Since the beginning of the Industrial Era about 1750, humans have been producing energy by burning fossil fuels (coal,oil and gas) which release increasing amounts of carbon dioxide (CO2) into the atmosphere. The amount of fossil fuel CO2 emitted to the atmosphere can be estimated with an accuracy of about 5 to 10% for recent decades from statistics of fossil fuel use. Total cumulative emissions between 1750 and 2011 amount to 375 ± 30 PgC, including a contribution of 8 PgC from the production of cement.

The second major source of anthropogenic CO2 emissions to the atmosphere is caused by changes in land use (mainly deforestation), which causes a net reduction in land carbon storage. Estimation of the effect of land use change on atmospheric CO2 requires knowledge of changes in land area as well as estimates of the carbon stored per area before and after the land use change. Other effects such as the decomposition of soil organic matter after land use change, also have to be taken into account.

In 1750 the estimated cropland and pasture area amounted to 7.5 to 9 million square km. It is estimated that since 1750, anthropogenic land use changes have resulted into about 50 million square km being used for cropland and pasture, corresponding to about 38% of the total ice-free land area. The cumulative net CO2 emissions from land use changes between 1750 and 2011 are estimated at 180 ± 80 PgC.

The rising atmospheric CO2 content creates a disequilibrium in the exchange fluxes between the atmosphere and the land and oceans. The rising CO2 concentration implies a rising atmospheric CO2 partial pressure (pCO2) that induces a globally averaged net air-to-sea flux and thus an ocean sink for CO2. On land, the rising atmospheric CO2 concentration fosters photosynthesis via the CO2 fertilization effect.

Atmospheric carbon reservoirs

In the atmosphere, CO2 is the dominant trace gas containing carbon. In 2011 its concentration was 390.5 ppm, which corresponds to a mass of 828 PgC. Additional carbon containing trace gases include methane, carbon monoxide, and still smaller amounts of hydrocarbons, black carbon aerosols and organic compounds.

Terrestrial carbon reservoirs

The terrestrial biosphere reservoir contains carbon in living vegetation and in dead organic matter in litter and soils. There is an additional amount of old soil carbon in wetland soils and in permafrost soils.

Atmosphere-land carbon exchange

The atmosphere exchanges CO2 with the land and the oceans. The most important land process for removing CO2 from the atmosphere is plant photosynthesis (Gross Primary Production). The process takes CO2, water and light as inputs and produces sugars and oxygen (CO2 + H2O + light -> sugar + O2). The sugar is then used by plants in a variety of organic chemical processes to produce plant structures and tissues. Ultimately carbon fixed in plant tissues becomes litter and soil carbon. Soil carbon is released back into the atmosphere by plant, soil microbial and animal respiration and by sporadic fires.

Plant respiration is the process of oxidizing organic compounds to produce CO2, water and energy (organic compounds + O2 -> CO2 + H2O + energy). Because CO2 uptake by photosynthesis occurs only during the growing season, whereas CO2 release by respiration occurs nearly year-round, the greater land mass in the Northern Hemisphere where growth is seasonal is responsible for the annual global ‘sawtooth’ pattern in atmospheric CO2.

The anthropogenic effect has resulted in increased CO2 in the atmosphere. More CO2 in the atmosphere results in more CO2 being taken up by the biosphere (CO2 fertilization) and concomitantly, more CO2 being released back into the atmosphere by respiration and fires. The pre-industrial land carbon balance was very close with a net flux of about 1.7 PgC/yr being transferred annually to land. The anthropogenic effect has increased that by 2.6 PgC/yr to 4.2 PgC/yr. However, this is not enough to offset increased anthropogenic emissions of 8.9 PgC/yr due to fossil fuel combustion and land use change. The disequilibrium results in an transfer of CO2 to the atmosphere.

Oceanic carbon reservoirs

In the ocean, carbon is available predominantly as Dissolved Inorganic Carbon (DIC) comprised of carbonic acid, bicarbonate and carbonate ions. In addition, the ocean contains a pool of Dissolved Organic Carbon (DOC) of which a substantial fraction remains dissolved for a very long time, with a turnover time of 1000 years or longer.

The marine biota, predominantly phytoplankton and other microorganisms, represent a small organic carbon pool, which is turned over very rapidly in days to a few weeks.

