Annual CH4 Emissions Research Articles

Methane emissions from terrestrial plants over China and their effects on methane concentrations in lower troposphere

Methane (CH4) is the most important greenhouse gas and reactive trace gas in the atmosphere. Recently, it has been reported that terrestrial plants can emit CH4 under aerobic conditions, which may call for reevaluation of the inventory of CH4 emissions in China. In this paper, those emissions over China and their effects on CH4 concentrations in lower troposphere were investigated. Firstly, based on the work of Keppler et al., the aerobic plant CH4 emission model (PLANTCH4) for China was established. And by using the database of normalized difference vegetation index (NDVI) derived from NOAA/AVHRR, the distribution of net primary productivity (NPP) over China was simulated, and thereby, for the first time, the amount and distribution of the newly identified source in China were estimated. Secondly, with the aid of the three-dimensional atmospheric chemistry model system (MM5-CALGRID), the effects of the emissions were studied. The results show that the annual aerobic plant CH4 emissions over China amount to 11.83 Tg, i.e. nearly 24% of Chinese total CH4 emissions. And the major fraction (about 43%) comes from forests. When those emissions are considered in modeling, computed countrywide mean surface concentration of CH4 is 29.9% higher than without them, with a maximum increase of 69.61 μg·m−3 in the south of Yunnan Province. In conclusion, to study CH4 emissions from terrestrial plants over China may have important implications for correctly estimating the contribution of China to global CH4 budget, and may call for a reconsideration of the role of CH4 in global and regional environment and climate change.

Methane emissions from the upwelling area off Mauritania (NW Africa)

Abstract. Coastal upwelling regions have been identified as sites of enhanced CH4 emissions to the atmosphere. The coastal upwelling area off Mauritania (NW Africa) is one of the most biologically productive regions of the world's ocean but its CH4 emissions have not been quantified so far. More than 1000 measurements of atmospheric and dissolved CH4 in the surface layer in the upwelling area off Mauritania were performed as part of the German SOPRAN (Surface Ocean Processes in the Anthropocene) study during two cruises in March/April 2005 (P320/1) and February 2007 (P348). During P348 enhanced CH4 saturations of up to 200% were found close to the coast and were associated with upwelling of South Atlantic Central Water. An area-weighted, seasonally adjusted estimate yielded overall annual CH4 emissions in the range from 1.6 to 2.9 Gg CH4. Thus the upwelling area off Mauritania represents a regional hot spot of CH4 emissions but seems to be of minor importance for the global oceanic CH4 emissions.

N2O and CH4 fluxes from a volcanic grassland soil in Nasu, Japan: Comparison between manure plus fertilizer plot and fertilizer-only plot

We examined the effects of manure + fertilizer application and fertilizer-only application on nitrous oxide (N2O) and methane (CH4) fluxes from a volcanic grassland soil in Nasu, Japan. In the manure + fertilizer applied plot (manure plot), the sum of N mineralized from the manure and N applied as ammonium sulfate was adjusted to 210 kg N ha−1 year−1. In the fertilizer-only applied plot (fertilizer plot), 210 kg N ha−1 year−1 was applied as ammonium sulfate. The manure was applied to the manure plot in November and the fertilizer was applied to both plots in March, May, July and September. From November 2004 to November 2006, we regularly measured N2O and CH4 fluxes using closed chambers. Annual N2O emissions from the manure and fertilizer plots ranged from 7.0 to 11.0 and from 4.7 to 9.1 kg N ha−1, respectively. Annual N2O emissions were greater from the manure plot than from the fertilizer plot (P < 0.05). This difference could be attributed to N2O emissions following manure application. N2O fluxes were correlated with soil temperature (R = 0.70, P < 0.001), NH+ 4 concentration in the soil (R = 0.67, P < 0.001), soil pH (R = –0.46, P < 0.001) and NO− 3 concentration in the soil (R = 0.40, P < 0.001). When included in the multiple regression model (R = 0.72, P < 0.001), however, the following variables were significant: NH+ 4 concentration in the soil (β = 0.52, P < 0.001), soil temperature (β = 0.36, P < 0.001) and soil moisture content (β = 0.26, P < 0.001). Annual CH4 emissions from the manure and fertilizer plots ranged from –0.74 to –0.16 and from –0.84 to –0.52 kg C ha−1, respectively. No significant difference was observed in annual CH4 emissions between the plots. During the third grass-growing period from July to September, however, cumulative CH4 emissions were greater from the manure plot than from the fertilizer plot (P < 0.05). CH4 fluxes were correlated with NH+ 4 concentration in the soil (R = 0.21, P < 0.05) and soil moisture content (R = 0.20, P < 0.05). When included in the multiple regression model (R = 0.29, P < 0.05), both NH+ 4 concentration in the soil (β = 0.20, P < 0.05) and soil moisture content (β = 0.20, P < 0.05) were significant.

