US scientists have figured out how icebergs break off Antarctica and Greenland, a finding that may help predict rising sea levels as the climate warms.

Writing in Friday's edition of the journal Science, they said icebergs formed fast when parent ice sheets spread out quickly over the sea.

"It won't help the Titanic, but a newly derived, simple law may help scientists improve their climate models" and predict ice sheet break-up, they said in a statement.

The Titanic sank in 1912 after hitting an iceberg, killing 1,500 people.

Ice cracking off into the ocean from Antarctica and Greenland could be the main contributor to global sea level rises in the future. If all the ice in Greenland and Antarctica melted, seas would rise by more than 60 metres.

The formation rate of icebergs was less linked to factors such as ice thickness, width of the ice flow, distance from land or waves, the scientists said.

Ice sheets are giant frozen rivers, caused by snowfall, that slowly flow to the sea and then break up.

In Antarctica, the Ross Ice Shelf extends 800 kilometres over the ocean before the edges snap off and form icebergs. Many other ice sheets stretch just a kilometre or two.

Computer models that predict how ice sheets behave in warmer weather generally gloss over exactly how icebergs break off because researchers have failed to understand the mechanism, known as calving.

"For iceberg calving, the important variable - the one that accounts for the largest portion of when the iceberg breaks - is the rate at which ice shelves spread," the study said.

A fast spread means cracks form throughout the shelf and make it crack up. A slower spread means that deep cracks do not form as fast and the ice sticks together.

"The problem of when things break is a really hard problem because there is so much variability," lead author Richard Alley, of Pennsylvania State University, said.

"Anyone who has dropped a coffee cup knows this. Sometimes the coffee cup breaks and sometimes it bounces," he said of the problems of understanding cracking.

The UN Climate Panel predicts seas will rise by 18 to 59 centimetres this century because of warming stoked by human use of fossil fuels.

- Reuters