with dire consequences for people and the planet.

The report, prepared by more than 100 scientists and published as a summary on Wednesday, found warmer oceans and icy places are killing off marine life and speeding up climate change.

Even in best-case scenarios for reducing greenhouse gas emissions this century, extreme floods that hit once every hundred years will, by 2050, be expected to strike some coastal cities and small-island nations annually.

"Although the oceans and cryosphere [frozen parts of the world] seem to be far away from most people, they are linked to everyone," said Lijing Cheng, an oceanographer at the Chinese Academy of Sciences and a lead author of the report.

"The key conclusion is the two big systems are changing — and changing quite rapidly — and have already serious impacts on people."

Almost all coral reefs are set to suffer local extinctions even if we cut carbon emissions

As waters warm and some marine life dies, fishermen and women may struggle to get by

Key Findings

- The rate of ocean warming has more than doubled since 1993 and sea level rise is accelerating because of Greenland and Antarctic ice loss.

- Under high-emission scenarios, mean sea levels are projected to rise 0.84m by the end of the century, 0.1m higher than the previously thought. Cutting emissions could halve this.

- Hotter oceans raise risk of "severe" impacts on nature and coastal ecosystems, even if global warming is limited to 1.5 degrees Celsius.

- Coordinated adaptation must be coupled with "urgent and ambitious" emissions reductions to avoid the worst effects of climate change.

Polar bears are among Arctic animals losing land to live in

Melting ice, rising seas

About one in every 10 humans lives less than 10m (33 ft) above mean sea level — and many have been struck by stronger storms and more frequent floods than their parents or grandparents.

Read more: What is the IPCC and what does it do?

Some of the report's authors warned that ocean warming effects will ripple inland, too, shrinking food supplies and forcing some people on exposed coasts to flee their homes.

Higher seas would make storms stronger and water saltier in Southeast Asia's Mekong Delta, for instance, potentially crippling harvests and raising food prices even in landlocked countries in other continents.

Meanwhile, thawing permafrost in the Arctic and Siberia pumps methane and carbon dioxide into the atmosphere, speeding up global heating in a potentially vicious feedback loop.

And a third of the ice in the Hindu Kush Himalaya mountain range — whose rivers nourish almost 2 billion people — is projected to vanish by the time today's children have grown.

The last five years have been the warmest on record in the ocean

Melting glaciers will first send freshwater rushing into the oceans, overwhelming coastal cities and low-lying islands, and then, once exhausted, dry up rivers, risking droughts downstream.

"Water is the connecting element," said Zita Sebesvari of the United Nations University, a lead author of the report specializing in what sea level rise means for coasts and islands. "What is happening is relocation of water on a large scale, from the frozen part of the planet to the ocean. And it causes problems at both ends."

'There is no time to wait'

Oceans, which have absorbed most excess heat from global warming, react slowly to changes in the climate. This means historic emissions will continue to warm seas even if we stop burning fossil fuels and cutting forests.

"As we cannot reduce the climate to its original state, there is also the need to adapt," said Hans-Otto Pörtner, a climatologist at the Helmholtz Center for Polar and Marine Research in Germany and co-chair of the working group that prepared the report. "There is no time to wait."

Coastal defenses such as Jakarta's giant seawall are designed to protect against sea level rise

Kiribati is among island nations who scientists fear may reach the limits of adaptation

Limits to adaptation

Caught on the front lines of a one-sided battle with nature, coastal cities and atoll islands are already acting.

Megacities such as Jakarta and Shanghai have built enormous walls to protect people from rising seas and greater storm surges. Sparsely-populated island nations such as Fiji are relocating entire communities in what scientists call a "managed retreat" — abandoning homes.

Small island states and low-lying islands are particularly at risk because they lack resources, said Helene Jacot Des Combes, a Fiji-based expert in disaster risk management at the University of the South Pacific and a lead author of the report.

"Whatever successful solution [they find] will need to be done in cooperation with the global community."

To protect coasts, the report also highlights the importance of preserving natural tidal barriers, such as mangroves and wetlands.

But there are "limits" to adaptation if emissions continue to rise, said Matthias Garschagen, a geographer at Ludwig Maximilian University of Munich and a lead author of the report.

"The modeling shows you only stand a chance of successful adaptation in low emission scenarios," he added.

Glacial retreat and a fall in snow have hurt farmers in the Himalayas

As the world's lowest-lying country, the Maldives is particularly pressed to adapt to rising seas and stronger storms

Who is the most vulnerable?

The scientists, brought together by the UN's Intergovernmental Panel on Climate Change, highlight that those least responsible for climate change are most threatened by changes in the ocean and cryosphere.

Melting ice in mountains and the Arctic has mostly had negative impacts on people's health, livelihoods, and even their ability to eat and drink, the report says. The risk of disaster to human settlements is expected to rise, as people and buildings become more exposed to hazards such as floods and avalanches.

The loss of fish, as oceans acidify, could threaten the diets of hundreds of millions of people — many of whom already struggle to feed themselves. Fish make up 17% of the animal protein we eat, according to the UN's Food and Agriculture Organization.

Some indigenous people in mountains will see their water sources dry up.

"There's very little positive to see in this," said John Tanzer, an expert in oceans at conservation charity World Wildlife Fund. "I don't know if the penny's dropped [in rich countries] about the extent of the possible humanitarian effects of this, which we can expect to accelerate as well."

But by taking action now, governments can limit damage. Coastal protection can reduce flood risk by tens to hundreds of times this century, the report says, if governments invest up to hundreds of billions of US dollars. It adds "profound" economic and institutional changes will enable sustainable development in the face of changing oceans and icy regions.

"Realizing this potential depends on transformative change," the authors wrote.