Atmospheric carbon cycle
[3] Human activities, primarily the extraction and burning of fossil carbon from Earth's lithosphere starting with the Industrial Revolution, have disturbed the previous balance of the atmospheric carbon cycle and have been mostly responsible for the ongoing rapid growth in CO2 and CH4 concentrations.
Assuming the growth trend in emissions continues, the CO2 concentration is on a path to at least double by the latter half of this century.
[6] The atmospheric carbon cycle also strongly influences Earth's energy balance through the greenhouse effect, and affects the acidity or alkalinity of the planet's surface waters and soils.
Despite comprising less than 0.05% of all atmospheric gases by mole fraction,[7] the recent rise in carbon concentrations has caused substantial global heating and ocean acidification.
[10] Methane (CH4) is one of the more potent greenhouse gases and is mainly produced by the digestion or decay of biological organisms.
[16] Because methane reacts fairly quickly with other compounds, it does not stay in the atmosphere as long as many other greenhouse gases, e.g. carbon dioxide.
Although individual CO2 molecules have a short residence time in the atmosphere, it takes an extremely long time for carbon dioxide levels to sink after sudden rises, due to e.g. volcanic eruptions or human activity[17] and among the many long-lasting greenhouse gases, it is the most important because it makes up the largest fraction of the atmosphere.
The increased carbon dioxide concentration strengthens the greenhouse effect, causing changes to the global climate.
Of the increased amounts of carbon dioxide that are introduced to the atmosphere each year, approximately 80% are from the combustion of fossil fuels and cement production.
[19] The residence time of carbon in the terrestrial biosphere varies and is dependent on a large number of factors.
This water sinks down and brings the carbon into the deeper ocean levels, where it can stay for anywhere between decades and several centuries.
For example, during El Nino events there is less deep ocean upwelling, leading to lower outgassing of carbon dioxide into the atmosphere.
Two exceptions are volcanic eruptions and the combustion of fossil fuels, both of which release high amounts carbon into the atmosphere very quickly.
The burning of fossil fuels and cement production are the main reasons for the increase in atmospheric CO2 since the beginning of the industrial era.
[19] The ratio of the annual increase in atmospheric CO2 compared to CO2 emissions from fossil fuel and cement manufactured is called the "airborne fraction.".
[26] The airborne fraction has been around 60% since the 1950s, indicating that about 60% of the new carbon dioxide in the atmosphere each year originated from human sources.
