Kelliher and Clark (2010) came to similar conclusions; these authors estimated that methane emissions from bison in the 10 states encompassing the historical range of this animal were close to the present-day methane emissions by cattle in these states (2.2 vs. 2.5 Tg methane/yr, respectively). It has to be pointed out that our bison population (plains bison) estimates include bison in the Canadian Prairie Provinces (Alberta, Saskatchewan, and Manitoba), which would likely overestimate methane emission for the contiguous United States. According to Reynolds’ distribution map (Reynolds et al., 2003), however, the Canadian portion of the plains bison range was relatively small compared to the distribution range in the contiguous United States.
Overall, methane emissions from bison, elk, and deer in the pre-settlement period in the contiguous United States were about 70% (medium bison population size) of the current emissions from farmed ruminants in the U.S.; data for current (2008) methane and GHG emissions in the U.S. are from the EPA “Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 – 2008” report. If the high bison population estimate is taken for this comparison, wild ruminants in the pre-settlement period emitted as much methane as the current domestic ruminants in the United States. Present-day livestock methane emissions are primarily from cattle; the contribution of sheep and goats to the total emissions is miniscule (about 1%). Estimates for methane emissions from horses (about 0.17 Tg/yr) and swine (about 0.10 Tg/yr) were published by EPA but are not included in this analysis. It is worth mentioning that in this most recent EPA report, GHG emissions from agriculture made up about 6% of the total GHG emission in the U.S. for 2008 (427.5 vs. 6,956.8 Tg CO2 Eq./yr, respectively). Methane from enteric fermentation was 140.8 Tg CO2 Eq./yr, representing 25% of total methane emissions in the U.S. Emissions of methane and nitrous oxide from enteric fermentation and manure management (i.e., the total livestock contribution to GHG emissions) was 202.9 Tg CO2 Eq./yr, or 47% of the agricultural emission, but only 2.9% of all GHG emissions in the U.S.
Present-day methane emissions from wild ruminants (excluding moose, mountain sheep, goat, antelope, and caribou) were also estimated. Population data were from the Feldhamer book (bison and deer), or the RMEF (elk). Body weight, DMI, and methane emission per unit of DMI were the same as for the pre-settlement calculations. With these assumptions, it can be estimated that present-day methane emission from the major wild ruminant species in the U.S. are about 0.23 Tg/yr, which is only 3.6% of the emissions from domestic ruminants. Due to its population size (estimated at 25 million; Miller et al., 2003) the white-tailed deer is the largest present-day wild ruminant contributor to GHG emissions in the contiguous United States.