The carbon cycle describes the movement of carbon through Earth's biosphere, lithosphere, hydrosphere, and atmosphere. Carbon is exchanged between the atmosphere, living organisms, oceans, soil, rocks through various processes like photosynthesis, respiration, decomposition, combustion of fossil fuels and weathering of rocks. Human activities like burning fossil fuels and deforestation have increased atmospheric CO2 levels, impacting Earth's climate by enhancing the greenhouse effect.

1. Ecosystem ecology: Biogeochemical
cycles including water, nitrogen, carbon
and phosphorus cycles

2. Carbon Cycle
Carbon: building block and fuel source
• Carbon is an essential element in the bodies of living organisms. About 18% of your body consists of carbon
atoms by mass, and those carbon atoms are pretty key to your existence.
• Without carbon, you wouldn't have the plasma membranes of your cells, the sugar molecules you use for fuel, or
even the DNA that carries instructions to build and run your body.
• Carbon dioxide (CO2) from the atmosphere is taken up by photosynthetic organisms (producers) and used to
make organic molecules, which travel through food chains. In the end, the carbon atoms are released as CO2 in
respiration.
• Slow geological processes, including the formation of sedimentary rock and fossil fuels, contribute to the carbon
cycle over long timescales.
• It is also economically important to modern humans, in the form of fossil fuels. Some human activities, such as
burning of fossil fuels and deforestation, increase atmospheric CO2 and affect Earth's climate and oceans.
• Carbon is part of our bodies, but it's also part of our modern-day industries. Carbon compounds from long-ago
plants and algae make up the fossil fuels, such as coal and natural gas, that we use today as energy sources.
When these fossil fuels are burned, CO2 is released into the air, leading to higher and higher levels of
atmospheric CO2. This increase in CO2 levels affects Earth's climate and is a major environmental concern
worldwide – Greenhouse gas effects.

3. Carbon Cycle
• The carbon cycle is most easily studied as two interconnected
subcycles:
i. One dealing with rapid carbon exchange among living organisms
ii. One dealing with long-term cycling of carbon through geologic
processes

5. Various sources of Carbon Cycling
(Adapted from Science Learning Hub, NZ)
1. Atmosphere (750 billion tonnes of carbon)
• Carbon in the atmosphere is mostly in the form of carbon dioxide (CO2) with
some methane and hydrofluorocarbons. The amount of CO2 in the atmosphere is increasing.
Acknowledgement: NASA
2. Vegetation (600 billion tonnes of carbon)
• Plants store carbon as carbohydrates made from CO2 the atmosphere. Land plants take up
about a quarter of all CO2 that enters the atmosphere.
Acknowledgement: Public domain
3. Soil and organic matter (1,600 billion tonnes of carbon)
• Soil contains a lot of carbon in the form of dead plant material and in the many bacteria and
other small organisms that live there.
Acknowledgement: Public domain
4. Coal, oil, gas (3,300 billion tonnes of carbon)
• Carbon has been locked up in fossil fuels, built up from once-living things, for millions of years.
Acknowledgement: Public domain

6. 5. Sediments and sedimentary rock (1,000,000,000 billion tonnes of
carbon)
• The carbon cycle overlaps the rock cycle. Ocean sediments and the rocks
they turn into contain huge amounts of carbon. This is mostly
in calcite and limestone.
Acknowledgement: Bordalier institute
6. Ocean surface (1,000 billion tonnes of carbon)
• Exchange of CO2 between the ocean and the atmosphere takes place at
the surface.
Acknowledgement: NASA
7. Deep ocean (40,000 billion tonnes of carbon)
• Most of the carbon entering the ocean ends up in the deep ocean where it
can be carried by currents for hundreds of years or be lost in sediments.
Acknowledgement: Public domain
8. Burning fossil fuels
• In fossil fuels, the carbon is stored in long-chain hydrocarbons, and then
through combustion with oxygen in our cars or in factories, the carbon is
converted to CO2, which is released to the atmosphere. And in addition, a
number of other by products are also produced through inefficiencies in
combustion like CO which are atmospheric pollutants.
Acknowledgements: Dr Sara Mikaloff-Fletcher, National Institute of Water and Atmospheric Research
(NIWA)

7. 9. Respiration
• Plants and animals release carbon dioxide into the atmosphere through the process of respiration. Respiration releases energy
from carbohydrates, with CO2 as a waste product.
Acknowledgement: The University of Waikato
10. Burning
• All the mass of all the trees that you’re seeing around is mostly coming from atmospheric CO2. So when we cut down forests and
burn forests, then we are releasing CO2 into the atmosphere.
Acknowledgements:
Dr Sara Mikaloff-Fletcher, National Institute of Water and Atmospheric Research (NIWA)
Scrub fire footage, courtesy Scion
11. Photosynthesis
• Plants take in CO2 from the atmosphere during photosynthesis. They use energy from the sun to combine the CO2 and water to
form carbohydrates.
Acknowledgement: The University of Waikato
12. Decay
• In the soil, decomposers (such as microbes and soil animals) break down dead plant material. As well as
making nutrients available for living plants, this process also releases CO2 into the atmosphere.
Acknowledgement: Maria Minor and Alastair Robertson (Massey University)
13. Carbon dioxide exchange
• There is a constant vigorous exchange that is going two ways between the ocean and the atmosphere. So once the CO2 goes
into the ocean then that CO2 forms carbonic acid, bicarbonate and carbonate ion, and it means that the ocean can take up more
CO2, so at present, there is about 60 times as much carbon in the ocean as there is in the atmosphere.
Acknowledgements: Dr Sara Mikaloff-Fletcher, National Institute of Water and Atmospheric Research (NIWA)

