Abstract
Livestock play an important role in carbon cycling through consumption of biomass and emissions of methane. Recent research suggests that existing bottom-up inventories of livestock methane emissions in the US, such as those made using 2006 IPCC Tier 1 livestock emissions factors, are too low. This may be due to outdated information used to develop these emissions factors. In this study, we update information for cattle and swine by region, based on reported recent changes in animal body mass, feed quality and quantity, milk productivity, and management of animals and manure. We then use this updated information to calculate new livestock methane emissions factors for enteric fermentation in cattle, and for manure management in cattle and swine. Using the new emissions factors, we estimate global livestock emissions of 119.1 ± 18.2 Tg methane in 2011; this quantity is 11% greater than that obtained using the IPCC 2006 emissions factors, encompassing an 8.4% increase in enteric fermentation methane, a 36.7% increase in manure management methane, and notable variability among regions and sources. For example, revised manure management methane emissions for 2011 in the US increased by 71.8%. For years through 2013, we present (a) annual livestock methane emissions, (b) complete annual livestock carbon budgets, including carbon dioxide emissions, and (c) spatial distributions of livestock methane and other carbon fluxes, downscaled to 0.05 × 0.05 degree resolution. Our revised bottom-up estimates of global livestock methane emissions are comparable to recently reported top-down global estimates for recent years, and account for a significant part of the increase in annual methane emissions since 2007. Our results suggest that livestock methane emissions, while not the dominant overall source of global methane emissions, may be a major contributor to the observed annual emissions increases over the 2000s to 2010s. Differences at regional and local scales may help distinguish livestock methane emissions from those of other sectors in future top-down studies. The revised estimates allow improved reconciliation of top-down and bottom-up estimates of methane emissions, will facilitate the development and evaluation of Earth system models, and provide consistent regional and global Tier 1 estimates for environmental assessments.
Generated Summary
This research presents a study on the revision of methane emissions factors for livestock and the spatially distributed annual carbon fluxes. The study revises existing bottom-up inventories of livestock methane emissions, particularly those derived from the IPCC 2006 guidelines, which may underestimate emissions due to outdated information. The methodology involves updating regional information for cattle and swine, based on factors like animal body mass, feed quality, milk productivity, and management practices. The revised data were then used to calculate new livestock methane emissions factors for enteric fermentation in cattle and manure management for both cattle and swine. The scope of the study includes estimating global livestock emissions, annual carbon budgets, and spatial distributions of methane and other carbon fluxes, with a focus on comparing the revised estimates with existing top-down and bottom-up estimates.
Key Findings & Statistics
- The study estimates global livestock emissions of 119.1 ± 18.2 Tg methane in 2011.
- This quantity is 11% greater than that obtained using the IPCC 2006 emissions factors.
- The increase encompasses an 8.4% increase in enteric fermentation methane, a 36.7% increase in manure management methane.
- Revised manure management methane emissions for 2011 in the US increased by 71.8%.
- For the year 2011, total livestock CH4 C emissions were 89.4 ± 13.7 Tg C (119.1 ± 18.2 Tg CH4), an increase of 11% over the estimates made using IPCC 2006 emissions factors.
- Changes in dairy cow manure management emissions factors ranged from a 68% decrease in Oceania to a 158% increase in the US and Canada region.
- Changes in swine manure management emissions factors, relative to IPCC 2006 reported values, range from -4% (W. Europe) to +1800% in Latin America.
- The revised percent of global livestock intake supplied by forage was 58.4% in 2011.
Other Important Findings
- The study’s revised bottom-up estimates of global livestock methane emissions are comparable to recently reported top-down global estimates for recent years.
- The revised estimates account for a significant part of the increase in annual methane emissions since 2007.
- The study highlights the importance of considering recent changes in regional livestock systems, such as changes in dairy cow body mass, milk productivity, and manure management practices.
- The study suggests that livestock methane emissions, while not the dominant overall source of global methane emissions, may be a major contributor to the observed annual emissions increases over the 2000s to 2010s.
- The differences at regional and local scales may help distinguish livestock methane emissions from those of other sectors in future top-down studies.
- Changes in swine manure management emissions factors, relative to IPCC 2006 reported values, range from -4% (W. Europe) to +1800% in Latin America.
Limitations Noted in the Document
- The revised emissions factors were developed for global analyses, and the transition from IPCC 2006 was made linearly over a long time period (1990–2012), which may not fully capture temporal dynamics.
- Variability at subregional and interannual scales are embedded in the estimates.
- The revised emissions factors may not provide the best representation of emissions at local scales and/or for earlier years during the transition.
- Emissions factors for poultry manure management were not revised in this study.
- The study acknowledges that the estimates could be too low due to underreported usage of anaerobic manure treatment lagoons, recent increases in local temperatures, and/or uncertainties in MCF values.
- The study notes that for meat cattle, the weights at the time of slaughter for meat animals are also determined by management decisions, which may vary with changing economic or environmental factors.
Conclusion
The study’s findings underscore the necessity for updated livestock CH4 emissions coefficients in order to achieve more accurate assessments. The study revises emissions factors, reflecting changes in livestock systems such as mature animal weight, feed quality, milk production, and manure management practices, as well as the percentage of animals that are stall-fed. The study’s findings suggest that livestock emissions are a significant contributor to observed emissions increases. The research emphasizes that while the global contribution of livestock to total CH4 emissions is approximately 3%, its contribution to rising atmospheric methane concentrations since 2007 is more considerable, and the new estimates presented offer enhanced capacity to reconcile top-down and bottom-up CH4 production estimates, and enable consistent emission estimations across regional, national and global levels. The need for updated livestock CH4 emissions coefficients is also highlighted. The revised estimates enable improved reconciliation of top-down and bottom-up estimates of methane emissions, will facilitate the development and evaluation of Earth system models, and provide consistent regional and global Tier 1 estimates for environmental assessments.