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The Carbon Cycle
Carbon is exchanged,
or "cycled" among Earth's oceans, atmosphere, ecosystem, and geosphere.
All living organisms are built of carbon compounds. It is the fundamental
building block of life and an important component of many chemical processes. It
is present in the atmosphere primarily as carbon dioxide (CO2), but also as
other less abundant but climatically significant gases, such as methane (CH4).
Sources and
Sinks
Because life
processes are fueled by carbon compounds which are oxidized to CO2, the latter
is exhaled by all animals and plants. Conversely, CO2 is assimilated by plants
during photosynthesis to build new carbon compounds. CO2 is produced by the
burning of fossil fuels, which derive from the preserved products of ancient
photosynthesis. The atmophere exhanges CO2 continuously with the oceans. Regions
or processes that predominatly produce CO2 are called sources of atmospheric
CO2, while those that absorb CO2 are called sinks.
CO2 in the
atmosphere acts like a blanket over the planet by trapping longwave radiation,
which would otherwise radiate heat away from the planet. As the amount of CO2
increases, so will its warming effect. CO2 is the largest contributor (currently
63%) to this effect by long-lived gases and its role increases each year. The
additional burden of CO2 in the atmosphere will remain for a very long time, of
the order of thousands of years, if we have to rely on the natural mechanisms of
erosion and sedimentation to process the added CO2.
Carbon (C), the fourth most
abundant element in the Universe, after hydrogen (H), helium (He), and oxygen
(O), is the building block of life.
It’s the element that
anchors all organic substances, from fossil fuels to DNA. On Earth, carbon
cycles through the land, ocean, atmosphere, and the Earth’s interior in a
major biogeochemical cycle (the circulation of chemical components through the
biosphere from or to the lithosphere, atmosphere, and hydrosphere). The global
carbon cycle can be divided into two categories: the geological, which operates
over large time scales (millions of years), and the biological/physical, which
operates at shorter time scales (days to thousands of years).
Carbon is the essential element
for life on Earth. Not only is carbon found in all living things, the element is
present in the atmosphere, in the layers of limestone sediment on the ocean
floor, and in fossil fuels like coal. (Illustration by Robert Simmon, NASA GSFC)
Geological Carbon Cycle Billions
of years ago, as planetesimals (small bodies that formed from the solar nebula)
and carbon-containing meteorites bombarded our planet’s surface, the carbon
content of the solid Earth steadily increased.
Since those times,
carbonic acid (a weak acid derived from the reaction between atmospheric carbon
dioxide [CO2] and water) has slowly but continuously combined with calcium and
magnesium in the Earth’s crust to form insoluble carbonates (carbon-containing
chemical compounds) through a process called weathering. Then, through the
process of erosion, the carbonates are washed into the ocean and eventually
settle to the bottom. The cycle continues as these materials are drawn into
Earth’s mantle by subduction (a process in which one lithospheric plate
descends beneath another, often as a result of folding or faulting) at the edges
of continental plates. The carbon is then returned to the atmosphere as carbon
dioxide during volcanic eruptions.
In the geological
carbon cycle, carbon moves between rocks and minerals, seawater, and the
atmosphere. Carbon dioxide in the atmosphere reacts with some minerals to form
the mineral calcium carbonate (limestone). This mineral is then dissolved by
rainwater and carried to the oceans. Once there, it can precipitate out of the
ocean water, forming layers of sediment on the sea floor. As the Earth’s
plates move, through the processes of plate tectonics, these sediments are
subducted underneath the continents. Under the great heat and pressure far below
the Earth’s surface, the limestone melts and reacts with other minerals,
releasing carbon dioxide. The carbon dioxide is then re-emitted into the
atmosphere through volcanic eruptions. (Illustration by Robert Simmon, NASA GSFC)
In any given year,
tens of billions of tons of carbon move between the atmosphere, hydrosphere, and
geosphere. Human activities add about 5.5 billion tons per year of carbon
dioxide to the atmosphere. The illustration above shows total amounts of stored
carbon in black, and annual carbon fluxes in purple. (Illustration courtesy NASA
Earth Science Enterprise)
The balance between
weathering, subduction, and volcanism controls atmospheric carbon dioxide
concentrations over time periods of hundreds of millions of years. The oldest
geologic sediments suggest that, before life evolved, the concentration of
atmospheric carbon dioxide may have been one-hundred times that of the present,
providing a substantial greenhouse effect during a time of low solar output. On
the other hand, ice core samples taken in Antarctica and Greenland have led
scientists to hypothesize that carbon dioxide concentrations during the last ice
age (20,000 years ago) were only half of what they are today.
Credit: NASA,
NOAA, EPA,UCAR
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