The world's oceans are faced with an unprecedented loss of species comparable to the great mass extinctions of prehistory, a major report suggests.

The seas are degenerating faster because of the cumulative impact of severe individual stresses, ranging from climate warming and sea-water acidification to chemical pollution and gross overfishing.

The coming together of these factors is now threatening the marine environment with a catastrophe "unprecedented in human history", according to the report by a panel of leading marine scientists brought together in Oxford this year by the International Programme on the State of the Ocean and the International Union for the Conservation of Nature.

The panel's stark suggestion is that the potential extinction of species, from large fish at one end of the scale to tiny corals at the other, is directly comparable to the five great mass extinctions in the geological record, during each of which much of the world's life died out.

The 27 scientists concluded that a "combination of stressors is creating the conditions associated with every previous major extinction of species in earth's history".

"The findings are shocking," said Dr Alex Rogers, professor of conservation biology at Oxford University and IPSO's scientific director. "This is a very serious situation demanding unequivocal action at every level. We are looking at consequences for humankind that will impact in our lifetime, and worse, in the lifetime of our children and generations beyond that."

The experts "found firm evidence" that the effects of climate change, coupled with other human-induced impacts such as overfishing and nutrient run-off, have already caused a dramatic decline in ocean health.

The report says: "Increasing hypoxia [low oxygen levels] and anoxia [absence of oxygen, known as ocean dead zones], combined with warming of the ocean and acidification, are the three factors which have been present in every mass extinction event in earth's history."

* The speed and rate of degeneration of the oceans is far faster than anyone had predicted.
* Many of the negative impacts identified are greater than the worst predictions.
* The first steps to globally significant extinction may have already begun.

- Independent