Handout A – Carbon cycle
Overview of the Carbon Cycle Carbon is an important element for life on Earth and can be found in all four major spheres of the planet: biosphere, atmosphere, hydrosphere, and geosphere/lithosphere. Carbon is found in both the living and non‐
living parts of the planet, as a component in organisms, atmospheric gases, water, and rocks. The carbon moves from one sphere to another in an ongoing process known as the carbon cycle. The carbon cycle influences crucial life processes such as photosynthesis and respiration, contributes to fossil fuel formation, and impacts the earth’s climate. Besides the relatively small additions of carbon from meteorites, the total carbon on Earth is stable. But, the amount of carbon in any given sphere of the planet can increase or decrease depending on the fluctuations of the carbon cycle. The cycle can be thought of in terms of reservoirs (places where carbon is stored) and flows (the movement between reservoirs). The atmosphere (the gases surrounding the Earth), the biosphere (the parts of the land, sea, and atmosphere in which life exists), the hydrosphere (all of Earth’s water), and the geosphere/lithosphere (rocky outer layer of the Earth) are the reservoirs and the processes by which carbon moves from one reservoir to another are the flows. Although carbon is relatively common on earth, pure carbon is not. Carbon is usually bound to other elements in compounds. The carbon cycle includes many carbon‐
containing compounds, such as carbon dioxide, sugars, and methane. Carbon cycles both quickly and slowly The many processes that move carbon from one place to another happen on different time scales. Some of them happen on short time scales, such as photosynthesis, which moves carbon from the atmosphere into the biosphere as plants extract carbon from the atmosphere. Some carbon cycle processes happen over much longer time scales. For example, when marine organisms with calcium carbonate skeletons and shells die, some of their remains sink towards the ocean floor. There, the carbon that was stored in their bodies becomes part of the carbon‐rich sediment and is eventually carried along, via plate tectonic movement, to subduction zones where it is converted into metamorphic rock. These two examples show the extreme variety of processes that take place in the carbon cycle. In general, the short‐term carbon cycle encompasses photosynthesis, respiration, and predator‐prey transfer of carbon. On land, there is a flow of carbon from the atmosphere to plants with photosynthesis and then a flow back to the atmosphere with plant and animal respiration and decomposition. For aquatic plants, photosynthesis involves taking carbon from carbon dioxide dissolved in the water around them. Carbon dioxide is also constantly moving between the atmosphere and water via diffusion. The long‐term carbon cycle involves more of the lithospheric processes. It includes the weathering and erosion of carbon‐containing rocks, the accumulation of carbon‐rich plant and animal material in sediments, and the slow movement of those sediments through the rock cycle. Humans affect the carbon cycle There are natural fluctuations in the carbon cycle, but humans have been changing the carbon flows on Earth at an unnatural rate. The major human‐induced changes result in increased carbon dioxide in the atmosphere. The largest source of this change is burning fossil fuels, but other actions such as deforestation and cement manufacturing also contribute to this change in the carbon cycle. Because carbon dioxide and methane are greenhouse gases that help to control the temperature of the planet, the human‐induced increase in atmospheric carbon levels is resulting in a host of climatic changes on our planet. As discussed above, the natural carbon cycle is important to learn because it is crucial to many of Earth’s processes, but an understanding of the carbon cycle is especially important at this time in human history because of the dramatic and consequential alterations we are making to the cycle. Source: http://www.calacademy.org/educators/lesson‐plans/carbon‐cycle‐role‐play Handout B: Carbon Sinks & Sources
Carbon Dioxide Sinks
Carbon dioxide is constantly being removed from the atmosphere as part of the carbon cycle. If
this did not happen, the world would heat up and become a lot like Venus. The mechanisms
that take carbon out of the atmosphere are called "carbon sinks."
The forests of the world are a big carbon sink. Deforestation is reducing the size of this sink,
allowing more carbon dioxide to remain in the atmosphere. On the other hand, recent
technological developments allow humans to build a new kind of carbon sink through carbon
capture and storage.
Let's look at carbon sinks and how they work.
Diffusion into Oceans
Carbon dioxide is the gas that is usually used to make the “fizz” in soda drinks. It is most soluble at high
pressures and low temperatures, which is why soda is normally served cold from cans or bottles. Some
CO2 will remain dissolved with the can or bottle open and at room temperature. This is also why colder
oceans can absorb more CO2 than warmer waters.
Molecules of carbon dioxide are continuously being exchanged between the atmosphere and water through
a process called diffusion. Diffusion of carbon dioxide into the oceans accounts for nearly half of the
carbon extracted from the atmosphere.
