What is a Carbon Footprint

by Stephen Shafer on December 2, 2017


Part 1.  What is a “carbon footprint”  and how do we use it?   First in the series,  directly below   

Part 2.  Comparing the carbon footprints  of world and American agriculture.      

Part 3.  Fossil Fuels  in the Carbon Footprint of American Agriculture 

Part 4.   Carbon Tax 

Part 5.  Carbon foodprints in American agriculture

Part 6. Carbon-sequestration and storage-in-soil/






photo credit James Shupp

Abstract  I define the terms “carbon footprint,” “greenhouse gas,”  “global warming potential”  and “CO2 equivalent.”   An   illustration of  two different imaginary  activities  with the same carbon footprint in metric tons CO2-e/yr shows the use  of breaking down a carbon footprint for an activity or sector  to see the relative contribution of the three major greenhouse gases to the whole.

Introduction   What is a “carbon footprint”  and  how do we use it?         

            Knowing the  “carbon footprint”  of  an activity helps to see how it might be shrunk to reduce the pace of global warming.   It also predicts  the impact on that activity  of a proposed  fee on carbonaceous raw materials like oil.  I will delineate the carbon footprint of  agriculture in the USA after pinning down   four  terms not always sharply defined.  “Carbon footprint”   tallies  emissions of  “greenhouse gases.”  Its magnitude  is typically  expressed as  the gases’  joint  effect of heat-trapping,  transformed into  a  mass  called “CO2-equivalent.”  The value of  CO2-e  is  derived from  Global Warming Potentials.  A few words on these terms.      

            A carbon footprint  belongs to  a specified   geophysical or operational unit of observation that   can be big (e.g.  Planet Earth,  a nation, a sector of the economy, an industry, a city) or  be a small business or a single household.  It may be a quantity like a ton of wheat.  The footprint  represents the heat-trapping effect  of  the combined releases of greenhouse gases (GHGs) to the air from producing or operating that unit.  GHGs   in the atmosphere  prevent re-radiation of  infra-red energy from our planet into space.  They act as greenhouse glass does to trap solar heat, warming the air inside;  thus the name “greenhouse gases.”  The principal GHGs  are as follows in order of abundance:  Water vapor (H2O), Carbon dioxide (CO2), Methane (CH4),   Nitrous Oxide (N2O),   Ozone (O3),  and  Chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), sulfur hexafluoride (SF6), hydrochlorofluorocarbons (HCFCs), perfluorocarbons                                                 

            As is recognized by nearly all scientists,  GHGs  have been accumulating in the atmosphere in the last several hundred years.  Their growing presence threatens the health of   terrestrial life.  For more background, see Appendix 1  in the <click here to read more> section at the end of this text.

          The greenhouse gases most germane to “carbon footprint” are carbon dioxide (CO2), methane  (CH4) and nitrous oxide (N2O).  Water vapor causes up to  60% of  heat-trapping in the atmosphere but is a passive vehicle driven by warming due to the other three gases and will not be considered further here.   Each of the other three has a characteristic capacity to block infra-red radiation (IR) from escaping back into space..  Take methane: averaged over a 100 year span, a kg of methane released this year will stop  about  34  times as much outbound infra-red radiation (IR) as a kg of CO2 emitted this  year.  Over that same span, a kg of N2O will prevent the escape of about 300 times as much  IR as a kg of CO2.  A  ratio like 34:1 or 300:1, simplified to a value like 34 or 300,  is the  Global Warming Potential (GWP) of that gas.

          Multiplying the  emitted  physical tonnage  of  each  of  the  three major greenhouse gases by its global warming potential,  then summing the products,  yields  “CO2 equivalent (abbreviated CO2-e)”  in which a carbon footprint is typically expressed. 

            For example, imagine facility A that releases one metric ton  (mt)/yr  of CO2 +  .04 mt/yr  of methane  (CH4) + .01  mt/yr of nitrous oxide (N2O).    Its  carbon footprint is   1*1 + .04*34 + .01 + 298 = 1 + 1.36 + 2.98 = 5.34  metric tons of CO2-e/yr. 

            Now picture facility B  with annual releases as shown in Table 1. 1

Facility       CO2  mt/y                      CH4 mt/y       N2O mt/y       CO2-e mt/y

A                     1                                  .04                   .01                   5.34

B                     4.87                             .01                   .001                 5.34    

Table 1.1   Calculating the carbon footprint for two imaginary facilities

            Both A and B have  a carbon footprint of 5.34 mt CO2-e/yr.    When each tries to lower its footprint by (say) 20% in two years, they will need different approaches.   A  carbon tax would hit B much harder.  Today there is no plan  in any carbon tax proposal to fee methane releases from agriculture, nor  nitrous oxide releases related to synthetic nitrogen fertilizers.

