Annual CH4 Emissions Research Articles

Using a crop/soil simulation model and GIS techniques to assess methane emissions from rice fields in Asia. IV. Upscaling to national levels.

The process-based crop/soil model MERES (Methane Emissions from Rice EcoSystems) was used together with daily weather data, spatial soil data, and rice-growing statistics to estimate the annual methane (CH4) emissions from China, India, Indonesia, Philippines, and Thailand under various crop management scenarios. Four crop management scenarios were considered: (a) a 'baseline' scenario assuming no addition of organic amendments or field drainage during the growing season, (b) addition of 3,000 kg DM ha−1 of green manure at the start of the season but no field drainage, (c) no organic amendments but drainage of the field for a 14-d period in the middle of the season and again at the end of the season, and (d) addition of 3,000 kg DM ha−1 of green manure and field drainage in the middle and end of the season. For each scenario, simulations were made at each location for irrigated and rainfed rice ecosystems in the main rice-growing season, and for irrigated rice in the second (or 'dry') season. Overall annual emissions (Tg CH4 yr−1) for a province/district were calculated by multiplying the rates of CH4 emission (kg CH4 ha−1 yr−1) by the area of rice grown in each ecosystem and in each season obtained from the Huke and Huke (1997) database of rice production. Using the baseline scenario, annual CH4 emissions for China, India, Indonesia, Philippines, and Thailand were calculated to be 3.73, 2.14, 1.65, 0.14, and 0.18 Tg CH4 yr−1, respectively. Addition of 3,000 kg DM ha−1 green manure at the start of the season increased emissions by an average of 128% across the five countries, with a range of 74–259%. Drainage of the field in the middle and at the end of the season reduced emissions by an average of 13% across the five countries, with a range of −10% to −39%. The combination of organic amendments and field drainage resulted in an increase in emissions by an average of 86% across the five countries, with a range of 15–176%. The sum of CH4 emissions from these five countries, comprising about 70% of the global rice area, ranged from 6.49 to 17.42 Tg CH4 yr−1, depending on the crop management scenario.

Methane emissions from wetlands on the Qinghai-Tibet Plateau

The areal extent of cold freshwater wetlands on the Qinghai-Tibet Plateau (QTP) is estimated at 0.133 × 106 km2, suggesting a significant methane potential. Methane fluxes from wet alpine meadows, peatlands,Hippuris vulgaris mires and secondary marshes were 43.18, 12.96, −0.28 and 45.90 mg · m−2 · d−1, respectively based on the transect flux studies at the Huashixia Permafrost Station (HPS) from July to August 1996. Average CH4 fluxes in the thaw season were extrapolated at 5.68 g · m−2 according to the areal percentage of wetland areas in the Huashixia region. The CH4 fluxes at four fixed sites, representative of similar ecosystems, ranged from −19.384–347.15 mg · m−2 · d−1, and the average CH4 fluxes varied from 6.54 to 71.97 mg · m−2 · d−1 at each site during the observation period from April to September 1997. The CH4 emissions at each site during the entire thaw season was estimated from 1.21 to 10.65 g · m−2, displaying strong spatial variations. Seasonal variations of CH4 fluxes also were observed at four sites: CH4 outbursts occurred upon the spring thaw in May and June, CH4 fluxes increased afterwards with rising soil temperatures. Episodic fluxes were observed in summer, which influenced the average CH4 fluxes considerably. Annual CH4 emissions from cold wetlands on the QTP were estimated at about 0.7–0.9 Tg based on the distribution of wetlands, representative CH4 fluxes, and number of thaw days. The centers of CH4 releasing were located in the sources of the Yangtze and Yellow rivers, and Zoige Peatlands.

Seasonal Patterns and Controls on Methane and Carbon Dioxide Fluxes in Forested Swamp Pools

We measured seasonal patterns in the fluxes of CH4 and CO2 in a forested wetland in central New York State from 1992 to 1995. The site was a red maple-hemlock swamp with permanent pools (~30cm deep), except when the pools dried during severe drought in 1995. Annual CH4 emissions from the pools to the atmosphere averaged 4.3 (+/- 2.4) mol m-2 during 1993 to 1995 and ranged from 1.9 mol m-2 in 1993 to 6.7 mol m-2 in 1995. CO2 emissions from the pools during 1993 and 1994 were 10.5 and 11.9 mol m-2, accounting for 84.6% and 75.6% of the gaseous carbon export during 1993 and 1994, respectively. A combination of temperature and carbon availability influenced seasonal CH4 and CO2 fluxes more than other environmental factors (temperature, water column depth, O2, CH4, dissolved organic carbon, NH4+, or dissolved inorganic carbon). Based on an observed 12 C threshold temperature, linear regression models explained from 31% to 77% of seasonal gas flux variability. Compared with other forested wetlands, this site produced substantially greater CH4 fluxes, and the CH4 flux accounted for a greater proportion of the total carbon flux from the system.

