ocean uptake of co2: are the oceans acidifying?
TRANSCRIPT
B R S S E M I N A R S E R I E S P R E S E N T S :
Friday 8 July
Ocean uptake of CO2:Are the Oceans Acidifying?
Dr Steve Widdicombe,Plymouth Marine Laboratories, UK
This presentation will introduce the process of ocean acidification, highlight someof the key environmental concerns and discuss some of the mitigation strategiesthat have been suggested. With world primary energy demand projected to rise atan average of 1.7% annually over the next 30 years this means an increase in therelease of CO2. Of all the predicted impacts attributed to this inevitable rise inatmospheric CO2 concentration (and the associated rise in temperature), one ofthe most pressing is the acidification of surface waters through the absorption ofthe atmospheric CO2 and its reaction with seawater to form carbonic acid. It ispredicted that this process may lead to a surface ocean pH reduction of 0.7 unitsby the end of the century. It is clear that the growing emissions of CO2 from humanprocesses could pose a distinct threat to the global environment. Howeverquantifying the consequences of CO2 release is problematic as many physical andbiogeochemical processes combine to create a complex set of interactions.
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Reviewing the Impact of Increased Atmospheric CO2 onOceanic pH and the Marine Ecosystem:
“Ocean Acidification”
Dr Steve WiddicombePlymouth Marine Laboratory
Benthic Ecologist
I have worked at PML for the past 15 years
Main interests are the factors that affect the communities that live in/on the seafloorand the ecosystem functions these communities perform.
Particular area of expertise is the use of large experiments to explore theseinterests.
The Plymouth Marine Laboratory [PML] is an independent and impartial collaborativecentre of the UK Natural Environment Research Council [NERC].
First involvement with ocean acidification was 4 years ago through a EuropeanNetwork of Excellence “CO2 GeoNet”
I now run 2 large interdisciplinary projects exploring the potential impacts of oceanacidification and the ecological risks associated with geological storage.
Carbon in the Ocean
Pre industrially, the oceansand the organisms that live inthem contained about 38 000Gt C (1 Gt = 1015 grams or 1billion tonnes)
Oceans are a carbon sink andtake up 2 Gt C per year
This 95% of all the carbon thatis in the oceans, atmosphereand on land.
6 Gt C per year released into the atmosphereby human activities
Atmospheric concentrations are higher today than at any timefor at least the last 420 000 years
Burning fossil fuels is releasing CO2 that wouldotherwise be locked up in geological reservoirs.
About one half (48%) of all the CO2 produced by fossil fuel burningand cement production in the past 200 years (1800 – 1994) hasbeen absorbed by the oceans.
A total of 118 ± 18 Gt C (1800 – 1994)
Today that figure is nearer 140 Gt C(over 500 Gt CO2)
Human impact on the carbon cycle
Oceanic AcidificationAtmosphere
CO2 (g) CO2 + H2O ´(H2CO3)´ HCO3
- + H+ ´ CO32- + 2H+
Surface Ocean
CO2 dissolves into the seawater to form Dissolved Inorganic Carbon(DIC) which consists of:
1. aqueous CO2 (including carbonic acid) – 1%
2. bicarbonate HCO3- - 91%
3. carbonate CO32- - 8%
All 3 forms of DIC are important for biological processes (e.g.photosynthesis and calcification)
DIC operates as a natural buffer to the addition of hydrogen ions –known as the “carbonate buffer”
BUT we now know there is a limit to how much CO2 the carbonatebuffer can deal with.
Henry’s Law
Oceanic pH
Acid solutions have an excess of H+ ions and a pH less than 7
Alkaline solutions have an excess of OH- ions and a pH more than 7
The term pH describes the acidity of a liquid.
pH = -log10[H+]
H20 _ H+ + OH-
Concentration of H+ and OH- are roughly equal (10-7 mole per litre)This means a neutral solution has a pH = 7
This measure is negative logarithmic – if H+ concentration increase10 fold, pH decreases by one unit.
Wolf-Gladrow, Riebesell, Burkhardt, Bijma (1999) Tellus 51B, 461
The Oceans are becoming more acid as they take up more CO2
Oceanic pH
The oceans are currently alkaline pH 8.1 ± 0.3
CO2 + H2O ´ HCO3- + H+ ´ CO3
2- + 2H+
Net effect of dissolving CO2 in seawater is to increase H2CO3, H+ andHCO3
- concentrations & decrease CO32- concentrations.
