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 This may be due to outdated information used to develop these emissions factors.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.
Generated Summary
This research presents a study focused on revising methane (CH4) emission factors and estimating spatially distributed annual carbon fluxes for global livestock. The study updates information for cattle and swine by region, incorporating changes in animal body mass, feed quality and quantity, milk productivity, and management of animals and manure. Using these new emission factors, the study estimates global livestock emissions and analyzes the dynamics and impacts of these emissions in relation to global and regional trends. The research utilizes a bottom-up approach, employing IPCC 2006 guidelines but updating input data to reflect more recent changes in livestock management and production. The scope includes the development of revised emissions factors, estimation of global and regional livestock carbon (C) fluxes, and downscaling and spatial distribution of these fluxes.
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.
- The increase also encompasses a 36.7% increase in manure management methane.
- Revised manure management methane emissions for 2011 in the US increased by 71.8%.
- Using the revised emissions factors, 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.
- Revised enteric fermentation emissions factors for dairy cows range from 7% smaller (E. Europe and W. and Central Asia) to 125% larger (E. and S.E. Asia) than 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.
- Revised global total CH4 C emission quantities for 2011 are 89.4 ± 13.7 Tg C (119.1 ± 18.2 Tg CH4), an increase of 11% over estimates made using IPCC 2006 emissions factors.
- This change encompasses an 8.4% increase in enteric fermentation CH4 C and a 36.7% increase in manure management CH4 C.
- In the US and Canada, where 2011 total livestock emissions were 24.2% greater than when calculated with IPCC 2006 emissions factors, including a 12.3% increase in enteric fermentation CH4 C and a 71.8% increase in manure management CH4 C.
- Total livestock CH4 emissions account for ca. 3% of total livestock C fluxes.
- The revised percent of global livestock intake supplied by forage was 58.4% in 2011.
- Annual global populations of most livestock species did not change greatly over the years between 1990 and 2013, except for goats and chickens.
- Revised total livestock methane emissions for 2011 were 11% larger than estimates made using IPCC 2006 emissions factors.
Other Important Findings
- The study’s revised bottom-up estimates of global livestock methane emissions are comparable to recent top-down global estimates.
- The 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.
- The revised emissions 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.
- The study’s findings highlight important recent changes in regional livestock systems, with mature dairy cattle body mass and milk productivity greater in all global regions than IPCC 2006 default values.
- Meat/other cattle populations increased dramatically in Latin America during the early 2000s, and swine population in E. and S.E. Asia has continued to increase.
- The largest increases in livestock CH4 emissions over time are between 30N and the equator (Northern tropics).
- The study shows that livestock feed intake increased in the US and Canada in 1995 and after 2009, possibly related to drought and increased use of corn for bioethanol production.
- The study emphasizes that in the US and Canada, the percentage of intake from forage increased sharply in 1995 and again after 2009.
Limitations Noted in the Document
- The revised per-animal emissions factors and/or total CH4 emissions reported may differ from recent national self-reported emissions factors due to several factors.
- These factors include the inclusion of dairy calves and heifers with mature dairy cow populations.
- Interannual and sub-regional variation in diet and other factors were not fully accounted for.
- The switch from IPCC 2006 was made linearly over a long time period (1990-2012) because information about their temporal dynamics was lacking.
- Variability at subregional and interannual scales are embedded in the estimates, and 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 CH4 were not revised in this study, but they should be reevaluated in future work.
- The study’s focus on regional-level calculations may not accurately represent emissions at local scales.
Conclusion
The study revises livestock methane emissions factors and estimates spatially distributed annual carbon fluxes. The revised estimates of global livestock methane emissions for 2011 are 11% higher than those based on IPCC 2006 guidelines. The study highlights the importance of recent changes in livestock systems, particularly in mature dairy cattle and swine populations. The study’s findings support the idea that livestock CH4 emissions, while not the dominant overall source of global CH4 emissions, may be a major contributor to the observed increases in emissions in the 2000s to 2010s. The study emphasizes the need for accurate, up-to-date estimates of livestock emissions at national, regional, and global levels. The study also suggests that the largest increases in livestock CH4 emissions over time are between 30N and the equator (Northern tropics). The study notes that the revised estimates allow for improved reconciliation of top-down and bottom-up estimates of methane emissions, and can be used to improve Earth system models and environmental assessments. The need for further research on per-animal emissions and the impact of local conditions on methane emissions is also highlighted. The study highlights the increasing contribution of livestock to global methane emissions and calls for more detailed and localized assessments.