The shells of tiny ocean animals known as foraminifera—specifically Globigerina bulloides—are shrinking as a result of the slowly acidifying waters of the Southern Ocean near Antarctica. The reason behind the rising acidity: Higher carbon dioxide (CO 2 ) levels in the atmosphere, making these shells more proof that climate change is making life tougher for the seas' shell-builders.



Marine scientist Andrew Moy at the Antarctic Climate & Ecosystems Cooperative Research Center (ACE) in Hobart, Tasmania, and his Australian colleagues report in Nature Geoscience this week that they made this finding after comparing G. bulloides shells in ocean cores collected along the South Tasman Rise in 1995 with samples from traps collected between 1997 and 2004. The cores provide records that stretch back 50,000 years.



"We knew there were changes in carbonate chemistry of the surface ocean associated with the large-scale glacial-interglacial cycles in CO 2 [levels], and that these past changes were of similar magnitude to the anthropogenic changes we are seeing now," says study co-author William Howard, a marine geologist at ACE. "The Southern [Ocean] works well [to study this issue] as it is a region where anthropogenic CO 2 uptake, and thus acidification, has progressed more than in other regions. Other variables, such as temperature, have changed, but not as much."



The researchers found that modern G. bulloides could not build shells as large as the ones their ancestors formed as recently as century ago. In fact, modern shells were 35 percent smaller than in the relatively recent past—an average of 17.4 micrograms compared with 26.8 micrograms before industrialization. (One microgram is one millionth of a gram; there are 28.3 grams in an ounce.)



"We don't yet know what impact this will have on the organisms' health or survival," Howard says, but one thing seems clear: the tiny animals won't be storing as much CO 2 in their shells in the form of carbonate. "If the shell-making is reduced, the storage of carbon in the ocean might be, as well."



That's bad news for the climate, because the ocean is responsible for absorbing at least one quarter of the CO 2 that humans load into the air through fossil fuel burning and other activities—and it is the action of foraminifera and other tiny shell-building animals, along with plants like algae that lock it away safely for millennia.



It will be harder to get such a clear sign in a shell from other ocean regions—as variables like temperature and the amount of minerals available can significantly change the chemistry of a given ocean region. As Howard noted, the Southern Ocean has absorbed lots of manmade CO 2 while temperatures and nutrients have not changed as much, making it more ideal for studying ocean acidification than other areas. Scientists examining foraminifera in the Arabian Sea, however, have found similar results, and Howard speculates the situation may be similar in the North Atlantic region, because it also absorbs a significant chunk of manmade CO 2 .



Howard says that CO 2 emissions must be cut or captured and stored permanently in some fashion to halt this gradual acidification of the world's oceans. In the meantime, he adds, it's likely that many of the other shell-building oceanic animals are suffering similar fates as G. bulloides.