Balanced Carbon Cycle and Human Activity

Question 1

The natural greenhouse effect

Answer 1

The Sun radiates short-wavelength, high-frequency ultraviolet radiation towards Earth. Some of this radiation is reflected by the atmosphere, however, some passes through the atmosphere and is absorbed by Earth’s surface, warming it. Some is reflected as longer-wavelength, lower-frequency infrared radiation, which either passes straight through the atmosphere and radiates into space or is reabsorbed and re-emitted towards Earth’s surface

Question 2

The enhanced greenhouse effect

Answer 2

Increased atmospheric greenhouse gases such as carbon dioxide, methane and water vapour result in more infrared radiation being reabsorbed and re-emitted towards Earth’s surface, causing global warming

Question 3

The role of soil

Answer 3

Soil stores biological carbon in the form of dead matter as well as waste. This matter is respired by microorganisms which release carbon dioxide into the atmosphere. This process can be rapid in some ecosystems or incredibly slow in other ecosystems (such as mangroves or tundras)

Question 4

Mangrove soils

Answer 4

Due to the anaerobic conditions in mangrove soils, which are covered by high tides twice per day, decomposition is very slow. This means very little carbon is released into the atmosphere, instead, it remains trapped by tree roots as energy-dense peat. Drainage or clearance of mangroves for tourism, shrimp farming and aquaculture could accelerate decomposition. A loss of just 2% of the world’s mangroves could release a volume of carbon 50x the natural sequestration rate.

As of today, 50% of the world’s original mangroves have been cleared.

In Gazi Bay, Southern Kenya, locals are attempting to reduce deforestation of mangrove trees for fuel wood. Instead, they are exploiting peat, which generates $60,000 per year for local communities.

Question 5

Tundra soils

Answer 5

Tundra soils consist of an active layer above a permafrost layer. The active layer temporarily thaws during the Summer. However, melting of the ice in the active layer causes the soil to become waterlogged. This prevents aerobic bacteria from respiring dead matter and releasing carbon dioxide. This makes tundra soils an excellent carbon sink.

Increasing greenhouse gas emissions lead to global warming, causing permafrost to melt. This extends the active layer, allowing for decomposition of dead matter, releasing carbon dioxide as well as allowing pockets of trapped methane to be released into the atmosphere. This is an example of a positive feedback loop as it leads to higher greenhouse gas concentration in the atmosphere, resulting in more global warming and melting of tundra soils.

This could lead tundras to become a carbon source instead of a carbon sink. Despite this, the higher temperatures may allow more vegetation to grow in tundras to absorb more atmospheric carbon dioxide by photosynthesis. However, it is believed this will only offset 20% of the increased carbon in the atmosphere caused by melting permafrost.

Question 6

Greenhosue gases in the atmosphere

Answer 6

Carbon dioxide makes up 0.04% of the Earth’s atmosphere. 20% of it stays in the atmosphere for 800 years. It accounts for 89% of greenhouse gases produced. Its main sources are burning fossil fuels and deforestation. It has seen a 30% increase in the atmosphere since 1850.

Methane makes up 7% of all greenhouse gases in the atmosphere. It is released due to rice farming, cattle ranching and pipeline leaks and is believed to be 21x more powerful than carbon dioxide. It has seen a 250% increase in the atmosphere since 1850

Nitrous oxides account for 3% of greenhouse gases produced and are released from jet engines, cars, lorries, fertilisers and sewage. They can form smog and acid rain and are 250x more powerful than carbon dioxide. They have seen a 16% increase in the atmosphere since 1850

Halocarbons account for 1% of greenhouse gases and are produced in industry . They are 3000 times more powerful than carbon dioxide.

Carbon dioxide concentration (pre-industrial) = 280ppm
Carbon dioxide concentration (2025) = 425 ppm

Methane concentration (pre-industrial) = 725ppb
Methane concentration (2025) = 1942 ppb