LONDON -- Rising levels of carbon dioxide are harming all forms of marine life because the oceans are acidifying as they absorb the gas, German researchers found.
Mollusks, corals and a class of creatures called echinoderms that includes starfish and sea urchins are the worst affected by the uptake of CO2 by the seas, according to a study Sunday in the journal Nature Climate Change by researchers at the Alfred Wegener Institute in Bremerhaven.
The gas forms carbonic acid when it dissolves in the oceans, lowering their pH level.
Creatures that show negative effects from acidification include commercial species such as oysters and cod. Given the pace at which carbon dioxide emissions are growing, human emissions threaten to trigger extinctions at a faster pace than die-outs millions of years ago, according to the researchers.
"There is a danger that we're pushing things too fast and too hard toward an evolutionary crisis," Hans-Otto Poertner, one of the authors, said in an interview.
The research will be fed into the United Nations' most detailed study into the science of climate change, which is being published in three parts and an overall summary by the end of 2014, and is designed to inform international climate treaty negotiations.
Sunday's study will be input for the second part of that report, by the U.N.'s Intergovernmental Panel on Climate Change, due to be published at the end of March. The first part is scheduled for publication on Sept. 27.
The researchers examined 167 previous studies about the effects of acidifying oceans on 153 species, analyzing their findings and using forecasts of future emissions to predict how they might be affected as carbon dioxide emissions into the atmosphere grow. The oceans absorb more than a quarter of man-made CO2 emissions.
They found that at concentrations of CO2 in the atmosphere of 500 parts per million to 650 parts per million, negative effects outweighed positive ones for corals, echinoderms, mollusks and fish, though not for crustaceans.
At higher concentrations, all categories of creatures were harmed. CO2 is currently just under 400 parts per million, rising about 2 ppm to 3 ppm a year.
"All animal groups we considered are affected negatively by higher carbon dioxide concentrations," Astrid Wittmann, a biologist at the institute and the report's other author, said in a statement. "Corals, echinoderms and mollusks above all react very sensitively to a decline in the pH value."
Negative effects include behavioral and sensory changes that make fish less fearful of predators, altered metabolism, and a slowing of the rate at which mollusks can form shells.
Similar sensitivity to rising CO2 can be observed in the fossil records of extinctions that took place 55 million years ago and 250 million years ago, Mr. Poertner said.
He cautioned that the study has limitations because "you cannot do sufficiently long studies to really mimic what will happen in 50 years."
The research was designed to look solely at the effects of the acidification caused by the carbon dioxide emissions, according to Mr. Poertner.
When the warming effects of the gas are also factored in, it could accelerate negative effects because the temperature a species can withstand in more acidic conditions may be lower, he said.
"We are at a risk of causing extinctions," Mr. Poertner said.
"We cannot give with any certainty the year when people will start to report extinctions due to climate change. It depends on what temperature change and CO2 concentrations we allow."