Industrial-age carbon dioxide found in oceans

By Sandi Doughton
Seattle Times staff reporter

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Almost half the carbon dioxide produced by the burning of oil, gas and coal since the start of the industrial age has wound up in the oceans instead of the air, say studies published today in the journal Science.

Carbon dioxide is the primary "greenhouse" gas that most experts believe is trapping heat around the planet and threatening to transform its climate.

"The oceans are performing a tremendous service to the world by reducing the amount of carbon dioxide that remains in the atmosphere," said Christopher Sabine, of the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle.

But that service could come at a cost to sea-dwelling creatures, said co-author Richard Feely, also of the environmental lab.

The dissolved carbon dioxide is already making the oceans slightly more acidic, which, if current trends continue, will break down coral reefs, mollusk shells and the protective coverings of plankton.

"These organisms form the base of the food chain. We're not really sure what the ecological implications will be," Feely said.

Researchers from France, Australia, Japan and several other nations joined in what was a 15-year project to solve one of the long-standing mysteries of global warming.

Scientists can estimate how much carbon dioxide has been unleashed by human activity, and they know that the level in the atmosphere has increased from 280 parts per million in 1800 to 380 ppm today. But when they do the math, there's a lot of missing gas.

"What's accumulating in the atmosphere is only about half of what we're producing," Sabine said. "The question for the past couple of decades has been: Where's the rest going?"

Forests and other vegetation, which gain nourishment from carbon dioxide and sunlight, are also absorbing a big chunk of the man-made carbon dioxide today, but the new analysis shows that in the long run, only the oceans have been a consistent "sink."
 
The scientists calculate that over the past two centuries, nearly 120 billion metric tons of carbon dioxide from human activity was absorbed by the oceans — a process that continues at a rate of 20 million tons a day.

Coming up with those numbers took 10 years of ocean measurements, gathered during 95 research cruises crisscrossing the planet's seas from Greenland to Tahiti and the Antarctic. Another five years was spent analyzing the data, collected from 72,000 different sites.

"It was an unprecedented effort," Sabine said.

As world leaders debate the validity of global-warming projections and consider how — or whether — to reduce emissions, the new understanding of the ocean's role will be valuable, said Taro Takahashi, an oceanographer at the Lamont-Doherty Earth Observatory at Columbia University who was not involved in the project.

"This is important information for policy makers charting the future course of the Earth," he said.

The main unanswered question is whether the oceans will continue to soak up carbon dioxide at the same rate as in the past, Takahashi added.

Preliminary evidence is pessimistic. The Seattle team found signs that the amount of carbon dioxide dissolving in the ocean may be declining, possibly as a result of rising ocean temperatures.

Just as a warm beer loses its fizz, warmer ocean waters won't absorb as much carbon dioxide, Takahashi said.

That could be important in considering some of the schemes that have been proposed to counter global warming, such as adding iron fertilizer to the oceans to increase carbon-dioxide uptake by marine plants.

Most of the dissolved carbon dioxide is in the ocean's upper layers, where single-celled plants and animals called plankton flourish. Laboratory tests have shown that high levels of dissolved carbon dioxide can prevent plankton and other creatures from forming shells, Feely said.

He estimates the oceans will reach those levels by the end of this century, but more subtle ecological impacts may appear before then.

"The organisms that live in that chemical environment will respond to these changes very, very quickly," he said.

The project was funded by NOAA, the National Science Foundation and the Department of Energy.

A second round of measurements are already underway.

"We hope to do a reanalysis every 10 years," Sabine said.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com