Fish killer: Dead fish in a "dead zone" caused by an algal bloom in the USA's Chautauqua National Wildlife Refuge.
Credit: NOAA
PARIS: Global warming may cause vast "dead zones" in the ocean that would be devoid of fish and seafood and endure for thousands of years.
The authors of a new paper on the phenomenon 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.
For the study, published this week in the journal Nature Geoscience, scientists in Denmark built a computer model to simulate climate change over the next 100,000 years.
Devoid of life
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 case 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 549 ppm 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 ºC over pre-industrial times under the worst scenario. Under the other scenario, there would be warming of roughly between two to four degrees.
Slow reversal
Either scenario spells bad news for the ocean, said study co-author 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," said Pedersen. "But because of the inertia in the ocean, once you have the process going, it's not feasible to reverse it again just like that, so it would continue for hundreds of years."
"Even if after a hundred years, if you stopped all carbon emissions, the ocean would still need hundreds of more years to cool," he added. "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.
Wide oxygen depletion of the ocean, though, poses a far greater threat, touching at the heart of biodiversity, the paper warns.
Mass extinction
Around 250 million years ago, a chemical change of the seas led to a massive wipeout of marine species.
Lead scientist Gary Shaffer of the Niels Bohr Institute at the University of Copenhagen said it was unclear, in the grim light of this study, whether future generations could look to the oceans as a major reserve of food.
"Reduced fossil-fuel emissions are needed over the next few generations to limit ongoing ocean oxygen depletion and acidification and their long-term adverse effects," he said.
Since 1900, the mean global atmospheric temperature has risen by 0.8 ºC the U.N.'s Intergovernmental Panel on Climate Change (IPCC) said in 2007. It forecast warming of 1.8 to 4.0 ºC by 2100 over 1980 to 99 levels, but said "positive feedback" triggers that could amplify warming remain unclear.
