What is the Carbon Cycle? What is the science behind it?




*For the new assessment of the last decade of understanding the carbon cycle, see the Second State of the Carbon Cycle Report (2018).*

Carbon Cycle North America figure from SOCCR2

What is the carbon cycle? What are the different pools and fluxes of carbon? Why are they important? This page provides a compilation of information and relevant links to help answer some of these questions.

The Carbon CycleWhat is the Carbon Cycle? What is the fast and slow cycle and how are they influenced?

Carbon Measurement Approaches and Accounting Frameworks: Approaches and methods for carbon stock and flow estimations, measurements, and accounting

The North American Carbon Cycle: The latest (2018) assessment and budget

Webinar Series Videos: 'The State of the Carbon Cycle: From Science to Solutions'

The Global Carbon Budget : The Global Carbon Budget as calculated by a global group of scientists

Frequently asked questions and their answersAnswers to commonly asked questions such as the following are listed here: Can you quantify the sources and sinks of the global carbon cycle? How much carbon is stored in the different ecosystems? In terms of mass, how much carbon does 1 part per million by volume of atmospheric CO2 represent? What percentage of the CO2 in the atmosphere has been produced by human beings through the burning of fossil fuels?  

Values in parentheses are estimates of the main carbon reservoirs in gigatons (GT) as reported in Houghton (2007)


The Carbon Cycle

 (Original Source: NASA Earth Observatory)

'Carbon is the backbone of life on Earth. We are made of carbon, we eat carbon, and our civilizations—our economies, our homes, our means of transport—are built on carbon. We need carbon, but that need is also entwined with one of the most serious problems facing us today: global climate change.....'

  • What is the carbon cycle?  'Carbon flows between each reservoir in an exchange called the carbon cycle, which has slow and fast components. Any change in the cycle that shifts carbon out of one reservoir puts more carbon in the other reservoirs. Changes that put carbon gases into the atmosphere result in warmer temperatures on Earth....'
  • The Slow Carbon Cycle 'Through a series of chemical reactions and tectonic activity, carbon takes between 100-200 million years to move between rocks, soil, ocean, and atmosphere in the slow carbon cycle. On average, 1013 to 1014 grams (10–100 million metric tons) of carbon move through the slow carbon cycle every year. In comparison, human emissions of carbon to the atmosphere are on the order of 1015 grams, whereas the fast carbon cycle moves 1016 to 1017 grams of carbon per year....'
  • The Fast Carbon Cycle: '...Plants and phytoplankton are the main components of the fast carbon cycle. Phytoplankton (microscopic organisms in the ocean) and plants take carbon dioxide from the atmosphere by absorbing it into their cells. Using energy from the Sun, both plants and plankton combine carbon dioxide (CO2) and water to form sugar (CH2O) and oxygen. The chemical reaction looks like this:

CO2 + H2O + energy = CH2O + O2                      

Four things can happen to move carbon from a plant and return it to the atmosphere, but all involve the same chemical reaction. Plants break down the sugar to get the energy they need to grow. Animals (including people) eat the plants or plankton, and break down the plant sugar to get energy. Plants and plankton die and decay (are eaten by bacteria) at the end of the growing season. Or fire consumes plants. In each case, oxygen combines with sugar to release water, carbon dioxide, and energy. The basic chemical reaction looks like this:

CH2O + O2 = CO2 + H2O + energy

This figure from the National Climate Assessment (2014) depicts different biogeochemical cycles, including the carbon cycle, as influenced by different factors. 'The top panel shows the impact of the alteration of the carbon cycle alone on radiative forcing. The bottom panel shows the impacts of the alteration of carbon, nitrogen, and sulfur cycles on radiative forcing. SO2 and NH3 increase aerosols and decrease radiative forcing. NH3 is likely to increase plant biomass, and consequently decrease forcing. NOx is likely to increase the formation of tropospheric ozone (O3) and increase radiative forcing. Ozone has a negative effect on plant growth/biomass, which might increase radiative forcing. CO2 and NH3 act synergistically to increase plant growth, and therefore decrease radiative forcing. SO2 is likely to reduce plant growth, perhaps through the leaching of soil nutrients, and consequently increase radiative forcing. NOx is likely to reduce plant growth directly and through the leaching of soil nutrients, therefore increasing radiative forcing. However, it could act as a fertilizer that would have the opposite effect.' (National Climate Assessment, 2014)

In all four processes, the carbon dioxide released in the reaction usually ends up in the atmosphere. The fast carbon cycle is so tightly tied to plant life that the growing season can be seen by the way carbon dioxide fluctuates in the atmosphere. In the Northern Hemisphere winter, when few land plants are growing and many are decaying, atmospheric carbon dioxide concentrations climb. During the spring, when plants begin growing again, concentrations drop. It is as if the Earth is breathing. The ebb and flow of the fast carbon cycle is visible in the changing seasons. As the large land masses of Northern Hemisphere green in the spring and summer, they draw carbon out of the atmosphere. This graph shows the difference in carbon dioxide levels from the previous month, with the long-term trend removed. This cycle peaks in August, with about 2 parts per million of carbon dioxide drawn out of the atmosphere. In the fall and winter, as vegetation dies back in the northern hemisphere, decomposition and respiration returns carbon dioxide to the atmosphere. These maps show net primary productivity (the amount of carbon consumed by plants) on land (green) and in the oceans (blue) during August and December, 2010. In August, the green areas of North America, Europe, and Asia represent plants using carbon from the atmosphere to grow. In December, net primary productivity at high latitudes is negative, which outweighs the seasonal increase in vegetation in the southern hemisphere. As a result, the amount of carbon dioxide in the atmosphere increases....'


