The Carbon Cycle
No discussion about the Earth is complete without taking a look at the topic
of Global Warming. It seems to be of such great concern, that only this past
spring has a national council of pastors convened to raise awareness of the
increased amount of greenhouse gases and the observed rise in global temperatures
with a result potentially disastrous rise in sea levels. Indeed, my aging Uncle
Bill is truly concerned that the entire state of Florida might find itself underwater
if predictions come to pass. To understand global warming, you must first understand
how carbon is recycled and what greenhouses gases are.
The concentration of carbon in living matter (18%) is almost 100 times greater
than its concentration in the earth (0.19%). So living things continually extract
carbon from their nonliving environment and return it to that nonliving environment
throughout the natural process of life and death. For life to continue, this
carbon must be recycled. It is the recycling of carbon on a global scale that
makes life on this planet possible. At present, there is not another place in
the Universe where carbon is recycled because Earth is the only place where
geological process that are responsible for this recycling happen.
Carbon exists in the nonliving environment as: a) carbon dioxide (CO2) in
the atmosphere and dissolved in water (forming HCO3-), b) carbonate rocks (limestone
and coral = CaCO3), c) deposits of coal, petroleum, and natural gas derived
from once-living things, and d) dead organic matter, e.g., humus in the soil
Carbon enters the biotic world through the action of autotrophs: primarily
photoautotrophs, like plants and algae, and to a small extent, chemoautotrophic
bacteria. Since this lesson is focused for Astronomy students more than AP Biology
students, a few definitions here might be helpful. Photoautotrophs are organisms
that take energy from the Sun and carbon from atmospheric carbon dioxide fixed
carbon dioxide into glucose. Chemoautotrophs are bacteria that get their energy
from chemical reactions and their carbon from atmospheric carbon dioxide. Humans,
and all other multicellular animals are chemoheterotrophs ... meaning we get
ourenergy from chemical reactions and out carbon from consuming organic foods.
Carbon returns to the atmosphere and water in large quantities at plate tectonic
spreading centers and to a somewhat lesser extent when volcanoes erupt. the
interplay of carbon and the living world finds carbon returning to the atmosphere
from: 1) respiration (as CO2), 2) burning, and 3) decay (producing CO2 if oxygen
is present, methane (CH4) if it is not.
From a geologic standpoint, carbon also leaves the botic world when it is
removed from the atmosphere by dissolving in water and forming carbonic acid
CO2 + H2O --> H2CO3 (carbonic acid)
Carbonic acid is used to weather rocks, yielding bicarbonate ions, other ions,
H2CO3 + H2O + silicate minerals -> HCO3- + cations (Ca++, Fe++, Na+, etc.)
Calcium carbonate is precipitated from calcium and bicarbonate ions in seawater
by marine organisms like coral
Ca++ + 2HCO3- -> CaCO3 + CO2 + H2O and thus, the carbon is now stored on
the seafloor in layers of limestone. Thinking this through, every time it rains
some of the Earth's supply of carbon dioxide is lost to a seemingly permanent
place in rocks. Were it not for seafloor spreading and volcanism, the Earthh
would have depleted its entire available supply of carbon dioxide a long time
ago ... trapping it forever in rock formations.
Fortunately, some of this carbon is returned to the atmosphere via metamorphism
of limestone at depth in subduction zones or in orogenic belts
CaCO3 + SiO2 -> CO2 + CaSiO3 followed by outgassing at the Ring of Fire.
It is precisely this global recycling of carbon dioxide from the activity
of Earth's plate tectonics that returns carbon dioxide to the atmosphere so
that plants can convert it into glucose and release oxygen as a by-product of
photosynthesis. WITHOUT THE GLOBAL RECYCLING OF CARBON DIOXIDE, LIFE HERE ON
EARTH WOULD SURELY END, AND WITHOUT ACTIVE PLATE TECTONICS, A MECHANISM TO RETURN
CARBON THAT HAS BEEN TRAPPED AS CARBONATE IN THE ROCK, LIFE WOULD BE IMPOSSIBLE.
