CARBON SEQUESTRATION VIA DIRECT
INJECTION INTO THE OCEAN
E. E. Adams, Massachusetts Institute of Technology,
Cambridge, MA, USA
K. Caldeira, Stanford University, Stanford, CA, USA
& 2009 Elsevier Ltd. All rights reserved.
Introduction
Global climate change, triggered by a buildup of
greenhouse gases, is emerging as perhaps the most
serious environmental challenge in the twenty-first
century. The primary greenhouse gas is CO 2 , whose
concentration in the atmosphere has climbed from its
preindustrial level of c. 280 to 4380 ppm. Stabilization at no more than 500–550 ppm is a target
frequently discussed to avoid major climatic impact.
The primary source of CO 2 is the burning of fossil
fuels – specifically gas, oil, and coal – so stabilization
of atmospheric CO 2 concentration will clearly require substantial reductions in CO 2 emissions from
these sources. For example, one commonly discussed
scenario to stabilize at 500 ppm by the mid-twentyfirst century suggests that about 640 Gt CO 2
(c. 175 Gt C) would need to be avoided over 50
years, with further emission reductions beyond 50
years. As references, a 1000 MW pulverized coal
plant produces 6–8 Mt CO 2 (c. 2 Mt C) per year,
while an oil-fired single-cycle plant produces about
two-thirds this amount and a natural gas combined
cycle plant produces about half this amount. Thus
the above scenario would require that the atmospheric emissions from the equivalent of 2000–4000
large power plants be avoided by approximately the
year 2050.
Such changes will require a dramatic reduction in
our current dependence on fossil fuels through increased conservation and improved efficiency, as well
as the introduction of nonfossil energy sources like
solar, wind, and nuclear. While these strategies will
slow the buildup of atmospheric CO 2 , it is probable
that they will not reduce emissions to the required
level. In other words, fossil fuels, which currently
supply over 85% of the world’s energy needs, are
likely to remain our primary energy source for the
foreseeable future. This has led to increased interest
in a new strategy termed carbon capture and storage,
or sequestration. The importance of this option for
mitigating climate change is highlighted by the recent
Special Report on Carbon Dioxide Capture and
Storage published by the Intergovernmental Panel on
Climate Change, to which the reader is referred for
more information.
Carbon sequestration is often associated with the
planting of trees. As they mature, the trees remove
carbon from the atmosphere. As long as the forest
remains in place, the carbon is effectively sequestered. Another type of sequestration involves capturing CO 2 from large, stationary sources, such as a
power plant or chemical factory, and storing the CO 2
in underground reservoirs or the deep ocean, the
latter being the focus of this article. There has been
much attention paid recently to underground storage
with several large-scale field sites in operation or
being planned. Conversely, while there have been
many studies regarding use of the deep ocean as a
sink for atmospheric carbon, there have been only a
few small-scale field studies.
Why is the ocean of interest as a sink for anthropogenic CO 2 ? The ocean already contains an estimated 40 000 Gt C compared with about 800 Gt C in
the atmosphere and 2200 Gt C in the land biosphere.
As a result, the amount of carbon that would cause a
doubling of the atmospheric concentration would
only change the ocean concentration by about 2%.
In addition, natural chemical equilibration between
the atmosphere and ocean would result in about
80% of present-day emissions ultimately residing in
the ocean. Discharging CO 2 directly to the ocean
would accelerate this slow, natural process, thus reducing both peak atmospheric CO 2 concentrations
and their rate of increase. It is noted that a related
strategy for sequestration – not discussed here –
would be to enhance the biological sink using nutrients such as iron to fertilize portions of the world’s
oceans, thus stimulating phytoplankton growth. The
phytoplankton would increase the rate of biological
uptake of CO 2 , and a portion of the CO 2 would be
transported to ocean depths when the plankton die.
The indirect flux of CO 2 to the ocean from the
atmosphere is already quite apparent: since preindustrial times, the pH of the surface ocean has been
reduced by about 0.1 units, from an initial surface
pH of about 8.2. Figure 1 illustrates what could
happen to ocean pH under conditions of continued
atmospheric release of CO 2 . Under the conditions
simulated, the pH of the surface would drop by over
0.7 units. Conversely, by injecting some of the CO 2
315
INJECTION INTO THE OCEAN
E. E. Adams, Massachusetts Institute of Technology,
Cambridge, MA, USA
K. Caldeira, Stanford University, Stanford, CA, USA
& 2009 Elsevier Ltd. All rights reserved.