  • Changes in the Carbon Cycle 'Left unperturbed, the fast and slow carbon cycles maintain a relatively steady concentration of carbon in the atmosphere, land, plants, and ocean. But when anything changes the amount of carbon in one reservoir, the effect ripples through the others....'
  • Effects of Changing the Carbon Cycle 'It is significant that so much carbon dioxide stays in the atmosphere because CO2 is the most important gas for controlling Earth’s temperature. Carbon dioxide, methane, and halocarbons are greenhouse gases that absorb a wide range of energy—including infrared energy (heat) emitted by the Earth—and then re-emit it. The re-emitted energy travels out in all directions, but some returns to Earth, where it heats the surface. Without greenhouse gases, Earth would be a frozen -18 degrees Celsius (0 degrees Fahrenheit). With too many greenhouse gases, Earth would be like Venus, where the greenhouse atmosphere keeps temperatures around 400 degrees Celsius (750 Fahrenheit)....'
  • Studying the Carbon Cycle 'What will those changes look like? What will happen to plants as temperatures increase and climate changes? Will they remove more carbon from the atmosphere than they put back? Will they become less productive? How much extra carbon will melting permafrost put into the atmosphere, and how much will that amplify warming? Will ocean circulation or warming change the rate at which the ocean takes up carbon? Will ocean life become less productive? How much will the ocean acidify, and what effects will that have?....' (Original Source: NASA Earth Observatory



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Carbon measurement Approaches and Accounting Frameworks

From the State of the Carbon Cycle Report (USGCRP, 2018) Preface (Shrestha et al, 2018):

'Three observational, analytical, and modeling methods are used to estimate carbon stocks and fluxes: 1) inventory measurements or “bottom-up” methods, 2) atmospheric measurements or “top-down” methods, and 3) ecosystem models (see Appendix D for details). “Bottom-up” estimates of carbon exchange with the atmosphere depend on measurements of carbon contained in biomass, soils, and water, as well as measurements of CO2 and CH4 exchange among the land, water, and atmosphere. Examples include direct measurement of power plant carbon emissions; remote-sensing and field measurements repeated over time to estimate changes in ecosystem stocks; measurements of the amount of carbon gases emitted from land and water ecosystems to the atmosphere (in chambers or, at larger scales, using sensors on towers); and combined urban demographic and activity data (e.g., population and building floor areas) with “emissions factors” to estimate the amount of CO2 released per unit of activity.

Top-down approaches infer fluxes from the terrestrial land surface and ocean by coupling atmospheric gas measurements (using air sampling instruments on the ground, towers, buildings, balloons, and aircraft or remote sensors on satellites) with carbon isotope methods, tracer techniques, and simulations of how these gases move in the atmosphere. The network of GHG measurements, types of measurement techniques, and diversity of gases measured has grown exponentially since SOCCR1 (CCSP 2007), providing improved estimates of CO2 and CH4 emissions and increased temporal resolution at regional to local scales across North America.

Ecosystem models are used to estimate carbon stocks and fluxes with mathematical representations of essential processes, such as photosynthesis and respiration, and how these processes respond to external factors, such as temperature, precipitation, solar radiation, and water movement. Models also are used with top-down atmospheric measurements to attribute observed GHG fluxes to specific terrestrial or ocean features or locations.'

For details, see the SOCCR2 Preface (Shrestha et al. 2018) and Appendix D (Birdsey et al. 2018).


Shrestha, G., N. Cavallaro, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, N. P. Gurwick, P. J. Marcotullio, and J. Field, 2018: Preface. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 5-20,

Birdsey, R., N. P. Gurwick, K. R. Gurney, G. Shrestha, M. A. Mayes, R. G. Najjar, S. C. Reed, and P. RomeroLankao, 2018: Appendix D. Carbon measurement approaches and accounting frameworks. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 834-838, doi: https://

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The North American Carbon Cycle and Budget

SOCCR2 Figure 2.3: For each component, estimates are shown for average annual stock changes (boxes), fluxes (vertical arrows), and lateral transfers (horizontal arrows) from ca. 2004 to 2013, the approximately 10-year period since the First State of the Carbon Cycle Report (CCSP 2007). All values are reported as teragrams of carbon (Tg C) per year. The sum of all fluxes between the atmosphere and the land or water components equals the increase in atmospheric carbon, so none of the lateral fluxes are counted as exchange with the atmosphere. Mathematical rounding accounts for the difference between this figure’s estimated 1,009 Tg C per year added to the atmosphere over North America and the net carbon source estimate of 1,008 Tg C per year given in Table 2.2. The net ecosystem flux of 959 Tg C per year from the atmosphere into land ecosystems is inferred from all the other fluxes based on the principle of conserving the overall mass balance of the different components. [Data sources: Data and certainty estimates are compiled and synthesized from the various chapters in this report. See Preface section titled “Treatment of Uncertainty in SOCCR2,” for an explanation of asterisks (i.e., certainty estimates).]

