The Real Announcement of THE
END OF THE WORLD
IT WON’T LOOK LIKE THIS
IT WILL LOOK A LOT LIKE THIS
Ocean Acidification – a new kind of ‘sea sickness’.
The Global Crisis that you haven’t heard about (yet)!
Worldwide emissions of carbon dioxide
from fossil fuel burning are dramatically
altering ocean chemistry and threatening marine organisms,
including corals, that secrete skeletal
structures and support oceanic
biodiversity.
The oceans – worldwide – have absorbed
approximately 11 billion metric tons of
carbon between 1800 and 1994.
The addition of all this
CO2 to the upper
oceans is changing
its chemistry, and
making it more
‘acidic’.
CO2 + H2O ⇌ H2CO3 (or ‘carbonic acid’)
It is what makes your Coke/Pepsi fizz.
Some basic chemistry first
The term pH describes the acidity of a liquid. It is defined as:
pH = –log10 [H+]
Which means that if hydrogen ions increase by X 10, the
pH decreases (becomes MORE acidic) by 1 unit.
Water (H2O) splits into [H+] and [OH-], and in pure water
these are equal (and equal to 10-7 moles/liter) and the pH is
therefore 7.0
H2O => [H+] + [OH-]
Acid solutions are <7.0. Alkaline (basic) solutions have
excess [OH-] and are > 7.0
(up to pH of 14).
On the logarithmic pH scale used to measure acidity, seawater near
the ocean surface averages about 8.2.
Pure water—neither acidic nor basic—has a pH close to 7.0.
coffee, has a pH about 5.0;
lemon juice has a pH of around 2.4;
laundry bleach, a pH around 12.5.
About 50% of the CO2 that humans put into the
atmosphere by burning fossil fuels ends up
being quickly dissolved in the oceans.
The remaining 50% of the CO2 stays in the
atmosphere, causing the increase in this
greenhouse gas by about ONLY 2 ppm per year.
If the Ocean didn’t absorb this large
quantity, atmospheric CO2 levels would
rise twice as fast –
at 4 ppm per year instead of 2 ppm.
Carbon dioxide dissolved in seawater first reacts with the
water molecule (H2O) to form carbonic acid (H2CO3).
CO2 + H2O = H2CO3
BUT, not all the CO2 dissolved in seawater reacts to make
carbonic acid and therefore seawater contains dissolved
gaseous CO2 (as in carbonated soda drinks).
Carbonic acid is an acid and splits up into its constituents,
releasing an excess of H+ to solution and so driving pH
towards lower values.
Carbonic acid splits up by adding one [H+] to solution along
with HCO3– (a bicarbonate ion):
[H2CO3] ⇒[H+] + [HCO3–]
Dissolved inorganic carbon (DIC) in seawater
As CO2 dissolves in seawater, the reaction with water
produces both [H+] and two negatively charged forms of
dissolved carbon; i.e.,
HCO3- (bicarbonate)
and
CO3-2 (carbonate)
At the typical pH of seawater (about 8.2), these are present as
90% as HCO3- (bicarbonate)
9% as CO3-2 (carbonate)
<1% as CO2 (carbon dioxide gas)
90% as HCO3-
9% as CO3-2
<1% as CO2
We are here
We are going here
Relative proportions of the three inorganic forms of CO2 dissolved in seawater.
Note the ordinatescale (vertical axis) is plotted logarithmically.
The increase in [H+] causes some CO32– (called
carbonate ion) to react with [H+] to become HCO3– :
[H+] + [CO32–] ⇒[HCO3–] (bicarbonate)
IMPORTANT conclusion…
The net effect of the dissolution of CO2 in seawater is to
increase concentrations of [H+], H2CO3 and HCO3–,
while decreasing concentrations of CO32–.
The decrease in carbonate ion concentration [CO32–] has
important consequences for the chemistry of carbonate
minerals commonly used by marine biota to form shells
or skeletons.
The formation and dissolution of carbonate minerals can be
represented as:
←mineral formation (the animal does this)
CaCO3 ⇔[Ca2+] + [CO32–]
dissolution→ (thermodynamics does this)
Because the dissolution of CO2 in seawater decreases
[CO32–], this reaction moves to the right, impeding the
formation of carbonate minerals and promoting their
dissolution.
Note that the dissolution of carbonate minerals tends to
decrease [H+] (i.e., increase pH), counteracting some of
the pH effects of added CO2.
The “carbonate buffer” effect.
The term ‘carbonate buffer’ describes how the dissolved inorganic
carbon system in seawater acts to diminish changes in ocean [H+]
concentration, and thus pH.
