Scientists
have long expected that recent high atmospheric CO2
levels will make ocean water more acidic, but the impact
was expected to be small. But a Swiss team suggests that
by 2050, more than half of the surface waters in the California
current off the US west coast will become too acidic for
shell-building marine life to thrive or survive—at
least at certain times of the year. When that eventually
happens, essentially all of the water in the bottom layer
will not be capable of supporting oysters and other shell-building
organisms.
When
CO2 is dissolved in water, it creates a weak
acid with some hydrogen ions. Acidity measures how many
hydrogen ions are present; and when more are present, an
organism expends more work to build a shell. Many marine
organisms, including oysters, crabs, lobsters, periwinkles,
and corals, use calcium carbonate (CaCo3) to
build shells, which are essential for their life. Above
a certain level of acidity, it is no longer possible for
them to build shells, and in fact shells may dissolve.
The
atmospheric level of CO2 has become higher over
the last century. The impact on the oceans is still small,
but in certain areas where deep water wells up to the surface,
CO2 is already naturally high. In these areas
the CO2 level in the water is closer to the “saturation
point” of carbonate.
In the California
current, as in west-coast currents around the world, deep
cold water rises up to the surface. The water is rich in
nutrients from organic matter that decomposed in the depths
or on the bottom. Decomposition releases CO2,
which makes water more acidic and closer to being undersaturated.
There have been incidents of so-called “corrosive”
undersaturated water on the Oregon coast that led to failures
of oyster hatcheries (see one report here1).
To date, excess atmospheric CO2 has reduced oceanic
pH by about 0.1 unit and the saturation state of carbonate
by 0.4 units for the ocean as a whole.
A
team from ETH in Zurich, Switzerland,
explored where and when acidification might result from
elevated CO2 levels. They simulated the ocean
chemistry of the California current to the year 2050, in
a model based on two common scenarios of CO2
emissions from economic activities. Their model was able
to depict details of the current and the nearshore waters,
unlike global ocean models which are too coarse to depict
such fine details.
Continues . . .
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The
Swiss team reported2 that large areas (about one-half)
of California surface waters are expected to become undersaturated,
and thus “acidic” in the summer, by 2050. If this
happens, sensitive organisms would be forced to live in less
than half of their current habitat area. Water becomes more
acidic the deeper one goes: in twenty to thirty years from
now, “essentially all the waters above the shelf sediments
will be undersaturated” and water on the sea floor would
be undersaturated all year long.
The
impact of acidification depends very much on the atmospheric
level of CO2. When the level reaches 400 parts
per million (ppm) (the current level in 2012 is 390 ppm),
“substantial parts of the twilight zone . . . and habitats
along the sea floor on the shelf become undersaturated.”
Many scientists expected that the Arctic Ocean would be the
first region to experience profound acidification, but the
California waters will become acidic before the Arctic Ocean
will. When the atmospheric CO2 reaches 500 ppm,
surface waters in the sunlit zone would start to be acidic
for extended periods.
The
Swiss cautioned that organisms differ greatly in their tolerance
of acidity: for some, undersaturated water is not a problem.
And some species have evolved to tolerate acidic water for
certain periods of time.
The
authors conclude that the California current is rapidly moving
toward conditions “well outside the natural range”
with frequently undersaturated water. The impact of this can
be magnified if further warming or low oxygen levels (“dead
zones”) accompany the acidic water. Both have been noted
in the California current.
Jane
Lubchenco, administrator of the US National Oceanic and Atmospheric
Administration (NOAA), remarked to the Associated Press that
the speed by which the oceans have become more acid caught
scientists off-guard, and the problem is now considered to
be the “equally evil twin” of climate change in
the atmosphere.
CITATIONS
1. “Rising
acidity brings an ocean of trouble,” news focus,
Science, vol. 337, 146-148, 13 July 2012.
2. “Rapid
progression of ocean acidification in the California Current
system” by N. Gruber, C. Hauri, Z. Lachkar, D. Loher,
T. Frolicher, G-K. Plattner, Science, vol. 337, 220-223, 13
July 2012.
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