Excerpt from the Second State of the Carbon Cycle Report (SOCCR2, USGCRP 2018) Chapter 2 (Hayes et. al, 2018):

'Since the Industrial Revolution, human activity has released into the atmosphere unprecedented amounts of carbon-containing greenhouse gases (GHGs), such as carbon dioxide (CO2) and methane (CH4), that have influenced the global carbon cycle. For the past three centuries, North America has been recognized as a net source of CO2 emissions to the atmosphere (Houghton 1999, 2003; Houghton and Hackler 2000; Hurtt et al., 2002). Now there is greater interest in including in this picture emissions of CH4 because it has 28 times the global warming potential of CO2 over a 100-year time horizon (Myhre et al., 2013; NAS 2018).

The major continental sources of CO2 and CH4 are 1) fossil fuel emissions, 2) wildfire and other disturbances, and 3) land-use change. Globally, continental carbon sources are partially offset by sinks from natural and managed ecosystems via plant photosynthesis that converts CO2 into biomass. The terrestrial carbon sink in North America is known to offset a substantial proportion of the continent’s cumulative carbon sources. Although uncertain, quantitative estimates of this offset over the last two decades range from as low as 16% to as high as 52% (King et al., 2015). Highlighted in this chapter are persistent challenges in unravelling CH4 dynamics across North America that arise from the need to fully quantify multiple sources and sinks, both natural (Warner et al., 2017) and anthropogenic (Hendrick et al., 2016; Turner et al., 2016a; NAS 2018). Adding to the challenge is disagreement on whether the reported magnitudes of CH4 sources and sinks in the United States are underestimated (Bruhwiler et al., 2017; Miller et al., 2013; Turner et al., 2016a).

At the global scale, about 50% of annual anthropogenic carbon emissions are sequestered in marine and terrestrial ecosystems (Le Quéré et al., 2016). Temporal patterns indicate that fossil carbon emissions have increased from 3.3 petagrams of carbon (Pg C) per year to almost 10 Pg C over the past 50 years (Le Quéré et al., 2015). However, considerable uncertainty remains in the spatial patterns of emissions at finer scales over which carbon management decisions are made. Most importantly, the sensitivity of terrestrial sources and sinks to variability and trends in the biophysical factors driving the carbon cycle is not understood well enough to provide good confidence in projections of the future performance of the North American carbon balance (Friedlingstein et al., 2006; McGuire et al., 2016; Tian et al., 2016).'

For further details, see the latest decadal assessment of North American Carbon Cyle, the Second State of the Carbon Cycle Report.


Hayes, D. J., R. Vargas, S. R. Alin, R. T. Conant, L. R. Hutyra, A. R. Jacobson, W. A. Kurz, S. Liu, A. D. McGuire, B. Poulter, and C. W. Woodall, 2018: Chapter 2: The North American carbon budget. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 71-108,

USGCRP, 2018: Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report. [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, 878 pp.,

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Webinar Series Videos: 'The State of the Carbon Cycle: From Science to Solutions' and others

Recorded webinars describing what is the carbon cycle, focusing on the Second State of the Carbon Cycle Report science findings and pertinent scientific and societally-relevant activities, are posted on our YouTube Channel. The series desciption is here.

Link to recordings of all webinars on the Youtube Channel of the U.S. Carbon Cycle Science Program

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Global carbon budget 

Note: For the latest annual global carbon and methane budgets, please see the Global Carbon Project.


The adjacent figure on the left represents recent global carbon budget estimates of annual carbon flows averaged from 2002 to 2011 , as provided in the Global Carbon Project's 2013 report. (Values in gigatons of carbon per year)


1 GtC = 1 gigaton of carbon (or British-French 1 gigatonne of carbon)
= 109 metric ton carbon or 1 billion tons of carbon
= 1 PgC = 1 petagram of carbon = 1015 g of carbon 

1 metric ton = 1000 Kg = 10

(The metric ton is also written as tonne in the British and French systems, as in this Global Carbon Budget figure.)