Oceanic pH
7.4
7.6
7.8
8
8.2
8.4
8.6
-25 -20 -15 -10 -5 0 5
time (million years before present)
pH
Caldeira and Wickett (2003)
Past (Pearson and Palmer,2000) and predictedvariability of marine pH.
2000 AD
2050 AD
2100 AD
Continued CO2 Emissions and Ocean Acidification
pH has changed by 0.1 pHunits since pre-industrialtimes.
This equates to a 30%increase in the concentrationof hydrogen ions in the last200 years.
0.5 – 2.5km Corals, Molluscs (Pteropods),
Macroalgae
Aragonite
1.5 – 5km Foraminifera, Coccolithophores,
Echinoderms, Molluscs,
Macroalgae
Calcite
Saturation HorizonMarine Groups
Marine organisms that construct calcium carbonate structures dependon the presence of carbonate.
CaCO3 will dissolve unless there is a sufficiently high concentration ofcarbonate (CO3
2-) ions.
CaCO3 becomes more soluble with decreasing temperature andincreasing pressure.
Creates a boundary called a “saturation horizon”.
Calcium Carbonate
Ca2+ + 2HCO3- _ CaCO3 + CO2 + H2O _ H2CO3 + CO3
2- _ 2HCO3-
Some what does a change inpH mean for the oceans andthe organisms that live in it?
Effects on micro organisms
Photosynthesis
Growth and composition
Metal speciation
Nutrient speciation
50% of global primary production is carried out in theoceans by plankton <10mm
- Small _
- Little effect
- Effect unknown
Effect of Reducing pH on Key Nutrients
Changes in speciation of phosphate, silicate and ammonia with pH(from Reebe and Wolf-Gladrow 2001)
Speciation: of key nutrients for phytoplanktonand bacterial growth are pH dependent
Nitrogen: in particularammonia-ammonium, ispH dependent, with theconcentration of NH3 (Aq)decreasing as pHdeclines
Inorganic phosphate: aspH falls to 7.8, theconcentration of H2PO4
-
rises as that of PO43-
decreases
Changes in the relative proportions ofphosphorus, nitrogen and trace metal speciesmay have an effect on plankton diversity
Effects on bacterial processesLow pH inhibition of nitrification
Change in Nitrification Rate with pH
y = 0.1508x2 - 1.5944x + 4.1302
R2 = 0.984
0
0.2
0.4
0.6
0.8
1
1.2
5.5 6 6.5 7 7.5 8 8.5pH
Re
lati
ve
Nit
rifi
ca
tio
nra
te
now
WCS
Experimentally derived response curve (Huesemann et al, 2002)
The shift away from NH3 willinhibit nitrification, which inturn will inhibit denitrification.
Nitrification / Denitrification
-3
-2234 NONOONNHNH �ÆÆ�+
N2
nitrificationDenitrificationto atmosphere
Oxidation of ammonia to nitrate / nitrite
Low pHÁ
Spatial variability in ecosystem response
Denitrification parameterisedfor sandy sediments, mayunder estimate muddysediments.
10% decrease is consistentacross the domain.
60
50
40
30
20
10
60
50
40
30
20
10
6
5
4
3
2
1
Compares with measurements / estimates of15-150 mmol N .m-2.y-1. (Lohse 93,96)
2000 simulation
WCS simulation
Difference in denitrification loss
WCS - 2000
Effects on larger organisms
Decreased motility, inhibition of feeding,
reduced growth, reduced recruitment,
respiratory distress, decrease in population size,
shell dissolution, mortality
increased susceptibility to infection
destruction of chemosensory systems
Effects of Low pH on Zooplankton: the Food of Fish
Yamada & Ikeda 1999; Heath 1995; Shiramura et al. 2002; Kurihara et al. 2004
Evidence to indicate that marine zooplankton(crustacea) passing through plumes of CO2enriched seawater suffer high mortalities – butvery little research
Some zooplankton have calcium carbonate shells(molluscs) and will be vulnerable. This pteropodis an important part of the Antarctic food web
Reduced fertilization of copepod eggs at CO2
levels beyond 1000 ppm (2100 worst casescenario)
Limacina helicina antartica
Adult female copepod bearing eggs
Crustacean zooplankton
Impact on Fish and Squid
Portner and Reipschlager (1996); Portner et al. (2004)
Squid are an important food resourcefor humans, whales etc.
