Recent Greenhouse Gas Concentrations

DOI: 10.3334/CDIAC/atg.032

Updated April 2016

Investigator

T.J. Blasing

Gases typically measured in parts per million (ppm), parts per billion (ppb) or parts per trillion (ppt) are presented separately to facilitate comparison of numbers. Global Warming Potentials (GWPs) and atmospheric lifetimes are from the Intergovernmental Panel on Climate Change (IPCC, 2013, Table 8.A.1), except for the atmospheric lifetime of carbon dioxide (CO₂) which is explained in footnote 4. Additional material on greenhouse gases can be found in CDIAC's Reference Tools. To find out how CFCs, HFCs, HCFCs, and halons are named, see Name that compound: The numbers game for CFCs, HFCs, HCFCs, and Halons. Concentrations given apply to the lower 75-80 percent of the atmosphere, known as the troposphere.

Sources of the current and preindustrial concentrations of the atmospheric gases listed in the table below are given in the footnotes. Investigators at the National Oceanic and Atmospheric Administration have provided the recent concentrations. Much of the data provided results from the work of various investigators at institutions other than CDIAC, and represent considerable effort on their part. We ask as a basic professional courtesy that you acknowledge the primary sources, indicated in the footnotes below, or in the links given in the footnotes. Concentrations of ozone and water vapor are spatially and temporally variable due to their short atmospheric lifetimes. A vertically and horizontally averaged water vapor concentration is about 5,000 ppm. Globally averaged water vapor concentration is difficult to measure precisely because it varies from one place to another and from one season to the next. This precludes a precise determination of changes in water vapor since pre-industrial time. However, a warmer atmosphere will likely contain more water vapor than at present. For a more detailed statement on water vapor from the National Oceanic and Atmospheric Administration, see the "water vapor" page at http://lwf.ncdc.noaa.gov/oa/climate/gases.html

Footnotes

1. Preindustrial (1750) concentrations of CO₂, CH₄, N₂O are taken from Chapter 8.3.2 of IPCC (2013). Global-scale trace-gas concentrations from prior to 1750 are assumed to be practically uninfluenced by human activities such as increasingly specialized agriculture, land clearing, and combustion of fossil fuels. However, effects of agriculture are possibly responsible for the increase in methane concentration around 1800 and perhaps some of the much smaller increases that occurred earlier. See Macfarling-Meure, et al., (2006) for a 2000-year ice-core record CO₂, CH₄ and N₂O concentrations. Preindustrial concentrations of industrially manufactured compounds are given as zero. The short atmospheric lifetime of ozone (hours-days) together with the spatial variability of its sources precludes a globally or vertically homogeneous distribution, so that a fractional unit such as parts per billion would not apply over a range of altitudes or geographical locations. Therefore a different unit is used to integrate the varying concentrations of ozone. The total mass of ozone in the troposphere is estimated in units of teragrams (Tg). A Tg is 10¹² grams, or a million metric tons. Preindustrial and recent O₃ amounts are taken from Chapter 8.2.3.1 of IPCC (2013).
2. Because atmospheric concentrations of most gases tend to vary systematically over the course of a year, figures given represent averages over a specific 12-month period for all gases except ozone (O₃), for which a current tropospheric total amount has been more broadly estimated (IPCC, 2013, page 670). The CO₂ concentration given is the average for year 2015, taken from the National Oceanic and Atmospheric Administration, Earth System Research Laboratory, website maintained by Dr. Pieter Tans. CH₄ concentration is the average of preliminary monthly concentrations, taken from a similar web site http://www.esrl.noaa.gov/gmd/ccgg/trends_ch4/ maintained by Dr. Ed Dlugokencky.
3. Concentrations of the other gaseous species are also from the National Oceanic and Atmospheric Administration (NOAA), courtesy of Dr. Steve Montzka.
4. The Global Warming Potential (GWP) provides a simple measure of the radiative effects of emissions of various greenhouse gases, integrated over a specified time horizon, relative to an equal mass of CO₂ emissions. The GWP with respect to CO₂ is calculated using the formula:
   
