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Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. It is often referred to by its formula CO2. It is present in the Earth's atmosphere at a low concentration and acts as a greenhouse gas. In its solid state, it is called dry ice. It is a major component of the carbon cycle. Atmospheric carbon dioxide
derives from multiple natural sources including volcanic outgassing,
the combustion of organic matter, and the respiration processes of
living aerobic organisms; man-made sources of carbon dioxide come
mainly from the burning of various fossil fuels for power generation
and transport use. It is also produced by various microorganisms
from fermentation and cellular respiration. Plants utilize carbon
dioxide during photosynthesis, using both the carbon and the oxygen
to construct carbohydrates. In addition, plants also release oxygen
to the atmosphere, which is subsequently used for respiration by
heterotrophic organisms, forming a cycle.
Carbon dioxide is used to produce carbonated soft drinks and soda water. Traditionally, the carbonation in beer and sparkling wine comes about through natural fermentation, but some manufacturers carbonate these drinks artificially. A candy called Pop Rocks is pressurized with carbon
dioxide gas at about 600 PSI. When placed in the mouth, it dissolves
(just like other hard candy) and releases the gas bubbles with an
audible "pop."
The leavening agents used in baking produce carbon
dioxide to cause dough to rise. Baker's yeast produces carbon
dioxide by fermentation within the dough, while chemical leaveners
such as baking powder and baking soda release carbon dioxide when
heated or exposed to acids.
Carbon dioxide is often used as an inexpensive,
nonflammable pressurized gas. Life jackets often contain canisters
of pressured carbon dioxide for quick inflation. Steel capsules are
also sold as supplies of compressed gas for airguns, paintball
markers, for inflating bicycle tires, and for making seltzer. Rapid
vaporization of liquid CO2 is used for blasting in coal mines.
Carbon dioxide extinguishes flames, and some fire
extinguishers, especially those designed for electrical fires,
contain liquid carbon dioxide under pressure. Carbon dioxide also
finds use as an atmosphere for welding, although in the welding arc,
it reacts to oxidize most metals.
Use in the automotive industry is common despite
significant evidence that welds made in carbon dioxide are brittler
than those made in more inert atmospheres, and that such weld joints
deteriorate over time because of the formation of carbonic acid. It
is used as a welding gas primarily because it is much less expensive
than more inert gases such as argon or helium.
Liquid carbon dioxide is a good solvent for many
organic compounds, and is used to remove caffeine from coffee.
First, the green coffee beans are soaked in water. The beans are
placed in the top of a column seventy feet (21 metres) high. The
carbon dioxide fluid at about 93 degrees Celsius enters at the
bottom of the column. The caffeine diffuses out of the beans and
into the carbon dioxide.
Plants require carbon dioxide to conduct
photosynthesis, and greenhouses may enrich their atmospheres with
additional CO2 to boost plant growth. It has been proposed that
carbon dioxide from power generation be bubbled into ponds to grow
algae that could then be converted into biodiesel fuel. High levels
of carbon dioxide in the atmosphere effectively exterminate many
pests. Greenhouses will raise the level of CO2 to 10,000 ppm (1%)
for several hours to eliminate pests such as whiteflies, spider
mites, and others.
In medicine, up to 5% carbon dioxide is added to
pure oxygen for stimulation of breathing after apnea and to
stabilize the O2/CO2 balance in blood.
A common type of industrial gas laser, the carbon
dioxide laser, uses carbon dioxide as a medium.
Carbon dioxide can also be combined with limonene
from orange peels or other epoxides to create polymers and
plastics.
Carbon dioxide is commonly injected into or adjacent
to producing oil wells. It will act as both a pressurizing agent
and, when dissolved into the underground crude oil, will
significantly reduce its viscosity, enabling the oil to flow more
rapidly through the earth to the removal well. In mature oil fields,
extensive pipe networks are used to carry the carbon dioxide to the
injection points.
Liquid and solid carbon dioxide are important refrigerants, especially in the food industry, where they are employed during the transportation and storage of ice cream and other frozen foods. Solid carbon dioxide is called "dry ice" and is used for small shipments where refrigeration equipment is not practical. Liquid carbon dioxide was used
as a refrigerant prior to the discovery of R-12 and may be enjoying
something of a renaissance due to environmental concerns. Its
physical properties are not favorable, having a low critical
temperature of 88F/31C (the maximum temperature at which it will
condense from gas to liquid) and high critical pressure of 1070 psi
(the pressure required for phase change at the critical
temperature). These properties necessitate the use of very strong
refrigeration plumbing to contain the operating pressure of ~1400
psi, in contrast to pressures of ~300 psi for R-134a systems.