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Frequently asked questions and their answers about the carbon cycle

(Source: Carbon Dioxide Information Analysis Center, CDIAC)


 Q. What are the present tropospheric concentrations, global warming potentials (100 year time horizon), and atmospheric lifetimes of CO2, CH4, N2O, CFC-11, CFC-12, CFC-113, CCl4, methyl chloroform, HCFC-22, sulphur hexafluoride, trifluoromethyl sulphur pentafluoride, perfluoroethane, and surface ozone?

A. View a table presenting data and source for current greenhouse gas concentrations.

Q. Can you quantify the sources and sinks of the global carbon cycle?


A. Read a discussion of the global carbon cycle. You may also view the figures here (adapted by CDIAC from the IPCC Fourth Assessment Report: Climate Change 2007 and the Woods Hole Research Center.)

Note: GtC = gigaton of carbon and giga = 109 and Pg C = petagram of carbon and Peta = 1015

Find the latest carbon budget estimates. Source: Global Carbon Project

And, click here to see figures summarizing the global cycles of biologically active elements. Source: William S. Reeburgh, Professor Marine and Terrestrial Biogeochemistry, University of California.


Q. How much carbon is stored in the different ecosystems?

A. View an illustration of the major world ecosystem complexes ranked by carbon in live vegetation. 


Q. In terms of mass, how much carbon does 1 part per million by volume of atmospheric CO2 represent?

A. Using 5.137 x 1018 kg as the mass of the atmosphere (Trenberth, 1981 JGR 86:5238-46), 1 ppmv of CO2= 2.13 Gt of carbon.

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Q. What percentage of the CO2 in the atmosphere has been produced by human beings through the burning of fossil fuels?


A. Anthropogenic CO2 comes from fossil fuel combustion, changes in land use (e.g., forest clearing), and cement manufacture. Houghton and Hackler have estimated land-use changes from 1850-2000, so it is convenient to use 1850 as our starting point for the following discussion. Atmospheric CO2 concentrations had not changed appreciably over the preceding 850 years (IPCC; The Scientific Basis) so it may be safely assumed that they would not have changed appreciably in the 150 years from 1850 to 2000 in the absence of human intervention.

In the following calculations, we will express atmospheric concentrations of CO2 in units of parts per million by volume (ppmv). Each ppmv represents 2.13 X1015 grams, or 2.13 petagrams of carbon (PgC) in the atmosphere. According to Houghton and Hackler, land-use changes from 1850-2000 resulted in a net transfer of 154 PgC to the atmosphere. During that same period, 282 PgC were released by combustion of fossil fuels, and 5.5 additional PgC were released to the atmosphere from cement manufacture. This adds up to 154 + 282 + 5.5 = 441.5 PgC, of which 282/444.1 = 64% is due to fossil-fuel combustion.

Atmospheric CO2 concentrations rose from 288 ppmv in 1850 to 369.5 ppmv in 2000, for an increase of 81.5 ppmv, or 174 PgC. In other words, about 40% (174/441.5) of the additional carbon has remained in the atmosphere, while the remaining 60% has been transferred to the oceans and terrestrial biosphere.

The 369.5 ppmv of carbon in the atmosphere, in the form of CO2, translates into 787 PgC, of which 174 PgC has been added since 1850. From the second paragraph above, we see that 64% of that 174 PgC, or 111 PgC, can be attributed to fossil-fuel combustion. This represents about 14% (111/787) of the carbon in the atmosphere in the form of CO2.

See the lastest State of the Carbon Cycle Report for details.

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 Q. How much carbon dioxide is produced from the combustion of 1000 cubic feet of natural gas?

A. If we start with 1000 cubic feet of natural gas (and assuming it is pure methane or CH4) at STP (standard temperature and pressure, i.e., temperature of 273°K = 0°C = 32°F and pressure of 1 atm = 14.7 psia = 760 torr), and burn it completely, here's what we come up with:

1 cubic foot (cf) = 0.0283165 cubic meters (m3)
and 1 m3 = 1000 liters (L)
so 1 cf = 28.31685 L
and 1000 cf = 28316.85 L

Since 1 mole of a gas occupies 22.4 L at STP, 28316.85 L of CH4 contains 28316.85/22.4 = 1264.145 moles of CH4 (each mole of CH4 = approx. 16 g)

If we burn CH4 completely, it follows this equation:
CH4 + 2O2 => CO2 + 2H20

That is, for each mole of methane we get one mole of carbon dioxide.

One mole of CO2 has a mass of approx. 44 g, so 1264.145 moles of CO2 has a mass of approx. 1264.145 x 44 or 55622.38 g

A pound is about equivalent to 454 g, so 55622.38 g is about equivalent to 55622.38/454 or 122 lb

That is, the complete combustion of 1000 cubic feet at STP of natural gas results in the production of about 122 lb of carbon dioxide.

