London - The world's floating ice is in "constant retreat", showing an instability which will increase global sea levels, according to a report published in Geophysical Research Letters on Wednesday.
Floating ice had disappeared at a steady rate over the past 10 years, according to the first measurement of its kind.
"It's a large number," said Professor Andrew Shepherd of the University of Leeds, lead author of the paper, estimating the net loss of floating sea ice and ice shelves in the last decade at 7 420km³.
That is greater than the loss of ice over land from the Greenland and Antarctic ice sheets over the same time period, highlighting the impact of warming oceans on floating ice.
Ice melt ebbs and flows from winter to summer. The report's calculations referred to the net loss over the past decade.
"There's a constant rate of retreat (annually)," said Shepherd. "It's a rapid process and there's no reason why it won't increase over the next century."
The study did not shed new light on how soon the North Pole may be ice-free in summer, which many climate experts say could happen by 2050, perhaps even earlier.
Glaciers the problem
Melting of floating sea ice and ice shelves adds little to sea level rise, because their entire mass is already in the water. By contrast, ice on land which melts into the sea will add to levels according to the equivalent of its entire weight.
If all the world's floating ice melted it would add about 4cm to sea levels.
But this could have a bigger effect by unblocking glaciers over land, which could then slide faster into the sea, and also because open water reflects less sunlight than ice, warming the local area.
If all the world's polar ice melted it would raise sea levels by about 70m, scientists estimate. "We're moving into an era where the sea ice and ice shelves are being eroded away because of temperature rise," said Shepherd.
Floating ice adds very little to sea levels, because it does not add to the total weight of water already in the sea, but it does add a little because ice contains no salt and so dilutes the ocean as it melts, causing the sea to expand in volume.
Melt of floating ice in the past decade had increased the volume of the world's seas by 193km³ in this way, said Shepherd.
Directly, that would add to sea level rise by the width of a few human hairs, he added.