Distributions, Land-source Input and Atmospheric Fluxes of Methane in Jiaozhou Bay

CH4 concentrations in both the surface and bottom waters of Jiaozhou Bay were determined during four surveys in 2003, which showed variability with both seasons and tidal cycles. Atmospheric fluxes of CH4 in Jiaozhou Bay showed obvious seasonal and spatial variations, with the highest values occurring in summer and the lowest in winter. The annual emission of CH4 from Jiaozhou Bay was estimated to be \( 7.98 \times 10^{6} - 18.0 \times 10^{6} \;{\text{mol}}/{\text{year}} \). CH4 in the water column of Jiaozhou Bay was found to come from several land-sources including riverine water input, sewage water input and groundwater input. The spatial and temporal variation in distributions and atmospheric fluxes of CH4 in Jiaozhou Bay was influenced mainly by the input of polluted river waters and the sewage effluents along the eastern coast, which highlights the effects of human impacts on CH4 emission rates.

Methane and nitrous oxide emissions from three paddy rice based cultivation systems in Southwest China

To understand methane (CH4) and nitrous oxide (N2O) emissions from permanently flooded rice paddy fields and to develop mitigation options, a field experiment was conducted in situ for two years (from late 2002 to early 2005) in three rice-based cultivation systems, which are a permanently flooded rice field cultivated with a single time and followed by a, non-rice season (PF), a rice-wheat rotation system (RW) and a rice-rapeseed rotation system (RR) in a hilly area in Southwest China. The results showed that the total CH4 emissions from PF were 646.3+/-52.1 and 215.0+/-45.4 kg CH4 hm(-2) during the rice-growing period and non-rice period, respectively. Both values were much lower than many previous reports from similar regions in Southwest China. The CH4 emissions in the rice-growing season were more intensive in PF, as compared to RW and RR. Only 33% of the total annual CH4 emission in PF occurred in the non-rice season, though the duration of this season is two times longer than the rice season. The annual mean N2O flux in PF was 4.5+/-0.6 kg N2O hm(-2) yr(-1). The N2O emission in the rice-growing season was also more intensive than in the non-rice season, with only 16% of the total annual emission occurring in the non-rice season. The amounts of N2O emission in PF were ignorable compared to the CH4 emission in terms of the global warming potential (GWP). Changing PF to RW or RR not only eliminated CH4 emissions in the non-rice season, but also substantially reduced the CH4 emission during the following rice-growing period (ca. 58%, P >RR approximate to RW. The GWP of PF is higher than that of RW and RR by a factor of 2.6 and 2.7, respectively. Of the total GWP of CH4 and N2O emissions, CH4 emission contributed to 93%, 65% and 59% in PF, RW and RR, respectively. These results suggest that changing PF to RW and RR can substantially reduce not only CH4 emission but also the total GWP of the CH4 and N2O emissions.

How northern peatlands influence the Earth's radiative budget: Sustained methane emission versus sustained carbon sequestration

Northern peatlands sequester carbon and emit methane, and thus have both cooling and warming impacts on the climate system through their influence on atmospheric burdens of CO2 and CH4. These competing impacts are usually compared by the global warming potential (GWP) methodology, which determines the equivalent CO2 annual emission that would have the same integrated radiative forcing impact over a chosen time horizon as the annual CH4 emission. We use a simple model of CH4 and CO2 pools in the atmosphere to extend this analysis to quantify the dynamics, over years to millennia, of the net radiative forcing impact of a peatland that continuously emits CH4 and sequesters C. We find that for observed ratios of CH4 emission to C sequestration (roughly 0.1–2 mol mol−1), the radiative forcing impact of a northern peatland begins, at peatland formation, as a net warming that peaks after about 50 years, remains a diminishing net warming for the next several hundred to several thousand years, depending on the rate of C sequestration, and thereafter is or will be an ever increasing net cooling impact. We then use the model to evaluate the radiative forcing impact of various changes in CH4 and/or CO2 emissions. In all cases, the impact of a change in CH4 emissions dominates the radiative forcing impact in the first few decades, and then the impact of the change in CO2 emissions slowly exerts its influence.

