Oslo: Tides affect the speed at which an Antarctic ice sheet bigger than the Netherlands is sliding toward the sea, adding a surprise piece to a puzzle about ocean levels and global warming, a study showed on Wednesday.
The Rutford Ice Stream of western Antarctica slips about a meter (three feet) a day toward the sea but the rate varies 20 per cent in tandem with two-week tidal cycles, it said. And the effect is felt even on ice more than 40 km (25 miles) inland.
"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 an Icelandic glaciologist at the British Antarctic Survey, Hilmar Gudmundsson.
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.
"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," he said of a report published in the scientific journal Nature.
The speed of other ice streams may also change with tides. Computer models of how Antarctica's ice may be affected by rising seas and global warming, widely blamed on human use of fossil fuels, will now have to factor in tides, he said.
"We have to be careful when we make measurements that we know that an ice stream can speed up or slow down as that's just part of its dynamics and natural variability," he said.
Some past scientific reports have wrongly interpreted changes in the rate of the ice slide as part of longer-term shifts, he said.
Gudmundsson said that 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 1.07-0.95 meters.
"That was the furthest inland measurement but I expect the tidal effect could be felt 75 km inland," Gudmundsson said.
Gudmundsson also said that 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.
Around Antarctica, the tidal effect may be strongest around the Ronne Ice Shelf, where there is a big twice-daily rise and fall in tides.
The Rutford Ice Stream is bigger than the Netherlands or U.S. states such as Maryland or Hawaii.
"The next thing to do is to follow up and to measure this on other ice streams," he said adding, "If the sea level changes then we want to know how sensitive the system is."
Climate scientists who advise the United Nations project that seas will rise by nine centimetre and 88 cm by 2100 because of a warming they say will also spur more droughts, heat waves, desertification and floods.