   
   where a_i is the instantaneous radiative forcing due to the release of a unit mass of trace gas, i, into the atmosphere, at time TR, C_i is the amount of that unit mass remaining in the atmosphere at time, t, after its release and TH is TR plus the time horizon over which the calculation is performed (100 years in this table). The formula is adapted from page 210 of IPCC (2007). The GWPs given are from Table 8.A.1 of IPCC (2013). The short lifetime of ozone (hours-days) precludes a meaningful calculation of global warming potential on the time horizons (20, 100, and 500 years) listed in IPCC documents.
5. The atmospheric lifetime is used to characterize the decay of an instantaneous pulse input to the atmosphere, and can be likened to the time it takes that pulse input to decay to 0.368 (=1/e) of its original value. The analogy would be strictly correct if every gas decayed according to a simple exponential curve, which is seldom the case. For example, CH₄ is removed from the atmosphere by a single process, oxidation by the hydroxyl radical (OH), but the effect of an increase in atmospheric concentration of CH₄ is to reduce the OH concentration, which, in turn, reduces destruction of additional methane, effectively lengthening its atmospheric lifetime. An opposite kind of feedback may shorten the atmospheric lifetime of N₂O (IPCC 2007, Section 2.10.3). For CO₂ the specification of an atmospheric lifetime is complicated by temporary removal processes which store carbon in the biosphere before it is returned to the atmosphere as CO₂ via respiration or, as a combustion product, in fires. This necessitates complex modeling of the decay curve. Because the modelled decay curve depends on the model used and the assumptions incorporated therein, it is difficult to specify an exact atmospheric lifetime for CO₂. Most estimates fall in the 100-300-year range. The above-described processes are all accounted for in the derivation of the atmospheric lifetimes given in the above table, taken from Table 8.A.1 in IPCC (2013).
6. Changes in radiative forcing since 1750 represent changes in the rate, per square meter, at which energy is supplied to the atmosphere below the stratosphere. Note from Figure TS.6 (top) in the Technical Summary of IPCC (2013) that aerosols frequently have the effect of decreasing this radiative forcing. Energy is measured in Joules; the rate at which it is made available is in Joules/second, or Watts; hence, radiative forcing is measured in Watts per square meter (W/m²). Values for increased radiative forcing are based on the 2015 concentrations and the 1750 concentrations given in the above table. Radiative forcing for tropospheric ozone is taken from the 5^th column of Table 8.6 of IPCC (2013). http://www.climatechange2013.org/images/report/WG1AR5_Chapter08_FINAL.pdf The "current" value in that table refers to a global average. Note, in the row immediately below the number for tropospheric forcing, the stratospheric forcing is given as negative 0.05 W/m². Note also the uncertainty ranges given in the tables. In the above table, radiative forcings of gases expressed in concentrations of parts per trillion apply to the average global concentrations given in the 3^rd column of the table, and are based on the radiative efficiencies given in Table 8.A.1 of IPCC 2013. For these gases, it is assumed that concentrations and radiative forcings prior to 1750 were zero because their only source is manufacture after that time. Calculations for these gases assume that the radiative efficiencies have not changed with time, for these small concentrations (cf. Mitchell 1989). Radiative forcing estimates of one investigator may differ slightly from those of another due to differences in assumed preindustrial values, radiative efficiencies, or values used as recent atmospheric concentrations. For comparison, see the radiative forcings given by the National Oceanic and Atmospheric Administration (NOAA) at NOAAs Annual Greenhouse Gas Index site, which also gives the equations used in the calculations of radiative forcings. For CFC-11 we used the radiative efficiency given by IPCC (2013), Table 8.A.1 (which is 0.26) instead of the value 0.25 used by NOAA in their calculation of the Annual Greenhouse Gas Index.
