The Carbon cycle

Question 1

Lithosphere

Answer 1

Over 99.9% of the carbon on Earth is stored in sedimentary rocks such as limestone.
About 0.004% of the carbon on Earth is stored in fossil fuels, such as coal and oil, in the lithosphere.

Question 2

Atmosphere

Answer 2

Carbon is stored as carbon dioxide (CO,) and in smaller quantities as methane (CH) in the atmosphere.
The atmosphere contains about 0.001% of the Earth’s carbon.

Question 3

Hydrosphere

Answer 3

Carbon dioxide (CO2) is dissolved in rivers, lakes and oceans.
The oceans are the second-largest carbon store on Earth, containing approximately 0.04% of the Earth’s carbon. The majority of carbon here is found deep in the ocean in the form of dissolved inorganic carbon.
A small amount is found at the ocean surface where it is exchanged with the atmosphere.

Question 4

Biosphere

Answer 4

Carbon is stored in the tissues of living organisms. It is transferred to the soil when living organisms die and decay.
The biosphere contains approximately 0.004% of the Earth’s total carbon.

Question 5

Cryosphere

Answer 5

The cryosphere contains less than 0.01% of Earth’s carbon. Most of the carbon in the cryosphere is in the soil in areas of permafrost (permanently frozen ground) where decomposing plants and animals have frozen into the ground.

Question 6

The carbon cycle

Answer 6

The carbon cycle is a closed system there are inputs and outputs of energy, but the amount of carbon in the system remains the same. However, some carbon is locked away (sequestered - see next page) in long-term stores, e.g. rock and fossil fuels deep underground. If these are released by e.g. burning fossil fuels, they are effectively inputs.

Question 7

Photosynthesis

Answer 7

Photosynthesis transfers carbon stored in the atmosphere to biomass.
Plants and phytoplankton use energy from the Sun to change carbon dioxide and water into glucose and oxygen. This enables plants to grow.
Carbon is passed through the food chain and released through respiration and decomposition

Question 8

Combustion

Answer 8

Combustion transfers carbon stored in living, dead or decomposed biomass (including peaty soils) to the atmosphere by burning.
Wildfires cause carbon flow.

Question 9

Ocean uptake and loss

Answer 9

CO2 is directly dissolved from the atmosphere into the ocean. It is also transferred to the oceans when it is taken up by organisms that live in them (e.g. plankton).
Carbon is also transferred from the ocean to the atmosphere when carbon-rich water from deep in the oceans rises to the surface and releases CO2

Question 10

Sequestration

Answer 10

Carbon from the atmosphere can be sequestered (captured and held) in sedimentary rocks or as fossil fuels. Rocks and fossil fuels form over millions of years when dead animal and plant material in the ocean falls to the floor and is compacted.
Carbon in fossil fuels is sequestered until we burn them (combustion)

Question 10

Respiration

Answer 10

Respiration transfers carbon from living organisms to the atmosphere.
Plants and animals break down glucose for energy, releasing carbon dioxide and methane (a gas containing carbon) in the process.

Question 11

Decomposition

Answer 11

Decomposition transfers carbon from dead biomass to the atmosphere and the soil.
After death, bacteria and fungi break organisms down. CO, and methane are released.
Some carbon is transferred to the soil in the form of humus.

Question 12

Weathering

Answer 12

Chemical weathering transfers carbon from the atmosphere to the hydrosphere and biosphere.
Atmospheric carbon reacts with water vapour to form acid rain. When this acid rain falls onto rocks, a chemical reaction occurs which dissolves the rocks. The molecules resulting from this reaction may be washed into the sea. Here, they react with CO₂ dissolved in the water to form calcium carbonate, which is used by sea creatures, e.g. to make shells.

Question 13

Fast Carbon Flow

Answer 13

Fast carbon flows quickly transfer carbon between sources. It only takes a matter of minutes, hours or days. Photosynthesis, respiration, combustion and decomposition are examples of fast carbon flows.

