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Livestock methane emission: measurement methods, inventory estimation, and the global methane cycle

Livestock methane emission: measurement methods, inventory estimation, and the global methane cycle. K eith R. Lassey (k.lassey@niwa.co.nz) NIWA, Wellington, New Zealand. Meeting of the CAgM Expert Team on “Contribution of Agriculture to the State of Climate”

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Livestock methane emission: measurement methods, inventory estimation, and the global methane cycle

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  1. Livestock methane emission: measurement methods,inventory estimation,and the global methane cycle Keith R. Lassey (k.lassey@niwa.co.nz) NIWA, Wellington, New Zealand Meeting of the CAgM Expert Team on“Contribution of Agriculture to the State of Climate” Ottawa, Canada, 27–30 Sep 2004 Work supported by the New Zealand Foundation for Research, Science & Technology

  2. Summary • I shall present: • Climatic role of CH4: a 2-slide primer • 2-slide overview of NZ’s ghg emission profile — the role of agricultural CH4 • Summary of the “SF6 tracer technique” for measuring CH4 from livestock • “Top-down” approaches towards emission verification

  3. Summary, contd • National emission inventories — how can small-scale experiments improve inventory quality? • Extrapolation to global inventory for livestock CH4, and origins of uncertainty • How does this inventory mesh with our knowledge of the global methane budget? Most recent references cited are available as pdf computer files

  4. Why the fuss about methane?

  5. ~2.43 W m-2 forcing from well-mixed greenhouse gases: 60% CO2, 20% CH4, 6% N2O Anthropogenic Radiative Forcing Data source: IPCC Third Assessment Report (TAR), 2001

  6. Corollary: 2.43 W m-2 forcing ~1.2°C rise at steady state What does “Radiative Forcing” mean? • Earth receives approximately 342 W m-2 of solar energy (mean over seasons, surface area) • ~31% reflected away, remaining 235 W m-2 absorbed • Surface emits 390 W m-2 as long-wave radiation, 90% is absorbed & re-emitted by atmospheric greenhouse gases, with 235 W m-2 emergent from top of atmosphere • Radiative forcing is the net increase in emergent radiation that results from a change in atmospheric composition • Consequence: 1 W m-2 extra forcing  ~0.5° mean temp increase

  7. New Zealand's greenhouse gas emission profile

  8. 85% enteric fermentation 96% agric. soils NZ’s CO2-equivalent emission profile, 2001, by gas Data source: NZ Climate Change Office, April 2003

  9. 65% CH4, enteric fermentation 34% N2O, agricultural soils NZ’s CO2-equivalent emission profile, by sector, 2001 Thus, enteric fermentation by itself accounts for one third of NZ’s inventory! Data source: NZ Climate Change Office, April 2003

  10. Measuring CH4 emission from individual animals:the SF6 tracer technique Johnson et al., 1994, Environ. Sci. Technol. 28: 359–362. Lassey et al., 1997, Atmos. Environ. 31: 2905–2914.

  11. inlet Calibrated SF6 permeation tube in rumen Evacuated yoke and capillary. The SF6 Tracer Technique • An evacuated yoke is placed on animal and left for 24 hrs to collect sample. • [CH4] and [SF6] are measured by GC. • Daily CH4 emission is calculated from SF6 release rate and [CH4]/[SF6].

  12. New Zealand Friesian cow being “breathalysed”. The capillary plumbing that delivers samples to the PVC “yoke” is clearly visible.

  13. New Zealand Friesian cow ready to graze.

  14. New Zealand Romney-cross wether being “breathalysed”. The straps around the body hold a bag in place for faeces collection.

  15. Flock of sheep, kitted up for breath sampling and for faeces collection, being introduced to fresh pasture.

  16. The SF6 permeation tube:the key to the technique, with properties that are incompletely understood Lassey et al., 2001, Chemosphere—Glob. Change Sci. 3: 367–376.

  17. Validation of the SF6 technique — compares favourably with: • chamber methodsBoadi et al., 2002, Can J. Anim. Sci. 82: 125–131. • micrometeorological methods Judd et al., 1999, Global Change Biology 5: 647–657.Leuning et al., 1999, Atmos. Environ., 33: 1357–1365.

