Abstract:

Hristov et al. (1) argue that our study “pro- vides a comprehensive, quantitative analysis of anthropogenic methane sources,” but that the conclusion “that US EPA [US Environ- mental Protection Agency] estimates for live- stock methane emissions are grossly under- estimated appears to be unsubstantiated by . . . [a] ‘bottom-up’ approach” outlined in their letter.

In this reply, we discuss the information provided by atmospheric methane data about methane emissions, and comment on the chal- lenge of connecting “bottom-up” and “top- down” estimates, a conclusion shared Hristov et al. (1).

Our study (2) used both near-surface and airborne atmospheric measurements of CH4 concentrations to characterize the total mass of methane added to the atmosphere by sur- face emissions, discretized in space and time. We conclude that total United States meth- ane emissions in 2007–2008 were 33.4 ± 1.4 TgC/yr (44.5 TgCH4/yr), 45–57% above the most recent US EPA baseline estimate for those years (3). Furthermore, we estimate “the magnitude of emissions with spatial pat- terns similar to animal husbandry and ma- nure” (2) at 12.7 ± 5.0 TgC/yr (16.9 TgCH4/ yr), 11–156% above baseline EPA estimates for those sectors (best estimate 84% above EPA). Our conclusions are generally consis- tent with previous more limited top-down studies examining total United States (e.g., ref. 4) and regional livestock/manure meth- ane emissions (e.g., ref. 5).

Hristov et al. (1) argue that “the validity of this ‘top-down’ approach can be verified by a relatively simple ‘bottom-up’ method using current livestock inventories and enteric or

manure methane emission factors.” The authors build this estimate for enteric fermen- tation by multiplying the US Department of Agricuture (USDA) livestock inventory esti- mates for 2013 (note that our study covers 2007–2008), by “assumed” feed dry matter intake and “assumed” methane production rates. “With the above assumptions,” Hristov et al. estimate methane emissions from en- teric fermentation comparable to the US EPA’s inventory for 2011. Similarly, the authors use USDA livestock inventories and Intergovernmetal Panel on Climate Change (IPCC) (6) manure methane emissions fac- tors to estimate United States manure emis- sions that are 35% lower than EPA inven- tory numbers.

The estimates of Hristov et al. (1) there- fore require a series of assumptions, for which errors compound as several factors are multiplied and added. Feed matter intake and emission factors both have substantial uncertainties (6), as do the IPCC manure methane emission factors (6). Given these uncertainties, which are inherent in all bot- tom-up inventories, we strongly disagree that “the validity of [our] ‘top-down’ ap- proach can be verified” using the Hristov et al. estimates (1).

The method we applied is especially suited to quantifying large-scale total emissions, and uncertainties increase for sector- and region- specific estimates [as outlined above and in our study (2)]. Even in light of these uncer- tainties, the total emissions with spatial pat- terns consistent with animal husbandry are still likely to be substantially above EPA esti- mates. Conversely, bottom-up inventories are strongest at detailing individual emission

types, but uncertainties compound at larger scales, such as the national scale examined here. This difference is precisely why we ar- gue that careful, detailed assessments are needed to reconcile the emissions clearly vis- ible from atmospheric observations with bot- tom-up emissions inventories. Hristov et al. (1) also note a “need for a detailed inven- tory . . . to more accurately estimate . . . emis- sions.” On this point we strongly agree.

Scot M. Millera,1, Anna M. Michalakb, and Steven C. Wofsya
aDepartment of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138; and bDepartment of Global Ecology, Carnegie Institution for Science, Stanford, CA 94305