A few months ago, the clear blue Pacific Ocean waters off the coast of Oregon suddenly turned a thick greenish brown. A swell of nutrients produced a bizarre blooming of plankton that reached levels never seen before by scientists. Then the plankton died and sank, causing oxygen levels in the water to plummet to zero.

The living ocean was transformed into a dead zone. Scientists conducted a submarine survey and found only the bodies of crabs and marine worms scattered across the ocean floor. There were no signs of any fish. Nothing had survived the cataclysm.

Nor has this been the only such disaster to strike a marine ecosystem in recent years. As scientists reported at the annual meeting of the American Association for the Advancement of Science in San Francisco yesterday, unprecedented changes to ocean currents are having a devastating effect on finely balanced marine ecosystems all over the globe. Similar upheavals have been recorded in other parts of the world, particularly off South America and Africa.

Marine researchers are convinced the evidence points to one culprit: global warming. Man-made changes to the climate are throwing previously predictable seasonal winds out of kilter. 'We finger the winds as the important culprit, but we do not know definitively why these winds are changing,' said Professor Jane Lubchenco from Oregon State University. 'However, we know the changes are what would be expected under climate change scenarios, and climate change is a viable hypothesis. We should expect more surprises.'

Seasonal winds blowing across the sea affect ocean currents by pushing away surface water, which is then replaced by colder water from below. But warmer land temperatures result in higher pressures and stronger winds, which in turn have an impact on currents, said the scientists. Normally these effects were predictable, but recently the system had become unstable and volatile - a pattern that mirrors climate change models. 'Wild fluctuations in the intensity of ocean upwellings are wreaking havoc with ecosystems,' added Lubchenco. 'We're seeing extreme distortions on both sides of the norm. This is a system that is out of kilter. It's fluctuating rapidly.'

Up to five decades of data have shown that these events were unprecedented, she said, pointing out that similar ocean current disruption had been seen in other regions, particularly off Peru, Chile and parts of Africa.

Last year's ecosystem collapse on the Oregon coast was the second to strike there in as many years. In 2005, a nutrient-rich ocean current that normally appears off northern California and Oregon in spring was delayed by a month. This led to a loss of plankton, the microscopic plant organisms upon which larger animals depend for food. Salmon, which normally take to the sea at this time, starved. The effects rippled through the food web as predators, including sea birds, went hungry and died. Huge numbers of dead birds washed up on the shores.

'Beaches were littered with the bodies of dead sea birds,' said Dr Julia Parrish, from the University of Washington in Seattle. Many of the starving survivors have been unable to breed since then, she added.

Then, a year later, in 2006, the dead zone appeared and remained for nearly 17 weeks. 'It grew to an area the size of the state of Ohio and lasted much longer than we thought would be possible, from something that we tracked day to day for months on end,' said Dr Francis Chan, from Oregon State University in Corvallis. 'It went from a low-oxygen system to a no-oxygen system. This had a dramatic effect on marine life.'