Ocean Basin Ocean Acidification

by Bill Chameides | January 26th, 2010
posted by Erica Rowell (Editor)

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The 274-foot R/V Thomas G. Thompson, operated by the University of Washington for the U.S. Navy, was used in 2006 to document rising ocean acidity in the North Pacific Ocean. (Image courtesy of the University of Washington)

A new study shows that rising acidification in the world’s oceans is widespread.

Carbon dioxide (CO2) is more than a global warmer; it’s also an ocean acidifier. The ocean is a huge reservoir of dissolved CO2 — with nearly 40,000 billion tons of carbon dissolved as CO2 in its vast waters, compared to 750 billion tons in the atmosphere.

We’re fortunate that the ocean is capable of absorbing even more (see my earlier posts here and here). Within about a year of being emitted, some 25 percent of the CO2 put into the atmosphere from burning fossil fuels and deforestation is transferred into the ocean. Eventually, within centuries, a large fraction of the remaining emitted CO2 will find its way into the ocean depths.

Oceans’ Ability to Absorb CO2 Provides a Buffer From Even Warmer Climes

Why are we fortunate? If it weren’t for the ocean uptake of CO2, the climate impact of CO2 emissions would be a lot more intense and longer lived. But as with many things in this world, there’s a catch to the ocean buffer. Basic solution chemistry tells us that when CO2 is dissolved in water, it turns into an acid — namely, carbonic acid. And so it’s expected that CO2 dissolved in seawater would render the water more acidic.

Theory tells us that the change in the ocean’s acidity from anthropogenic CO2 will be small — for those of you familiar with the pH scale of acidity, we’re talking so far about a decrease in pH of 0.1 unit since the Industrial Revolution (a change of about -0.3 pH is expected by mid-century).

While small, these acidity changes are by no means insignificant. The reason: increasing acidity makes it harder for calcareous organisms like plankton, mollusks, and corals to form their carbonate shells and skeletons. As acidity increases, these creatures’ lives become more precarious. The long-term implications for ocean fisheries and coral reefs, indeed all marine life, are uncertain but could very well be dire (more on this here and here).

Measurements Confirm Rising Ocean Acidity

OK, you say, but that’s all theory. How do you know it’s really happening? Good question and one that has spurred ocean chemists to take to the seas with instruments in tow to find out what’s really happening.

Their ocean measurements in a host of locations around the world have confirmed that ocean’s acidity is increasing. Other observations, as well, albeit much more limited ones, indicate that the acidity change is affecting some of the ocean’s acid-sensitive organisms (examples here and here).

Ocean Acidity Found to Be Widespread

Now another link in the evidence chain has been found — data show that the acidity change is basin-wide.

In 1991 as part of the World Ocean Circulation Experiment or WOCE, scientists, including Robert H. Byrne of the University of South Florida (the lead author of the new paper), completed a transect across the North Pacific Ocean extending from 22°–56° N along the 150°W meridian (see figure).

Ocean transect (referred to among oceanographers as P16N) used by Robert H. Byrne and colleagues to assess ocean acidity in 1991 and again in 2006.

Among the measurements taken during this cruise were those testing acidity levels using a new spectrophotometric method involving an added pH-sensitive dye to seawater samples to see how much light the dye absorbs.

The advantage of the new method is its precision (about 0.001 pH units) — more than adequate to document pH changes from the expected changes from the increase in atmospheric CO2 concentrations that has occurred since 1991.

In 2006, Byrne and colleagues repeated the acidity measurements along the P16N transect as part of an international research project called Climate Variability and Predictability (CLIVAR).

A comparison of their acidity measurements with those of 1991 is reported in last week’s edition of Geophysical Research Letters.

The results are pretty striking: ocean acidity has increased for virtually all of the surface waters to a depth of 150 meters along the transect, and down to 500 meters for much of the transect.

Using accompanying measurements of other parameters, such as the levels of dissolved organic carbon, the authors argue that much of the acidity change is attributable to anthropogenic increases in atmospheric CO2.

The change in ocean acidity found in the surface waters of the transect was as large as 0.06 pH units and averaged about 0.02 units — essentially what would be expected thermodynamically given the increase in atmospheric CO2 concentrations between 1991 and 2006.

One Surprise, but Overall the Acid Test Confirmed Rising CO2 and Rising Ocean Acidity

There was one surprise in their observations. The authors found little or no change in acidities along the transect at depths 150-500 meters north of 38°N. (South of 38°N, the acidity change extended down to 500 meters.)

They speculate that this lack of change may have been caused by a slowdown of ocean mixing that has been postulated to have occurred in this region in the 1990s.

These data make it hard to deny that our dependence on fossil fuels is causing widespread ocean acidification, just as it’s hard to deny that global temperatures are on the rise. What will be the impacts of the change in ocean acidity? We don’t know for sure — although they could be catastrophic — much like the impacts of global warming. But, don’t despair. It looks like we are hellbent on finding out.

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