Scientists have warned that marine life will be irreversibly changed unless CO2 emissions are drastically cut.
Writing in Science, experts say the oceans are heating, losing oxygen and becoming more acidic because of CO2.
They warn that the 2C maximum temperature rise for climate change agreed by governments will not prevent dramatic impacts on ocean systems.
And they say the range of options is dwindling as the cost of those options is skyrocketing.
Twenty-two world-leading marine scientists have collaborated in the synthesis report in a special section of Science journal. They say the oceans are at parlous risk from the combination of threats related to CO2.
They believe politicians trying to solve climate change have paid far too little attention to the impacts of climate change on the oceans.
It is clear, they say, that CO2 from burning fossil fuels is changing the chemistry of the seas faster than at any time since a cataclysmic natural event known as the Great Dying 250 million years ago.
They warn that the ocean has absorbed nearly 30% of the carbon dioxide we have produced since 1750 and, as CO2 is a mildly acidic gas, it is making seawater more acidic.
It has also buffered climate change by absorbing over 90% of the additional heat created by industrial society since 1970. The extra heat makes it harder for the ocean to hold oxygen.
Several recent experiments suggest that many organisms can withstand the future warming that CO2 is expected to bring, or the decrease in pH, or lower oxygen… but not all at once.
Jean-Pierre Gattuso, lead author of the study, said: “The ocean has been minimally considered at previous climate negotiations. Our study provides compelling arguments for a radical change at the UN conference (in Paris) on climate change”.
They warn that the carbon we emit today may change the earth system irreversibly for many generations to come.
Carol Turley, of Plymouth Marine Laboratory, a co-author, said: “The ocean is at the frontline of climate change with its physics and chemistry being altered at an unprecedented rate so much so that ecosystems and organisms are already changing and will continue to do so as we emit more CO2.
“The ocean provides us with food, energy, minerals, drugs and half the oxygen in the atmosphere, and it regulates our climate and weather.
“We are asking policy makers to recognise the potential consequences of these dramatic changes and raise the profile of the ocean in international talks where, up to now, it has barely got a mention.”
The scientists say ocean acidification is likely to impact reproduction, larval survival and feeding, and growth rates of marine organisms - especially those with calcium carbonate shells or skeletons.
The authors say when the multiple stressors work together they occasionally cancel each other out, but more often they multiply negative effects.
The experts say coastal protection, fisheries, aquaculture and human health and tourism will all be affected by the changes.
They warn: “Immediate and substantial reduction of CO2 emissions is required in order to prevent the massive and effectively irreversible impacts on ocean ecosystems and their services”.
Professor Manuel Barange, director of science at the Plymouth Marine Laboratory, said: “Climate change will continue to affect ocean ecosystems in very significant ways, and society needs to take notice and respond.
“Some ecosystems and their services will benefit from climate change, especially in the short term, but overall the impacts are predominately negative.
“Negative impacts are particularly expected in tropical and developing regions, thus potentially increasing existing challenges in terms of food and livelihood security.
"We are allowing ourselves to travel a uniquely dangerous path, and we are doing so without an appreciation for the consequences that lie ahead."
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