Within the ocean, carbon is cycled through different forms by processes called marine pumps, the most important of which are the biological and carbonate pumps.

The marine biological pump is a process by which biological organisms form shells from calcium and carbonate. In forming shells, two bicarbonate ions are split into one carbonate and one dissolved CO2 molecules, which increases the partial CO2 pressure in surface waters and drives release of CO2 into the atmosphere.

The marine carbonate pump is a process in which shells created in surface waters by oceanic microorganisms sink and are re-mineralized back into DIC and calcium ions in the ocean depths. Only a small fraction (~0.2 PgC yr) of the carbon exported by biological and carbonate pumps from the surface reaches the sea floor to be captured in sediments for millennia and longer.

Atmosphere-ocean carbon exchange

Atmospheric CO2 dissolves into the ocean surface through gas exchange. This exchange is driven by the partial CO2 pressure difference between the air and the sea. In the pre-industrial era, the balance between the amount of CO2 being absorbed by the oceans and the CO2 released back into the atmosphere from the oceans was very close, with an estimated net of 0.7 PgC/yr released into the atmosphere from the oceans.

The anthropogenic effect has resulted in more CO2 being exchanged with the oceans with a net balance of 1.6 PgC/yr being transferred from the atmosphere to the oceans. The net anthropogenic effect is that since 1750 the oceans have become a carbon sink absorbing significant quantities of CO2 from the atmosphere.

Land-ocean carbon exchange

A significant amount of terrestrial carbon (1.7 PgC/yr) is transported from soils to streams and ultimately to rivers. A fraction of this carbon is outgassed as CO2 by rivers and lakes to the atmosphere, a fraction is buried in freshwater organic sediments and the remaining amount (~0.9 PgC/yr) is delivered by rivers to the coastal ocean as dissolved inorganic carbon, dissolved organic carbon and particulate organic carbon.

Net atmosphere and land + ocean carbon exchange

The net response to rising CO2 is to increase cumulative land and ocean uptake, regardless of the time scale. The increased takeup of CO2 by land and the oceans is not sufficient to compensate for fossil fuel combustion and land use change.

Methane

Methane (CH4) absorbs more heat (long wavelength radiation) per molecule than CO2 making it is a more effective greenhouse gas, but methane remains less than 10 years in the atmosphere compared to 100 years for CO2. Global methane mass is measured as teragrams of CH4 (TgCH4). One teragram is a billion tonnes of CH4.

The Earth contains vast reserves of methane, in the form of natural gas or as hydrates. The global size of these reservoirs is difficult to estimate.

A very large geological stock, globally 1500 to 7000 PgC, (with “low confidence in estimates”) exists in the form of frozen hydrate deposits (‘clathrates’) in shallow ocean sediments and on the slopes of continental shelves, and permafrost soils. These CH4 hydrates are stable under conditions of low temperature and high pressure. Warming or changes in pressure could render some of these hydrates unstable with a potential release of CH4 to the overlying soil/ocean and/or
atmosphere.

Global methane cycle

Figure Global methane cycle Annual fluxes in TgCH4/yr estimated for the time period 2000–2009 for the atmosphere and three geological reservoirs (hydrates on land and in the ocean floor and gas reserves). Black arrows denote ‘natural’ fluxes, that is, fluxes that are not directly caused by human activities since 1750, red arrows anthropogenic fluxes, and the light brown arrow denotes a combined natural + anthropogenic flux. The atmospheric inventories have been calculated using a conversion factor of 2.7476 TgCH per ppb. The assumed preindustrial annual mean globally averaged CH4 concentration was 722 ± 25 ppb taking the average of the Antarctic Law Dome ice core observations and the measurements from the GRIP ice core in Greenland. The atmospheric inventory in the year 2011 is based on an annual globally averaged CH4 concentration of 1803 ± 4 ppb in the year 2011. It is the sum of the atmospheric increase between 1750 and 2011 (in red) and of the pre-industrial inventory (in black). The average atmospheric increase each year, also called growth rate, is based on a measured concentration increase of 2.2 ppb/yr during the time period 2000–2009.

The sources of CH4 at the surface of the Earth include

• Natural emissions of fossil CH4 and calcium carbonate (CaCO) of sea floor sediments
• Emissions caused by leakages from fossil fuel extraction and use (natural gas, coal and oil industry).