Methane Emission and Uptake from Soils as Influenced by Excreta Deposition from Grazing Animals

AbstractThere have been only a few recent field‐based measurements to estimate the impact of excreta from grazing animals on methane (CH4) exchange. We report measurements of CH4 exchange from cattle dung and urine deposited in a simulated seasonal grazing pattern. Dung and urine samples were deposited on six separate experimental plots at different times of the year to simulate different stages of normal grazing under UK conditions to study the effects of environmental factors on CH4 exchange rates. During all the experiments, dung pats were the main emitter of CH4 with an estimated mean daily rate of 0.72 (range 0.21 to1.46) g CH4‐C per cow. Both net emission and consumption of CH4 from urine patches, however, were observed during the different experiments with a negligible mean daily emission of 0.02 (range −0.05 to 0.15) g CH4‐C per cow. Significant variations were observed between the experiments, despite the similarity of application rates and the composition of the excreta. These variations were mainly due to climatic differences when excreta were deposited: ambient temperature and rainfall were the most important factors. The estimated total annual emission of CH4 from cattle excreta deposition during the grazing season in the UK ranged between 0.42 and 3.26 (mean = 1.57) kt CH4‐C. Our work confirms other studies that suggest that excreta patches are insignificant sources of CH4 compared to the rumens of grazing cattle, and that climatic factors have a significant impact on CH4 emissions.

Determinants of spatial variability of methane emissions from wet grasslands on peat soil

Methane (CH4) emissions from soils, representing the consequence of CH4 production, CH4 consumption and CH4 transport, are poorly characterised and show a large spatial variability. This study aimed to assess the determinants of field-scale spatial variability of CH4 emissions from wet grasslands on peat soil. Mean CH4 emission rates of a three-year experiment at 18 plots distributed over three sites in the nature preserve “Nieuwkoopse Plassen” on peat soil in the Netherlands were related to CH4 production and CH4 consumption capacities of soil layers, and to soil and vegetation characteristics. Spatial variability of CH4 emissions and possible determining factors was high. Annual CH4 emissions ranged from 3 to 37 g CH4 m−2 yr−1. Coefficients of variation (CV) of CH4 emissions were on average 37% among sites and 83% within sites. Most important determinants of spatial variability were CH4 production capacity (average: 211 ng CH4 g−1 dry soil h−1; CV: 131%) and aboveground biomass of sedges (Carex spp.) (average: 0.45 g dm−2; CV: 127%) (P<0.01). Sedges may affect CH4 emissions by stimulating CH4 transport from anaerobic layers to the surface via their vascular system and/or by serving as substrate for methanogens. For extrapolation of CH4 emissions to larger areas, best results will be obtained by using factors that are easy to determine, like vegetation.

Seasonal Study of Methane and Nitrous Oxide in the Coastal Waters of the Southern Baltic Sea

Atmospheric and dissolved methane (CH4) and nitrous oxide (N2O) were measured in the unique coastal ecosystem of theBoddenwaters, including the western Oder estuary, (southern Baltic Sea) during five campaigns between 1994 and 1997. The CH4saturations, ranging from 105–15500%, showed great spatial and temporal variability with maximum values in September and minimum values in December. The N2O saturations were in the range of 91–312% with a maximum in March. Enhanced concentrations of both gases were observed only in the western Oder estuary near the mouth of the Peene River. Thus, we conclude that the distributions of CH4and N2O in the investigatedBoddenwaters are, directly or indirectly, linked to the Peene River runoff and not to the Oder River. Our estimate of the annual CH4emissions from theBoddenwaters to the atmosphere indicates a significant contribution (c.17%) to the overall CH4emissions from the Baltic Sea. In contrast, theBoddenwaters represent only a small source for atmospheric N2O.CH4production rates estimated from sediment slurry experiments revealed a significant spatial variability and indicated that methanogenic activity was related to acetate consumption in the surface sediment layer. Sedimentary CH4production might depend on different amounts of accumulation of organic material.