Of course, the mass of the methane in 1000 cubic feet will vary if the temperature and pressure are NOT as assumed above, and this will affect the mass of CO2 produced. According to the Ideal Gas Law:

PV = nRT

Where       P = pressure

                    V = volume

                    n = moles of gas

                    T = temperature   

                    R = constant (0.08206 L atm/mole K or 62.36 L torr/mole K)

 At STP, 1000 cf contains

n = PV/RT moles of methane

  = (1 atm)(28316.85 L)/(0.08206 L atm/mole K)(273°K)

  = 1264 moles CH4  (the value given in the example above)
In the energy industry, however, 1 standard cubic foot (scf) of natural gas is defined at 60°F (= 15.6°C = 288.6°K) and 14.7 psia, rather than at STP (Handbook of Formulae, Equations and Conversion Factors for the Energy Professional, JOB Publications, Tallahassee, FL;). Solving again at this higher (relative to STP) temperature, we get:

 n = (1 atm)(28316.85 L)/(0.08206 L atm/mole K)(288.6°K) = 1196 moles CH4

That is, at the higher temperature, a given volume of gas will contain fewer moles, and less mass. Going again through the calculation for CO2 emitted, but using the value of 1196 moles of CH4, results in an answer of approximately 115 lb of carbon dioxide.

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 Q. Why do some estimates of CO2 emissions seem to be about 3 1/2 times as large as others?

A. When looking at CO2 emissions estimates, it is important to look at the units in which they are expressed. The numbers are sometimes expressed as mass of CO2 but are listed in all of our estimates only in terms of the mass of the C (carbon). Because C cycles through the atmosphere, oceans, plants, fuels, etc. and changes the ways in which it is combined with other elements, it is often easier to keep track only of the flows of carbon. Emissions expressed in units of C can be easily converted to emissions in CO2 units by adjusting for the mass of the attached oxygen atoms, that is by multiplying by the ratios of the molecular weights, 44/12, or 3.67. 

Q. Why is the sum of all national and regional CO2 emission estimates less than the global totals?

A. The difference between the sum of the individual countries (or regions) and the global estimates is generally less than 5%. There are four primary reasons for this.

  1. global totals include emissions from bunker fuels whereas these are not included in national (or regional) totals. Bunker fuels are fuels used by ships and aircraft in international transportation,
  2. global totals include estimates for the oxidation of non-fuel hydrocarbon products (e.g., asphalt, lubricants, petroleum waxes, etc.) whereas national totals do not,
  3. national totals include annual changes in fuel stocks whereas the global total does not, and
  4. due to statistical differences in the international statistics, the sum of exports from all exporters is not identical to the sum of all imports by all importers.

Q. Why do some smaller nations have larger per capita emission estimates than industrialized nations like the US?

A. Often it is difficult to attribute emissions to a source. Many small island nations have military bases that are used for re-fueling or have large tourist industries. Who do you assign the emissions to; the US whose military planes are re-fueling on the Wake Island with aviation and jet fuel or the Wake Island? The accounting practices used within the UN Energy Statistics Database assign this fuel consumption to the Wake Island thus elevating the Wake Island's per capita estimate. The same is true for tourist nations like Aruba who are assigned the fuels used in the commercial planes carrying tourists back to their native countries. Although this distorts the per capita emission estimates it makes it easier from an accounting standpoint than trying to trace each plane or ship to its final destination. One should be cautious in using only the per capita CO2 emission estimates.

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Q. What is the greenhouse effect? Is it the same as the ozone hole issue?

A. No, they are two different (but related) issues.The greenhouse effect issue concerns the warming of the lower part of the atmosphere, the troposphere (the layer in which temperature drops with height; it is about 10-15 kilometers thick, varying with latitude and season), by increasing concentrations of the so-called greenhouse gases (carbon dioxide, methane, nitrous oxide, ozone, and others) in the troposphere. This warming occurs because the greenhouse gases, while they are transparent to incoming solar radiation, absorb infrared (heat) radiation from the Earth that would otherwise escape from the atmosphere into space; the greenhouse gases then re-radiate some of this heat back towards the surface of the Earth.

The ozone hole issue concerns the loss of ozone in the upper part of the atmosphere, the stratosphere, resulting from increasing concentrations of certain halogenated hydrocarbons (such as chlorinated fluorocarbons, known as CFCs). Through a series of chemical reactions in the stratosphere, the halogenated hydrocarbons destroy ozone in the stratosphere. This is of concern because the ozone blocks incoming ultraviolet radiation from the Sun, and portions of the ultraviolet radiation spectrum have been found to have adverse biological effects.