8. 14. Phytoplankton
• Plants take up CO2 which they essentially breathe it in and use it
to build their physical structures.
• Phytoplankton are basically teeny tiny microscopic plants that
live in the surface of the ocean, so essentially you have in the
surface of the ocean these little tiny microscopic plants are taking
carbon from the water and using it to build parts of their bodies.
• As the phytoplankton mature, there are a couple of different
things that can happen. One thing that can happen is they can be
recycled back into the biological processes of the surface ocean
– so maybe zooplankton eats them, maybe they die and they are
returned to their component minerals by natural processes and
that carbon is taken up by some other phytoplankton. But the
other thing that can happen is that, when they die, they can
precipitate down into the deep ocean and when that happens
because ocean circulation is so slow that carbon can be stored
or sequestered in the ocean for a very long period of time.
Acknowledgements:
Dr Sara Mikaloff-Fletcher, National Institute of Water and Atmospheric Research (NIWA)

9. 15. Sinking sediment
• When phytoplankton die, many sink and take their carbon (calcium carbonate shells) to form sediments at the bottom of
the ocean. This is called a biological pump, removing carbon from the ocean and atmosphere systems for long periods
of time.
Acknowledgement: National Oceanic Atmospheric
16. Deep circulation
• Some carbon from phytoplankton is carried by deep ocean currents that might not return it to the surface for hundreds
of years.
Acknowledgement: Public domain
17. Rock formation
• Sediments containing lots of calcium carbonate from shells can be turned into rock over millions of years. This
limestone rock can be pushed up to form land by tectonic Earth movements. Some carbon trapped in the sediments
forms gas and oil.
Acknowledgement: Verity Coomer
18. Weathering and run-off
• Rain slowly dissolves minerals from rocks – a process called weathering. These minerals eventually get washed into
the sea, where some minerals, such as calcium carbonate, add to sediments at the bottom of the ocean. A lot of organic
matter from the soil also gets carried to sea by rivers.
Acknowledgement: Terry and Janice Dowdeswell

10. Video: Carbon Cycle

11. Human impacts on the carbon cycle
• Global demand for Earth’s limited fossil fuel reserves has risen since the beginning of
the Industrial Revolution (started in Europe and the US; 1700’s and early 1800’s).
• Fossil fuels are considered as a non-renewable resource (exp: coal,
petroleum) because they are being used up much faster than they can be produced by
geological processes (it takes million of years to be build up!).
• When fossil fuels are burned, carbon dioxide is released into the air. Increasing use of
fossil fuels has led to elevated levels of atmospheric CO2.
• Deforestation: The cutting-down of forests is also a major contributor to
increasing CO2 levels. Trees and other parts of a forest ecosystem sequester carbon, and
much of the carbon is released as CO2 if the forest is cleared.
• Some of the extra CO2 produced by human activities is taken up by plants or absorbed by
the ocean, but these processes don't fully counteract the increase.
• So, atmospheric CO2 levels have risen and continue to rise. CO2 levels naturally rise and
fall in cycles over long periods of time, but they are higher now than they have been in the
past 400,000 years, as shown in the graph below:

12. Image credit: "Threats to biodiversity: Figure 1" by OpenStax College, Biology, CC BY 4.0

13. Why does it matter that there is lots
of CO2 in the atmosphere?
• Carbon dioxide is a greenhouse gas. When in the atmosphere, it traps heat and keeps it from
radiating into space.
• The elevated levels of CO2 and other greenhouse gases (water vapour, carbon dioxide, methane,
nitrous oxide, ozone, chlorofluorocarbon, hydrofluorocarbons) are causing pronounced changes in
Earth's climate. Without decisive changes to reduce emissions, Earth's temperature is projected to
increase by 1 to 5°C by the year 2100.
• Also, while uptake of excess carbon dioxide by the oceans might seem good from a greenhouse
gas perspective, it may not be good at all from the perspective of sea life.
• The CO2 dissolved in seawater can react with water molecules to release H+ ions. So, dissolving
more CO2 in water causes the water to become more acidic. More acidic water can, in turn, reduce
carbonate concentrations and make it harder for marine organisms to build and maintain their shells
of CaCO3 (calcium carbonate).
• ​Both increasing temperatures and higher acidity can harm sea life and have been linked to coral
bleaching.

15. HEALTHY CORALS
BLEACHED CORALS