Methane emissions from above ground open manure slurry tanks

Estimates of methane (CH4) emissions from slurry tanks or lagoons in the past have been made primarily by using chamber techniques, which are point specific and interfere with conditions at the slurry‐atmosphere interface. This study is based on the use of sulphur hexafluoride (SF6) as a tracer gas to estimate CH4 transport from the slurry surface. The tracer was released from the surface of the swine slurry, and air samples were taken at the downwind rim of the tank over a 165 day period from June 12 to November 20, 1995, at the McGiIl University ‐ Macdonald Campus Farm, Ste‐Anne‐de‐Bellevue, Quebec, and analyzed for CH4 and SF6 concentrations (C). Knowing the SF6 source strength (Q), CH4 source strength was then determined from the measured downwind concentrations using the C/Q ratio. Using this method, annual CH4 emission from the swine slurry tank was estimated at 56.5 kg CH4 m−2 tank surface yr−1 (± 20%). Assuming that diffusion processes at the nearby dairy tank were similar to those at the swine tank, the same C/Q ratio was used to determine CH4 emissions from the dairy slurry based on downwind CH4 concentrations measured over the same period as at the swine tank. Annual methane emission from the dairy tank was estimated at 74 kg CH4m−2 tank surface yr−1 (± 45%). On the basis of these estimates, CH4 emissions from outdoor holding tanks for swine and dairy slurry in Canada were approximated at 0.71 Tg CH4 yr−1 (± 40%) and 0.24 Tg CH4 yr−1 (± 70%), respectively, giving a combined annual emission of approximately 0.95 Tg yr−1(± 50%).

Methane release from stands of water horsetail (Equisetum fluviatile) in a boreal lake

Summary1. Annual and diel variations in methane (CH4) release in stands of Equisetum fluviatile were measured from June to November in Lake Pääjärvi, southern Finland, where E. fluviatile is the dominant emergent macrophyte. An estimate of total annual release of CH4 from stands of E. fluviatile in this lake was also made. Diel variation was measured twice (June and August), whereas measurements for annual variation were performed monthly. The hypothesis that a relationship exists between the productivity of stands and CH4 release was also tested, whereupon net ecosystem exchange (NEE) of CO2 as well as standing stock of E. fluviatile were determined, in addition to simultaneous recordings of air temperature and solar radiation.2. Seasonal variations in CH4 release were pronounced, with the highest release rate of 813 mg m–2 day–1 measured in July and the lowest 6.5 mg m–2 day–1 in November, when the shoreline was already frozen.3. Methane release rates were strongly correlated with mean air temperature in the measuring chambers and with total solar radiation. There was no significant correlation between the instantaneous radiation and CH4 release rates.4. The seasonal patterns of CH4 release and NEE of CO2 resembled each other, except in July when NEE suddenly dropped. The decrease in NEE coincided with the highest CH4 release rate measured and the highest temperature during the measuring period, i.e. 32 °C outside and 37 °C inside the chamber. Excluding this date, daily CH4 release was strongly correlated with NEE (r2 = 0.971).5. No diel changes in CH4 release rates were detected. In June and August the maximum release rates were 11.4 and 16.8 mg CH4 m–2 h–1, respectively.6. The standing stock of E. fluviatile at different times of the growing season was not correlated with CH4 efflux; the CH4 release rates could be related neither to the number of shoots, i.e. sufficient conduits for gas transport were always present, nor to the shoot biomass in the measuring chambers.7. For an estimate of the annual release, the monthly values measured at noon were integrated over the entire growing season; this resulted in 43.7 g CH4 m–2 for the annual emission. The total annual emission of CH4 from the area covered with E. fluviatile in Lake Pääjärvi was calculated to be ≈ 5000 kg.8. Significant amounts of CH4 are released from stands of E. fluviatile in boreal lakes. The CH4 release rate follows a seasonal pattern but there is no diel pattern. Methane release rate can be related to temperature, solar radiation and NEE of CO2, but not to the standing stock of E. fluviatile or the number of shoots.

