Broken ice floats in the Arctic Ocean off the northern coast of Greenland in this 2004 true-color NASA image

CHICAGO — The nightmare global warming scenario which provided the plot for a Hollywood blockbuster -- the Atlantic Ocean current that keeps Europe warm shuts down and triggers rapid climate change -- has long worried scientists.

But a study published Thursday in the journal Science found it may not occur as quickly as previously feared.

There is evidence that this current has shut down with some regularity in the past -- and sometimes quite rapidly -- in response to large influxes of fresh water from melting glaciers.

However, it appears as though the current rate of glacial melt is occurring at a more gradual pace which will "give ecosystems more time to adjust to new conditions," said study coauthor Peter Clark, a professor of geosciences at Oregon State University.

"Our data still show that current is slowing, and may decline by 30 percent by the end of this century," Clark said.

"That's very significant, and it could cause substantial climate change. But it's not as abrupt as some concerns that it could shut down within a few decades."

Clark and his colleagues constructed a massive computer model which simulated the atmospheric and oceanic conditions of the height of the last ice age and the changes which resulted in the Earth's last major global warming some 14,500 years ago.

The simulation presented results that are in line with the fossil and geological record and confirms the accuracy of some models of future climate change scenarios.

It found that the "climate dominos" began to fall when the glaciers which blanketed most of North America began to melt, said co-author Zhengyu Liu, director of the Center for Climatic Research at the University of Wisconsin-Madison.

Massive amounts of water poured into the North Atlantic and lowered the ocean's salt levels, which are key to powering the convection current that carries warm tropical surface water north and cooler, heavier subsurface water south.

The fresh water stopped the current, cooling the North Atlantic region and causing heat to build up in southern waters.

The cooler temperatures slowed the glacial melt and eventually the ocean current restarted. But there was a much larger reserve of heat to haul north.

"All that stored heat is released like a volcano, and poured out over decades," Liu said in a statement.

"That warmed up Greenland and melted (arctic) sea ice."

While the model was not able to determine the rate of ice melt during the period, it found that both a slow decrease and a sudden end to melt run-off resulted in the same consequence: a 15-degree warming.

"That happened in the past," Liu said. "The question is, in the future, if you have a global warming and Greenland melts, will it happen again?"

The researchers continue to run the massive program, which has simulated about a third of the last 21,000 years.

It will eventually run up to the present and then 200 years into the future.

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