The earth's oceans and forests serve as its primary carbon dioxide
sinks. Precise figures are not available, and estimates are widely
disputed, but many sources indicate that the ocean (including not only
the water, but the small organisms therein) takes in approximately
one-third of the carbon dioxide that is emitted. Another major carbon
dioxide sink is provided by the vegetation of earth's forests. The
remaining one-third is absorbed by soil and rocks, or stays in the
atmosphere. Some figures include the soil and rocks as part of the
An estimated 228 gigatons of carbon dioxide is emitted each year. If
we accept this estimate, and use the estimate that one-third of the
carbon emissions are absorbed by oceans and one-third by the
photosynthesis of forest vegetation, this would put the amount
absorbed by oceans and forests at approximately 76 gigatons each. It
should be noted that most of the 228 gigatons of carbon dioxide is
produced by natural processes, but the activities of human beings are
responsible for a significant portion:
"Scientists estimate that 225 billion tons of CO2 are removed annually
from the atmosphere by forests and oceans. However 228 billion tons
are added, of which 5.5 billion are from the burning of fossil fuels
and 2.2 billion are released by deforestation."
Yale University: Global Change for The Basic Mathematics Student
"The ecosystem naturally recycles an estimated 225 billion tons of
carbon dioxide each year from processes like respiration, decay, and
fires. Most of the carbon is removed from the atmosphere by forests
and by microorganisms in the oceans. Human activity has tipped the
balance by adding 5.5 billion tons of carbon dioxide to the air each
year from fossil fuel use. Deforestation contributes another 2.2
billion tons of carbon a year to the atmosphere while simultaneously
reducing the total amount of forests available to remove carbon.
Although the amount of carbon dioxide we add to the atmosphere seems
small by comparison to the amount recycled by nature every year, we
are nevertheless greatly increasing the proportion of carbon dioxide
in the atmosphere."
Hollowtop Outdoor Primitive School: Carbon Dioxide
"A 1995 report from the Intergovernmental Panel on Climate Change
(IPCC)... wrote that some 90 billion tons of carbon as carbon dioxide
annually circulate between the earth's ocean and the atmosphere, and
another 60 billion tons exchange between the vegetation and the
atmosphere. Compared to man-made sources' emission of about 5 to 6
billion tons per year, the natural sources would then account for more
than 95 percent of all atmospheric carbon dioxide."
Ohio State University: GLOBAL WARMING NATURAL, MAY END WITHIN 20 YEARS
"Each year, about a third of the amount of carbon dioxide that is
released by human activity is taken up by the oceans, while another
third is taken up by vegetation on land. Only a third remains in the
atmosphere. The existence of these sinks is therefore critical in
slowing the rate of rise of CO2 and of subsequent climate change, and
we need to understand the processes giving rise to both of them.
Currently, though we have a fairly accurate knowledge of the long-term
averages of these sinks, we know little about how or why they vary
from year to year. Atmospheric carbon isotope measurements suggest
that the ocean sink is quite variable, but most oceanographers believe
the variation should be smaller. Almost all our knowledge about the
size of the still-enigmatic land sink for CO2 is dependent on the
accuracy with which we can calculate the ocean uptake. The vegetation
sink cannot be directly measured or predicted, and can only be
calculated by 'difference' once all other sources and sinks are
University of East Anglia: Investigating ocean uptake of atmospheric
carbon dioxide using merchant ships
"Fossil fuel emissions contribute a small amount of carbon to the
atmosphere in comparison to the amount of carbon exchanged between the
atmosphere and the land. However, Post [Wilfred Mac Post, of Oak Ridge
National Laboratory's Environmental Sciences Division] noted, there is
no clear balance for these fossil fuel emissions, as they are a
man-made impact on the carbon cycle.
Worldwide, plants take up about 100 to 120 gigatons of carbon each
year through photosynthesis, and that carbon is returned to the
atmosphere through plant respiration and the decay of organic matter.
According to computer models, Post said, burning fossil fuels and
clearing land are currently contributing about 8 gigatons of carbon to
the atmosphere each year, and not all of this carbon returns to the
land. Post estimated that the ocean takes up about one-third of these
releases, the land takes up another third, and the final third stays
in the atmosphere. Thus, fossil fuel burning and land clearing are
creating a net gain in atmospheric carbon each year, he said."
Oak Ridger: Scientist studying carbon cycle
"There are three important labile pools of carbon: in the oceans, the
atmosphere, and the terrestrial biosphere. Of these, the oceans
contain by far the most carbon, about 38,000 Gigatons. (1 gigaton of
carbon, GtC is equal to 1012 kg, which is the mass of one km3 of
water). Most of this is in the deep ocean as dissolved bicarbonate ion
(HCO3-), and can only interact with the atmosphere on timescales of
thousands of years required for deep ocean mixing and ventilation.
Organic matter on land (plants, soils, microbes, animals, people)
accounts for perhaps 2,000 GtC, and atmospheric CO2 comprises the
smallest labile reservoir at about 780 GtC."
Biocycle, Colorado State University: Global Carbon Cycle
"Globally, fossil fuel emissions were about 5.4 billion tons per year,
and emissions from net deforestation were about 1.6 billion tons per
year. Of that, approximately 3.4 billion tons per year accumulated in
the atmosphere, approximately 2 billion tons annually accumulated in
the oceans, and 1.6 billion tons is presumed to have gone into land
sinks, such as the U.S. sink."
University of New Hampshire: Scientists Estimate Size of the U.S.
"When we try to account for sources and sinks for carbon dioxide in
the atmosphere we uncover some mysteries. For example, notice in
Figure 1 (schematic of the carbon cycle) that fossil fuel burning
releases roughly 5.5 gigatons of carbon (GtC [giga=1 billion]) per
year into the atmosphere and that land-use changes such as
deforestation contribute roughly 1.6 GtC per year. Measurements of
atmospheric carbon dioxide levels (going on since 1957) suggest that
of the approximate total amount of 7.1 GtC released per year by human
activities, approximately 3.2 GtC remain in the atmosphere, resulting
in an increase in atmospheric carbon dioxide. In addition,
approximately 2 GtC diffuses into the worlds oceans, thus leaving 1.9
GtC unaccounted for."
NASA Earth Observatory: The Carbon Cycle
Some interesting reading:
Global Warming: Study Casts Doubt On Uses Of Carbon Sinks
UniSci: Forests Storing 700 Million Tons Of Carbon Per Year
National Geographic: Studies Measure Capacity of "Carbon Sinks"
IEA Greenhouse: Carbon Dioxide Utilisation
Google search strategy:
Google Web Search: "carbon dioxide sinks"
Google Web Search: "carbon sinks"
Google Web Search: "carbon cycle"
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