The greenhouse effect and ozone hole issues are, however, related. For example, CFCs are involved in both issues: CFCs, in addition to destroying stratosphere ozone, are also greenhouse gases. It has traditionally been thought there is not much mixing of the troposphere and stratosphere. But there is recent evidence of transport of stratospheric ozone into the troposphere (see "Ozone-rich transients in the upper equatorial Atlantic troposphere," by Suhre et al., Nature , Vol. 388, 14 August 1997, pages 661-663, and the related discussion paper, "Ozone clouds over the Atlantic," by Crutzen and Lawrence, on pages 625-626 in the same issue of Nature ). So ozone depletion in the stratosphere could result in reduced concentrations of this greenhouse gas in the troposphere. Conversely, global climate change could also affect ozone depletion through changes in stratospheric temperature and water vapor (see "The effect of climate change on ozone depletion through changes in stratospheric water vapour," by Kirk-Davidoff et al., Nature, Vol. 402, 25 November 1999, pages 399-401). [RMC]

Q. Should we be concerned with human breathing as a source of CO2?

A. No. While people do exhale carbon dioxide (the rate is approximately 1 kg per day, and it depends strongly on the person's activity level), this carbon dioxide includes carbon that was originally taken out of the carbon dioxide in the air by plants through photosynthesis - whether you eat the plants directly or animals that eat the plants. Thus, there is a closed loop, with no net addition to the atmosphere. Of course, the agriculture, food processing, and marketing industries use energy (in many cases based on the combustion of fossil fuels), but their emissions of carbon dioxide are captured in our estimates as emissions from solid, liquid, or gaseous fuels.

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 Q. How does the oxygen cycle relate to the greenhouse effect and global warming?

A. With recent developments it is now feasible to measure variations in the oxygen content of the atmosphere at the parts per million (ppm) level. Regular measurements of changes in atmospheric oxygen (O2) are currently being made at a number of locations around the world using two independent techniques, one based on interferometry and one based on stable isotope mass spectroscopy. Oxygen measurements can inform us about fundamental aspects of the global carbon cycle. Oxygen is generated by green plants in photosynthesis and converted to carbon dioxide (CO2) in animal and human respiration. Carbon dioxide is the greenhouse gas of most concern due to its abundance in the atmosphere (~ 360 ppm) and anthropogenic sources. Variations in atmospheric O2 are controlled largely by fluxes of carbon (e.g., photosynthesis and respiration CO2 + H2O <=> CH2O + O2).

For further reading, we suggest:

Keeling, R.F., D.A. Najjar, M.L. Bender, P.P. Tans. 1993. What Atmospheric Oxygen Measurements Can Tell Us About The Global Carbon Cycle. Global Biogeochemical Cycles 7:37-67.

Moore, B. III, and B.H. Braswell. 1994. The lifetime of excess atmospheric carbon dioxide. Global Biogeochemical Cycles 8:23-38.

Keeling, R.F. and S.R. Shertz. 1992. Seasonal and interannual variations in atmospheric oxygen and implications for the global carbon cycle. Nature 358:723-727.

Broecker, W. and J.P.Severinghaus. 1992. Diminishing Oxygen. Nature 358:710-711. Back to top 

 Q. How long does it take for the oceans and terrestrial biosphere to take up carbon after it is burned?

A. With over 800 billion metric tons of carbon in the atmosphere and an annual exchange with the biosphere and oceans equal to around 200 billion metric tons, an average atom of carbon spends only about 4 years in the atmosphere before it goes into the oceans or the terrestrial biosphere. We can think of this as the average residence time for a carbon atom in the atmosphere. However, the oceans and terrestrial biosphere not only take up carbon from the atmosphere (e.g., absorption by the oceans and photosynthesis by plants) but they also give it back (e.g., emission from oceans and respiration by animals). That is, most of these carbon atoms are “recycled” so the atmosphere is not entirely rid of them. The time it takes for a carbon atom to make it out of this recycling system and to get into the deep ocean is about 100 years. The figure below, provided by Ken Caldiera of the Carnegie Institution for Science, shows how an instantaneous doubling of pre-industrial carbon dioxide (from 280 parts per million to 560 parts per million) would be removed from the atmosphere-biosphere system. About 50% of the added CO2 would be removed after about 200 years and about 80% of it would be removed after about 1000 years, but complete removal of the remaining 20% to the deep ocean and carbonate rocks would have to rely on geological processes operating over much longer time periods.

 Q. How much CO2 is emitted as a result of my using specific electrical appliances?

A. For this answer, we refer you to an excellent article, "Your Contribution to Global Warming," by George Barnwell, which appeared on p. 53 of the February-March 1990 issue of National Wildlife, the magazine of the National Wildlife Federation. The article, assuming that your electricity comes from coal, calculates CO2 emissions corresponding to the use of various electrical appliances. For example, one hour's use of a color television produces 0.64 pounds (lb) of CO2, and each use of a toaster produces 0.12 lbCO2, whereas a day's use of a waterbed heater produces 24 lb CO2.

In general, the coefficient is about 2.3 lb CO2 per kilowatt-hour (kWh) of electricity. You can calculate the kWh of electricity by multiplying the number of watts (W) the appliance uses times the number of hours (h) it is used, then dividing by 1000. For example a 60-W light bulb operated for 24 h uses (60 W) x (24 h) / (1000) = 1.44 kWh.

This use of electricity would produce an emission of (1.44 kWh) x (2.3 lb CO2 per kWh) = 3.3 lb CO2 if the electricity is derived from the combustion of coal.

