A major United Nations report yesterday painted a bleak picture of the future, warning of accelerating sea-level rise, increased coastal flooding, faster melting of ice caps and mountain glaciers, and warmer and more acidic oceans harming fisheries and reefs.
Adding to a growing number of recent scientific warnings, the report from the Intergovernmental Panel on Climate Change (IPCC) highlights the urgent need to radically cut greenhouse gas emissions and adapt to the changes already locked in.
Dr Debra Roberts, co-chair of the IPCC Working Group II, told reporters yesterday that climate change has a major impact on the systems that humans depend on - from the top of mountains to the depths of oceans.
"These changes will continue for generations to come. This new report highlights the urgency in prioritising timely, ambitious and coordinated action to address widespread and enduring changes in the ocean and cryosphere," she added.
The cryosphere refers to the frozen parts of the planet.
Higher temperatures are already heating up oceans and causing them to expand and raise sea levels. Warmer oceans and air are, in turn, melting ice sheets in Antarctica and Greenland, further raising sea levels.
High mountain glaciers in the Hindu Kush Himalayan region, which provide water for about two billion people, are melting quickly and this could accelerate, threatening drinking water supplies, agriculture and hydropower generation.
"The open sea, the Arctic, the Antarctic and the high mountains may seem far away to many people," said IPCC chair Lee Hoesung. "But we depend on and are influenced by them directly and indirectly in many ways - for weather and climate, for food and water, for energy, trade, transport, recreation and tourism, for health and well-being, for culture and identity."
The amount of sea-level rise during the 20th century.
The amount that sea levels could rise if greenhouse gas emissions continue to increase strongly.
The amount of excess heat that oceans have absorbed from the climate system.
The frequency of marine heatwaves if emissions keep rising, which would destroy most reefs.
The amount of near-surface permafrost that could melt if emissions keep increasing strongly.
More than 100 scientists from 36 nations compiled the report from nearly 7,000 publications, with the aim of summarising the latest science for policymakers worldwide.
So far, the ocean has been a friend to humanity by soaking up about a quarter of mankind's carbon dioxide (CO2) emissions and absorbing about 90 per cent of the excess heat trapped in the atmosphere by the thickening blanket of CO2.
But as emissions keep rising, the oceans are becoming more acidic, holding less oxygen, and are heating up and fuelling stronger storms.
If carbon emissions keep rising at current rates, the oceans will turn into foe, threatening low-lying islands and cities and driving up costs of coastal protection and flooding.
Sea levels rose 15cm during the last century and the process has accelerated. They could further rise by 1.1m if greenhouse gas emissions continue to increase strongly.
Ice loss from Greenland and the Antarctic is speeding up and is expected to accelerate from the middle of this century, the report said.
Under the most pessimistic scenario of rising emissions, sea-level rise is projected to exceed rates of several centimetres per year, resulting in a multi-metre rise by 2300.
The report also raises deep concerns over the accelerating melting of Arctic permafrost, the thin layer of frozen ground that holds large amounts of carbon and methane, a far more powerful greenhouse gas than CO2 that could exacerbate the warming. If greenhouse gas emissions continue to increase strongly, around 70 per cent of near-surface permafrost could be lost, it adds.
Marine heatwaves are projected to further increase in frequency, duration, extent and intensity. Their frequency will be 20 times higher at 2 deg C warming, compared with pre-industrial levels. They would occur 50 times more often if emissions continue to increase strongly, a rate that would destroy most reefs.
Referring to the global youth climate movement that saw four million students go on a school strike last Friday, Dr Hans-Otto Portner, co-chair of IPCC Working Group II, said young people are aware of the urgency to tackle climate change.
He said: "They see the future, and the future of the world that should still be there for them to deliver the services it currently still does. They are concerned, and are building their concerns and expressions of concern on the science from the IPCC's assessments.
"So the message is received, and society and policy have the choice to take rapid action as needed to keep those changes under control."