Science News - November 24, 2004

Global warming’s other effects on the oceans

The often overheated debate over the effect of anthropogenic CO₂ on the earth’s atmosphere has distracted scientists from considering how this greenhouse gas might be affecting other global systems, warn some researchers. Two related papers published in Science in July estimated that as much as one-half of the anthropogenic CO₂ is being sucked up by the oceans, increasing ocean carbon levels and lowering the average pH from a pre-industrial level of 8.16 to 8.04 today (Science 2004, 305, 362–366; 367–371). If the CO₂ concentration doubles from present levels by 2100, as some models predict, then the pH could drop to a level between 7.6 and 7.8.

Victoria J. Fabry Tiny, photosynthetic plankton, such as this coccolithophorid, form the base of the ocean's food chain. As the carbon dioxide pours into the oceans, the water becomes more acidic, and these creatures find it more difficult to form a calcium skeleton.

How this lower pH will affect calcification in marine organisms is unclear, but scientists in the United States are now scrambling to plan a conference for the spring of 2005 to discuss the impact on corals. Meanwhile, British researchers are working on a Royal Society report on this topic to be released in the summer.

“It’s one of those things that has taken a long time to gel,” says Peter Brewer, a senior scientist at the Monterey Bay Aquarium Research Institute. Brewer says he wrote one of the very first articles showing that rising CO₂ levels would acidify the oceans. With the publication of the recent papers in Science, he says, “it’s now gotten out there, and a light bulb sort of went off.”

One reason this issue has been overlooked is because various disciplines often speak in different languages. Brewer says that his work was often ignored at conferences because he would give values in moles or tons. “When I finally converted these numbers to pH, a lot of biologists finally took notice,” he says. “So I think it’s dialogue. Once people started seeing it in terms they can understand, it finally started to sink in.”

Christopher Sabine, an oceanographer with the National Oceanic and Atmospheric Administration and coauthor on one of the Science papers, agrees. He adds that it is confusing that some scientists speak about petagrams (Pg) of CO₂, while others relate values to petagrams of carbon. “It’s carbon we’re really interested in,” he says.

Another problem was the poor quality of the data on carbon storage in the ocean. To get at the issue, Sabine and his co-workers measured carbon levels from 1989 to 1998 at various depths for 9618 sites worldwide, amassing a database of about 72,000 measurements. Using these data, Sabine’s group estimates that from 1800 to 1994, humans added 118 Pg of carbon to oceans; each petagram is equivalent to a billion metric tons. “This new study presented for the very first time just how much CO₂ has accumulated in the oceans,” he says.

Using values that countries report annually for the total amount of anthropogenic carbon spewed out by fossil-fuel combustion and cement production, Sabine’s group estimates that about half of this CO₂ has ended up in the oceans. However, some tricky accounting issues arise. Land management practices also cause carbon emissions, but the calculated amounts show a wide range of 100–180 Pg. Sabine says that this leads some scientists to estimate that one-third of the CO₂ is going into the ocean.

Whatever the actual fraction is, Sabine’s group found that half the oceanic CO₂ from fossil fuels remains in the top 400 meters of water, a sign that the oceans have not fully mixed the carbon. “That’s why I say that the ocean has only used up about a third of its potential to take up CO₂,” says Sabine. “Most of the deep water hasn’t been exposed to the high levels of CO₂ [since the industrial revolution],” he says.

As CO₂ pours into the ocean and lowers the pH, it also begins to lower the saturation state of calcium carbonate particles, which are found in several mineral forms, including aragonite and calcite. Aragonite forms the skeletal structure of corals and is less soluble than calcite, which is produced by a host of planktonic organisms, the most prominent of which are coccolithophorids.

Last year, Chris Langdon published a paper showing that an oceanic pH of 7.8 slowed coral growth by 30% (Global Biogeochem. Cycles 2003, 17, 1–14). “We found that the coral rate of calcification or skeletal growth is directly proportional to the concentration of carbonate ion in the water,” says Langdon. “So if you reduce the carbonate by 30%, which is what you get when you double the CO₂, then you see a 30% decline in coral growth.”

Another clue comes from the work of Victoria Fabry, a professor of oceanography at California State University, San Marcos. Her group has studied coccolithophorids and found that increased CO₂ levels cause decreased calcification and lead to thinner, malformed skeletons in these plankton.

But even one bloom may have 15–290 different strains of genetically different coccolithophorids, says Ulf Riebesell, professor of marine biogeochemistry at the Leibniz Institute of Marine Sciences at Kiel University (Germany). “They may respond differently,” he says.

In a fjord near Bergen (Norway), Riebesell conducted mesocosm studies with large sacks enclosing 20 cubic meters of water. These large tents allowed him to capture the entire pelagic ecosystem, including coccolithophorids, dinoflagellates, cyanobacteria, and diatoms. He found that increased levels of CO₂ led to declines in calcification similar to those found under lab conditions.

However, he did find one interesting difference. For the past 50 years, the Redfield ratio has described nutrient uptake in oceans as occurring in a definite stoichiometry between carbon, nitrogen, and phosphorous. Riebesell found that under high CO₂ concentrations, these standard ratios—C:N:P = 106:16:1—no longer hold true. “It’s an interesting feedback that we don’t yet understand, but if you add more CO₂, the biology reacts by absorbing more CO₂,” he says.

But nobody knows what might happen if increased CO₂ levels are accompanied by higher temperatures, he and others point out. “I know from coral work that has not yet been published that temperature and CO₂ can compensate each other or reinforce each other,” he says. “It’s confusing at the moment, so we don’t really know what to expect in a global-warming, ocean-acidification scenario. This work has to be done.”

Scientists on both sides of the Atlantic are preparing to tackle this problem. Langdon and Sabine are putting together a research conference on acidifying oceans and corals to be held in the spring. Meanwhile, Europeans are focusing on a detailed plan to study the problem. John Raven, professor of biology at the University of Dundee (U.K.), is leading a group of British scientists who are producing a Royal Society report on acidifying oceans. He expects the report to be completed in the spring, just in time for the 2005 G8 summit in the United Kingdom. “Since Prime Minister Blair claims he’s putting the environment up front, we’d like his advisors to have this report on hand,” he says.

The acidifying ocean effect also raises interesting issues about carbon sequestration. For years, scientists have studied ways to lower levels of CO₂ by pumping it underground or into the oceans.

“The ocean invasion rate is now at around 1 million tons an hour, and everybody knows this,” says Brewer. “But for some reason, there is all this alarm over a scientist building a pipe and pumping it in.”

But Scott Klara, technology manager for sequestration at the U.S. Department of Energy’s National Energy Technology Laboratory, says, “The oceans are a very sensitive ecosystem, and nobody is exploring this as a serious option.”

Researchers say that interest in this topic is growing but that the public and scientific communities are still not aware of what is happening. “The math and chemistry on this are very clear,” says Fabry. “We are changing the chemistry of our sea water, and it will impact organisms in ways that will be negative. We just don’t know what will happen.” —PAUL D. THACKER