CARBON DIOXIDE (CO 2 ) CYCLE
T. Takahashi, Lamont Doherty Earth Observatory,
Columbia University, Palisades, NY, USA
Copyright & 2001 Elsevier Ltd.
Introduction
The oceans, the terrestrial biosphere, and the atmosphere are the three major dynamic reservoirs for carbon on the earth. Through the exchange of CO 2
between them, the atmospheric concentration of CO 2
that affects the heat balance of the earth, and hence
the climate, is regulated. Since carbon is one of the
fundamental constituents of living matter, how it
cycles through these natural reservoirs has been one of
the fundamental questions in environmental sciences.
The oceans contain about 50 times as much carbon
(about 40 000 Pg-C or10
15 g as carbon) as the atmosphere (about 750Pg-C). The terrestrial biosphere
contains about three times as muchcarbon (610 Pg-C
in living vegetation and 1580 Pg-C in soil organic
matter) as the atmosphere. The air–sea exchange of
CO 2 occurs via gas exchange processes across the sea
surface; the natural air-to-sea and sea-to-air fluxes
have been estimated to be about 90 Pg-C y
À1 each.
The unperturbed uptake flux of CO 2 by global terrestrial photosynthesis is roughly balanced with the
release flux by respiration, and both have been estimated to be about 60 Pg-C y
À1
. Accordingly, atmospheric CO 2 is cycled through the ocean and terrestrial
biosphere with a time scale of about 7 years.
The lithosphere contains a huge amount of carbon
(about 100 000 000 Pg-C) in the form of limestones
((Ca, Mg) CO 3 ), coal, petroleum, and other formsof
organic matter, and exchanges carbon slowly with
the other carbon reservoirs via such natural processes
as chemical weathering and burial of carbonate
andorganic carbon. The rate of removal of atmospheric CO 2 by chemical weathering has been estimated to be of the order of 1 Pg-Cy
À1 . Since the
industrial revolution in the nineteenth century, the
combustion of fossil fuels and the manufacturing of
cement have transferred the lithospheric carbon into
the atmosphere at rates comparable to the natural
CO 2 exchange fluxes between the major carbon
reservoirs, and thus have perturbed the natural balance significantly (6 Pg-Cy
À1 is about an order of
magnitude less than the natural exchanges with
the oceans (90 Pg-C y
À1 and land (60 Pg-C y
À1
)).
The industrial carbon emission rate has been about
6 Pg-C y
À1 for the 1990s, and the cumulative industrial emissions since the nineteenth century to the
end of the twentieth century have been estimated to
be about 250 Pg-C. Presently, the atmospheric CO 2
content is increasing at a rate ofabout 3.5 Pg-C y
À1
(equivalent to about 50% of theannual emission) and
the remainder of the CO 2 emitted into the atmosphere is absorbed by the oceans and terrestrial biosphere in approximately equal proportions. These
industrial CO 2 emissions have caused the atmospheric CO 2 concentration to increase by as much as
30% from about 280 ppm (parts per million mole
fraction in dry air) in the pre-industrial year 1850 to
about 362 ppm in the year 2000. Theatmospheric
CO 2 concentration may reach 580 ppm, double
thepre-industrial value, by the mid-twenty first century. This represents a significant change that is
wholly attributable to human activities on the Earth.
It is well known that the oceans play an important
role in regulating our living environment by providing
water vapor into the atmosphere and transporting
heat from the tropics to high latitude areas. In addition
to these physical influences, the oceans partially
ameliorate the potential CO 2 -induced climate changes
by absorbing industrial CO 2 in the atmosphere.
Therefore, it is important to understand howthe
oceans take up CO 2 from the atmosphere and how
they store CO 2 in circulating ocean water. Furthermore, in order to predict the future course of the
atmospheric CO 2 changes, we need to understand
how the capacity of the ocean carbon reservoir might
be changed in response to the Earth’s climate changes, that may, in turn, alter the circulation of ocean
water. Since the capacity of the ocean carbon reservoir is governed by complex interactions of physical,
biological, and chemical processes, it is presently not
possible to identify and predict reliably various climate feedback mechanisms that affect the ocean CO 2
storage capacity.
Units
In scientific and technical literature, the amount of
carbon has often been expressed in three different
units: giga tons of carbon (Gt-C), petagrams of
carbon (Pg-C) and moles of carbon or CO 2 . Their
relationships are: 1Gt-C ¼ 1 Pg-C ¼ 1 Â 10
15 g of carbon ¼ 1000 million metric tonnes of carbon ¼
(1/12) Â 10
15 moles of carbon. The equivalent quantity as CO 2 may be obtained by multiplying the above
numbers by 3.67 ( ¼ 44/12 ¼ the molecular weight of
CO 2 divided by the atomic weight of carbon).
