The oceans play a key role in absorbing carbon dioxide, but international climate talks have only "minimally considered" the impact the gas is having as it turns these bodies of water acidic, scientists say in a new study.
More than 20 marine scientists worked to analyze two potential futures.
One involves meeting the target set out by the UN-established Intergovernmental Panel on Climate Change (IPCC) of limiting the rise in global warming to 2 C through CO2 cuts (the RCP2.6 scenario). The other (RCP8.5) reflects the current trajectory of business-as-usual CO2 emissions.
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The study, published last week in the journal Science, calls for negotiators at climate talks to stress the impact CO2 is having on ocean acidification.
They say their goal is to speak directly to policymakers, adding the study has the backing of the Oceans 2015 Initiative.
That group of experts says that each day, more than a quarter of the carbon dioxide emitted by human activity is absorbed by the oceans. Without this dynamic, the effects of climate change would be far greater.
By 2100, scientists predict that the oceans' pH level will have fallen by 0.4 units — or an increase of 150 per cent in ocean acidity — compared to its mid-19th-century level. That corresponds to a three-fold increase in the water's acidity.
Acidification makes it more difficult for marine animals to produce skeletons and shells, and puts the essential ocean habitat, coral reefs, at risk of eroding faster than they can be rebuilt.
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Researchers taking part in the study say ocean warming and acidification are expected to "act synergistically to push corals and coral reefs into conditions that are unfavourable for coral reef ecosystems."
While they praised the goal of cutting emissions in order to keep warming to no more than 2 C above the pre-industrial level, they also said that scenario could still leave important marine ecosystems "substantially" altered.
"If CO2 levels are kept to the RCP2.6 scenario, by 2100, the risk of impact increases to high for warm-water corals and mid-latitude bivalves," the study said.
"Projections with RCP8.5 indicate a very high risk of impact on most marine organisms considered."
The researchers said that to avoid that kind of risk requires limiting the increase in global surface temperature between 1990 and 2100 to below 2°C and the increase in SST (sea surface temperature) below 1.2°C.
Their analysis, they said, shows "immediate and substantial" reduction of CO2 emissions is required in order to prevent the "massive and effectively irreversible impacts" on ocean ecosystems that are projected with emissions scenarios more severe than those under the two-degree warming limit.