Acidifying Oceans Add Urgency To Carbon Dioxide Cuts

ScienceDaily (July 6, 2008) — It's not just about climate change

anymore. Besides loading the atmosphere with heat-trapping greenhouse

gases, human emissions of carbon dioxide have also begun to alter the

chemistry of the ocean--often called the cradle of life on Earth.

The ecological and economic consequences are difficult to predict but

possibly calamitous, warn a team of chemical oceanographers in the

July 4 issue of Science, and halting the changes already underway

will likely require even steeper cuts in carbon emissions than those

currently proposed to curb climate change.

Ken Caldeira of the Carnegie Institution's Department of Global

Ecology, writing with lead author Richard Zeebe of the University of

Hawaii and two co-authors*, note that the oceans have absorbed about

40% of the carbon dioxide (CO2) emitted by humans over the past two

centuries. This has slowed global warming, but at a serious cost: the

extra carbon dioxide has caused the ocean's average surface pH (a

measure of water's acidity) to shift by about 0.1 unit from pre-

industrial levels. Depending on the rate and magnitude of future

emissions, the ocean's pH could drop by as much as 0.35 units by the

mid-21st century.

This acidification can damage marine organisms. Experiments have

shown that changes of as little as 0.2-0.3 units can hamper the

ability of key marine organisms such as corals and some plankton to

calcify their skeletons, which are built from pH-sensitive carbonate

minerals. Large areas of the ocean are in danger of exceeding these

levels of pH change by mid-century, including reef habitats such as

Australia's Great Barrier Reef.

Most marine organisms live in the ocean's sunlit surface waters,

which are also the waters most vulnerable to CO2-induced

acidification over the next century as emissions continue. To prevent

the pH of surface waters from declining more than 0.2 units, the

current limit set by the U.S. Environmental Protection Agency in

1976, carbon dioxide emissions would have to be reduced immediately.

"In contrast to climate model predictions, such future ocean

chemistry projections are largely model-independent on a time scale

of a few centuries," the authors write, "mainly because the chemistry

of CO2 in seawater is well known and changes in surface ocean

carbonate chemistry closely track changes in atmospheric CO2."

Although the ocean's chemical response to higher carbon dioxide

levels is relatively predictable, the biological response is more

uncertain. The ocean's pH and carbonate chemistry has been remarkably

stable for millions of years--much more stable than temperature.

"We know that ocean acidification will damage corals and other

organisms, but there's just no experimental data on how most species

might be affected," says Caldeira. "Most experiments have been done

in the lab with just a few individuals. While the results are

alarming, it's nearly impossible to predict how this unprecedented

acidification will affect entire ecosystems." Reduced calcification

will surely hurt shellfish such as oysters and mussels, with big

effects on commercial fisheries. Other organisms may flourish in the

new conditions, but this may include undesirable "weedy" species or

disease organisms.

Though most of the scientific and public focus has been on the

climate impacts of human carbon emissions, ocean acidification is as

imminent and potentially severe a crisis, the authors argue.

"We need to consider ocean chemistry effects, and not just the

climate effects, of CO2 emissions. That means we need to work much

harder to decrease CO2 emissions," says Caldeira. "While a doubling

of atmospheric CO2 may seem a realistic target for climate goals,

such a level may mean the end of coral reefs and other valuable

marine resources."

• James Zachos, University of California, Santa Cruz, and Toby

Tyrrell, Southampton University, U.K.

posted to ClimateConcern by Lily Anselm