Spatial analysis of methane emissions from paddy soils in China and the potential for emissions reduction

China is a major source of anthropogenic methane (CH4) emissions because it is the world's largest producer of rice grain, nearly all of which is grown in irrigated paddies. This study sought to reduce the uncertainty in estimates of CH4 emissions from rice cultivation in China by improving the quantification of the effects of management practices (intermittent drainage and fertilizer inputs) on emissions. These results were spatially extrapolated with digital maps of type of rice using new estimates of organic matter and fertilizer inputs, as well as estimates of soil drainage. The estimated total annual CH4 emissions from rice agriculture in China in 1990 were 9.9 ± 3.0 × 1012 g. If intermittent drainage practices were adopted on 33% of the poorly drained soils used for rice cultivation in southern China, the estimated emissions would be 8.9 ± 2.7 × 1012 g CH4 yr-1. Reducing projected organic matter inputs by 50% as a sensitivity analysis to reflect the trend for reduced use of organic fertilizer, resulted in emissions of 9.6 ± 2.9 × 1012 g CH4 yr-1, with 8.7 ± 6 × 1012 yr-1 emitted with 33% adoption of intermittent drainage on poorly drained paddies. Although intermittent drainage has been shown to reduce emissions by 50%, the area of rice that is relatively easy to drain and re-flood is not very large. The use of intermittent drainage with better drained paddies is limited because of problems with re-flooding and it is also limited with very poorly drained paddies that are difficult to drain. The 10% emission reduction predicted with 33% adoption of intermittent drainage practices, while not large, is conservative and may be possible to realize. These CH4 emissions results are relative estimates because the uncertainty remains large due to a lack of emissions measurements from paddies in more regions and a lack of detailed information about organic fertilizer application rates.

Global methane emissions from minor anthropogenic sources and biofuel combustion in residential stoves

Most global methane (CH4) budgets have failed to include emissions from a diverse group of minor anthropogenic sources. Individually, these minor sources emit small quantities of CH4, but collectively, their contributions to the budget may be significant. In this paper, CH4 emissions are estimated for a wide variety of individual minor emissions sources on a country‐specific basis. Emissions from biomass combustion in the residential sector are also examined. The minor sources examined include fuel combustion in furnaces, vehicles, aircraft, ship, rail systems, industrial waste treatment and combustion processes, various industrial manufacturing processes (e.g., chemical manufacturing), on‐site residential waste burning, forest wildfires, and prescribed burning activities, oil refining, and the storage/distribution of oil‐derived products, coke production, and charcoal production. Country‐specific emissions associated with residential wood, charcoal, and dung combustion are also estimated. The total annual CH4 emissions from all sources examined here are estimated to be about 40 Tg. Almost half of this total is due to residential fossil fuel and biofuels combustion.

Potential methane emission from north‐temperate lakes following ice melt

Methane, a radiatively active “greenhouse” gas, is emitted from lakes to the atmosphere throughout the open‐water season. However, annual lake CH4 emissions calculated solely from open‐water measurements that exclude the time of spring ice melt may substantially underestimate the lake CH4 source strength. We estimated potential spring CH4 emission at the time of ice melt for 19 lakes in northern Minnesota and Wisconsin. Lakes ranged in area from 2.7 to 57,300 ha and varied in littoral zone sediment type. Regression analyses indicated that lake area explained 38% of the variance in potential CH4 emission for relatively undisturbed lakes; as lake area increases potential CH4 emission per unit area decreases. Inclusion of a second term accounting for the presence or absence of soft organic‐rich littoral‐zone sediments explained 83% of the variance in potential spring CH4 emission. Total estimated spring CH4 emission for 1993 for all Minnesota lakes north of 45° with areas ≥4 ha was 1.5 × 108 mol CH4 assuming a 1 : 1 ratio of soft littoral sediment to hard littoral sediment lakes. Emission estimates ranged from 5.3 × 107 mol assuming no lakes have soft organic‐rich littoral sediments to 4.5 × 108 mol assuming all lakes have soft organic‐rich littoral sediments. This spring CH4 pulse may make up as much as 40% of the CH4 annually emitted to the atmosphere by small lakes.

