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EDUCATION: GLOBAL METHANE INVENTORYInventory of Methane Emissions1. The Methane Inventory ProjectThe scientific objectives of this research are to understand the global methane cycle, including how its sources and sinks have changed over the last ten years, how they might change in future decades, and what the implications are for climate. 2. Project BackgroundThe International Global Atmospheric Chemistry (IGAC) project of IGBP (International Geosphere-Biosphere Programme) initiated the Global Emissions Inventory Activity (GEIA) in 1991. The purpose of IGAC is to measure, understand, and predict changes in global atmospheric chemistry (Graedel, 1994); the purpose of GEIA is to develop global, widely recognized emissions inventories of a variety of gases and particles in support of atmospheric chemistry models. The modeling community recognized the advantages of supporting a consensus data effort such as GEIA. For example, providing a standardized, authoritative dataset of methane emissions for use in atmospheric chemistry models means that differences among results of various models come from differences among the models themselves and not from a combination of different models and different input data. On a practical level, standardized datasets reduce the effort spent by individual research groups to develop distributions of emissions for their modeling work. Atmospheric concentrations of methane, as well as its sources and their seasonal and spatial distribution, are reasonably well known. datasets of methane emissions were produced prior to GEIA by Elaine Matthews, Inez Fung, and Jean Lerner, at the NASA Goddard Institute for Space Studies (GISS). Some of these datasets are available via data centers, institutional anonymous FTP, and the World Wide Web. Major limitations to distribution of the complete inventory are:
In 1993, GEIA's methane working group presented recommendations for estimating global inventories of the following natural and anthropogenic methane sources: coal mining and processing, natural gas production, transmission, and distribution, landfills, rice cultivation, wetlands and animals. (Emissions from biomass burning are the subject of another GEIA activity (BIBEX)). These recommendations make use of existing datasets where possible, and include specifics about methodologies, uncertainties, and data availability. 3. Project ObjectivesThe overall objective of this research is to compile and distribute a current, complete, and internationally-recognized inventory of methane sources and associated emissions. This research has the following components:
4. The Project4.1 Methane Emission from Rice CultivationThe focus of the 1996 summer project was on estimating global methane emission from wetland rice cultivation in 1990 using the default IPCC methodology for that source. Methodology. The IPCC methodology was developed according to the concept that ideal conditions for methane emission from rice cultivation are those in which flooding is continuous throughout the growing season for the rice. Such continuous flooding is considered to occur only under artificially irrigated conditions; rainfed rice cultivation is prevalent is some countries but under these conditions, fields typically are subject to intermittent flooding during the growing season. Dryland rice produces no methane. Based on measurements of methane emission from rice fields, the IPCC methodology assumes that methane emission from rice under ideal conditions (continuous flooding under irrigation) is 20 grams of methane per square meter per year (g CH4/m2/yr) and that fields managed under other water regimes (rainfed) produce methane at lower rates. In the calculation, emission factors are associated with water regimes and define the value used to "adjust" the maximum emission value of 20 g CH4/m2/yr. The emission factor for irrigated fields is 1.0 × 20 g CH4/m2/yr. For rainfed rice fields the factor is 0.8 × 20 g CH4/m2/yr if under continuous flooding, 0.4 if under intermittent flooding, and 0.6 if the flooding regime is not specified (undifferentiated). To calculate the total methane emissions from an area, the emissions produced under each water regime are calculated and summed. The total is multiplied by 1 #215;105 to convert from 107 g to 1012 g (teragrams).
The methane emission for each water regime is found by taking the product of the Total Harvested Area [At], the % Area Irrigated [Ai] or Rainfed (undifferentiated [Aru] or continuous flooding [Arc] or intermittent flooding [Ari]), the appropriate emission factor for each water regime (Fi, Fru, Frc, Fri ), and the IPCC default emission value of 20 g CH4/m2/yr. The total is multiplied by 1×105 to convert to teragrams.
(Note: The factor of .01 in the calculation is to convert the number for area irrigated or rainfed into a percentage.)
An example is shown below:
Methane emissions from continuously flooded fields:
Methane emissions from undifferentiated rainfed fields:
Total Emissions = .19 Tg: The inventory was accomplished by obtaining information on rice-harvest areas for all rice-producing countries of the world reflecting the GEIA reference year of 1990. The country data on harvested rice areas were obtained from the 1990 UN Food and Agriculture Production Yearbook (FAO, 1990). Subnational statistics for harvested rice areas were also obtained for the United States, India, China, and Brazil from statistical yearbooks. Using geographic information on locations of rice-cultivation from a land-use dataset already developed (Matthews, 1991) and following the default IPCC methodology for estimating emission from this source, a global inventory of methane emission from rice cultivation was developed. Additional regional data was obtained from statistical yearbooks by country. Results and Discussion. The 1990 emission of methane from rice was calculated to be 19.1 Tg. A regional summary of the results is shown in Table 3-1. Figure 3-1 shows the global distribution of results in g CH4/m2/yr. The overwhelming proportion of the total area (90%) and emissions (95%) comes from East Asia. Mean rates of methane emission for Africa, South and Central America are one third to one half the maximum rate of 20 g CH4/m2/yr. Fields in East Asia emit at a rate of about 14 g CH4/m2/yr, 75% of the maximum. Rice fields in the remaining regions are almost exclusively irrigated and therefore produce methane at the maximum rate.