If a process, such as CO2 dissolution, adds [H+] to seawater, some
of the added [H+] reacts with carbonate (CO32–) ion to convert it to
bicarbonate (HCO3–). Because most of the added [H+] would be
consumed in this way, the change in pH is much less than it would
otherwise be.
But this process also consumes some carbonate ion; therefore this
pH buffering capacity would diminish as CO2 concentrations
increase. Because CO2 is absorbed at the sea surface, it is the
surface oceans that are most affected.
On the longer time scales of ocean mixing, interaction with CaCO3 - rich
sediments tends to buffer the chemistry of the seawater so that changes in
pH are lessened. For example, if the deep oceans start to become more
acidic such as through the addition of CO2, which decreases concentrations
of CO32–, some carbonate ion will be dissolved from sediments.
Diagram of the carbonate
buffer and biological pump
in the surface oceans.
After absorption of CO2 into
the oceans, it is converted
by the carbonate buffer.
seafloor
Calcification in the oceans also releases CO2 , some of which is returned to the
atmosphere. The biological pump (descending wiggly arrows) converts CO2 from the
atmosphere into organic carbon (Corg) and CaCO3 and transfers it to the deep ocean
waters and sediments.
(i.e., removes it from the upper ocean and ATM inventories).
Calcite dissolution occurs at depths in the range of about 1.5 to 5 km and aragonite
dissolves at depths in the range of about 0.5 to 2.5 km.
There is a critical concentration of carbonate ions in seawater (the
saturation concentration) below which CaCO3 will start to dissolve
(the CCD).
Because CaCO3 solubility increases with
decreasing temperature and increasing pressure,
the critical concentration occurs at a depth, the ‘saturation
horizon’,
below which seawater is under-saturated and CaCO3 will tend to
dissolve and above which seawater is super-saturated and
CaCO3 will tend to be preserved.
With increasing
atmosphere CO2, the
ocean becomes more
acidic, and the CCD
(where sediments can
store carbonate)
becomes shallower.
Because added CO2 decreases the carbonate ion concentration,
the saturation horizons will become shallower with increasing
releases of human derived CO2 to the atmosphere.
That is – the CCD will rise to shallower depths – and it is.
During the 19th and 20th centuries, the surface ocean’s
uptake capacity for CO2 was large and this allowed the ocean to absorb enormous amounts of
CO2 from the atmosphere - without a proportional increase
of the pCO2 of the ocean’s surface waters.
The amount of CO2 taken up
by the oceans per year since
1850..
in gigatons/year.
The average American emits about 120 pounds of
CO2 per day. About 5 times the world average.
120 x 365 days = 43,800 lbs = 21.9 tons.
PREVIOUSLY, the oceans could absorb could
about 40 pounds of CO2 per day per person.
But this is all
starting to change.
RECENT INCREASES IN CO2 VALUES:
upper => ATM: lower => OCEAN
Present surface seawater pH values from all oceans (pH calculated from
dissolved inorganic carbon and alkalinity). The majority of the data fall into
narrow pH range of 8.1 ± 0.1.
Also shown are typical pH ranges of glacial, pre-industrial, present, and future
(year 2100) surface seawaters resulting from the observed and predicted
increase in atmospheric CO2 levels (blue line with exponential increase) as
obtained by simple scenario calculation.
PAST
PRESENT
Future
Map of mixed surface layer (@50 m) pH values in global oceans for
1994. Low values are upwelling regions (e.g., Equatorial Pacific, Arabian
Sea) where subsurface waters with lower pH values are brought to the
surface. Highest values are regions of high biological production and
export.
This is NOT a subtle effect and is easily observed.
Column inventory of anthropogenic CO2 in the ocean.
Because this newly
added anthropogenic
carbon has a different
isotopic signature
than natural carbon, it
is easy to map the
distribution of this
CO2 absorption in the
upper oceans.
The increased ocean acidity lowers the concentration of
carbonate ion, a building block of the calcium carbonate
that many marine organisms use to grow their skeletons
and create coral reef structures.
A simpler equation….
1. too acid, no calcium
carbonate precipitation.
2. no calcium carbonate, no
reef.
3. No reefs, less fish.
Importance of micro organisms
(phytoplankton and nonphotosynthetic zooplankton and
microbial cells) and of larger
animals in the marine carbon
cycle.
The thickness of the lines
indicates relative carbon flow
through the pathway.
The cycle assumes no net input
of carbon or loss from the
oceans.
HOW BIG AN IMPACT? Greenhouse gases have lowered
the pH value of seawater by about 0.1 unit since 1700.