Although carbon dioxide is non-inflammable and non-toxic it is an
asphyxiant, which raises safety concerns in the case of leaks in
enclosed spaces or system rupture in the case of vehicle accident.
Despite these issues Coca-Cola has fielded CO2-based beverage
coolers and the US Army and others have expressed
interest.
As of 2006, the earth's atmosphere is about 0.038% by volume (381 µL/L or ppmv) or 0.057% by weight CO2. This represents about 2.97 × 1012 tonnes of CO2. Because of the greater land area, and therefore greater plant life, in the northern hemisphere as compared to the southern hemisphere, there is an annual fluctuation of about 5 µL/L, peaking in May and reaching a minimum in October at the end of the northern hemisphere growing season, when the quantity of biomass on the planet is greatest. The latest data, as of March 2006, shows CO2 levels
now stand at 381 parts per million (ppm) - 100ppm above the
pre-industrial average.
Despite its small concentration, CO2 is a very
important component of Earth's atmosphere, because it absorbs
infrared radiation and enhances the greenhouse effect.
The initial carbon dioxide in the atmosphere of the
young Earth was produced by volcanic activity; this was essential
for a warm and stable climate conducive to life. Volcanic activity
now releases about 130 to 230 teragrams (145 million to 255 million
short tons) of carbon dioxide each year. Volcanic releases are about
1% of the amount which is released by human activities.
Since the start of the Industrial Revolution, the
atmospheric CO2 concentration has increased by approximately 110
µL/L or about 40%, most of it released since 1945. Monthly
measurements taken at Mauna Loa since 1958 show an increase from 316
µL/L in that year to 376 µL/L in 2003, an overall increase of 60
µL/L during the 44-year history of the measurements. Burning fossil
fuels such as coal and petroleum is the leading cause of increased
man-made CO2; deforestation is the
second major cause. Around 24,000 million tonnes of
CO2 are released per year worldwide, equivalent to about 6500
million tonnes of carbon.
In 1997, Indonesian peat fires may have released
13%-40% as much carbon as fossil fuel burning does. Various
techniques have been proposed for removing excess carbon dioxide
from the atmosphere in carbon dioxide sinks. Not all the emitted CO2
remains in the atmosphere; some is absorbed in the oceans or
biosphere. The ratio of the emitted CO2 to the increase in
atmospheric CO2 is known as the airborne fraction (Keeling et al.,
1995); this varies for short-term averages but is typically 57% over
longer (5 year) periods.
The Global Warming Theory (GWT)
predicts that increased amounts of CO2 in the atmosphere tend to
enhance the greenhouse effect and thus contribute to global warming.
The effect of combustion-produced carbon dioxide on climate is
called the Callendar effect.
CO2 concentrations over the last 400,000 years  The most direct method for measuring atmospheric carbon dioxide concentrations for periods before direct sampling is to measure bubbles of air (fluid or gas inclusions) trapped in the Antarctic or Greenland ice caps. The most widely accepted of such studies come from a variety of Antarctic cores and indicate that atmospheric CO2 levels were about 260-280µL/L immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years. The longest ice core record comes from East
Antarctica, where ice has been sampled to an age of 800,000 years
before the present. During this time, the atmospheric carbon dioxide
concentration has varied between 180-210 µL/L during ice ages,
increasing to 280-300 µL/L during warmer interglacials. The data can
be accessed at
http://www.ncdc.noaa.gov/paleo/icecore/antarctica/vostok/vostok_data.html.
Some studies have disputed the claim of stable CO2
levels during the present interglacial (the last 10 kyr). Based on
an analysis of fossil leaves, Wagner et al. argued that CO2 levels
during the period 7-10 kyr ago were significantly higher (~300 µL/L)
and contained substantial variations that may be correlated to
climate variations. Others have disputed such claims, suggesting
they are more likely to reflect calibration problems than actual
changes in CO2. Relevant to this dispute is the observation that
Greenland ice cores often report higher and more variable CO2 values
than similar measurements in Antarctica. However, the groups
responsible for such measurements believe the variations in
Greenland cores result from in situ decomposition of calcium
carbonate dust found in the ice. When dust levels in Greenland cores
are low, as they nearly always are in Antarctic cores, the
researchers report good agreement between Antarctic and Greenland
CO2 measurements.
