Global warming may create 'dead zones' in the ocean that would be devoid of fish and seafood and endure for up to two millennia, according to a study published yesterday.

Its authors say deep cuts in the world's carbon emissions are needed to brake a trend capable of wrecking the marine ecosystem and depriving future generations of the harvest of the seas, according to our sister newspaper Gulf Daily News.

In a study published online by the journal Nature Geoscience, scientists in Denmark built a computer model to simulate climate change over the next 100,000 years.

At the heart of their model are two well-used scenarios which use atmospheric levels of carbon dioxide (CO2), the main greenhouse gas, as an indicator of temperature rise. Under the worst scenario, CO2 concentrations would rise to 1,168 parts per million (ppm) by 2100, or about triple today's level. Under the more optimistic model, CO2 would reach 549ppm by 2100, or roughly 50 per cent more than today.

The temperature rise that either would yield depends on several factors: when the peak in carbon emissions is reached and how quickly it falls, and whether the warming unleashes natural triggers, or tipping points, that enhance or prolong the warming in turn.

Taking such factors into account, the scientists predict a possible rise of around five to seven degrees Celsius (nine to 16 degrees Fahrenheit) over pre-industrial times under the worst scenario. Under the other scenario, there would be warming of roughly between two and four C (3.7-7.2 F).

Either scenario spells bad news for the ocean, said Jens Olaf Pepke Pedersen, a physicist at the Technical University of Denmark.

Under the worst scenario, warmer seas and a slowdown of ocean circulation would lower marine oxygen levels, creating 'dead zones' that could not support fish, shellfish and other higher forms of marine life - and may not revive for 1,500 to 2,000 years.

'They would start slowly by the end of this century, it's not something that would happen tomorrow or in the near future but over the next few generations,' Pedersen said.

'Even if after a hundred years, if you stopped all carbon emissions, the ocean would still need hundreds of more years to cool. These low-oxygen areas would continue to expand and they would peak around 2,000 years from now. The ocean would then slowly recover as it cools.'

Even under the less gloomy scenario, there would still be significant, long-term expansion of oxygen-starved zones.

Marine 'dead zones' already exist today, in shallow areas next to the coast, where runoff from agricultural fertiliser causes an explosion in oxygen-gobbling algae.