EARTH IS THE ONLY PLANET OR MOON AMONGTHE 153 PRESENTLY NAMED MAJOR SOLAR SYSTEM
BODIES THAT POSSESSES ACTIVE TECTONIC PLATES.
The Climate Buffer
Because of the role of CO2 in climate, feedbacks in the carbon cycle act to
maintain global temperatures within certain bounds so that the climate never
gets too hot or too cold to support life on Earth. The process is a large-scale
example of LeChatelier's Principle. This chemical principle states that if a
reaction at equilibrium is perturbed by the addition or removal of a product
or reactant, the reaction will adjust so as to attempt to bring that chemical
species back to its original concentration. For example, as carbonic acid is
removed from solution by weathering of rocks, the reaction will adjust by producing
more carbonic acid. And since the dissolved CO2 is in equilibrium with atmospheric
CO2, more CO2 is removed from the atmosphere to replace that removed from solution
If CO2 concentration increases in the atmosphere because of an increased rate
of outgassing, global temperature will rise. Rising temperature and more dissolved
CO2 will lead to increased weathering of crustal rocks as a result of faster
reaction rates (temperature effect) and greater acidity. Enhanced weathering
will use up the excess CO2 thereby cooling the climate.
If global temperature cools as a result of some astronomical forcing or tectonic/ocean
circulation effect, the lower temperatures will result in lower rates of chemical
weathering. Decreased weathering means less CO2 being drawn from the atmosphere
by weathering reactions, leaving more CO2 in the atmosphere to increase temperatures.
If more rocks become available for rapid weathering as a result of mountain
uplift the enhanced weathering will draw down atmospheric CO2 and decrease global
temperatures. But the decreased temperatures will slow reaction rates, thereby
using less CO2, thus allowing temperatures to moderate.
Presently, the uptake and return of CO2 are not in balance.
The carbon dioxide content of the atmosphere is gradually and steadily increasing.
The graph below shows the CO2 concentration at the summit of Mauna Loa in Hawaii
from 1958 through 1999. The values are in parts per million (ppm). The seasonal
fluctuation is caused by the increased uptake of CO2 by plants in the summer.
The increase in CO2 probably began with the start of the industrial revolution.
Samples of air trapped over the centuries in the glacial ice of Greenland show
no change in CO2 content until 300 years ago. Since measurements of atmospheric
CO2 began late in the nineteenth century, its concentration has risen over 20%.
This increase is surely "anthropogenic"; that is, caused by human
activities such as burning fossil fuels (coal, oil, natural gas) which returns
to the atmosphere carbon that has been locked within the earth for millions
of years. Additionally, South American nations have taken to clearing and burning
of forests, especially in the tropics. In recent decades, large areas of the
Amazon rain forest have been cleared for agriculture and cattle grazing.
Where is the missing carbon?
Curiously, the increase in atmospheric CO2 is only about one-half of what
would have been expected from the amount of fossil fuel consumption and forest
Where has the rest gone?
Research has shown that increased CO2 levels lead to increased net production
by photoautotrophs. There is some evidence that the missing CO2 has been incorporated
increased growth of forests, especially in North America and increased amounts
of phytoplankton in the oceans. This was mentioned earlier in the Climate Buffer
The Greenhouse Effect and Global Warming
Despite these "sinks" for our greatly increased CO2 production,
the concentration of atmospheric CO2 continues to rise? Should we be worried?
Carbon dioxide is transparent to light but rather opaque to heat rays. Therefore,
CO2 in the atmosphere retards the radiation of heat from the earth back into
space the "greenhouse effect".
Has the increase in carbon dioxide led to global warming?
Average temperatures do seem to have increased slightly (~0.6°C) in the
Careful monitoring of both ocean and land temperatures.
Many glaciers and ice sheets are receding.
