What is the best known and most significant human alteration of the carbon cycle?
Answers
The human impact on the Carbon Cycle
The carbon cycle is a natural process, and has been ongoing throughout Earth’s history. Left unperturbed (by natural or human processes) it maintains a stable concentration in the atmosphere, biosphere, hydrosphere, and lithosphere (see the table above). As the reservoirs are linked (either directly or indirectly), a change in any of the carbon reservoirs causes changes in the others. Actions by humans have resulted in the removal of carbon from carbon sinks (such as the oil and coal deposits mentioned above), directly adding it to the atmosphere. This has been most notable since the Industrial Revolution in the 18th and 19th Century.
The two main human impacts on the carbon cycle are:
1. Burning of fossil fuels.
Under natural conditions the release of carbon from fossil fuels occurs slowly, as they are subducted into the mantle, and CO2 is released through volcanic activity. However, humans are heavily reliant on fossil fuels, and extract it from the lithosphere in great quantities. Burning coal, oil, natural gas, and other fossil fuels – for industrial activity and power generation for example, removes the carbon from the fossil fuels and emits it as CO2 into the atmosphere.
2. Land use and land cover change (e.g. deforestation)
Large amounts of carbon are stored in living plants (c.1,000 gigtones). Therefore, land use changes, especially the clearance of forests (which are very densely inhabited by plants, and therefore contain a large amount of carbon), can influence the carbon cycle in two ways. Firstly, the removal of vegetation eliminates plants which would otherwise be capturing carbon from the atmosphere through photosynthesis. Secondly, as dense forests are replaced by crops/pasture land/built environments, there is usually a net decrease in the carbon store, as smaller plants (and worse still, concrete) store far less carbon than large trees. Deforestation also allows much more soil to be eroded, and carbon stored in the soil is rapidly taken into rivers.
Because of the cyclical nature of the carbon cycle, the impacts humans cause can lead to a number of amplifications and feedbacks. Increasing atmospheric CO2 and CH4 (along with other greenhouse gases) causes higher global air temperatures which in turn increases decomposition in soil, thereby releasing more CO2 to the atmosphere. Increases in global temperature also affect ocean temperatures, modifying oceanic ecosystems and having the potential to disrupt the oceanic carbon cycle, limiting the ocean’s ability to absorb and store carbon.