Factors influencing methane emission from peat soils: Comparison of tropical and temperate wetlands

Methane (CH4) emissions from peat soils in tropical and temperate wetlands were compared. Annual CH4 emission rates in Ozegahara, the largest wetland on Honshu main island, Japan, were higher than in drained forest wetland areas examined in Indonesia. Methane emissions from the lowland paddy fields examined in Indonesia were higher than those of peaty paddy fields in Japan. There was generally a positive correlation (r2 = 0.09; P < 0.1) between CH4 emissions and CH4 production activities in wetland soils. In Ozegahara, there was a positive relationship (r2 = 0.80; P < 0.01) between CH4 production activities and soil pH, but there was no such relationship in Indonesia. The range of soil pH in Ozegahara was 5–7, while pH values in the Indonesian sites were lower than 5. There was a positive response of CH4 emission with respect to groundwater level in all of these areas. In Indonesia, land-use change from swamp and drained forest to cassava or coconut field lowered groundwater levels and decreased CH4 emission, while change to lowland paddy raised the groundwater level and increased CH4 emission. Addition of acetate generally inhibited CH4 production during the early period (until 2 weeks) of incubation, then enhanced it afterward in both Ozegahara and Indonesian wetland soils. Addition of hydrogen mostly enhanced CH4 production. From the results of this study, CH4 fluxes from peat soil to the atmosphere were positively correlated with CH4 production activities, and CH4 production activity in peat soil was regulated by soil pH, while land-use change from wetland to upland crop lowered groundwater level and thus reduced CH4 production and enhanced CH4 oxidation.

Methanotrophic Activity, Abundance, and Diversity in Forested Swamp Pools: Spatiotemporal Dynamics and Influences on Methane Fluxes

Methane oxidation (methanotrophy) in the water column and sediments of forested swamp pools likely control seasonal and annual emission of CH4 from these systems, but the methanotrophic microbial communities, their activities, locations, and overall impact, is poorly understood. Several techniques including 14CH4 oxidation assays, culture-based most probable number (MPN) estimates of methane-oxidizing bacteria (MOB) and protozoan abundance, MOB strain isolation and characterization, and PCR techniques were used to investigate methanotrophy at a forested swamp near Ithaca, New York. The greatest methanotrophic activity and largest numbers of MOB occurred predominantly at the low oxygen sediment/water interface in the water column. Seasonally, methanotrophic activity was very dynamic, ranging from 0.1 to 61.9 μ moles CH4 d−1 g−1 dry sediment, and correlated most strongly with dissolved inorganic carbon (r = 0.896). Incorporation of methanotrophic variables (abundance and activity) into existing CH4 flux regression models improved model fit, particularly during mid summer when CH4 fluxes were most dynamic. Annually integrated methane flux and methanotrophic activity measurements indicate that differences in methanotrophic activity at the sediment/water interface likely accounted for differences in the annual CH4 emission from the field site. Direct isolations of MOB resulted in the repeated isolation of organisms most closely related to Methylomonas methanica S1. A single acidophilic, type II MOB related to Methylocella palustris K was also isolated. Using a PCR-based MPN method and 16S rRNA genome copy number from isolates and control strains, type I and type II MOB were enumerated and revealed type I dominance of the sediment-associated MOB community.

