Attempts to quantify methane emissions from wild ruminants have been made in the past. Crutzen et al. (1986), for example, estimated that wild ruminants produce about 0.37 Tg/yr (1 teragram = 1012 grams) of methane. McAllister et al. (1996) estimated wild ruminants (bison, elk, caribou, deer, sheep) in Canada alone produce 0.15 Tg/yr, which on average comes to about 49 g of methane (or about 1 kg of CO2 Eq.; the global warming potential of GHG is expressed as CO2 equivalents)/animal/d, a figure close to the 41 g (or 861 g of CO2 Eq.)/animal/d estimated for deer (various species of deer and caribou) by Crutzen et al. (1986). For comparison, the average U.S. car emitted between 15 and 22 kg of CO2/d in 2010 (http://www.fueleconomy.gov/feg/pdfs/guides/FEG2010.pdf).
There are several challenges in calculating methane emissions from wild ruminants (or any wild animal): (1) the population size is not exactly known and (2) the emission per animal or per unit of feed intake is also not exactly known. The reasons for #1 are obvious. The uncertainty about emissions per animal or unit of feed intake stems from uncertainties about: (1) the type of feed consumed by the wild animal, (2) the amount of methane produced per unit of these feeds, and (3) the daily amount of feed consumed by the animal. All these combined make the estimations of methane production from wild ruminants only an approximation. For example, in the study that most closely resembled a scientific effort to estimate methane emissions from bison, elk, and deer, that of Galbraith et al. (1998), the animals were fed sun-cured alfalfa pellets. This kind of diet, of course, is not even close to the natural diet of these species in the wild. In addition, in this study methane production was measured in chambers, the effect of which on a wild animal behavior and metabolism is unknown.
The analysis we present here is not an exception and we had to make several assumptions and approximations to be able to compute wild ruminants’ emissions for the United States in the past and present. Table 1 footnotes summarize most of these assumptions. For example, historical population numbers are approximations, at best. Present day populations are known with a much greater degree of certainty and still are approximations. Historical (pre-European settlement) population estimates were mostly from the 2003 edition of the Feldhamer et al. book “Wild Mammals of North America” (The John Hopkins University Press, Baltimore, MD) - in our humble opinion, an excellent, encyclopedic review of the biology of North American mammals.