CH4 is also generated by incomplete burning of fossil fuels and plant biomass (both natural and anthropogenic fires. Methane emissions also are generated from wetlands, fresh water bodies, and from termites. There are also very small emissions from the ocean. Anthropogenic methane emissions are generated by rice paddy agriculture, ruminant livestock, landfills, man-made lakes and wetlands and waste treatment.

Atmospheric CH4 is removed primarily by photochemistry, through atmospheric chemistry reactions with the OH radicals. Other smaller removal processes of atmospheric CH4 take place in the stratosphere through reaction with chlorine and oxygen radicals, by oxidation in well aerated soils, and possibly by reaction with chlorine in the marine boundary layer.

Source	Methane processes	Natural(TgCH4/yr)	Anthropogenic(TgCH4/yr)	Total(TgCH4/yr)
Wetlands	Methanogenesis	177 to 188	–	177 to 188
Freshwaters	Methanogenesis	8 to 73	–	8 to 73
Termites	Cellulose digestion	2 to 22	–	2 to 22
Ocean clathrates	Destabilization	2 to 9	–	2 to 9
Geological sources	Volcanism	9 to 47	–	9 to 47
Atmosphere	Stratospheric hydroxl reaction	-16 to -84	–	-16 to -84
Atmosphere	Tropospheric hydroxyl reaction	-454 to -617	–	-454 to -617
Atmosphere	Tropospheric chlorine reaction	-13 to -17	–	-13 to -17
Atmosphere	Soil oxidation	-9 to -47	–	-9 to -47
Biomass	Burning	tbd	tbd	32 to 39
Drilling	Fossil fuels	–	85 to 105	85 to 105
Landfills and waste	Methanogenesis	–	67 to 90	67 to 90
Ruminant livestock	Digestion	–	87 to 94	87 to 94
Wetland rice cultivation	Methanogenesis	–	33 to 40	33 to 40

Atmospheric methane observations from ice cores show methane concentration increased by about 100 ppb between 5000 years ago and around 1750. During the Industrial Era the amount of methane in the atmosphere has more than doubled.

Atmospheric carbon reservoir	Prior to 1750(TgCH4)	Industrial Era(TgCH4)	Total(TgCH4)
Methane	1984	2970	4950

Important conclusions from AR5 about the carbon cycle

With a “very high level of confidence”, the increase in CO2 emissions from fossil fuel burning and from land use change are the dominant cause of the observed increase in atmospheric CO2 concentration. About half of the emissions have remained in the atmosphere (240 ± 10 PgC) since 1750. The rest has been removed from the atmosphere by sinks and stored in the natural carbon cycle reservoirs. The ocean reservoir stored 155 ± 30 PgC. Vegetation biomass and soils not affected by land use change stored 160±90 PgC.

Anthropogenic carbon reservoirs	Size (PgC)
Atmospheric CO2	240 ± 10
Oceans	155 ± 30
Land (vegetation and soils)	160 ± 90

During the past 800,000 years prior to 1750, atmospheric CO2 varied from 180 ppm during glacial (cold) up to 300 ppm during interglacial (warm) periods. This is “well established” from multiple ice core measurements.

Ice core records indicate CO2 values of ~180 to 200 ppm during glacial (cold) intervals. Interglacial CO2 values were 240 to 260 ppm until 420,000 years ago, after which they increased to 270 to 290 ppm.

Ice cores recovered from the Antarctic ice sheet reveal that the concentration of atmospheric CO2 at the Last Glacial Maximum (LGM) at 21,000 years ago was about one third lower than during the subsequent interglacial (Holocene) period which started about 11,700 years ago.

During the 7000 years prior to 1750, atmospheric CO2 from ice cores shows a very slow increase from 260 ppm to 280 ppm. The contribution of CO2 emissions from early anthropogenic land use is “unlikely” sufficient to explain the CO2 increase prior to 1750.

Atmospheric CH4 from ice cores increased by about 100 ppb between 5000 years ago and 1750. About as “likely as not”, this increase can be attributed to early human activities involving livestock, human-caused fires and rice cultivation.

IPCC Fifth Assessment Report Climate Change 2013: The Physical Science Basis Chapter 6 Carbon and Other Biogeochemical Cycles