 Q. Why do certain compounds, such as carbon dioxide, absorb and emit infrared energy?

A. Molecules can absorb and emit three kinds of energy: energy from the excitation of electrons, energy from rotational motion, and energy from vibrational motion. The first kind of energy is also exhibited by atoms, but the second and third are restricted to molecules. A molecule can rotate about its center of gravity (there are three mutually perpendicular axes through the center of gravity). Vibrational energy is gained and lost as the bonds between atoms, which may be thought of as springs, expand and contract and bend. The three kinds of energy are associated with different portions of the spectrum: electronic energy is typically in the visible and ultraviolet portions of the spectrum (for example, wavelength of 1 micrometer, vibrational energy in the near infrared and infrared (for example, wavelength of 3 micrometers), and rotational energy in the far infrared to microwave (for example, wavelength of 100 micrometers). The specific wavelength of absorption and emission depends on the type of bond and the type of group of atoms within a molecule. Thus, the stretching of the C-H bond in the CH2 and CH3 groups involves infrared energy with a wavelength of 3.3-3.4 micrometers. What makes certain gases, such as carbon dioxide, act as "greenhouse" gases is that they happen to have vibrational modes that absorb energy in the infrared wavelengths at which the earth radiates energy to space. In fact, the measured "peaks" of infrared absorbance are often broadened because of the overlap of several electronic, rotational, and vibrational energies from the several-to-many atoms and interatomic bonds in the molecules. (Information from "Basic Principles of Chemistry" by Harry B. Gray and Gilbert P. Haight, Jr., published 1967 by W. A. Benjamin, Inc., New York and Amsterdam)

Q. Is it possible to separate the carbon and oxygen from CO2 as is possible with other molecules?

A. The problem in separating the carbon and oxygen from CO2 is that CO2 is a VERY stable molecule, because of the bonds that hold the carbon and oxygen together, and it takes a lot of energy to separate them. Most schemes being considered now involve conversion to liquid or solids. One present concept for capturing CO2, such as from flue gases of boilers, involves chemical reaction with MEA (monoethanol amine). Other techniques include physical absorption; chemical reaction to methanol, polymers and copolymers, aromatic carboxylic acid, or urea; and reaction in plant photosynthetic systems (or synthetic versions thereof). Overcoming energetic hurdles is a major challenge; if the energy needed to drive these reactions comes from burning of fossil fuels, there may not be an overall gain. One aspect of the current research is the use of catalysts to promote the reactions. (In green plants, of course, chlorophyll is such a catalyst!) One area of current research is looking at using cellular components to imitate photosynthesis on an industrial scale. For example, see which describes research of the Chemical Technology Division at Oak Ridge National Laboratory.

The International Energy Agency's Greenhouse Gas R&D Programme has many activities in the area of separation and sequestration of CO2 - see their web site ( [RMC]

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 Q. I am curious about the global warming potential of water vapor. Do you know if estimates are done of this in the same way as global warming potentials are calculated for other greenhouse gases? I am also interested in why no mention is ever made of the enhanced greenhouse effect caused by anthropogenic emissions of water vapor. Are the anthropogenic emissions not significant?

A. Water vapor is indeed a very potent "greenhouse" gas, in terms of its absorbing and re-radiating outgoing infrared radiation. It is commonly not mentioned as an important factor in global warming, because it is not clear that the atmospheric concentration (as compared with CO2, methane, etc.) is rising. Some (Richard Lindzen at MIT, prominently) have argued that the uncertain potential feedbacks involving water vapor represent a serious shortcoming in models of climate warming. See the following online resource for a good discussion of this issue:

Q. Is there environmental impact/concern (greenhouse emissions) associated with technologies using CO2 (e.g. dry ice blasting, supercritical cleaning, painting, etc.,). If so, are there current or impending regulation specific to their use?

A. "Most of the CO2 used in these kinds of applications is recovered from processes like fermentation and it is either CO2 that it is being extracted from the atmosphere by plants or CO2 that would have been released from fossil fuel burning anyhow. In essence it passes through this kind of use rather than being emitted immediately and there is no extra CO2 produced".

 Q. Could you tell me, please, if I have 1 gallon of fuel in my car, how many (units?) of CO2 will be emitted? Is there any difference if the car 4 or 6 or 8 cylinders or in respect of horse power in percentage?

A. A good estimate is that you will discharge 19.6 pounds of CO2 from burning 1 gallon of gasoline. This does not depend on the power or configuration of the engine but depends only on the chemistry of the fuel. Of course if the car gets more miles per gallon of gasoline, you will get less CO2 per unit of service rendered (that is, less CO2 per mile traveled).[GM]

The U.S. Department of Energy and the U.S. Environmental Protection Agency recently launched a new Fuel Economy Web Site designed to help the public factor energy efficiency into their car buying decisions. This site offers information on the connection between fuel economy, advanced technology, and the environment.

 Q. How much CO2 do you get from combustion of fossil fuels? How can the mass of the CO2 be greater than the mass of the fuel burned?