This doesn’t sound like much, but this decrease of 0.1 unit
equates to a 30% increase in the concentration of hydrogen
ions;
or, as the 2005 Royal Society report puts it, "a considerable
acidification of the oceans."
too acid
23% of all coral reefs in the
ocean are now dead.
Another 20% have ‘bleached’
and are dying.
Projections indicate that by
the year 2030,
ALL coral reefs in the ocean
will be dead.
Using a mid-range scenario of greenhouse emissions, as
calculated by the IPCC, models estimate that near-surface
oceanic pH could drop by 0.4 unit by the year 2100.
Although this would leave the oceans still slightly alkaline, it
corresponds to a threefold increase in hydrogen ion concentration
since pre-industrial times.
And it may take tens of thousands of years before pH values return to
preindustrial levels.
Coccolithophorids are an algae which are
the Basis of the Food Chain in the oceans,
and they provide much of nutrition for
marine life.
They have calcareous outer shells.
If the oceans become too acidic, they can’t
form their exoskeletons and they die.
Are there any positive effects of CO2 fertilization of the upper ocean?
Photosynthesis of phytoplankton species differ in sensitivity to CO2. Most
species (here S. costatum and P. globosa) reach their maximum photosynthetic
rate under present-day ambient CO2 levels (14.7 μmol per liter), some species,
(i.e., E. huxleyi) show increased rates of photosynthesis when CO2 is increased
above present levels. This raises the possibility that coccolithophores may
benefit directly from the current increase in atmospheric CO2.
What happens to the marine biological community when CO2 levels increase
X2 and X3?
Three major areas of concern: (a) increased CO2 uptake by plankton will
accelerate the rate of ocean acidification in deeper layers, (b) lead to a
decrease in oxygen concentrations in the deeper ocean, and (c) will negatively
influence the nutritional quality of plankton.
The latter development can have consequences for entire ocean food webs.
So – as the oceans become more acidic, who wins, and who loses?
There are trade-offs, between the increased bio-production of a few
species, and the loss of species that have calcareous shells.
The formation of shells or plates of CaCO3, by calcification, is a
widespread phenomenon among marine organisms, such as most
molluscs, corals, echinoderms, foraminifera and calcareous algae.
Although it is not always clear what function this calcification has, it
seems integral to their biology; so any decrease in calcification, as
a result of increased CO2, is therefore likely to have significant
consequences such as the weakening of coral skeletons and reef
structures generally.
So if you have body parts made out of calcite or aragonite, a more
acidic ocean in the future will not be good for you.
If you are a coral, you are probably ‘toast’.
But 25% of all protein for Asian populations depends on fish that live in
coral reefs – about 1 billion people.
Moving from left to right (below) can’t be a good thing.
Calcium carbonate is heavier than seawater – and acts as
‘ballast’ – in making dead forams sink to the seafloor.
Removal of these species would slow the biological ‘pump’ that
helps sequester atmospheric CO2 into seafloor sediments;
and this slowing would cause the atmospheric CO2 inventory to
RISE faster than it is at the present.
Many other calcifying organisms—including marine plankton such as
pteropods, a planktonic marine snail—are negatively impacted by
these seawater chemistry changes.
Calcite shelled pteropods are an important food source for salmon,
mackerel, herring, and cod.
The Real Announcement of THE
END OF THE WORLD
IT WON’T LOOK LIKE THIS
IT WILL LOOK A LOT LIKE THIS
But recent scientific observations
indicate that the present absorption
of excess CO2 by the oceans can’t
continue at this rate…
And the rate of absorption of
atmospheric CO2 is actually
slowing down NOW.
Because of the thermodynamic effects of ocean acidification, a
larger portion of future CO2 emissions will remain in the
atmosphere,
thus enhancing the predicted global warming effects of
CO2 on climate on Earth.
Ocean Acidification is actually NOT the End of the World –
it has happened previously;
• 250 My (end Permian event),
• 65 My (end Cretaceous event) and
• 55 My (Eocene Hydrate Event) ago,
But these were all massive extinction ‘events’…
Can’t we just “wait it out?” How long will it last?
About 75% of CO2 emissions will have an average perturbation lifetime
of 1800 years and 25% have lifetimes >>5000 years.
This leads to the most dramatic changes in marine chemistry in at
least the past 650,000 years
CO2 + CO32- + H2O => 2HCO3In order to make a carbonate shell, an animal needs both Ca++ and
CO32- .
Adding CO2 to the equation decreases the CO32- and makes it
much harder for marine life to precipitate calcium carbonate shells.
Marine life - like corals.