Humanity's use of fossil fuels sends 35 billion metric tons of carbon dioxide into the atmosphere every year. That has already begun to change the fundamental chemistry of the world's oceans, steadily making them more acidic. Now, a new high resolution computer model reveals that over the next 4 decades, rising ocean acidity will likely have profound impacts on waters off the West Coast of the United States, home to one of the world's most diverse marine ecosystems and most important commercial fisheries. These impacts have the potential to upend the entire marine ecosystem and affect millions of people dependent upon it for food and jobs.
About one-third of the carbon dioxide (CO2) humans pump into the atmosphere eventually diffuses into the surface layer of the ocean. There, it reacts with water to create carbonic acid and release positively charged hydrogen ions that increase the acidity of the ocean. Since preindustrial times, ocean acidity has increased by 30%. By 2100, ocean acidity is expected to rise by as much as another 150%.
Declining pH of seawater reduces the amount of carbonate ions in the water, which many shell-building organisms combine with calcium to create the calcium carbonate that they use to build their shells and skeletons. The lower carbonate availability, in turn, decreases a measure known as the saturation state of aragonite, an easily dissolvable mineral form of calcium carbonate that organisms such as oyster larvae rely on to build their shells. If the aragonite saturation state falls below a value of 1, a condition known as undersaturation, all calcium carbonate shells will dissolve. But trouble starts well before that. If the aragonite saturation state falls below 1.5, some organisms such as oyster larvae are unable to harvest enough aragonite to build shells during the first days of their lives, and they typically succumb quickly.
These changes are particularly worrisome for global ocean regions known as eastern boundary upwelling zones. In these regions, such as those along much of the West Coast of the United States, winds push surface water away from the shore, causing water from the deep ocean to well up. This water typically already has naturally high levels of dissolved CO2, produced by microbes that eat decaying algae and other organic matter and then respire CO2. Along the central Oregon coast, for example, when summer winds blow surface ocean waters offshore, a measure of the amount of CO2 in the water known a partial pressure rises from a few hundred to over 2000, causing ocean acidity to spike.
But oceanographers still didn't have a good handle on how rising atmospheric CO2 levels would interact with CO2 rich waters that upwell naturally. So for their current study, researchers led by Nicolas Gruber, an ocean biogeochemist at the Swiss Federal Institute of Technology in Zurich, decided to look closely at what's likely to happen in an upwelling region known as the California Current System off the West Coast of the United States. They constructed a regional ocean model that ties together what's going on in the atmosphere and the ocean. Because this model focused on the California Current System, Gruber and colleagues were able to give it a resolution 400 times that of conventional global ocean models. In their model, the Swiss team considered different scenarios of CO2 emissions over the next 4 decades and linked these to CO2 produced in the ocean due to respiration.
The buildup of atmospheric CO2 will rapidly increase the amount of undersaturated waters in the upper 60 meters of ocean , where most organisms live, the team reports online today in Science. Prior to industrialization, undersaturation conditions essentially did not exist at this top layer in the ocean. Today, Gruber says, undersaturation conditions exist approximately 2% to 4% of the time. But by 2050, surface waters of the California Current System will be undersaturated for half of the year.
Perhaps just as bad, however, aragonite saturation will fall below 1.5 for large chunks of each year. This could spell doom for Pacific oysters, a $110 million-per-year industry on the West Coast, as well as for other shell-building organisms that are sensitive to changes in ocean acidity, says Sue Cudd, owner of the Whiskey Creek Shellfish Hatchery on Netarts Bay in Oregon. Another species likely to face difficulty are tiny sea snails known as pteropods, which are a vital food source for young salmon.
The new results are "alarming," says Richard Feely, a chemical oceanographer at the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle, Washington. "It's dramatic how fast these changes will take place."
George Waldbusser, an ocean ecologist and biogeochemist at Oregon State University, Corvallis, says it's not clear precisely how rising acidity will affect different organisms. However, he adds, the changes will likely be broad-based. "It shows us that the windows of opportunity for organisms to succeed get smaller and smaller. It will probably have important effects on fisheries, food supply, and general ocean ecology."