Shutting Down the Oceans
Act I: Acid Oceans
Global warming and acidification are damaging the phytoplankton at the basis
of the oceans’ enormous food web, putting the entire biosphere in jeopardy.
Dr. Mae-Wan Ho
A fully referenced
version of this article is posted on ISIS members’ website. Membership details
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Terminal oceans
Imagine vast expanses of oceans devoid of
life as far and wide as you can project your senses, no whales, no fish, no
seabirds and no corals beneath. The warm seawater is thick with floating plastic
wastes and slime, and the oppressive heavy air putrid with death and decay.
That’s not a scene from a science-fiction film, but a likely future scenario
unless we take appropriate action now: stop polluting and exploiting
the oceans and shift comprehensively to renewable energy options to drastically
reduce carbon emissions (Which
Energy?).
Increase in
carbon dioxide concentration in the atmosphere and global warming are threatening
the oceans’ phytoplankton that supports all marine life from zooplankton to
whales. Phytoplankton is also the fastest assimilator of carbon, clearing
carbon dioxide from the atmosphere to prevent it building up as a greenhouse
gas that warms the earth.
When phytoplankton
is in jeopardy, all life is in jeopardy, on land and at sea. Marine life will
literally starve to death, and the decay and decomposition that follow would
release enormous amounts of carbon dioxide from the estimated 800 Gt of the
ocean’s standing biomass, resulting in further global warming on a massive
scale.
A little girl
who was taken to the beach for the first time in her life was moved to declare
she loved the ocean because it was “always open”. It is unthinkable that the
final curtains may soon fall over the oceans.
Acid oceans
Oceans take up carbon dioxide passively by
dissolving it in the water of the surface layers, and as carbon dioxide increases
in the atmosphere, so too does the concentration of carbon dioxide in the
water. This makes the surface water more acidic and interferes with calcification
in organisms that make their shells or external skeletons from calcium carbonate
[1]. The ‘calicifers’ span the marine food web from phytoplankton that make
their own food by means of photosynthesis, to practically all other organisms
that depend directly or indirectly on phytoplankton for food. Califiers include
coccolithophores among the phytoplankton; foraminifera and pteropods (tiny
marine snails) among the zooplankton; and corals. Under normal conditions,
calcite and aragonite (forms of calcium carbonate) are stable in surface waters
where the carbonate ion is at supersaturating concentrations. However as the
water becomes more acidic, the concentration of carbonate ion falls, and structures
made of calcium carbonate can now dissolve.
Researchers
have already found that corals, coccolithophore
algae and pteropods have reduced calcification or enhanced dissolution of
their shells and skeletons when exposed to elevated levels of carbon dioxide.
Acidity is measured as pH, the
negative logarithm to the base 10 of the concentration of hydrogen ion, H+.
The scale of pH goes from 0 to 14, pH 7 is neutral, pH greater than 7 is alkaline
and less than 7 acidic. The pH of the oceans is slightly alkaline at 8.0 to
8.2; and has dropped 0.1 unit since the industrial revolution. By the end
of this century, it will become another 0.3-0.4 unit lower, representing a
100 to 150 percent increase in hydrogen ion concentration.
As pH drops,
so does the concentration of carbonate, making it more difficult for marine
organisms to form calcium carbonate. There is substantial experimental evidence
indicating that calcification rates will decrease in both low latitude corals
that form reefs out of aragonite, the metastable form of calcium carbonate,
and phytoplankton that form their shells out of calcite, the stable form of
calcium carbonate.
Theoretical predictions and experiments match up
An international team of 27 climate scientists
from France, the United States, Japan, Switzerland Germany, Australia, and
UK used 13 models of the ocean carbon cycle to assess calcium carbonate saturation
under the ‘business as usual’ scenario [1]. In their projections, the surface
water of the Southern Ocean (that which surrounds the Antarctica) will begin
to become under-saturated with respect to aragonite by 2050. By 2100, this
could extend throughout the entire Southern Ocean and into the sub-Arctic
Pacific Ocean. When live pteropods were exposed to the predicted level of
under-saturation during a two-day shipboard experiment, their aragonite shells
showed notable dissolution.
The researchers
said their findings indicate that conditions
harmful to ecosystems at high-latitude could develop within decades and not
centuries as previously thought.
They computed modern-day ocean
carbonate chemistry from the observed alkalinity and dissolved inorganic carbon
(DIC) from data collected during the CO2
Survey of
the World Ocean Circulation Experiment (WOCE), part of the World
Climate Research Programme (WCRP) which used resources from nearly 30 countries
to make in situ and satellite
observations of the global ocean between 1990 and 1998 [2], and the Joint Global Ocean Flux Study (JGOFS), an international
multi-disciplinary programme involving 20 nations to study carbon fluxes between
the atmosphere and the surface and interior of the oceans and their sensitivities
to climate change [3].