At 3 fold increases in atmospheric CO2, fish andother complex animals are likely to havedifficulty reducing internal CO2 concentrations,resulting in accumulation of CO2 andacidification of body tissues and fluids(hypercapnia)
The effects of lower level, long term increases inCO2 on reproduction and development of marineanimals is unknown and of concern
Squid very sensitive because of their highenergy and O2 demand for jet propulsiondecrease in pH of 0.25 having drastic effects(reduction of c. 50%) on their oxygen carryingcapacity
Effects on calcifyingorganisms
Coccolithophores
Others include: Foraminifera, echinoderms and molluscs
and Corals
Coccolithophores: Important Primary Producerson European Shelf Seas
Change in biogeochemistryand ecosystem
Holligan 1993; Archer et al. 2003
Extensive blooms (often 100,000’s km2)of these calcite forming phytoplanktonoccur on shelf seas
Reduction incoccolithophores couldchange phytoplanktonbiodiversity
Largest current producer ofcalcite on Earth
Important role in the global carbon cyclethrough the transport of calcium carbonateto deeper waters and sediments
Coccolithophores aremajor producers ofdimethyl sulphide (DMS)to the atmosphere –thought to be important incloud formation andhence a negativefeedback to climate
Effects of CO2 on Coccolithophores
Gephyrocapsa oceanica
Riebesell et al. Nature (2000)
300 ppm
780-850 ppm
Emiliania huxleyi
pCO2
Reducedcalcification
Coccolithophore model
depth
Julian day
Year 1WeakerstratificationConstant at 30m,
Year 2Strongerstratification,Warmer withdeepening out ofthe euphotic zone
Simulated coccolithophore biomass, compares withobservations of 44 mg C.m-3 at surface, (Burkill, 2002;Widdicombe, 2002).
Based on Tyrrell & Taylor (1996) and Merico et al (2004)
Coccolithophore results
0
400
800
1200
1600
Jan Feb Apr Jun Aug Oct Dec Feb Apr Jun Aug Oct Dec
Y2000
Y2050
Y2100
Ywcs
0
5
10
15
20
Jan Feb Apr Jun Aug Oct Dec Feb Apr Jun Aug Oct Dec
Y2000
Y2050
Y2100
Ywcs
Á Coccolithophore cell biomass (mgC.m-2)
Marked inhibition of coccolithophores is seenwith decreasing pH.
Inhibition strength is sensitive to the physicalconditions prevailing
Stronger light and nutrient limitation in year 2decrease the relative effect of calcificationinhibition
ÁNumber of liths per cell
Inhibition correlates with the degree of lithcoverage and hence the parameterisation ofmortality and grazing relative to the lithcoverage.
Parameterisation penalises <10 liths.cell-1
Warm Water Coral ReefsOnly 1.28 million squarekilometres (less than 1.2%of the worlds continentalshelf area)
BUT many millions ofpeople directly dependanton healthy coral reefs(tourism and fisheries).
Highly diverse ecosystem.
Coral reefsoccur in warm,alkaline, sunlitwaters with higharagonitesaturation.
Corals form a powerful mutualisticsymbiosis with tiny dinoflagellate
algae known as zooxanthellae.
Sitting in the tissues, the algalsymbionts photosynthesize and passmost of their production to the coral.
What are coral reefs ?
In return, the animal provides inorganic nutrients such asammonia and phosphate – from their waste metabolism.
The fate of corals
95% correlation with increases in seatemperature (1-2% above long-termsummer sea temperature maxima) andbleaching.
Backed up experimentally
Almost 30% of warm water corals have disappeared sincethe beginning of the 1980s
Largely due to increasingly frequent and intense periods ofwarm sea temperatures
This causes coral bleaching.
Pre-industrial (pCO2 – 280 ppm) 2060 (pCO2 – 517 ppm)
Required Aragonite saturation for “healthy” coral growth – Average min/max 3.28 – 4.06
High atmospheric CO2 is compounding the problem bylowering the aragonite saturation state of seawater
If atmospheric CO2 concentrations double, calcification rates coulddecrease by 10-30% (Gattuso et al., 1999; Kleypas et al., 1999b).
Some recent studies even suggest a decrease of 54%.
Coral loss = Financial loss
Globally, corals support millions of people through subsistence foodgathering and tourism.
Studies have been based on global warming only.
(Hoegh-Guldberg & Hoegh-Guldberg, 2004)
Reef associated revenue / goods contribute 68% of the grossregional product (AU$ 1.4 billion)
Most dependent region - North Queensland.Of the AU$900 million tourism revenue,AU$800 million was associated with having ahealthy coral reef.
