Meat Eaters Guide: Methodology - Environmental Working Group

d. Sensitivity Analysis: Each crop or animal production system may include up to 30 different inputs/
outputs with specific values. Moreover, it is generally true that there are strong correlations among
some inputs and outputs in any production system (i.e., a change in one might imply changes in others),
but these relationships are not easily quantifiable. Due to lack of data about uncertainties and
correlations, as well as the large number of input/output variables in each system, it has not been
possible or useful to conduct a comprehensive numerical sensitivity analysis on input/output values
for all production systems.
e. Uncertainty Associated with GHG Emissions Agricultural Production Systems and IPCC factors
In general, there is significant variability and uncertainty with respect to greenhouse gas emissions
from agricultural systems. This analysis relies on widely accepted IPCC emission factors for underlying
biochemical processes (such as methane from enteric fermentation and nitrous oxide from fertilizer
application). While these are tailored to specific agricultural systems and conditions, (dry vs.
temperate climates, grass- vs. grain-feed, dry vs. liquid manure storage, etc.), they are based on averages
and, in some cases, very few field measurements – and therefore actual emissions may vary
considerably depending on particular conditions.
Nitrous oxide emissions, in particular, are inherently highly variable and hard to measure with great
certainty, given different microbial, soil and weather conditions. Some have estimated that the nitrous
oxide emission factor could vary by as much as 50 percent. 43 Similarly the CO 2 associated with lime
application (a common feature in soybean production) is also known to have variability as high as 50
percent, according to the IPCC and other studies. 44 Whether this variability could significantly change
our GHG calculation depends on the relative contribution of corn feed (and N 2 0) to the overall footprint.
In the case of chicken, feed (mostly corn) represents 53 percent of pre-farmgate (production
emissions) and about half of those are N 2 0 soil emissions (see section D for details). However, since
feed comprises only 18 percent of total chicken emissions, reducing N 2 0 emissions significantly would
only make a small difference in the overall GHG footprint.
It should be noted that with respect to methane, estimates on feed conversion to methane and methane
emissions from manure tend to be less variable. Therefore, for production systems such as beef,
where methane constitutes the largest emission source, there is greater certainty as to the overall
carbon footprint. The uncertainty associated with GHG emissions from agriculture is the subject of an
ongoing debate and the best that we can do at the moment is rely on the most reasonable estimates
as developed by the IPCC.
D. Modeling Emissions from Meat Production and Consumption
This section describes the essential production, consumption and modeling details and emissions of
selected geographical meat production systems included in EWG Meat Eaters Guide. We provide key
calculations explaining how the waste factor is integrated into the model, as well as the key assumptions
behind choices for determining the edible portion of meat.
Environmental Working Group Meat Eaters Guide: Methodology 2011 21