Modern-day surface carbonate ions concentration varies with latitude from 105
mmol/kg in the Southern Ocean (all waters south of 60 S) to
240 mmol/kg in the tropics. Low temperature and large
amounts of upwelled deep water in the Southern Ocean containing high levels
of carbon dioxide from organic matter decomposition is responsible for the low
concentrations of carbonate there.
Carbon dioxide generated by human
activities has already reduced modern surface carbonate by more than 10 percent
since the industrial revolution (29 mmol/kg in the tropics and 18mmol in the
Southern Ocean.).
Dire predictions
By year 2100, as atmospheric carbon dioxide
reaches 788 ppm (parts per million) under the business as usual scenario,
average tropical surface carbonate will decline to 149 +14 mmol/kg, a 45 percent reduction relative
to pre-industrial levels, and that agrees with previous estimates. In the
Southern Ocean however, surface concentrations will dip to 55 + 5 mmol/kg, which is 18 percent below the threshold
(66 mmol/kg) at which aragonite becomes under-saturated. These
changes extend well below the sea surface. Throughout the Southern Ocean,
the entire water column becomes under-saturated with respect to aragonite.
The aragonite saturation horizon shifts from its present average depth of
730 m all the way to the surface. Simultaneously, in a portion of the sub-Arctic
Pacific, the aragonite saturation horizon moves from its present depth of
about 120 m to the surface. In the North Atlantic, surface waters remain saturated
with respect to aragonite, but the saturation horizon shallows dramatically.
North of 50 deg N, it shallows from 2 600 m to 115 m. The greater erosion
in the North Atlantic is due to deeper penetration and higher concentrations
of anthropogenic carbon dioxide, a tendency already evident in present-day
estimates based on data and in models.
The changes in aragonite concentration
could have severe consequences for calcifying organisms, particularly shelled
pteropods, the major planktonic producers of aragonite. Pteropod population
densities are high in polar and subpolar waters, but only 5 species typically
occur in such coldwater regions, and of these only one or two species are
common at the highest latitudes. High latitude pteropods have one or two generations
per year and form integral components of food webs, and are typically found
in the upper 300 m where they may reach densities of hundreds to thousands
of individuals per cubic millilitre. In the strongly seasonal high latitudes,
sedimentation pulses of pteropods frequently occur just after summer. In the
Ross Sea, pteropods account for the majority of the annual export flux of
both carbonate and organic carbon to the sea floor. South of the Antarctic
Polar Front, pteropods also dominate the export flux of calcium carbonate
to the sea floor.
Pteropods may already be unable
to maintain shells in waters under-saturated with respect to aragonite. Data
from sediment traps indicate that empty pteropod shells show pitting and partial
dissolution as soon as they fall below the agaronite saturation horizon. In
vitro measurements confirm the rapid rates of pteropod shell dissolution.
New experimental findings suggest that even the shells of live pteropods dissolve
rapidly once surface waters become under-saturated with aragonite. The authors
show that when live subartic pteropod Clio pyramidata is exposed to a level of under-saturation similar
to that predicted for Southern Ocean surface waters in the year 2100 under
business as usual scenario, a marked dissolution at the growing edges of the
shell occurs within 48 hours.
Marine food web at risk
The failure of pteropods
to thrive will affect many species, as they contribute to the diet of diverse
species of other zooplankton, small fishes, the North Pacific Salmon, mackerel
herring, cod and baleen whales.
Surface dwelling calcite plankton
such as foraminifera and coccolithophorids may fare better in the short term.
However, the beginning of high-latitude-calcite under-saturation will only
lag aragonite by 50 to 100 years. The diverse bottom-dwelling calcareous organisms
in high latitude regions may also be threatened, including cold-water corals,
which provide essential fish habitat. Cold-water corals seem much less abundant
in the North Pacific than in the North Atlantic where the aragonite saturation
horizon is much deeper. Some important groups in the Arctic and Antarctic
bottom-dwelling communities secrete magnesium calcite, which can be more soluble
than aragonite. These include gorgonians, coralline red algae and echinoderms.
At twice the normal concentration of carbon
dioxide, juvenile echinoderms stopped growing and produced more brittle and
fragile exoskeletons in a subtropical six-month experiment. Experimental
evidence from many lower-latitude, shallow-dwelling calcifiers reveals a reduced
ability to calcify with a decreasing carbonate saturation state. At twice
the normal carbon dioxide concentration, calcification rates in some shallow-dwelling
calcareous organisms may decline by up to 50 percent.
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