Greenhouse gas buildup seen as risk to oceans
Almost half the carbon dioxide from the past two centuries of human industry has been absorbed by the world's oceans, an injection of greenhouse gas that could change the acidity of the ocean in the future and threaten some sea creatures, according to a report published today.
Scientists have long known that the oceans played a key role in the storage of carbon dioxide, the abundant gas largely responsible for global warming. But today a team of researchers reported in the journal Science that they have taken the first widespread measurement of how much carbon dioxide the ocean is absorbing, and found this situation has the potential to cause harmful changes to the ocean's chemistry.
The information is crucial for understanding and predicting climate change, and for researching environmentally sound ways to store the carbon dioxide that continues to be pumped out.
''The oceans are already sequestering . . . tons of carbon," said Richard A. Feely, one of the two papers' lead authors and a scientist with the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle. ''What we are just beginning to understand and appreciate is that organisms are very sensitive to this uptake."
The researchers examined the total carbon dioxide emissions from fossil fuel burning and cement manufacturing -- two key manmade sources of the gas -- between 1800 and 1994. By taking readings from 9,600 ocean samples collected at different depths throughout the world, they found that ocean water had absorbed 48 percent of the total manmade carbon dioxide emitted. The rest went into the atmosphere.
Researchers said the study contains some good news: Much of the ocean's absorbed carbon is still near the surface, meaning the water has vast capacity to absorb more.
Oceans absorb carbon dioxide two ways: dissolving the gas out of the air, or trapping it when it is processed by phytoplankton and other sea vegetation. Virtually no human-generated carbon is now found at the ocean floor, because it can take hundreds of years for the surface water to mix with water in the oceans' deepest regions. Christopher Sabine, also of the Pacific lab and the other lead author of the study, said yesterday that the oceans have reached about a third of their long-term capacity to absorb carbon dioxide.
But the researchers also cautioned that they have little understanding of how increasing carbon dioxide absorption in the sea will affect marine life, particularly coral and shell-forming organisms. When carbon dioxide mixes with water, it essentially becomes an acid in the delicate ocean. The oceans now maintain alkaline conditions because of enormous reserves of calcium carbonate, in the form of shells, the skeletons of sea creatures, and their dissolved byproducts. That calcium carbonate -- the main ingredient in antacids like Tums -- gives the ocean its own antacid system.
But laboratory experiments indicate that the more carbon dioxide there is in ocean water, the less alkaline the ocean may become, making it more difficult for organisms to grow shells and, possibly, causing those shells to dissolve. Besides helping the ocean to regulate itself, many of these shelled animals are also crucial food for fish and other marine life.
''This process allows us to have more carbon dioxide taken up," Feely said. ''But there is a price to pay: The living organisms in the surface waters are very, very sensitive to [this]."
By the end of the century, the researchers predict, the ocean's surface waters will have increased their carbon intake by 12 percent, a situation that could change the ocean's acidity. Feely estimates that the pH level of the surface ocean could drop lower than it has been for more than 5 million years. (A lower pH is more acidic; higher is more alkaline.)
The study took more than a decade to conduct, involving 95 research cruises, and scientists from the United States, South Korea, Australia, Canada, Japan, Spain, and Germany. Sponsored by NOAA, the National Science Foundation, and the US Department of Energy, it is considered the first global ocean study since the 1970s.
The research may also have implications for attempts to slow global warming by storing the excess carbon dioxide humans emit. Some scientists have suggested trapping excess gas at factories and injecting liquid globs of it deep into the ocean. Others have suggested fertilizing the ocean with iron to stimulate the growth of ocean vegetation that could trap the gas. But scientists are still debating the potential biological effects of various trapping schemes
''It shows we are still finding out things about the ocean that might make life better in the future -- or not so good," said Richard Houghton, senior scientist for the Woods Hole Research Center in Falmouth.
Beth Daley can be reached by email at bdaley@globe.com 