Ocean Acidification The other CO2 problem

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Ocean AcidificationThe other CO2 problem
Keith Hunter Department of Chemistry University
of Otago
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CO2 concentrations in dated polar ice cores show
that the atmospheric increase began in the
mid-18th century (about 1750). It has been
constant around 280 µatm for at least the last
7,000 years.
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About half of the fossil fuel CO2 is missing
from the atmosphere
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Fossil fuels combusted 240 Atmosphere 160 Oce
an 120 Land - 40
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Penetration of anthropogenic CO2 into the
ocean Feely et al. Science 2004
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As atmospheric pCO2 and free CO2 increase, pH and
carbonate decrease CO2 CO32- H2O ?
2 HCO3-
Forms of CO2 in water Free CO2
beer, soda water Bicarbonate HCO3-
baking soda Carbonate CO32- limestone, washing
soda
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CO2(g) H2O CO32- ? 2 HCO3-
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Initially, carbonate is consumed CO2(g) H2O
CO32- ? 2 HCO3- Then calcium carbonate is
dissolved CO2(g) H2O CaCO3 ? 2 HCO3-
Ca2
The changeover occurs when CaCO3 becomes
undersaturated Ca2CO32- lt Ks
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The saturation state controls whether calcium
carbonate is stable or not (i.e. whether it
dissolves)
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pH 8.2
pH 7.4
Corals subjected to acidified water lose their
CaCO3 When transferred to normal seawater, they
calcify again Fine and Tchernov, Science 2007
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Observed and predicted Southern Ocean surface
acidification conditions for the 21st
century Predicts first tipping point for
aragonitic organisms by about 2030
McNeil B. I., Matear R. J. PNAS
200810518860-18864
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Otago Coastal Transect forpH and pCO2
measurements
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Otago Shelf TransectSubantarctic waters
Strong seasonal cycle driven by plankton
productivity Water is fully acidified in
winter Much less so in summer

• Annual pH change
• Equilibrium -0.0017 (Baring Head CO2 data)
• Subantarctic -0.0020 ? 0.0017 (standard error,
  n 28, probability 80)

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Spatial Variability
3 different water masses Neritic (0-18 km),
Subtropical (18-32 km) and Subantarctic (gt 32
km). Subantarctic water seems to be close to
equilibrium with atmospheric CO2 as assumed in
global models of CO2 uptake by the ocean. Neritic
and Southland Current waters are out of
equilibrium, less affected by anthropogenic CO2,
i.e. less acidified.
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