Short‐ and long‐term greenhouse gas and radiative forcing impacts of changing water management in Asian rice paddies

AbstractFertilized rice paddy soils emit methane while flooded, emit nitrous oxide during flooding and draining transitions, and can be a source or sink of carbon dioxide. Changing water management of rice paddies can affect net emissions of all three of these greenhouse gases. We used denitrification–decomposition (DNDC), a process‐based biogeochemistry model, to evaluate the annual emissions of CH4, N2O, and CO2 for continuously flooded, single‐, double‐, and triple‐cropped rice (three baseline scenarios), and in further simulations, the change in emissions with changing water management to midseason draining of the paddies, and to alternating crops of midseason drained rice and upland crops (two alternatives for each baseline scenario). We used a set of first‐order atmospheric models to track the atmospheric burden of each gas over 500 years. We evaluated the dynamics of the radiative forcing due to the changes in emissions of CH4, N2O, and CO2 (alternative minus baseline), and compared these with standard calculations of CO2‐equivalent emissions using global warming potentials (GWPs). All alternative scenarios had lower CH4 emissions and higher N2O emissions than their corresponding baseline cases, and all but one sequestered carbon in the soil more slowly. Because of differences in emissions, in radiative forcing per molecule, and in atmospheric time constants (lifetimes), the relative radiative impacts of CH4, N2O, and CO2 varied over the 500‐year simulations. In three of the six cases, the initial change in radiative forcing was dominated by reduced CH4 emissions (i.e. a cooling for the first few decades); in five of the six cases, the long‐term radiative forcing was dominated by increased N2O emissions (i.e. a warming over several centuries). The overall complexity of the radiative forcing response to changing water management could not easily be captured with conventional GWP calculations.

Estimation of Methane Emission from Rice Cultivation in Korea

This study focused on estimating a methane (CH4) inventory and developing mitigation options in South Korea, and was performed jointly in an integrated national research program on CH4 from rice fields conducted by three National Agricultural Research Institutes, under the Rural Development Administration during 1993–1997. Methane emissions were measured by a closed chamber method, in rice plots at three locations (Suwon, Iksan, and Milyang) with the single rice cropping system. All experimental data from 5 years of study were summarized and used for calculating nation-wide CH4 emissions. Temperature, soil type, cultural practices, water management, organic matter management, and cultivar selection significantly affected the fluctuations of CH4 emissions. The two most promising mitigation options for reducing emissions were altered water management, in particular mid-season aeration by short-term drainage, and improved organic matter management, by promotion of aerobic degradation through composing or soil incorporation. Annual total CH4 emission in Korea changed from 410 Gg in 1990 to 339 Gg in 2000, due mainly to a decrease in the cultivated area of paddy rice. If we convert annual CH4 emission to Global Warming Potential as CO2 equivalent, it amounts to 7.1 M CO2 t yr−1 of greenhouse gas emitted to the atmosphere in 2000 from the rice fields in Korea, which is just 5.3% of the annual CO2 emission from the industry and energy sector. More importantly, the balance between CO2 uptake by photosynthesis and CO2 emission is positive (a net sink), so that rice culture actually has net benefits for the global atmospheric carbon issue. Further reductions in emission amounts, by following recommendations in this article, could make these benefits even greater.

Global methane emissions from landfills: New methodology and annual estimates 1980–1996

Significant interannual variations in the growth rate of atmospheric CH4 justify the development of an improved methodology for landfill emissions, the largest anthropogenic source in many developed countries. A major problem is that reliable solid waste data often do not exist, especially for developing countries where emissions are increasing. Here we develop and apply a new proxy method to reconstruct historical estimates for annual CH4 emissions for the period 1980–1996. Using composited solid waste data from 1975–1995, we developed linear regressions for waste generation per capita based on energy consumption per capita, a surrogate which reflects population and affluence, the major determinants of solid waste generation rates. Using total population (developed countries) or urban population (developing countries), annual landfill CH4 emissions were estimated using a modified Intergovernmental Panel on Climate Change (IPCC) methodology. Methane recovery was modeled by fitting historic data to time‐dependent linear relationships. Two scenarios for global emissions using the surrogate were compared to two scenarios using an IPCC standard methodology. Results from all four scenarios range from 16 to 57 Tg CH4 yr−1, a similar range as previous estimates. We support the use of the lower energy surrogate scenario (A) with annual emissions of 16–20 Tg CH4 yr−1, both positive and negative annual variations, and commercial recovery &gt;15% by 1996. The surrogate provides a reasonable methodology for a large number of countries where data do not exist, a consistent methodology for both developed and developing countries, and a procedure which facilitates annual updates using readily available data.