  18. What information can the SF6 technique supply? Accurate estimates of CH4 emission rates from individual animals in a herd or flock • over times from hours to days • whether grazing or confined

  19. What is needed to get the ‘methane conversion rate’ (Ym)* as used in inventories? We need to measure or estimate the ‘gross energy intake’ (GEI)† for each animal, at same time as CH4 emitted. GEI is difficult to measure reliably for grazing animals. *Ym is the CH4 energy as a percent of GEI (eg, 4–9%) † GEI is related to dry matter intake (DMI), ~18.45 MJ/kg

  20. What techniques are available to determine GEI? • Use an inert ruminal marker of known release rate, and determine its concentration in faecesProblem: release rate is unreliable or variable; marker concentration can vary • Collect total faecal production and identify with indigestible feedProblem: amenable only to male animals; animal has to bear collection bag and load

  21. What techniques are available to determine GEI? — contd • Confine the animals, and analyse feed offered and rejectedProblem: modified feeding behaviour cf ad libitum grazing and feed selection • Estimate GEI using IPCC-like methodologyProblem: need detailed information on each animal and its production; need local validation of the methodology Result: GEI imposes considerable uncertainty on Ym

  22. ‘Top-down’ measurements of agricultural CH4 fluxes

  23. CH4 at each altitude depends upon flux over the “footprint” for that position Wind direction Schematic of upwind-downwind contrasts in methane mixing ratio

  24. Different spatial scales: • Paddock scale (~100m):micrometeorology — downwind mast ~6m high; footprint  paddock • Regional scale (~20–50km):sampling to BL mixing height (~1.5km), regional-scale footprint , complex mesoscale modelling • Sub-Farm scale (~0.5–5 km):intermediate complexity for both sampling and modelling

  25. Problems common to all scales: • Estimating emission fluxes and inferred per-animal emissions to sufficient precision • Does not provide reliable estimates for Ym values to guide inventory compilation

  26. At the regional scale … • The precision of the inferred flux is poor: ±60% @ 95% confidence • not good enough to verify reasonable claimed flux reductions of ~20% • Upwind-downwind profile contrasts increase at lower altitude (as expected) • contrast just gets interesting when plane reaches its lowest altitude!

  27. So … In the NZ context (complex topography, often windy), the intermediate ‘sub-farm’ scale seems a good compromise. But, it is still ‘work in progress’!

  28. Tethersonde measures: • wind speed • wind direction • humidity • temperature • air pressure ( altitude) • & transmits every 10–20 s Sampling tubes attached and carried aloft Sampling from kytoon, Battersea, Wairarapa, 27 Jun 2003

  29. Tethersonde receiver and mast, Battersea, Wairarapa, 27 Jun 2003

  30. A cow’s view of a helikite, Battersea, Wairarapa, March 2004.

  31. Extrapolating to a national inventory for enteric CH4

  32. It is not feasible to conduct small scale experiments for every represented livestock class under all feeding conditions for all seasons. So the compilation of national inventories uses a model that integrates available emission data with a knowledge of ruminant metabolism.

  33. There is no “theoretical” way to calculate Ym! So we must rely on experiments to quantify Ymand to identify its determinants. Standard inventory methodologies for enteric CH4 — such as IPCC Tier 2 — proceed by matching energy demand to energy supply: • Disaggregate into livestock categories and sub-categories • Estimate energy required for body maintenance, growth, pregnancy, lactation, work, etc • Assess the gross energy intake (GEI) required to match the demand, taking account of feed conversion efficiency • Estimate the CH4 emission as a specified portion Ym of the GEI

  34. So, Ym is … • the link between small-scale experiments and a reliable national emission inventory for enteric CH4. • what experiments need to get right for a range of livestock categories. This is tough!

