The Rutford Ice Stream of western Antarctica slips about 3 feet a day toward the sea but the rate changes 20 percent in tandem with two-week tidal cycles, according to the report.

    "We've known that (twice-daily) tides affect the motion of ice streams but we didn't know it happened on this two-weekly time scale," said Hilmar Gudmundsson, an Icelandic glaciologist at the British Antarctic Survey. "For such a large mass of ice to respond to ocean tides like this illustrates how sensitively the Antarctic Ice Sheet reacts to environmental changes."

    Tides rise and fall about twice a day but also vary in a two-week cycle of high "spring" tides, when the sun and the moon are aligned with the Earth, and low "neap" tides, when they are at right angles to the planet.

    These new findings mean computer models will now have to factor in tides, as well as rising seas and global warming, to predict the affect on the ice shelf. 

    "We have to be careful when we make measurements that we know that an ice stream can speed up or slow down -- that's just part of its dynamics and natural variability," Gudmundsson told Reuters.

    Some past scientific reports have incorrectly interpreted changes in the rate of the ice slide as part of longer-term shifts, he said.

    Gudmundsson said the speed of the Rutford ice when it left solid ground to become part of the floating Ronne Ice Shelf in the Weddell Sea was fastest just before spring tides at 1.2 meters a day and slowest before neap tides at 0.9 meters.

    Even 40 km inland, at a height of almost 200 meters above sea level, the ice's daily speed varied between 1.07 to 0.95 meters.

    "That was the furthest inland measurement but I expect the tidal effect could be felt 75 kilometers inland," he said.

    Gudmundsson said it was unclear whether a projected long-term rise in world sea levels, like a rising tide in slow motion, might accelerate a run-off of ice from Antarctica.

    "The next thing to do is to follow up and to measure this on other ice streams," he said. "If the sea level changes ... we want to know how sensitive the system is."

    Climate scientists who advise the United Nations project that seas will rise by 9 centimeters and 88 centimeters by 2100 because of a warming they say will also spur more droughts, heat waves, desertification and floods.

    The report was published in the scientific journal Nature.

    (Agencies)