A. Let us illustrate with the combustion of natural gas (methane).

C = carbon, atomic weight approximately 12
H = hydrogen, atomic weight approximately 1
O = oxygen, atomic weight approximately 16

CH4 = methane, molecular weight approximately 16
O2 = molecular oxygen, molecular weight approximately 32
CO2 = carbon dioxide, molecular weight approximately 44
H2O = water, molecular weight approximately 18

For combustion of methane

CH4 + 2O2 = CO2 + 2H2O

So, combustion of 16 mass units (grams, pounds, whatever) of methane produces 44 mass units of carbon dioxide and 36 mass units of water while consuming 64 mass units of oxygen. [GM]

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Q. Is there any ONE person who discovered global warming? If not, what year was global warming discovered?.

A. The first person to have predicted that emissions of carbon dioxide from the burning of fossil fuels would cause a global warming is considered to be S. Arrhenius, who published in 1896 the paper "On the influence of carbonic acid in the air upon the temperature of the ground." That atmospheric carbon dioxide was actually increasing was confirmed beginning in the 1930s, and convincingly so beginning in the late 1950s when highly accurate measurement techniques were developed (the most famous demonstration of this is in C.D. Keeling's record at Mauna Loa, Hawaii). By the 1990s, it was widely accepted (but not unanimously so) that the Earth's surface air temperature had warmed over the past century. An ongoing debate is whether such a warming can, in fact, be attributed to increasing carbon dioxide in the atmosphere.

 Q. Where may I obtain information on the properties of CO2?

A. National Institute of Standards and Technology's web site

 Q. I would like to know whether or not significant amounts of soil organic matter (SOM) and freshly fallen litter in a forest ecosystem can be degraded ABIOTICALLY (i.e., through chemical or physical processes) and consequently generate CO2. In other words, can I consider that entire CO2 emitted from soil is derived from biological processes?

A. Earliest quantitative measurements indicate that decomposition can be entirely attributed to biological processes.

SHANKS, R. E., and J. S. OLSON. 1961. First-year breakdown of leaf litter in southern Appalachian forests. Science 134(3473):194-195.

OLSON, J. S. 1963. Energy storage and the balance of producers and decomposers in ecological systems. Ecology 44(2):322-331.

There may be some photo-oxidation but it is likely to be minor. For a more modern treatment (but largely based on the model develobet over 30 years ago by Olson) see:

Bosatta, E. and Agren, G.I. 1985. Theoretical analysis of decomposition of heterogeneous substrates. Soil Biology and Biochemistry 17:601-610.

Bosatta, E. and Agren, G.I. 1995. The power and reactive continuum models as particular cases of the q-theory of organic matter dynamics. Geochemica et Cosmochimica Acta 59:3833-3835.

Agren, G.I. and Bosatta, E. 1996. Theoretical Ecosystem Ecology. Cambridge University Press. 

 Q. I understand that atmospheric concentrations of CO2 are increasing, but when I look at a graph (for example, Keeling's Mauna Loa data), the curve is squiggly. For half of each year, the concentrations increases, and for the other half it decreases. What is the reason for this?

A. The variations within each year are the result of the annual cycles of photosynthesis and respiration. Photosynthesis, in which plants take up carbon dioxide from the atmosphere and release oxygen, dominates during the warmer part of the year; respiration, by which plants and animals take up oxygen and release carbon dioxide, occurs all the time but dominates during the colder part of the year. Overall, then, carbon dioxide in the atmosphere decreases during the growing season and increases during the rest of the year. Because the seasons in the northern and southern hemispheres are opposite, carbon dioxide in the atmosphere is increasing in the north while decreasing in the south, and vice versa. The magnitude of this cycle is strongest nearer the poles and approaches zero towards the Equator, where it reverses sign. The cycle is more pronounced in the northern hemisphere (which has relatively more land mass and terrestrial vegetation) than in the southern hemisphere (which is more dominated by oceans). The Carbon Cycle Group of the NOAA Climate Monitoring and Diagnostics Laboratory (CMDL), has an excellent 3-dimensional illustration of how atmospheric CO2 varies with time year, season, and latitude.

Q. How may I perform CO2 calculations of the carbon dioxide system in seawater?

A. The Program Developed for CO2 System Calculations (ORNL/CDIAC-105), recently released by Ernie Lewis, Department of Applied Science, Brookhaven National Laboratory, and Doug Wallace, Abteilung Meereschemie, Institut fuer Meereskunde, was developed to help calculate inorganic carbon speciation in seawater.

This program, CO2SYS, performs calculations relating parameters of the carbon dioxide system in seawater and freshwater by using two of the four measurable parameters of the CO2 system [total alkalinity (TA), total inorganic CO2 (TCO2), pH, and either fugacity (fCO2) or partial pressure of CO2 (pCO2)] to calculate the other two parameters at a set of input conditions (temperature and pressure) and a set of output conditions chosen by the user.

(Please follow the original ORNL source of the above FAQs for updates. 

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