            Earlier I  noted  there  is no universal and precise definition of the “carbon footprint”  that fits every  socio-economic or geophysical unit.  For example, a personal calculator from Great Britain strongly emphasizes fossil fuel burning (a CO2 emitter)  in transportation and electricity generation, omitting CH4 and N2O.

          Indirect  emissions of  all  GHGs are usually ignored in household and small business calculators.  An eye-opening report from MIT  concluded, however,  that when  indirect emissions from municipal, state and federal services are counted, the carbon footprint of  the most sedentary and ascetic person in the US is double the global average of 4 mt  CO2-e/yr.   Some household calculators allow carbon offsets for persons who own trees or have planted them to buy offsets; some do not.

            Thus the carbon footprint   for  any unit of observation from nation/yr to a package of food  will depend heavily on the boundaries of  the analysis.  We will see in a later  essay of this series that much of the fossil fuel burnt by farmers and ranchers directly  is not charged in macroanalysis to “agriculture,” but to “transportation.”  Nor are emissions from fossil fuels combusted to  generate  electricity used on ranches and farms charged  to “agriculture.”  Even when these sources of CO2 release are reclassified, however, the heat-trapping effect of  actual CO2 from agriculture is not much compared to that of CH4 and N2O.  IT is more like facility A above.

           “Carbon footprint”  is an odd  term  in that nitrous oxide, an important  component of  agriculture’s carbon footprint, has no carbon.  The term  denotes  a quantity that could be more accurately,  though  less understandably,   be called  “contribution to radiative forcing.”

            The next essay, second  in the series,  looks at the  carbon footprint of agriculture in the world and in the USA.

Click here to continue to Appendix

Appendix 1

            Some greenhouse gases,  like the chlorofluorocarbons and hydrofluorocarbons, are entirely made by human activity.  The others have been in the atmosphere for millions of years but owing to human activities are now more prevalent than before the year 1750.   Carbon dioxide  has risen in a nearly straight line by 44% since 1750. Methane levels rose 13% 1749-1852; by another 40% over the next century, and by another 62% from 1950 to 2015. Nitrous oxide levels, stable for centuries,  climbed  7% between 1745 and 1954,  then by another 14% to 2015.     Ozone is the exception.  It was being depleted by the scarce super-potent GHG  chlorofluorocarbons  until the downslope was halted (to wide relief) by their decreased use in the 1980s.   The sixth group includes several score gases,  scarce even put together, all dating from the past hundred years.  They were used in aerosols, refrigerants and fire-extinguishers.  Their use is on the way out, but most will persist a long time.

         Excluding water vapor, the “greenhouse gases” comprise altogether  less than a tenth of one per cent of all gases in the atmosphere, in which nitrogen (N2) is the largest component (roughly 78%).  Oxygen (O2) makes up roughly 20%, and argon about 0.9% .  Water vapor varies from one to four per cent, depending on factors like season and latitude.     It may seem surprising that 0.1% (one part per thousand) can do so much harm, but many biological poisons are lethal at much lower concentrations.

            The greenhouse gases most pertinent to “carbon footprint” are carbon dioxide (CO2), methane  (CH4) and nitrous oxide (N2O).  Each of these has a characteristic capacity to block infra-red radiation from escaping back into space, trapping heat in the atmosphere.  Take methane: averaged over a 100 year span, a kg of methane released this year will stop  25-35 times as much outbound infra-red radiation (IR) as a kg of CO2 emitted this  year.  Over that same span, a kg of N2O will prevent the escape of about 300 times as much   (IR) radiation as a kg of CO2.

            The ratio of  IR blocking ability of a GHG  to that of CO2 is that GHG’s Global Warming Potential (GWP).  Figures for GWP are not set in stone.  See Table 1.2.

                                                100-yr GWP 4th AR IPCC          100-yr GWP 5th AR IPCC

  • Water vapor H2O                       varies                                                        varies
  • Carbon dioxide CO2                              1                                                         1
  • Methane CH4                                       25                                                        34
  • Nitrous Oxide N2O                            298                                                      298
  • Ozone O3                                             ?                                                           ?
  • Halocarbons et al                  many in thousands                          many in thousands

Table 1.2.   Global Warming Potentials (GWPs)  for  greenhouse gases  presented in two different reports by the International  Panel on Climate Change (IPCC).          






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