Reef degradation (600ppm by 2100) will cost the local economies ofcoastal Queensland a minimum of AU$2.5 billion over 19 years (2001-2020).
If atmospheric CO2 is higher (800ppm by 2100) financial cost could beas high as AU$14 billion.
The Caribbean
Hawaii
Caribbean reefs provide annual net benefits (fisheries, divetourism and shoreline protection) of between US$ 3.1 billion andUS$ 4.6 billion (Burke et al, 2004)
By 2015, loss of income due to reef degradation, thought to beseveral hundred million dollars per year.
Estimated that coral reefsgenerate US$ 364 millioneach year.
US$304 – snorkelling and diving40 - property7 – non-use2.5 - fisheries
NOTE: these costs do not include the ecological /environmental goods and services provided by the reef.
Cold Water CoralsOften referred to as “deep water” corals butexist at depths between 10m and over 1000m
Distribution probably controlled bytemperature.
Found all over the world.
Exact area unknown but recent studies indicate that cold water coralscould equal or even exceed warm water corals (Freiwald et al., 2004)
Non-photosynthetic, rely on organicmatter from above.
Long lived, slow growing – coloniesup to 8000 years old have been found.
Fossil records show they have beenaround for millions of years.
Support a diverse ecosystem.
Cold water corals such as Lophelia pertusa
Shallowing of thearagonite saturationhorizon will have abig impact on theseecosystems
Freiwald, www.sams.ac.uk, Orr in press
Location of recentlydiscovered Lophelia coralsaround the UK
The Marine Ecosystem
Hetero-trophs
Bacteria
Meso-Micro-
Particulates
Dissolved
Phytoplankton
Consumers
Pico-f
DiatomsFlagell-ates
NO3
PO4
NH4
Si
DIC
Nutrients
Cocco-liths
Meio-benthos
AnaerobicBacteria
AerobicBacteria
DepositFeeders
SuspensionFeeders
Detritus
NutrIents
OxygenatedLayer
ReducedLayer
RedoxDiscontinuity
Layer
AtmosphereO2 CO2 DMS
Ecosystem
Summary
The oceans are absorbing much of the CO2 weproduce and are becoming more acidic as a result.
Ocean acidification is a predictable response, more so than globalwarming.
Ocean acidification and global warming are intrinsically linked – a warmerocean absorbs less CO2
The oceans currently take up 1 tonne of human derived CO2 per year foreach person on the planet.
Almost half of all the human derived CO2 produced in the last 200years is now in the ocean.
Ocean pH has already changed by 0.1 pH units since 1800. (30%increase in hydrogen ions)
pH looks likely to decrease by at least another 0.5 units by 2100
Rate of change in ocean pH is at least 100 times greater than the worldhas experienced for millions of years.
Summary
Seawater pH and carbonate concentrations are critical in marinesystems.
Best scientific information suggests corals (warm and cold) will beadversely affected, along with the communities they support.
Many other organisms dependant on calcification will be affected.
Non-calcifying organisms could also be affected through physiologicalstress, reproductive success and resistance to infection.
Ecosystem functions e.g. nitrogen cycling / eutrophication, will beaffected.
Ecosystems will survive but will they contain the organisms we want orneed?
Ecosystems will be affected.
Sunset over a warming ocean now > 0.1 pH units lower than pre-industrial and which contains over 500 Gt of fossil fuel CO2
Ocean acidification: another argument for control of CO2 emissions
What can we do
about it ???
4 Solutions have beenproposed:
1. Drastically reduce the release of industrial CO2
2. Ocean Fertilisation
3. Ocean sequestration
4. Geological sequestration
0
surface ocean nitrate concentration (mol kg)m -1
5 10 15 20 25 30
Nitrate concentrations in surface water –the “HNLC” regions
Ironex I, Ironex II, Soiree, Eisenex I,Seeds, Series, Sofex, Eisenex II
Iron Fertilisation Experiments
• In the HNLC regions of the open ocean, addition of ironstimulates diatom blooms.
• The ecosystem is transformed from a low-particulate-exportto a high export system.
• There is depletion of inorganic nutrients and dissolved CO2
in the surface.
So is fertilisation the answer ?
• Depending on where fertilisation is done, this depletion canhave serious down stream effects that impact on globalcarbon fixation.
• Enhanced sinking flux leads to lower O2 concentrationsbelow thermocline, potentially N2O production.
From Hanisch(1998)
Typical 1990scartoonsketch ofocean CO2disposalscenarios.
Ocean Sequestration
Deep water sequestration experiments