Comparing carbon footprints of world and American agriculture

by Stephen Shafer on December 2, 2017








June grazing: photo by Stephen Shafer   

Abstract In the  world, and in the USA, crop and livestock agriculture  accounts for about 10% of total greenhouse gas emissions.  In both spheres, fossil fuel combustion (FFC)    is usually tagged with only  9-12% of agricultural GHG releases.  Ag-related sources not tabulated as “agriculture” when so treated  raise that proportion in America from 9% to > 19%.   Still, in  the USA,  nitrous oxide and  methane far outweigh carbon dioxide from FFC  in the carbon footprint of agriculture.   Suppose fossil fuel combustion could by a miracle be banished  from agriculture, lowering GHG releases by 120 M mt CO2-e/yr.   That would not make as much improvement  to  global warming  as agriculture could effect by lowering emissions by 60 M mt CO2-e/yr   and simultaneously  sequestering much more than 120 M mt CO2-e/yr in soil  through low-tech methods that ease off on fossil fuels, reduce nitrogen needs  and improve food security.

Introduction  The   first essay  of this  series on “carbon farming” covered  greenhouse gases (GHGs) and  carbon footprints.  In the immediate future every sector of the American and of the world economy must try strenuously  in good faith to lower its “carbon footprint.”  To see  how American agriculture will  do our part  it helps to profile GHG emissions in that sector. first  compared to those of the world,  then homing in on the USA.

            GHG by economic sector, whole world  In the whole world the agriculture sector (in Graph 1 called Ag & land use and in the source document called Agriculture Forestry and Other Land Use or AFOLU)  accounted as of  2010  for about 11.8 billion metric tons  (Gt)/yr of  CO2-equivalents,  24% of the global total.   “Crop and livestock agriculture”  by itself, however, accounted for only 6 Gt CO2-e,  ~ 12% of the global total of 49 billion from all sectors. 


Graph 2. 1  GHG releases 2010 whole world by economic sector in billions of metric tons CO2-equivalents (Gt CO2-e)   Source 

Note:  I made some minor changes of the values for residential and commercial, but their sum is not changed.     

          GHG by economic sector USA  In the USA,  the agriculture sector (i.e. crop and livestock) accounts  for only 8.4%  (566 mt CO2-e/6740 mt CO2-e) of  GHG,  less than the  12% that characterizes the world (Graph 2.1, above).   Source is the interactive tool on page one of this epa document  

                As  carbon pricing strategies would hit  agriculture with  rising prices for motor fuels,  I  decided to count in fossil fuel combustion in farm-operated vehicles doing farm work off the farm.  [ For methods click on the prompt to "Read More" at end of text. ]   The  modified distribution is depicted in  Graph 2.4 .  The ag sector thus added-to now accounts for 9.4% of the country’s GHG

 Graph 2.4   Greenhouse gas emissions (GHG) in million metric tons (M mt) by economic sector, USA 2014, agricultural* and transportation*  sectors modified to put  off farm  fossil fuel combustion into “agriculture.” original data from 2017 EPA web site          


      Sources of GHG releases in world agriculture

Graph 2. 5 World greenhouse gas releases in crop and livestock agriculture 2010,  by source   Note: figures recalculated from FAO document .   Abbreviations “ferment”= fermentation  “mgt” = management; “Syn fert N” = synthetic fertilizer nitrogen; “fossil fuel” = fossil fuel combustion in agricultural machinery.  Note savannah burning may belong to “Forestry and other land use” rather than crop and livestock, but I left it in. Total releases 6.31 billion metric tons. 

           The striking feature of the world picture (Graph 2. 5, above) is that about 71% of GHG, in CO2-e,  is  methane (CH4)  from animals and paddy rice.  Only 13% is from synthetic fertilizer nitrogen (nitrous oxide = N2O)  and only 13%  is  CO2 from fossil fuel combustion on farms.    In the USA, compared to the world ,  “soil management” (manufacture and application of synthetic nitrogen fertilizers) explains 44% of the national total of CO2-e released. Compare this to 13% in the world (Graph 2.5).  Enteric fermentation is the principal (39%)  source of GHG from world agriculture.  In the USA it ranks  second at 29% to soil management.  Manure management in the USA is just 15% of the total, less than the 22% characterizing world agriculture.   In both the USA (9% by my reckoning)  and the world (13%),  fossil fuel combustion (FFC) in mobile farm machinery seems a relatively small  contributor to agricultural greenhouse gases. 

              FFC in farm vehicles doing farm business on public roads should be counted, however,    when thinking of carbon tax impacts.  Unable to find a figure,  I had to guesstimate one, as mentioned above, and  added 70 M mt/CO2-e/yr.   Graph 2.6 shows the new distribution of GHG sources within Agriculture.  With this modification, fossil fuel combustion as a GHG source within this economic sector goes up from 9.4% to 19%.

Graph 2.6.  GHG emissions by source within Agricultural economic sector USA 2015 not including CO2 emissions from electricity generation or synthetic nitrogen fertilizer manufacture.  The national total is 643 M mt CO2-e/yr including   9 M mt CO2-e/yr from urea and liming.  Note Figures for columns 1,2, 3 and 5 are from  Chapter 5  of  a 633-page epa report.  Figure in col 5 is estimated by me for OFF farm vehicles  (see text) is not from EPA   Figure in column 4 for ON farm machinery   is for 2014. See reference .