Woody shrubs are now growing in areas of northern Alaska that 50 years ago were
Many angiosperms in temperate climates are flowering earlier in the spring than
they used to.
Many species of birds and butterflies are moving north and breeding earlier
in the spring.
Will continued increase in carbon dioxide lead to more global warming and, if
so, how much?
At this point, the answer depends on what assumptions you plug into your computer
models. But as the different models have been improved, they seem to be converging
on a consensus: a doubling of the CO2 concentration (expected by the end of
this century) will cause the earth to warm somewhere in the range of 2.53.5°C.
While we worry about possible global warming from the additional CO2 we put
into the atmosphere by burning fossil fuels, if there was no CO2 in the atmosphere
the global climate would be significantly cooler. While I clearly agree with
the research in the science literature that the global climate is warming, there
is ample evidence of repeated episodes of warming and cooling to ice ages in
the past. It is my contention that more is at play with the current warming
trend than merely human factors and cows passing gas in the fields. As an astronomer,
I believe scientists must take into account the vagaries of the Sunspot Cycle,
potential slight changes in the Earth's orbit (Milankovitch Theory), and unusual
changes in plate tectonic actitivy as well as mankind's contribution. More information
and other links can be found at the USGS
Carbon Cycle page.
I have prepared a special section on the topic of Global Warming ... it is
still under construction, but it is a central topic in any course on Earth Science.
Click on the title above to learn more details.
Other Greenhouse Gases
Although their levels in the atmosphere are much lower than that of CO2, methane
(CH4) and chlorofluorocarbons (CFCs) are also potent greenhouse gases. Methane
("marsh gas") is released by natural processes (e.g. from decay occurring
in swamps), but human activities may now account for over one-half of the total.
Some examples of human activites that raise global methane is the growing of
rice in paddies and the burning of forests. On a somewhat humorous note, raising
cattle (fermentation in their rumens produces methane that is expelled) collectively
adds an estimated 100 million tons a year to the atmosphere. But estimates can
be wrong. In 1990, the U.S. Environmental Protection Agency estimated that rice
paddies were also adding about 100 million tons a year; accurate measurements
later showed that this estimate was too high. And to add to the uncertainty,
the discovery that plants naturally release methane to the atmosphere was reported
in 2006. This previously-unrecognized source may account for 1030% of
So while the burning of the tropical rain forest adds to the atmospheric methane
budget by incomplete combustion during burning and release from the GI tract
of the cattle that are later placed on the cleared land, some of this may be
offset by the reduction in the natural production by the trees removed from
the forest. The methane concentration in the air is presently some 1.8 parts
per million (ppm) and is growing at a rate of 1% per year. Although this concentration
is far less than that of CO2, methane is 30 times as potent a greenhouse gas
and so may now be responsible for 1520% of the predicted global warming.
The marked warming of the earth that occurred at the end of the Paleocene
epoch is thought to have been caused by the release of large amounts of methane
from the sea floor. Today, geologists are discovering "fire ice" which
is frozen methane crystals in deep ocean sediments. Their existence may promise
a new resource for an energy-hungry world, but the substance is exceedingly
volatile. Perhaps large quantities of this form of methane erupted at the end
of the Paleocene!
Chlorofluorocarbons (CFCs) are synthetic gases in which the hydrogen atoms
of methane are replaced by atoms of fluorine and chlorine (e.g., CHF2Cl, CFCl3,
CF2Cl2).These gases are noninflammable, nontoxic, and very stable. They are
widely used in industry as refrigerants (e.g., in refrigerators and air conditioners),
solvents, propellants in aerosol cans (now banned in some countries), and in
the manufacture of plastic foams. They escape to the air from all of these uses
(e.g., from leaky and discarded refrigeration units). Their chemical inertness,
which makes CFCs so desirable for industry, also makes them a threat to the
atmosphere. Once in the atmosphere, it may take 60100 years for them to
decompose and disappear. In the meantime, they may contribute to as much as
25% of the greenhouse effect. But perhaps even more worrisome is the threat
they pose to the ozone shield. At the 1997 Nobel Conference at Gustavus College
in St Peter, MN I heard a lecture from Sherry Rowland about CFCs. He stated
that a single CFC molecule can degrade 300,000 molecules of Ozone. To learn
more of this threat, read below.