505
T. Takahashi, Lamont Doherty Earth Observatory,
Columbia University, Palisades, NY, USA
Copyright & 2001 Elsevier Ltd.
Introduction
The oceans, the terrestrial biosphere, and the atmosphere are the three major dynamic reservoirs for carbon on the earth. Through the exchange of CO 2
between them, the atmospheric concentration of CO 2
that affects the heat balance of the earth, and hence
the climate, is regulated. Since carbon is one of the
fundamental constituents of living matter, how it
cycles through these natural reservoirs has been one of
the fundamental questions in environmental sciences.
The oceans contain about 50 times as much carbon
(about 40 000 Pg-C or10
15 g as carbon) as the atmosphere (about 750Pg-C). The terrestrial biosphere
contains about three times as muchcarbon (610 Pg-C
in living vegetation and 1580 Pg-C in soil organic
matter) as the atmosphere. The air–sea exchange of
CO 2 occurs via gas exchange processes across the sea
surface; the natural air-to-sea and sea-to-air fluxes
have been estimated to be about 90 Pg-C y
À1 each.
The unperturbed uptake flux of CO 2 by global terrestrial photosynthesis is roughly balanced with the
release flux by respiration, and both have been estimated to be about 60 Pg-C y
À1
. Accordingly, atmospheric CO 2 is cycled through the ocean and terrestrial
biosphere with a time scale of about 7 years.
The lithosphere contains a huge amount of carbon
(about 100 000 000 Pg-C) in the form of limestones
((Ca, Mg) CO 3 ), coal, petroleum, and other formsof
organic matter, and exchanges carbon slowly with
the other carbon reservoirs via such natural processes
as chemical weathering and burial of carbonate
andorganic carbon. The rate of removal of atmospheric CO 2 by chemical weathering has been estimated to be of the order of 1 Pg-Cy
À1 . Since the
industrial revolution in the nineteenth century, the
combustion of fossil fuels and the manufacturing of
cement have transferred the lithospheric carbon into
the atmosphere at rates comparable to the natural
CO 2 exchange fluxes between the major carbon
reservoirs, and thus have perturbed the natural balance significantly (6 Pg-Cy
À1 is about an order of
magnitude less than the natural exchanges with
the oceans (90 Pg-C y
À1 and land (60 Pg-C y
À1
)).
The industrial carbon emission rate has been about
6 Pg-C y
À1 for the 1990s, and the cumulative industrial emissions since the nineteenth century to the
end of the twentieth century have been estimated to
be about 250 Pg-C. Presently, the atmospheric CO 2
content is increasing at a rate ofabout 3.5 Pg-C y
À1
(equivalent to about 50% of theannual emission) and
the remainder of the CO 2 emitted into the atmosphere is absorbed by the oceans and terrestrial biosphere in approximately equal proportions. These
industrial CO 2 emissions have caused the atmospheric CO 2 concentration to increase by as much as
30% from about 280 ppm (parts per million mole
fraction in dry air) in the pre-industrial year 1850 to
about 362 ppm in the year 2000. Theatmospheric
CO 2 concentration may reach 580 ppm, double
thepre-industrial value, by the mid-twenty first century. This represents a significant change that is
wholly attributable to human activities on the Earth.
It is well known that the oceans play an important
role in regulating our living environment by providing
water vapor into the atmosphere and transporting
heat from the tropics to high latitude areas. In addition
to these physical influences, the oceans partially
ameliorate the potential CO 2 -induced climate changes
by absorbing industrial CO 2 in the atmosphere.
Therefore, it is important to understand howthe
oceans take up CO 2 from the atmosphere and how
they store CO 2 in circulating ocean water. Furthermore, in order to predict the future course of the
atmospheric CO 2 changes, we need to understand
how the capacity of the ocean carbon reservoir might
be changed in response to the Earth’s climate changes, that may, in turn, alter the circulation of ocean
water. Since the capacity of the ocean carbon reservoir is governed by complex interactions of physical,
biological, and chemical processes, it is presently not
possible to identify and predict reliably various climate feedback mechanisms that affect the ocean CO 2
storage capacity.
Units
In scientific and technical literature, the amount of
carbon has often been expressed in three different
units: giga tons of carbon (Gt-C), petagrams of
carbon (Pg-C) and moles of carbon or CO 2 . Their
relationships are: 1Gt-C ¼ 1 Pg-C ¼ 1 Â 10
15 g of carbon ¼ 1000 million metric tonnes of carbon ¼
(1/12) Â 10
15 moles of carbon. The equivalent quantity as CO 2 may be obtained by multiplying the above
numbers by 3.67 ( ¼ 44/12 ¼ the molecular weight of
CO 2 divided by the atomic weight of carbon).
505