7. Blasing (1985) gave the range of best estimates of the dry-air mole fraction of CO₂ around year 1800 as between 275 and 285 parts per million. This was drawn from an extensive study of previous work by Gammon et al. (1985), which gave that range from within a broader possible range of 260-285 ppm. IPCC (2013, Technical Summary, page 50) gives a range of 273-283 ppm for year 1750; Chapter 8 of IPCC (2013) indicates a narrower range of 276-280. The Law Dome Ice core record available on the CDIAC web site, indicates a value of 277 for year 1750; IPCC (2013, p. 467) gives 278 ppm. These values are generally consistent with those from Neftel et al. From all this we conclude that estimates of preindustrial concentrations have been robust as new information has been obtained over the last 30 years or more. The slight differences from one person's estimate to the next lead to slight differences in estimated increases in radiative forcing since "preindustrial times" which are now taken as the radiative forcings in year 1750. Evidence of pre-industrial CO₂ concentrations comes from several sources, including concentrations in gases preserved in ice cores (Etheridge et al., 1996, MacFarling-Meure, 2006, Petit et al., 1999), in well-dated carbon-isotope signatures in annual tree rings (Stuiver et al. 1984), and in early measurements (Fonselius et al., 1956). Many of the early measurements indicated higher values, due to proximity of local sources, or large sources; the lowest values are considered representative of atmospheric background concentrationsat the remote locations where they are currently measured (e.g., the South Pole). Estimates of "pre-industrial" CO₂ can also be obtained by first calculating the ratio of the recent atmospheric CO₂ increases to recent fossil-fuel use, and using past records of fossil-fuel use to extrapolate past atmospheric CO₂ concentrations on an annual basis. Estimates of "pre-industrial" CO₂ concentrations obtained in this way are higher than those obtained by more direct indicators; this is believed to be because the effects of widespread land clearing are not accounted for. Ice-core data provide records of earlier concentrations. For over 400,000 years of ice-core record from Vostok, see J. M. Barnola et al. For ice-core records extending 800,000 years back in time, see CDIACs Gateway Page to CO₂ data.
8. Recent CO₂ concentration (399.5 ppm) is the 2015 average taken from globally averaged marine surface data given by the National Oceanic and Atmospheric Administration Earth System Research Laboratory website. Please read the material on that web page and reference Dr. Pieter Tans when citing this average. The oft-cited Mauna Loa average for 2015 is 400.83 ppm, which is a good approximation, although typically higher than the global average given above. Instrument records back to late 1959 are available.
9. Pre-industrial concentrations of CH₄ are evident in the 2000-year records from Law Dome, Antarctica and longer ice-core records found on CDIAC's collection of data access links to atmospheric trace gases. A spline function fit to those (Southern Hemisphere) data gives 697 ppb for year 1750, but this may be lower than the global average if agricultural sources in the Northern Hemisphere were already contributing nontrivially. For graphs of two-thousand-year records of CH₄, CO₂ and N₂O concentrations are found here.(Macfarling-Meure, et al., 2006).
10. The value given for year 1750, obtained from a spline fit to measured values in the ice core record from Law Dome, Antarctica, is 271 ppb.
11. For SF₆ data from January 2004 onward see this ftp area. For data from 1995 through 2004, see the National Oceanic and Atmospheric Administration (NOAA),Halogenated and other Atmospheric Trace Species (HATS). Concentrations of SF₆ from 1970 through 1999, obtained from Antarctic firn air samples, can be found in W. T. Sturges et al.

References

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• Etheridge, D. M., L. P. Steele, R. L. Langenfelds, R. J. Francey, J. M. Barnola, and V. I. Morgan. 1996. Natural and anthropogenic changes in atmospheric CO₂ over the last 1000 years from air in Antarctic ice and firn. J. Geophys. Res. Atmos., 4115–4128.
• Gammon, R.H., E.T. Sundquist and P.J Fraser. 1985. History of carbon dioxide in the atmosphere, pp. 25-62 IN: Atmospheric Carbon Dioxide and the Global Carbon Cycle, J.R. Trabalka, Ed. DOE/ER-239, U.S. Department of Energy, Washington, D.C.
• IPCC (Intergovernmental Panel on Climate Change). 2007. Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Solomon, S., D. Qin, M. Manning, Z. Chen, M,. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge United Kingdom and New York, NY, USA, 996 pp.
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