Conclusions Because most people think of CO2 first  and of the filling station pump,  the  biggest   impact of a price on carbon  that superficial analysis foresees right away would be higher motor fuel prices.  These   by rights should drive down fossil fuel use and hence CO2 emissions.  Today’s farm directly combusts  a lot of diesel along with natural gas, heating oil  and  gasoline.  Graph 2.6 shows,   however, that even in  the mechanized  USA  fossil fuel combustion as a source of GHG in agriculture   is not much  compared to “soil management” + {enteric fermentation and manure management}.

            These last three GHG sources ( in italics)  account jointly for 92%  of USA agricultural GHG  if fossil fuels  burnt off  the farm are not counted.  Only the first (fossil fuel burning in mobile farm machinery,  with its small 9% share )  is  an obvious  fit into predicting the impact of a carbon tax or fee on farmers.  Other  uses of fossil fuels, shown in bold above,  are ignored.   Reckoning the full size of a carbon tax  impact on farmers and ranchers, however,  means looking beyond motor fuels burnt in the fields  to the other ways that fossil fuels, combusted, underlie the resource-use budgets of ranches and farms.

            Suppose (and this is impossible near-term) American agriculture ended all fossil fuel combustion,  direct and indirect that I have brought in so far.  Reductions in GHG emissions would be relatively small,  maybe 19% of  of 643 M mt CO2-e/yr = 128 M mt CO2-e/yr.   The consequences, absent new carbon-neutral energy sources, would be  awful: farms idled, harvests shrunken.  There is, fortunately,  another way than rapidly ditching fossil fuel combustion for American farmers and ranchers to combat global warming and at the same time  improve our own lot and that of everyone who eats: carbon sequestration in soil and trees.

       The third, next,  essay of this series, “Fossil Fuels  in the Carbon Footprint of American Ag,”    continues the theme that fossil fuel combustion related to agriculture must be lowered in the near future but is not by itself the key to how farming  can help solve global warming and maintain food security.  It will also address  other sources  of fossil fuel combustion related to agriculture that have not come up so far, including electricity generation  and power for the manufacture of synthetic nitrogen fertilizers.


End of text click here to  go to appendix

Appendix on adding a figure made up by me for fossil fuel combustion in farm vehicles off the farm

               Using data from  Chapter 5  of  a 633-page epa report  I moved 70 M mt CO2-e/year  of transportation into agriculture to account [very very roughly] for  GHG releases from fossil fuel combustion  in farm-owned vehicles doing  farm business on public roads.  The national total, excluding territories,  remains 6703 M mt. CO2-e/yr.  The 70 M mt figure assumes that about one fifth of CO2-e emissions from light duty trucks are attributable to such public-road farm activity (e.g. hauling livestock, delivering hay).   In 2014, CO2-e emissions from light duty trucks were 338 Mmt.  See page two of  this reference. The redistributed figures are shown in Graph 2.4  .


Graph 2.4   USA  GHG for 2014. by source,  modified slightly from figures on an EPA web siteFigure in col 5 is estimated by me for OFF farm vehicles  (see text) is not from EPA   Figure in column 4 for ON farm machinery   is for 2014,  See reference . CO2 emissions from urea and liming (9 M mt/yr) omitted for graphic simplicity. Total including the 9 is 643 M mt Co2-e/yr.


Appendix 2   The volume of GHG releases from fossil fuel combustion in mobile equipment on farms  is  not small compared to other sectors of transportation, being slightly more than rail and than a surprising entrant, pipelines.  [I think it’s the compressor stations for natural gas pipelines that generate most of the emissions in that category]

See also


 Supplementary Materials

The excellent paper by Vermeulen et al [Vermeulen Sonja J, Bruce M Campbell, John S. I. Ingram  Climate Change and Food Systems Annual Review Environment and Resources 2012; 37: 195-222 ] places agriculture within food systems states.  The boundary is soft.   “food systems” contributed  in 2008 between 9.8 Gt and 16.9 Gt CO2-e , or 19 to 29% of global anthropogenic greenhouse gas emissions.  “Food systems”  include  

Pre-production with three sub-categories

A. fertilizer manufacture

B. animal feed production

C. pesticide production

Production (two sub-categories)  

     A. direct emissions from agriculture in range of 5 to 6 Gt/yr in 2005

     B. indirect emissions from agriculture,  wide range of 2 to 6.6 Gt    [Indirect emissions from land use dominate emissions in developing countries but are almost nil in US and Canada and in China. See figure 2 of Vermeulen]

Post-production( six subcategories)

A. primary and secondary processing

B. storage, packaging and transport

C. refrigeration  

D. retail activities

E. catering and domestic food management

F. waste disposal

I believe the authors consider the pre-production and production stages to be “agriculture.  They write   “The post-production stages of the food chain collectively emit GHG emissions equal to the production stages in high-income countries (Figure 1), although in middle-income and low-income countries, and hence globally, agriculture is by far the dominant source of emissions. (Table 1) Indirect and direct emissions from agriculture differ markedly in their contribution by region   (Figure 2).” 

<end of Vermuelen sidebar>

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