The global total of 19.4 Tg is lower than any other published estimates (IPCC, 1995). Since there is substantial uncertainty about the total amount of methane emitted from rice cultivation, this unexpectedly low value could be correct. However, the appropriate comparison is between the calculated emission rates for rice fields in various countries and field measurements of emission rates in major rice-growing countries (IPCC, 1995). Crude averages for methane emission, from a set of field measurements for China and for India, are about 60 and 25 g CH4/m2/yr, respectively (IPCC, 1995). Using the IPCC methodology, we calculated country means of 19 and 11 g CH4/m2/yr for China and India, respectively. Therefore, the measurement means for China are about three times the value calculated using the IPCC methods. In addition, measurements for India indicate that actual field emissions may be 25% higher than the maximum possible using the IPCC methods (and about 20 g CH4/m2/yr) and 2.5 times the mean value calculated in the present study for India using the IPCC methods. The latest IPCC methodology, which was used in this study, is a simplified version of an early one which had included data on crop growing seasons - number of days crops were growing, the months in which crops were grown, and number of crops grown in a year. However, since the GEIA inventories are designed as input for modeling of atmospheric chemistry, the seasonal cycle of emissions must be evaluated. We plan to include seasonal cropping calendars (Matthews et al, 1991) in our next calculation of methane emission from rice cultivation. 4.2 Methane Emission from LandfillsResearch at Mott Hall Junior High School 1996-1997 The topic of greenhouse effect and global warming is part of the 8th grade Earth science curriculum at Mott Hall Intermediate School and is taught within the context of the nature of solar energy and its interaction with the Earth's atmosphere and surface, and atmospheric chemical processes. The continuation of the project during the 1996-97 schoolyear built on the research carried out at GISS during the summer and extended the summer's demonstration product to encompass an additional methane source: landfills. During the 1996-97 schoolyear, 10 students and the instructor worked on developing an inventory for 1990 contributions of landfills to emission of methane to the atmosphere. They began with North America and the Caribbean where data are prevalent. The IPCC methodologies provide default values for several variables needed in the estimate including per capita refuse production, fraction of refuse placed in landfills, fraction of refuse decomposing anaerobically, methane potential of waste, and level of methane recovery. However, the students needed population statistics in order to estimate the total amount of waste generated, as well as to develop the geographic distribution of methane emission ultimately required by atmospheric chemistry modelers. Working as a team, with one student as a facilitator, each student selected several states/provinces/countries and compiled 1990 population statistics on the following: population of cities < 50,000; city latitudes/longitudes; and total, urban and rural populations for states of the United States, provinces of Canada, and other countries. Each student contributed his/her population statistics to the effort, and the team carried out the IPCC default calculation of methane emissions from landfills in North America and the Caribbean. The students worked together to produce a report and a science fair poster on their research (the Mott Hall science fair took place on 28 May, 1997). The report/posters describe the scientific background for the udy of methane emissions, sources of data, an overview of methane sources, details of the IPCC methodology and how it was implemented, sources of population data, and results of the study. Methodology. The IPCC methodology for calculating methane emissions from landfills was based on estimates of the factors influencing methane production in solid waste. Foremost among these is the degradable organic carbon (DOC) content of the waste. The percent of DOC is based on the composition of the waste. Since the composition can vary widely especially between developed and developing countries, data on the amount and composition of solid waste from various regions were collected. From this, default values for the per capita production of solid waste, the fraction to landfill and the fraction of DOC may be calculated for various countries. Where data for specific countries are sparse, assumptions are made based on regional data available. Calculating methane emissions from landfills requires data on the urban and rural population and the per capita solid waste production of a country from which the total amount of municipal solid waste (MSW) can be calculated. Methane emissions can then be calculated by applying the IPCC default values for the fraction of the total MSW to landfill and the fraction of DOC in that landfill that undergoes anaerobic decay and multiplying that by the methane potential. This procedure is as follows: Step 1 - Calculate the Total Municipal Solid Waste in Tg (109g)
divided by 1×106 to convert 103 kg to 1012 g = Terragrams.
Step 2 - Calculate the methane emissions in Tg 1012g)
Example:
Step 1 - Calculate the Total Municipal Solid Waste in Tg (109 g) MSWt = [ (Pu × Fu × 365) + (Pr × Fr × 365) ] / (1×106) MSWt = [ (2440 × 1.9 × 365) + (1601 × 1.9 × 365) ] / (1×106) = 2.802 Tg/yr Step 2 - Calculate the methane emissions in Tg (109 g) M = MSWt × Fl × Fa × Fmp × .001372 M = 2.802 × 0.7 × 1.0 × 57 × .001372 = 0.153 Tg/yr Results and Discussion. Methane emissions from landfills in the United States (50 states) for the 1990 reference year were calculated to be 9.4 Tg/yr (Table 3-2). This represents about 30% of the global total. Since the methodology is based on population, the five most populous states, California, New York, Texas, Florida, and Pennsylvania, (Figure 3-2) account for 36% of the population and 36% of the methane production.
The methodology used allows for a simple calculation of methane emissions from landfills globally. However, there are a number of uncertainties which affect these estimates. The time between the placement of waste and the production of methane, and the oxidation of methane as it diffuses to the surface of the landfill remain substantial uncertainties. Obtaining more data on the composition and amount of MSW, especially in developing countries, could make better estimates of methane emissions. This is a subject further discussed in the Student Involvement component. 4.3 Inventory of Animals and Associated Methane EmissionsDuring the summer program of 1997, the student and faculty member of the team are collecting 1990 data on animal populations by country, and by political bdivision for large countries. Animals to be included are: dairy and beef cattle, sheep, goats, pigs, water buffalo, and camels. When the data compilation is completed, the global methane emission from animals will be calculated using the IPCC default methodology; geographic distribution of the animal densities and associated emissions will be obtained using previously developed global, 1 degree resolution datasets of locations of animals (Lerner et al., 1988) in conjunction with the emissions from IPCC for countries. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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