One billion people will be affected by climate change as soon as 2050, according to UN scientists.
A major new report from the Intergovernmental Panel on Climate Change (IPCC) is entitled The Ocean and Cryosphere in a Changing Climate.
The cryosphere refers to the frozen parts of the planet.
The study, compiled by more than 100 leading climate experts from 36 countries over the past two years, is calling for urgent, ambitious, and coordinated action.
It says there is now "overwhelming evidence" that global warming is resulting in "profound consequences for ecosystems and people".
Experts have now raised their projections for future sea level rise as evidence grows that the Antarctic ice sheet is becoming unstable and vulnerable to breaking up.
Sea levels are now rising at more than twice the rate of the last century.
Melting ice, glaciers and snow are increasing the risks of landslides, avalanches, rockfalls and floods.
It predicts that "extreme sea level events" such as storm surges and floods that previously occurred "once a century" will happen at least once a year in many parts of the world by 2100.
However, it says such events will happen "at least annually" in low-lying large cities and small islands by 2050.
"Unprecedented transitions" needed
The experts are also warning of dramatic impacts on the ocean, which has absorbed 90% of the heat from global warming.
Debra Roberts, one of the IPCC authors, has urged world leaders to intensify efforts to reduce greenhouse gas emissions.
She said: “We will only be able to keep global warming to well below 2°C above pre-industrial levels if we effect unprecedented transitions in all aspects of society, including energy, land and ecosystems, urban and infrastructure as well as industry.
"The ambitious climate policies and emissions reductions required to deliver the Paris Agreement will also protect the ocean and cryosphere – and ultimately sustain all life on Earth".
Hoesung Lee, chair of the IPCC, added: "If greenhouse gas emissions continue to increase, global warming will drastically alter the ocean and the cryosphere.
"If we reduce emissions sharply, consequences for people and their livelihoods will still be challenging... but they will be potentially more manageable for those who are most vulnerable."
