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Changes in the carbon cycle: Human causes

Human causes of changes in the carbon cycle

90% of anthropogenic (human-related) carbon release comes from burning fossil fuels such as coal, oil and natural gas according to the Intergovernmental Panel on Climate Change (IPCC)

The remaining 10% comes from is caused by land-use change such as deforestation, agricultural practices and land drainage.

Approximately half the anthropogenic carbon is absorbed evenly between the oceans and vegetation. The remaining half is absorbed by the atmosphere.

Global concentrations of carbon dioxide in the atmosphere have increased from around 320 ppm to over 400 ppm since the 1960s. This is the highest level ever recorded.

Combustion of fossil fuels

Fossil fuels are very important long-term carbon stores. The majority of the world’s oil and gas is extracted from rocks that are 70-100 million years old. When burnt to generate energy and power, the carbon that has been stored is primarily released into the atmosphere as carbon dioxide.

Fossil fuels are hydrocarbons. This means they are mainly composed of carbon and hydrogen.

Methane is the main component of natural gas. Oil is more complex, comprising of carbon, hydrogen, nitrogen and sulphur.

When combustion occurs, reactions occur with oxygen releasing CO2 and water.

Carbon dioxide has been produced in increasing quantities since the Industrial Revolution. Once in the atmosphere carbon dioxide contributes to the natural greenhouse effect causing global temperatures to rise. Rapid industrialisation in developing nations such as China and continued demand from industrialised nations has led to a rapid increase in emissions since the 1950s.

Data illustrating the increasing concentration of carbon dioxide in the atmosphere has been used to support the concept of anthropogenic climate change and global warming.

Land-use change

Ten per cent of carbon released globally comes from changing land-use. This affects mainly short-term stores and has direct links to climate change.

Farming practices

There is a range of ways agricultural practices contribute to the release of carbon. Activities that release carbon include:

  • Ploughing and harvesting
  • Rearing livestock
  • Using machinery that burns fossil fuels
  • Using fertilisers that contain fossil fuels

Some farming practices lead to the release of methane in high quantities. Livestock release methane when they ruminate. Twenty per cent of the USA’s methane emissions, some 5.5 tonnes of methane, originate from cattle.

Methane is also a bi-product of rice cultivation. Methane is emitted from flooded rice fields as bacteria in the waterlogged soil produce it in large quantities. Nitrous oxide, commonly known as laughing gas, is also produced by soil microbes in rice fields. Scientists at the US-based advocacy group the Environmental Defense Fund suggest the short-term warming impact of these additional gases in the atmosphere could be equivalent to 1,200 coal power plants.

Studies suggest rice yields have increased by 25 per cent due to higher concentrations of carbon dioxide in the air. However, this has resulted in a 40 per cent increase in methane emissions. This is unlikely to fall in the near future as 50 per cent of the world’s population relies on rice as their primary food source.


Forests are effective stores of carbon. In a natural system when a tree dies it decomposes and slowly releases carbon dioxide over time. However, this is compensated by the growth of new vegetation, leading to the system being carbon neutral.

When deforestation occurs by burning, carbon dioxide is immediately released into the atmosphere. Where the land is then used for different purposes, such as cattle ranching, there is a reduction in the system’s ability to absorb carbon dioxide in the future.

Twenty per cent of global carbon dioxide emissions is caused by deforestation. Although deforestation is occurring around the world, it is particularly concentrated in tropical regions. Deforestation is occurring in tropical areas for building, mining, ranching and for commercial crops such as soya and palm oil.

Deforestation also occurs as timber is a valuable source of income through the production of wood products such as furniture. Forests are also harvested for firewood.


Urbanisation is the replacement of countryside with built-up areas. Urbanisation has a significant impact on local carbon cycles. Stores, such as vegetation, are replaced or covered up e.g. soils.

Around the world, only 2 per cent of the Earth’s land area is urbanised. However, these urban areas are responsible for the release of 97% of anthropogenic carbon dioxide emissions. Emissions originate from transport, industry, cement production and the transformation of land from natural to urban.

It is estimated that 2.4% of global carbon emissions, excluding burning fossil fuels, is caused by cement production. Carbon dioxide is a by-product of the chemical conversion of limestone to lime. It is also released through the burning of fossil fuels to generate energy to power the process.

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