Introduction
Global climate change, triggered by a buildup of
greenhouse gases, is emerging as perhaps the most
serious environmental challenge in the twenty-first
century. The primary greenhouse gas is CO 2 , whose
concentration in the atmosphere has climbed from its
preindustrial level of c. 280 to 4380 ppm. Stabilization at no more than 500–550 ppm is a target
frequently discussed to avoid major climatic impact.
The primary source of CO 2 is the burning of fossil
fuels – specifically gas, oil, and coal – so stabilization
of atmospheric CO 2 concentration will clearly require substantial reductions in CO 2 emissions from
these sources. For example, one commonly discussed
scenario to stabilize at 500 ppm by the mid-twentyfirst century suggests that about 640 Gt CO 2
(c. 175 Gt C) would need to be avoided over 50
years, with further emission reductions beyond 50
years. As references, a 1000 MW pulverized coal
plant produces 6–8 Mt CO 2 (c. 2 Mt C) per year,
while an oil-fired single-cycle plant produces about
two-thirds this amount and a natural gas combined
cycle plant produces about half this amount. Thus
the above scenario would require that the atmospheric emissions from the equivalent of 2000–4000
large power plants be avoided by approximately the
year 2050.
Such changes will require a dramatic reduction in
our current dependence on fossil fuels through increased conservation and improved efficiency, as well
as the introduction of nonfossil energy sources like
solar, wind, and nuclear. While these strategies will
slow the buildup of atmospheric CO 2 , it is probable
that they will not reduce emissions to the required
level. In other words, fossil fuels, which currently
supply over 85% of the world’s energy needs, are
likely to remain our primary energy source for the
foreseeable future. This has led to increased interest
in a new strategy termed carbon capture and storage,
or sequestration. The importance of this option for
mitigating climate change is highlighted by the recent
Special Report on Carbon Dioxide Capture and
Storage published by the Intergovernmental Panel on
Climate Change, to which the reader is referred for
more information.
Carbon sequestration is often associated with the
planting of trees. As they mature, the trees remove
carbon from the atmosphere. As long as the forest
remains in place, the carbon is effectively sequestered. Another type of sequestration involves capturing CO 2 from large, stationary sources, such as a
power plant or chemical factory, and storing the CO 2
in underground reservoirs or the deep ocean, the
latter being the focus of this article. There has been
much attention paid recently to underground storage
with several large-scale field sites in operation or
being planned. Conversely, while there have been
many studies regarding use of the deep ocean as a
sink for atmospheric carbon, there have been only a
few small-scale field studies.
Why is the ocean of interest as a sink for anthropogenic CO 2 ? The ocean already contains an estimated 40 000 Gt C compared with about 800 Gt C in
the atmosphere and 2200 Gt C in the land biosphere.
As a result, the amount of carbon that would cause a
doubling of the atmospheric concentration would
only change the ocean concentration by about 2%.
In addition, natural chemical equilibration between
the atmosphere and ocean would result in about
80% of present-day emissions ultimately residing in
the ocean. Discharging CO 2 directly to the ocean
would accelerate this slow, natural process, thus reducing both peak atmospheric CO 2 concentrations
and their rate of increase. It is noted that a related
strategy for sequestration – not discussed here –
would be to enhance the biological sink using nutrients such as iron to fertilize portions of the world’s
oceans, thus stimulating phytoplankton growth. The
phytoplankton would increase the rate of biological
uptake of CO 2 , and a portion of the CO 2 would be
transported to ocean depths when the plankton die.
The indirect flux of CO 2 to the ocean from the
atmosphere is already quite apparent: since preindustrial times, the pH of the surface ocean has been
reduced by about 0.1 units, from an initial surface
pH of about 8.2. Figure 1 illustrates what could
happen to ocean pH under conditions of continued
atmospheric release of CO 2 . Under the conditions
simulated, the pH of the surface would drop by over
0.7 units. Conversely, by injecting some of the CO 2
315