  35. Extrapolating to a global inventory for enteric CH4

  36. Extrapolating to a global inventory can proceed in one of two ways: • Use a common database for all countries (eg, FAO), apply a common methodology (eg, a common set of EFs), then aggregate • Sum the national inventories that are reported to UNFCCC, then account for non-reporting countries This approach treats all countries consistently — is the usual approach This approach is under analysis — but source categorization is often country-specific

  37. Some global estimates: • Crutzen et al.* made first global inventory: they estimated various EFs on the basis of “literature” Ym values: • 5.5–7.5% for cattle in developed countries • 9% for other cattle, buffalo • 6% for sheep • Est. global inventory 74 Tg/yr (based on FAO population data for ca 1982) * Crutzen et al., 1986, Tellus 38B, 271–284. (1 Tg = 1012 g; 1 g CH4 occupies 1.4 litre at STP)

  38. Some global estimates (contd): • IPCC SAR (1995) cited inventory estimate by Anastasi & Simpson* who combined Ym values of Crutzen et al. with FAO (1990) data: 84 Tg/yr • IPCC TAR (2001) included inventory estimate by Mosier et al.† who combined Ym values of Crutzen et al. (some re-categorization) with FAO (1994) data: 80 Tg/yr * Anastasi & Simpson, 1993, JGR 98, 7181–7186. † Mosier et al., 1998, Clim. Change 40, 39–80.

  39. Uncertainty in global estimates: • Ym values or equivalent EFs have uncertainty, both in magnitude and in application • Livestock categories are not fully captured in FAO statistics • Seasonality (pregnancy, lactation, birthing, feed quality, management) also not captured in FAO statistics • Global enteric CH4~80 Tg/yr, with uncertainty 65–100 Tg/yr (IPCC SAR)

  40. So, Ym is … • critical to reliable inventory evaluation at all scales • a major source of uncertainty, both in magnitude and through poor understanding of determinants. • still in need of more experiments to get it right!

  41. Context: the global CH4 budget

  42. first-order sink The global methane budget troposphere C(t)(Tg) multiple sinksl(t)C(t)(Tg/yr) aggregate source S(t)(Tg/yr) l-1is the tropospheric methane “lifetime”

  43. What we know about the current global budget • Tropospheric CH4: C 4900 Tggrowing at 10–20 Tg/yr • based on NOAA/CMDL global monitoring network • Sink strength due to OH*: l-1 10 yror lC  500 Tg/yr • based on OH inferred from AGAGE monitoring of CH3CCl3 • Minor sinks: 30(soils)+ 40(strat)Tg/yr • Aggregate source, S 590 ± 90 Tg/yr * OH radical (photolysis) is the principle CH4 scavenger

  44. Of the total CH4 source of ~590 Tg/yr — • approximately 220 Tg/yr is natural (mainly wetlands)* • enteric fermentation (~80 Tg/yr) accounts for ~20–25% of the anthropogenic source — arguably the largest single anthropogenic component * Houweling et al., 2000, JGR 105, 17,243–17,255.

  45. Summary and Conclusions

  46. Summary & Conclusions • The SF6 tracer technique is valuable as the only tool available for measuring CH4 emissions from individual grazing animals — however: • it is expensive: requires skilled animal management and skilled GC practitioners • it should be treated as a tool still under research rather than a routine protocol • permeation tube performance, in particular, should attract ongoing scrutiny

  47. Summary & Conclusions, contd • A critical target of small-scale experiments such as with SF6 tracer technique is to better quantify Ym and its determinants, eg throughstudying: • emission variability (among animals, over time) • feed inter-comparisons • juvenile vs mature livestock • a statistically large number of animals

  48. Summary & Conclusions, contd • ‘Top-down’ methods are useful for validation or verification and for up-scaling — provided that good precision can be attained (target: ~20%) • Maybe soon(?) satellites will be able to measure with precision CH4 plumes from very large regions (~102 km) — the largest-scale top-down experiment of all!

  49. Summary & Conclusions, contd • National inventories remain uncertain (~15–25%) due to uncertainties in disaggregation and in population and to uncertain Ym • Global inventories remain uncertain (>20%) due to even greater uncertainties in disaggregation and in population and to uncertain Ym • Enteric CH4 accounts for 20–25% of the global anthropogenic CH4 source

  50. Acknowledgments • NZ funding agency: • Foundation for Research, Science & Technology • SF6 tracer technique in NZ: • Washington State University developers • Marc Ulyatt, Harry Clark et al. at AgResearch (NZ) • Top-down measurements and modelling: • Neil Gimson, Gordon Brailsford, Tony Bromley, and other NIWA staff • Marek Uliasz (Col. State University) • National and global inventory analyses: • Harry Clark, Gerald Rys, Helen Plume (NZ)

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