Question 14

Slow carbon flow

Answer 14

Sequestration is a slow carbon flow. It takes millions of years for carbon to be sequestered in sedimentary rocks.

Question 15

How wildfires can change the carbon cycle

Answer 15

Wildfires rapidly transfer large quantities of carbon from biomass (or soil) to the atmosphere. Loss of vegetation decreases photosynthesis, so less carbon is removed from the atmosphere.
In the longer term, however, fires can encourage the growth of new plants, which take in carbon from the atmosphere for photosynthesis. Depending on the amount and type of regrowth, fires can have a neutral effect on the amount of atmospheric carbon.

Question 16

How volcanic activity change the carbon cycle

Answer 16

Carbon stored within the Earth in magma is released during volcanic eruptions. The majority enters the atmosphere as CO2.
Recent volcanic eruptions have released much less CO2 than human activities. However, there is the potential for a very large eruption to disrupt the carbon cycle significantly.

Question 17

Hydrocarbon(fossil fuel) extraction and use

Answer 17

Extracting and burning (combustion) of fossil fuels releases CO, into the atmosphere.
Without human intervention, the carbon would remain sequestered in the lithosphere for thousands or millions of years to come.

Question 18

Land use changes

Answer 18

Vegetation is removed to make way for buildings - this reduces carbon storage in the biosphere.

Concrete production releases lots of CO₂, and lots of concrete is used when urban areas expand.

Question 19

Farming Practices

Answer 19

Agricultural activities release carbon into the atmosphere:
Animals release CO2 and methane when they respire and digest food.
Ploughing can release CO, stored in soil.
Growing rice in rice paddies releases a lot of methane.

Question 20

What is the carbon budget

Answer 20

The carbon budget is the difference between the inputs of carbon into a subsystem and outputs of carbon from it.

Question 21

Carbon Source

Answer 21

The outputs of carbon outweigh the inputs, so it releases more carbon than it absorbs.

Question 22

Carbon Sink

Answer 22

The inputs of carbon outweigh the outputs, so it absorbs more carbon than it releases.

Question 23

The carbon cycle affects the atmosphere and climate

Answer 23

The carbon cycle affects the amount of gases containing carbon (e.g. CO, and methane) in the atmosphere. These are greenhouse gases - they trap some of the Sun’s energy, keeping some of the heat in and keeping the planet warm.
As the concentrations of greenhouse gases in the atmosphere increase (e.g. due to changes in the carbon cycle caused by human activities such as deforestation and the burning of fossil fuels), temperatures are expected to rise. This is global warming.
Changes in temperature across the globe will affect other aspects of the climate, e.g. more intense storms are predicted.

Question 24

The carbon cycle affects the land

Answer 24

The carbon cycle allows plants to grow - if there was no carbon in the atmosphere, plants could not photosynthesise. If there was no decomposition, dead plants would remain where they fell and their nutrients would never be recycled.
Changes in the carbon cycle can reduce the amount of carbon stored in the land, e.g. warmer temperatures caused by global warming are causing permafrost to melt. This releases carbon previously stored in the permafrost into the atmosphere.
An increase in global temperatures could also increase the frequency of wildfires

Question 25

The carbon cycle affects the oceans

Answer 25

As part of the carbon cycle, carbon dioxide is dissolved directly into the oceans from the atmosphere.
CO2 in oceans is used by organisms such as phytoplankton and seaweed during photosynthesis and by other marine organisms to form calcium carbonate shells and skeletons.
Increased levels of CO2 in the atmosphere can increase the acidity of the oceans because the oceans initially absorb more CO2. This can have adverse effects on marine life.
Global warming can also affect oceans. For example, organisms that are sensitive to temperature, e.g. phytoplankton, may not be able to survive at higher temperatures, so their numbers decrease. This means that less CO2 is used by them for photosynthesis, so less carbon is removed from the atmosphere.
Warmer water is also less able to absorb CO2, so as temperatures rise, the amount of CO2 that could potentially be dissolved in the sea decreases.