Carbonate Precipitation
Many forms of sea life extract carbon and oxygen from seawater and combine them with calcium to
produce calcium carbonate (CaCO3). This is used to produce shells and other hard body parts by a variety
of organisms, such as coral, clams, oysters, and some microscopic plants and animals. When these
organisms die, their shells and body parts sink to the seabed. Over very long periods of time, enormous
numbers of dead organisms can create thick layers of carbonate-rich deposits on the ocean floor. Over
millions of years, these deposits become buried by more and more carbonates and/or sand or mud,
creating heat and pressure that physically and chemically changes them into sedimentary rocks such as
limestone, marble, and chalk. These rocks may eventually be raised to become exposed land due to plate
tectonics—the movement of continents and oceans around the globe.
Under the right conditions of temperature and CO2 concentration, calcium carbonate may precipitate out
of ocean water directly, without the intervention of marine organisms.
The sedimentary rocks of the Earth contain about 2,000 times more carbon than there is in all of the
water, plants, and animals in today’s oceans. Marine organisms today continue to make skeletons and
shells that, millions of years from now, might become hard rock.
Some carbon is fixed into rocks over millions of years through organic processes on land and under the
sea. This creates fossil fuels such as peat and coal (from buried dead plants) and oil and gas (mostly from
buried dead microorganisms). This represents only a tiny proportion of global carbon—20,000 times less
than in sedimentary rocks.
Runoff
Some carbon is washed from the land into the oceans by water. Falling raindrops absorb some CO2 from
the atmosphere, creating very dilute carbonic acid. When this slightly acidic rain meets carbonate rocks
such as limestone or chalk, it dissolves some of the rock, which is then carried by streams and rivers back
to the ocean. This process causes the underground caves that are often found in limestone. Carbonate
solution by rainwater also contributes toward the erosion of buildings and statues made from limestone
and marble. Streams and rivers also carry particles of organic carbon from dead land plants and animals
out into the oceans.
Sources: http://www.planetseed.com/relatedarticle/carbon‐dioxide‐sources http://www.planetseed.com/relatedarticle/carbon‐dioxide‐sinks
Carbon Dioxide Sources
Carbon Dioxide (CO2) has been entering the atmosphere from many sources for millions of
years. One of these sources—the burning of fossil fuels by humans—began with the Industrial
Revolution. It is now a big source of carbon dioxide and is having a major effect on the balance
of the carbon cycle.
Let's see where the CO2 in the atmosphere is coming from.
Respiration
Animals extract oxygen from the air they breathe. It is combined with sugars producing energy, CO2, and
water. Fish and other water creatures extract the dissolved oxygen from the water in which they live.
Respiration occurs in plants as well. They need energy to grow. Green plants are both CO2 sources,
because of respiration, and CO2 sinks, because of photosynthesis.
Decay
When plants and animals die they begin to undergo chemical processes that break down the organic
compounds they are made of into simpler compounds, including CO2.
Volcanoes
Volcanic eruptions produce solid and gaseous emissions that include CO2.
Diffusion out of the Oceans
Dissolved CO2 is released into the air from the oceans. As explained above, this is part of a two-way
process in which CO2 is also absorbed by the oceans. Currently there is more CO2 being absorbed than is
being released. The oceans are a net CO2 sink at this time.
Human Interventions
Human activities have an effect on
the flow of CO2 into and out of the
atmosphere. Over the past 150 years
these effects have increased
significantly.
Burning fossil fuels
Coal is almost pure carbon, and oil
and natural gas (hydrocarbons) are
mostly carbon. We burn these in
oxygen (from the air) to produce
power for heat, light, and transport.
The burning of fossil fuels currently
releases about 26 billion metric tons
(28.7 billion U.S. tons) of CO2 into the
atmosphere every year.
Cement production
About 5% of human-related CO2
released into the atmosphere is from
the production of cement, which
involves crushing and baking
carbonate rocks such as limestone
and chalk. These rocks are also used
in the production of iron and steel and
other industrial processes.
Some human activities affect the amount of CO2 flowing into and out of
the atmosphere, as shown here: cement production, power plants that
burn fossil fuels, automobiles, and deforestation. The image depicts a
hypothetical situation in which the net flow of CO2 into and out of the
atmosphere is zero. That is, the flows are in balance and there is no net
increase or decrease in atmospheric CO2.
Click on the image for an animation that allows you to affect the balance.
Deforestation
For centuries, people have burned forests to clear land for agriculture. This affects the CO2 balance in two
ways. First, the burning releases CO2 into the atmosphere. Second, the number of trees that can remove
CO2 by photosynthesis is reduced. It is true that agricultural land is also a CO2 sink, but it is not as
effective as forests.
There is also significant deforestation as a result of cutting trees for lumber. This does not add CO2 to the
atmosphere because the trees are not burned, but the CO2 sink provided by forest cover is reduced.