A determination of the CH4, NO x and CO2 emissions from the Prudhoe Bay, Alaska oil development

In this paper we quantify the CH4, CO2 and NO x emissions during routine operations at a major oil and gas production facility, Prudhoe Bay, Alaska, using the concentrations of combustion by products measured at the NOAA-CMDL observatory at Barrow, Alaska and fuel consumption data from Prudhoe Bay. During the 1989 and 1990 measurement campaigns, 10 periods (called ‘events’) were unambiguously identified where surface winds carry the Prudhoe Bay emissions to Barrow (approximately 300 km). The events ranged in duration from 8–48 h and bring ambient air masses containing substantially elevated concentrations of CH4, CO2 and NO y to Barrow. Using the slope of the observed CH4 vs CO2 concentrations during the events and the CO2 emissions based on reported fuel consumption data, we calculate annual CH4 emissions of (24+/−8)×103 metric tons from the facility. In a similar manner, the annual NO x emissions are calculated to be (12+/−4)×103 metric tons, which is in agreement with an independently determined value. The calculated CH4 emissions represent the amount released during routine operations including leakage. However this quantity would not include CH4 released during non-routine operations, such as from venting or gas flaring.

Seasonal Variation in Methane Emission from Stored Slurry and Solid Manures

AbstractMethane (CH4) is an important greenhouse gas and recent inventories have suggested that livestock manure makes a significant contribution to global CH4 emissions. The emission of CH4 from stored pig slurry, cattle slurry, pig solid manure, and cattle solid manure was followed during a 1‐yr period. Methane emission was determined by dynamic chambers. Emission rates followed a In‐normal distribution for all four manures, indicating large spatial and seasonal variations. Monthly geometric means for pig slurry, cattle slurry, pig solid manure, and cattle solid manure varied from 0.4 to 35.8, 0.0 to 34.5, 0.4 to 142.1, and 0.1 to 42.7 g CH4 m−3 d−1, respectively. For slurries CH4 emission rates increased significantly with storage temperatures, the Q10 value ranging from 3.4 to 5.7 depending on slurry type. The presence of a natural surface crust reduced CH4 emission from slurry by a factor of 11 to 12. Surface crust effects declined with increasing slurry temperature. Solid manures stored in dungheaps showed significant heat production. Pig solid manure temperatures were maintained at 30 to 60 °C throughout most of the year, while cattle solid manure temperatures were close to ambient levels until late spring, when heat production was initiated. Methanogenesis in solid manure also increased with increasing temperatures. For pig solid manure, CH4 emission rates peaked at 35 to 45 °C. No distinct temperature optimum could be detected for cattle solid manure, however, temperatures rarely exceeded 45 °C. The Q10 values for dungheaps ranged from 2.7 to 10.3 depending on manure type and Q10 temperature interval. Annual CH4 emissions from pig slurry, cattle slurry, pig solid manure, and cattle solid manure were estimated at 8.9, 15.5, 27.3, and 5.3 kg animal−1 yr−1, respectively. The annual emission of CH4 from stored animal manure in Denmark is estimated at 28 000 Mg yr−1.

A methane flux transect along the trans-Alaska pipeline haul road

Methane flux measurements were made during the summer of 1987 at 10 km intervals along a north—south transect from Prudhoe Bay (70°13’N) to the Arctic Circle (66°30’N) in Alaska. A regional comparison showed that the mean CH4 flux from arctic tundra (52 mg m-2 d-1) was significantly greater than the mean from high latitude taiga (11 mg m-2 d-1). Sites occupied on the transect were classified into 6 categories based on differences in plant communities and CH4 emission was further compared. Mean CH4 fluxes (mg m-2 d-1) for each category were: wet tundra, 90; low brush-muskeg bog, 45; moist tundra, 31; freshwater ponds, 21; spruce forest, 4.6; and alpine tundra, 0.6. Differences among means were statistically significant. During the period of observation, CH4 emission was only weakly correlated with soil temperature, water table depth, thaw depth and organic layer depth. Extending these fluxes to appropriate areas and emission periods yields independent estimates of annual CH4 emission from global tundra and taiga environments of 38 Tg and 15 Tg. This is about 46% of the wetland emission term or 10% of the global atmospheric input.

