The health of the world's oceans is declining much faster than originally thought — under siege from pollution, overfishing and other man-made problems all at once — scientists say in a new report.

The mix of interacting ingredients is in place for a mass extinction in the world's oceans, said a report by a top panel of scientists that will be presented to the United Nations on Tuesday.

The report says the troubles from global warming and other factors are worse when they combine with each other. Factors include dead zones from farm run-off, an increase in acidity from too much carbon dioxide, habitat destruction and melting sea ice, along with overfishing.

"Things seem to be going wrong on several different levels," said Carl Lundin, director of global marine programs at the International Union for Conservation of Nature, which helped produce the report with the International Programme on the State of the Ocean. The conclusions follow an international meeting this spring in England to discuss the fate of the world's oceans.

Some of the changes affecting the world's seas — all of which have been warned about individually in the past — are happening faster than the worst case scenarios that were predicted just a few years ago, the report said.

"It was a more dire report than any of us thought because we look at our own little issues," Lundin said in an interview. "When you put them all together, it's a pretty bleak situation."

The combination of problems suggests there's a brewing worldwide die-off of species that would rival past mass extinctions, scientists said in the document. Coral deaths alone would be considered a mass extinction, according to study chief author Alex Rogers of the University of Oxford. A single bleaching event in 1998 killed one-sixth of the world's tropical coral reefs.

'Unprecedented' coral deaths

Lundin pointed to deaths of 1,000-year-old coral in the Indian Ocean and called it "really unprecedented."

"We now face losing marine species and entire marine ecosystems, such as coral reefs, within a single generation," the report said.

"Multiple high intensity" factors also led to the previous five mass extinction events in the past 600 million years, the scientists note.

The chief causes for extinctions at the moment are overfishing and habitat loss, but global warming is "increasingly adding to this," the report said.

Carbon dioxide from the burning of coal and other fossil fuels ends up sinking in the ocean which then becomes more acidic.

Warmer ocean temperatures also are shifting species from their normal habitats, Rogers said. Add to that melting sea ice and glaciers.

Chemicals and plastics from daily life are also causing problems for sea creatures, the report said. Overall, the world's oceans just can't bounce back from problems — such as oil spills — like they used to, scientists said.

However, Lundin said, "Some of these things are reversible if we change our behavior."

A separate study released Monday, unrelated to the international project, provided the most detailed look yet of sea level rise from global warming. It found the world's oceans have been rising significantly over the past century. The yearly rise is slightly less than one-tenth of an inch, but it adds up over decades, according to the study based on sediment cores from North Carolina marshes. That study was published in this week's Proceedings of the National Academy of Sciences.