Main Greenhouse Gases Notes

Notes for The Main Greenhouse Gases

¹  The atmospheric lifetime of carbon dioxide is difficult to define because it is exchanged with reservoirs having a wide range of turnover times; IPCC 2001, gives a range of 5-200 years. In contrast, most CH4 is removed from the atmosphere by a single process, oxidation by the hydroxyl radical (OH). The atmospheric lifetime of a gas is relatively easy to define when essentially all of its removal from the atmosphere involves a single process. However, some complications still arise. For example, the effect of an increase in atmospheric concentration of CH4 is to reduce the OH concentration, which, in turn, reduces destruction of the additional methane, effectively lengthening its atmospheric lifetime. An opposite sort of feedback applies to N2O: an increase induces chemical reactions leading to an increase in ultraviolet radiation available to photolyze the N2O, thereby shortening its atmospheric lifetime. Such feedbacks are accounted for in the above table.

²  The GWP provides a simple measure of the radiative effects of emissions of various greenhouse gases, integrated over a specified time horizon, relative to CO2 emissions. Unless otherwise indicated, GWP's taken from: IPCC 2001.

³  Following the convention of IPCC (2001), inferred 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.

⁴  For most gases, concentrations for year 2004 are given, as indicated more specifically in the footnotes below. Estimates for 1998, from IPCC (2001), are given for C2F6.  Atmospheric concentrations of some of these gases are not constant throughout the year. Global annual arithmetic averages are given.

⁵  The value given by IPCC 2001, page 185, is 280 ± 10 ppm. This is supported by measurements of CO2 in old, confined, and reasonably well-dated air.  Such air is found in bubbles trapped in annual layers of ice in Antarctica, in sealed brass buttons on old uniforms, airtight bottles of wine of known vintage, etc. Additional support comes from well-dated carbon-isotope signatures, for example, in annual treerings. 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 measurements; this is believed to be because the effects of widespread land clearing are not accounted for.

⁶  Recent CO2 concentration (377.3 ppm) is the average of the 2004 annual values at Barrow, Alaska; Mauna Loa, Hawaii, AmericanSamoa, and the South Pole (one high-latitude and one low-latitude station from each hemisphere).

⁷  Pre-industrial concentrations of CH4 are evident in the "1000-year" ice-core records.  However, those values need to be multiplied by a scaling factor of 1.0119 to make them compatible with the AGAGE measurements of current methane concentrations, which have already been adjusted to the Tohoku University scale.  Therefore, pre-industrial values calculated from the ice-core data have been multiplied by 1.0119 before insertion in the above table.

⁸  The first value represents Mace Head, Ireland, a mid-latitude Northern-Hemisphere site, and the second value represents Cape Grim, Tasmania, a mid-latitude Southern-Hemisphere site. For CH4, these values can be compared with the thousand-year ice-core records from Greenland and Antarctica, respectively, discussed in the preceding footnote. "Current" values given for these gases are annual arithmetic averages based on monthly non-pollution concentrations for year 2004.

⁹  Source: IPCC (2001), The pre-1750 value for N2O is consistent with ice-core records in IPCC2001.  Estimates of "current" (1998) concentrations of C2F6 are based on avariety of sources, including emissions rates and annual growth rates.

Source of graphical information and notes:
Blasing, T.J. ad K. Smith 2006.  "Recent Greenhouse Gas Concentrations."  In Trends: A Compendium of Data on Global Change.  Carbon Dioxide Information Analysis Cetner, Oak Ridge National Laboratory, US Department of Energy, Oak Ridge, TN, USA.  http://cdiac.ornl.gov/pns/current_ghg.html