The Ozone Problems
Ozone is a highly active form of oxygen (O3 rather than O2). Ozone is made
when a electric spark passes through air, and this accounts for the characteristic
odor give off by some electrical motors. Ozone presents two quite different
biological problems: too much at low levels of the atmosphere (the troposphere);
too little at high altitudes (the stratosphere).
Ozone in the Troposphere
Ozone is produced by the reaction of sunlight, oxygen, and automobile exhaust
(which contains hydrocarbons and nitrogen oxides). Ozone is largely responsible
for the discomfort associated with photochemical smog. This form of smog, long
familiar to people in the Los Angeles basin, is now common wherever sunlight
and stagnant air occur in urban areas (Mexico City is a dramatic example with
ozone levels that often exceed 100 ppb and sometimes rise above 350 ppb). High
levels of ozone during smog build-up can cause difficulty to people with respiratory
ailments like emphysema and asthma. Ozone also damages plants and may be an
important factor in the damage that is occurring to forests in Europe and North
Ozone in the Stratosphere
While we often have too much ozone around us, the concentration
of ozone high in the stratosphere (which begins about 7 miles up - where airliners
cruise) has declined over the past two decades. Satellite monitoring of the
stratosphere, which began in 1978, has revealed a marked decline. The most serious
decline occurs over Antarctica in spring (October) when a precipitous drop in
ozone causes an ozone hole. The figure (courtesy of NASA) shows a map of the
ozone hole measured over Antarctica on 5 October 1987 by a device carried on
the Nimbus 7 satellite. The tips of South America (upper right quadrant ) and
Africa (lower right) are drawn in, as is the outline of Australia and New Zealand
(lower left). The Dobson unit is a measure of the number of molecules of ozone
in a vertical column of the atmosphere. You can see that the concentration of
ozone decreases in ever-smaller concentric circles with the lowest reading centered
over the South Pole.
The Ozone Shield
The spreading of this ozone-depleted air may account for the more gradual and
more protracted declines that are being seen at midlatitudes. From 1978-1990,
average ozone levels declined 8% over Europe and about 5% over the United States.
This is ominous because ozone shields the earth's surface from much of the ultraviolet
radiation reaching the earth from the sun. Ultraviolet rays can cause skin cancer,
cataracts, and may depress the immune system. The graph (from C. R. Roy, et.
al., in Nature 347:235, 1990) shows measurements of the intensity of ultraviolet
light and the concentration of ozone on several sunny days in Melbourne, Australia
during December 1987 and January 1988. When ozone levels were low, ultraviolet
light was more intense and vice versa. The drop in ozone, which lasted about
a month, was probably caused by ozone-depleted air drifting in from the ozone
hole over the South Pole. Most of the ultraviolet light that reaches the earth
is ultraviolet "B" (UVB), which includes wavelengths from 290 to 320
Although some of the recent depletion of ozone in the stratosphere was probably
due to natural causes (volcanic eruptions, fewer sunspots), some is most likely
caused by manmade chlorofluorocarbons (CFCs). These gases escape from such sources
as aerosol spray cans, leaky or discarded refrigeration units, and a variety
of industrial processes. The U.S., Canada, and the Scandinavian countries stopped
using CFCs in aerosol cans over a decade ago, but this and other uses of CFCs
have continued to grow worldwide. However, a multi-nation agreement drawn up
in 1987 established a schedule for reducing the use of these materials.
And, in fact, monitoring shows that the concentration of CFCs in the stratosphere
has been decreasing since the mid-90s.
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