Rising, warming, acidifying oceans and melting ice could be affecting more than a billion people by as soon as 2050, warns a dire, sweeping new climate report.
The UN's Intergovernmental Panel on Climate Change (IPCC) found how our oceans had effectively stopped the planet from over-heating by sucking up more than 90 per cent of the excess heat in the climate system.
Yet, by the end of the century, oceans could be having to absorb two to four times what they had over the last four decades – or as much as seven times if warming couldn't be limited to the Paris Agreement's ambitious target of 2C above pre-industrial levels.
All the while, marine heatwaves, like that which helped fuel New Zealand's hottest-ever summer over 2016 and 2017, had doubled in frequency since the early 1980s.
They could be happening at a rate 20 times higher with 2C of warming - or 50 times higher if emissions kept climbing.
Oceans would also continue to acidify - an effect that had already shifted the abundance and distribution of fish populations.
And glaciers and ice sheets in polar and mountain regions were losing more and more mass, driving increasingly higher sea levels.
Over the 20th century, the world's oceans rose by around 15cm; now, it was lifting at a rate more than twice as fast – an estimated 3.6 mm per year – and accelerating.
By 2100, the sea level could have climbed by between 30cm and 60cm if the 2C line could be held – if not, they may be as much as 110cm higher.
Elsewhere in the report, the latest data showed the extent of Arctic sea ice was declining every month.
If nations somehow managed to stabilise climate change at the 1.5C mark, the Arctic Ocean would only be ice-free in September about once in every hundred years.
Under the 2C scenario, this could happen more often.
Further, permafrost ground that had long been frozen was warming, and a widespread thaw was expected to unfold next century.
Three quarters of it could be lost if emissions continued to soar, releasing vast stores of locked-up organic carbon and compounding the impacts from what was already in the atmosphere.
One of the report's lead authors, Debra Roberts, said those worst effects could only be averted if the world made "unprecedented transitions".
"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."
The economic costs of a metre of sea level rise would be measured in the tens of billions here in New Zealand - and in the trillions worldwide.
"That extreme sea levels which formerly occurred once-a-century may now become annual events should ring alarm bells," Niwa principal scientist Professor Cliff Law said.
"Coastal zones are where we interact with and gain most benefit from the oceans, yet these regions are in the front line of climate change."
Victoria University climate scientist Professor James Renwick said more than one billion people depended on glacier ice for their water supply, and those communities would be increasingly put at risk as the ice melted away.
Unless urgent action was taken, the tens of millions of people living in low-lying small island nations - and millions more living very close to sea level – could face displacement.
"If no action is taken, sea level rise could easily exceed one metre by 2100 and be on the way to several metres more, as large parts of the ice sheets in Antarctica and Greenland melt," Renwick said.
To stop global warming at the 1.5C mark, he said, global carbon dioxide emissions of carbon dioxide need to half in the next 10 years - and go to zero by 2050.
"If the wholesale transition to renewable energy does not start by 2020, we will miss that limit and be well on the way to two degrees of warming and beyond," he said.
"Such a warm future would bring serious disruption to global food security and water availability and would displace hundreds of millions of people.
"The economic and human costs are virtually incalculable."