In a study released on the sidelines of a key UN climate conference in the Kenyan capital, they said climatic changes had sparked rapid rises in sea levels, temperatures and acidity that pose severe dangers to humanity.

"Human activities are unleashing processes of change in the oceans that are without precedent in the past several million years," said the study "The Future of Oceans -- Warming Up, Rising High, Turning Sour."

"Ambitious climate protection measures are needed to limit the consequences of warming, acidification and sea-level rise for the marine environment and human society," said the report from the German Advisory Council on Global Change (WBGU).

Unlike their terrestrial counterparts, marine ecosystems are far more sensitive to climatic changes that may, for instance, spark shifts in sealife populations, alter food webs and species composition, it said.

Most of the world's reefs -- habitats for fish on which human depend -- may be destroyed within the next 30 to 50 years because many corals cannot survive in higher water temperatures, it added.

Thus, the survival of the fisheries sector is threatened with nefarious economic ramifications, it said.

The problem "is probably one of the most severe impacts of global warming on human beings," Stefan Rahmstorf, a physics professor and a member of WBGU, told reporters at a news conference.

Key to mitigating the damage is limiting the rate of temperature change to a maximum of 0.2 degrees Celsius and the near-surface air temperature to a maximum of two degrees.

Already, scientists say global temperatures have risen by 0.7 degrees since 1900, due to greenhouse gas emissions from the burning of fossil fuels mainly by industrialised countries.

Other steps to ease the looming crisis would be dedicating 20 to 30 percent of sea and ocean ecosystems to conservation, particularly reefs that provide coastal protection and are a source of protein, and halting some fishing subsidies, according to the report.

"The publicly subsidised overfishing of the oceans should be terminated, not least in order to strengthen the resilience of fish stocks to the impacts of climate change," it said.

It also called for improved knowledge of the relation between interference with marine life, biological diversity and the resilience of marine ecosystems.

"There is need to link nature conservation with coastal protection," said the study, released as some 6,000 participants from 189 countries entered the fourth day of talks seeking ways of limiting global carbon emissions.

Average global sea-level rise stood at between 1.5 and 2.0 centimetres per decade throughout the 20th century, according to the study that also says satellite measurements show the rate hit three centimetres in the past decade.

"If warming continues, there is further acceleration of sea-level rise," it said.

And Rahmstorf also warned that further temperature increases risk touching off stronger hurricanes in the future.

"If global warming continues, we will see stronger hurricanes in the future," Rahmstorf said.

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