Changes in carbon dioxide during the Phanerozoic (the last 542 million years). The recent period is located on the left-hand side of the plot, and it appears that much of the last 550 million years has experienced carbon dioxide concentrations significantly higher than the present day.
On longer timescales,
various proxy measurements have been used to attempt to determine
atmospheric carbon dioxide levels millions of years in the past. These include
boron and carbon isotope ratios in certain types of marine sediments,
and the number of stomata observed on fossil plant leaves.
While these measurements give much less precise estimates of
carbon dioxide concentration than ice cores, there is evidence
forveryhighCO2concentrations(>3,000 µL/L) between 600 and 400 Myr BP and
between 200 and 150 Myr BP. On long timescales, atmospheric CO2
content is determined by the balance among geochemical processes
including organic carbon burial in sediments, silicate rock
weathering, and vulcanism. The net effect of slight imbalances in
the carbon cycle over tens to hundreds of millions of years has been
to reduce atmospheric CO2. The rates of these processes are
extremely slow; hence they are of limited relevance to the
atmospheric CO2 response to emissions over the next hundred years.
In more recent times, atmospheric CO2 concentration continued to
fall after about 60 Myr BP, and there is geochemical evidence that
concentrations were <300 µL/L by about 20 Myr BP. Low CO2
concentrations may have been the stimulus that favored the evolution
of C4 plants, which increased greatly in abundance between 7 and 5
Myr BP. Although contemporary CO2 concentrations were exceeded
during earlier geological epochs, present carbon dioxide levels are
likely higher now than at any time during the past 20 million years
and at the same time lower than at any time in history if we look at
time scales longer than 50 million years. NOAA research estimates
that 97% of atmospheric CO2 created each year is from natural
sources and approximately 3% is from human activities.
Air-sea exchange of CO2  The Earth's oceans contain a huge amount of carbon dioxide in the form of bicarbonate and carbonate ions-much more than the amount in the atmosphere. The bicarbonate is produced in reactions between rock, water, and carbon dioxide. One example is the dissolution of calcium carbonate: CaCO3 + CO2 + H2O ? Ca2+ + 2 HCO3-
Reactions like this tend to buffer changes in
atmospheric CO2. Reactions between carbon dioxide and non-carbonate
rocks also add bicarbonate to the seas, which can later undergo the
reverse of the above reaction to form carbonate rocks, releasing
half of the bicarbonate as CO2. Over hundreds of millions of years
this has produced huge quantities of carbonate rocks. If all the
carbonate rocks in the earth's crust were to be converted back into
carbon dioxide, the resulting carbon dioxide would weigh 40 times as
much as the rest of the atmosphere.
The vast majority of CO2 added to the atmosphere
will eventually be absorbed by the oceans and become bicarbonate
ion, but the process takes on the order of a hundred years because
most seawater rarely comes near the surface.
Carbon dioxide was one of the first gases to be described as a substance distinct from air. In the sevententh century, the Flemish chemist Jan Baptist van Helmont observed that when he burned charcoal in a closed vessel, the mass of the resulting ash was much less than that of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" or "wild spirit" (spiritus sylvestre). Carbon dioxide's properties were studied more thoroughly in the 1750s by the Scottish physician Joseph Black. He found that limestone (calcium carbonate) could be heated or treated with acids to yield a gas he termed "fixed air." He observed that the fixed air was denser than air and did not support either flame or animal life. He also found that it would, when bubbled through an aqueous solution of lime (calcium hydroxide), precipitate calcium carbonate, and used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, Joseph Priestley used carbon dioxide produced from the action of sulfuric acid on limestone to prepare soda water, the first known instance of an artificially carbonated drink. Carbon dioxide was first liquefied (at elevated
pressures) in 1823 by Humphry Davy and Michael Faraday. The earliest
description of solid carbon dioxide was given by Charles Thilorier,
who in 1834 opened a pressurized container of liquid carbon dioxide,
only to find that the cooling produced by the rapid evaporation of
the liquid yielded a "snow" of solid CO2.
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