A 3‐year continuous record on the influence of daytime, season, and fertilizer treatment on methane emission rates from an Italian rice paddy

CH4 emission rates have been measured in an Italian rice paddy between 1984 and 1986, covering three vegetation periods. For these measurements a fully automated, computerized sampling and analyzing system was developed which allowed the simultaneous determination of CH4 emission rates at 16 different field plots. CH4 emission rates showed strong diurnal and seasonal variations. Diurnal changes correlated with changes in soil temperature. During the season, CH4 emission rates showed a first maximum in May–June before tillering and a second maximum in July during the reproductive stage of the rice plants. In 1985 and 1986 two maxima were observed during summer in addition to the first maximum in the rate of CH4 emission during spring. Application of mineral and/or organic fertilizer strongly influenced the CH4 emission rates, depending on the type, rate, and mode of fertilizer application. Thus the rates decreased by at most 40% and 60% after fertilization by deep incorporation with 200 kg N/ha urea and 200 kg N/ha ammonium sulfate, respectively. Application of 200 kg N/ha calcium cyanamide led to a reduction of the first maximum of CH4 emission but caused the second maximum to increase, the overall result being that the seasonally averaged CH4 emission rate was comparable to that observed in unfertilized fields. Application of rice straw at a rate of 12 t/ha enhanced the rate of CH4 emission by a factor of 2 compared with the control. Higher application rates of rice straw did not cause a further increase in CH4 emission. The complete records of CH4 emissions over three vegetation periods indicate an average seasonal CH4 emission rate from unfertilized fields of 0.28 g CH4/m2 d, with a range of 0.16–0.38 g CH4/m2 d. Based on this value and applying the observed temperature dependence of the CH4 emission rates, the global annual CH4 emission from rice paddies is estimated to be in the range of 50–150 Tg, with a likely average of 100 Tg. This figure represents between 19% and 25% of the global CH4 emission, indicating that rice paddies are one of the most important individual sources of atmospheric CH4.

Methane emissions to the troposphere from the Amazon floodplain

Methane emissions to the troposphere from the Amazon River floodplain have been determined. In total, 94 individual flux measurements were made along a 1,700‐km stretch of the river during the early falling water period of the flood cycle (July–August 1985). The overall average rate of CH4 emission from wetlands was 390 mg CH4 m−2 d−1. Water surfaces covered by aquatic macrophytes had the highest emissions, 590 mg CH4 m−2 d−1, while emissions from flooded forests were 110 mg CH4 m−2 d−1 and open lake areas averaged 120 mg CH4 m−2 d−1. Ebullition was the dominant mechanism of emission, accounting for 85% of the total. Surface water dissolved oxygen was always undersaturated, while dissolved CO2 concentrations were significantly supersaturated with respect to atmospheric equilibrium. Surface water CH4 concentrations were highly supersaturated, averaging 6.4 μM. The δ13C of the CH4 varied from −42‰ to −73‰ with an apparent trend toward heavier values downstream. Extrapolation to the entire Amazon floodplain results in estimated annual emissions in the troposphere of 10 Tg yr−1 and indicates the importance of the area in the global atmospheric CH4 cycle.

Methane emission during a cultivation period from an Italian rice paddy

CH4 release rates from rice paddies were measured in Vercelli, Italy, in 1983 during a complete vegetation period, using a static box system. The rice paddies were found to be a source of atmospheric methane during the time of flooding. The CH4 release rates range between a few milligrams of CH4 per square meter per hour and 51 mg CH4 m−2 h−1 and show a seasonal variation with maximum emission rates between tillering and flowering. Averaged over the complete vegetation period, the CH4 release rate accounts for 16 mg m−2 h−1. Significant differences in the CH4 release rates of unfertilized field plots and plots fertilized with mineral fertilizer (CaCN2) were not observed. The CH4 release rates show strong diurnal variations, with highest values in the late afternoon and lowest values in the early morning, which coincides with the temperature variation in the upper soil layer (1–10 cm). These variations must be taken into account in estimating reliable global CH4 emission rates from rice paddies. CH4 is almost exclusively emitted into the atmosphere by gas bubbles during the first 6 weeks after flooding the rice paddies, that is, on fields without vegetation. Afterwards, 80% of the observed CH4 transport from the paddy soil into the atmosphere was due to diffusion through the stems of the rice plants. Transport by diffusion through the paddy water appeared to be of minor importance. The global annual CH4 emission from rice paddies is estimated to be of the order of 39–94 Tg yr−1 (Tg = 1012 g) for 1940 conditions and 70–170 Tg yr−1 for 1979 conditions, indicating a secular trend of the CH4 emission by rice paddies of about 1.6% per year during the last 35 years.