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A dead fish coated in heavy oil floats near shore June 4, 2010 near East Grand Terre Island, Louisiana. (Win McNamee/Getty Images)
A dead fish coated in heavy oil floats near shore June 4, 2010 near East Grand Terre Island, Louisiana. (Win McNamee/Getty Images)

How the gushing oil in the Gulf will ripple around the world Add to ...

In heavy concentrations, oil can kill the grasses and vegetation that protect the land from the advancing sea. Louisiana is already losing an estimated 10,000 hectares of wetlands every year. That's the equivalent of a football field every minute. Since the 1930s, roughly 3,700 square kilometres of Louisiana shoreline has been submerged, due in large part to the dredging and channelling of the Mississippi.

Future storms could stir up oil trapped in the marshes, replaying the damage year after year.

"The amount of oil already loose in the Gulf means that significant changes are to be expected in the fundamental ecological engine of the northern Gulf of Mexico," Mr. Rader explained.

"The legacy will be with us for decades, and that's the best case. There is already so much material hitting so many key components in the system because of the scale of the gusher."

Some scientists say predictions about the effects of the spill are pointless because there's so much they don't know yet. Christopher Reddy, associate scientist at Woods Hole Oceanographic Institution in Massachusetts, has been studying oil spills for more than a decade. And none, is quite like the last one, he pointed out.

"Every oil spill is unpredictable," explained Mr. Reddy, who is now in the Gulf on a grant from the U.S. National Science Foundation. "Nature, and oil, don't take directions well."

When he's closer to home, Mr. Reddy loves taking visitors into the Wild Harbor salt marsh near West Falmouth, Mass. Near there, on a foggy night in 1969 an oil barge ran aground, dumping 200,000 gallons of fuel oil. By the early 1970s, the coastline had virtually returned to normal, at least to the naked eye.

Even now, 41 years later, dip a stick into the muck, lift it up, and the unmistakable smell of crude wafts up. A small fraction of the spilled oil remains trapped in the mud, deprived of oxygen, and still affecting crabs and vegetation.

"It can look postcard-perfect, but underneath, chemical warfare may be going on," Mr. Reddy said.

He hastened to add that a similar barge spill near Portland, Maine, in 1996 took barely a year to completely clean up.

"Some spills clean themselves up pretty quickly. Others have effects that last for decades," he said.

Jeffrey Short, a former research chemist at the U.S. National Oceanic and Atmospheric Administration, shares Mr. Reddy's concerns about making too many predictions. He and other scientists acknowledged there are so many unknowns, from wind patterns to the amount of oil in the water.

Mr. Short, who worked for the Alaska and U.S. governments on the Exxon Valdez assessment and cleanup, also knows how difficult and costly it is to get rid of oil once it makes contact with land and animals.

His greatest fear is the amount of oil getting into the marshes and wetlands along the Gulf coast. All it would take is for onshore winds to move the oil that's now drifting on the loop current back toward the marshes to cause significant, long-term damage.

"The process of getting it out can be very slow," he said. "It could create enough of an impact that could take a decade or more to get over."

At nearly 11-million gallons, the Exxon Valdez was the largest U.S. spill until the Deepwater Horizon. The Exxon Valdez, however, doesn't even rank among the top 10 worst spills in the world.

Dire predictions have been made after major disasters, only to be proven unfounded. Many scientists predicted an environmental apocalypse after the largest spill in history - Saddam Hussein's release of 240-million gallons of oil into the Persian Gulf in 1991. In the end, there was little permanent damage. Half the oil evaporated and another quarter washed ashore in Saudi Arabia.

In 1979, there was a spill eerily similar to the Deepwater Horizon, but even larger. An oil well in Mexico's Bay of Campeche collapsed when a pressure build-up sparked an explosion. In the 10 months it took to cap the well, 140-million gallons spilled into the Gulf, creating an 1,800 square kilomentres oil slick.

Mexican officials sprayed the slick with chemical dispersants. Some of the oil made it as far as the barrier islands of Texas. But three decades later, the accident is a distant memory.

Mr. Rader of the Environmental Defense Fund said the best thing that has come of this spill is it gets people thinking about better managing the Gulf for all the countries that depend on its bounty. That means more focus on protecting wetlands and coral reefs, and working to mitigate the dumping of nutrients into waterways.

"The best case would be that this is a bellwether that causes us to build a multilateral environmental management that takes into account changes to the planet that we know are coming," he said.

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