Factors controlling large scale variations in methane emissions from wetlands

Global wetlands are, at estimate ranging 115–237 Tg CH4/yr, the largest single atmospheric source of the greenhouse gas methane (CH4). We present a dataset on CH4 flux rates totaling 12 measurement years at sites from Greenland, Iceland, Scandinavia and Siberia. We find that temperature and microbial substrate availability (expressed as the organic acid concentration in peat water) combined explain almost 100% of the variations in mean annual CH4 emissions. The temperature sensitivity of the CH4 emissions shown suggests a feedback mechanism on climate change that could validate incorporation in further developments of global circulation models.

Annual CO2 exchange and CH4 fluxes on a subarctic palsa mire during climatically different years

Carbon dioxide and methane fluxes in a palsa mire characterized by dry (palsa) and wet surfaces in a subarctic zone in Finland were measured using a static chamber technique during two climatically different years, 1998 and 1999. Each of the 24 collars was individually studied for water table level, peat temperature, pH, vegetation, frost depth, CO2 exchange and CH4 fluxes. The annual gaseous carbon balance varied from −36.9 g C m−2 to −138.6 g C m−2 (uptake) on wet surfaces. During both years, palsa surfaces with shrub vegetation were sinks for carbon, whereas palsa surfaces with sparse vegetation lost carbon to the atmosphere. In 1999, a wet year, a lower NEE resulted mainly from a decrease in photosynthesis. The annual emissions of CH4 ranged from 1.0 g CH4‐C m−2 in the palsa to 24.7 g CH4‐C m−2 at the palsa margin. On wet surfaces, photosynthesis, respiration and CH4 fluxes were tightly linked. Annual NEE and CH4 fluxes were close to the values reported for boreal peatlands (fens).

Trace gas emissions from the production and use of domestic biofuels in Zambia measured by open‐path Fourier transform infrared spectroscopy

Domestic biomass fuels (biofuels) were recently estimated to be the second largest source of carbon emissions from global biomass burning. Wood and charcoal provide approximately 90% and 10% of domestic energy in tropical Africa. In September 2000, we used open‐path Fourier transform infrared (OP‐FTIR) spectroscopy to quantify 18 of the most abundant trace gases emitted by wood and charcoal cooking fires and an earthen charcoal‐making kiln in Zambia. These are the first in situ measurements of an extensive suite of trace gases emitted by tropical biofuel burning. We report emission ratios (ER) and emission factors (EF) for (in order of abundance) carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), acetic acid (CH3COOH), methanol (CH3OH), formaldehyde (HCHO), ethene (C2H4), ammonia (NH3), acetylene (C2H2), nitric oxide (NO), ethane (C2H6), phenol (C6H5OH), propene (C3H6), formic acid (HCOOH), nitrogen dioxide (NO2), hydroxyacetaldehyde (HOCH2CHO), and furan (C4H4O). Compared to previous work, our emissions of organic acids and NH3 are 3–6.5 times larger. Another significant finding is that reactive oxygenated organic compounds account for 70–80% of the total nonmethane organic compounds (NMOC). For most compounds, the combined emissions from charcoal production and charcoal burning are larger than the emissions from wood fires by factors of 3–10 per unit mass of fuel burned and ∼2 per unit energy released. We estimate that Zambian savanna fires produce more annual CO2, HCOOH, and NOx than Zambian biofuel use by factors of 2.5, 1.7, and 5, respectively. However, biofuels contribute larger annual emissions of CH4, CH3OH, C2H2, CH3COOH, HCHO, and NH3 by factors of 5.1, 3.9, 2.7, 2.4, 2.2, and 2.0, respectively. Annual CO and C2H4 emissions are approximately equal from both sources. Coupling our data with recent estimates of global biofuel consumption implies that global biomass burning emissions for several compounds are significantly larger than previously reported. Biofuel emissions are produced year‐round, disperse differently than savanna fire emissions, and could strongly impact the tropical troposphere.

Options for mitigating methane emission from a permanently flooded rice field

AbstractPermanently flooded rice fields, widely distributed in south and south‐west China, emit more CH4 than those drained in the winter crop season. For understanding CH4 emissions from permanently flooded rice fields and developing mitigation options, CH4 emission was measured year‐round for 6 years from 1995 to 2000, in a permanently flooded rice field in Chongqing, China, where two cultivations with four treatments were prepared as follows: plain‐cultivation, summer rice crop and winter fallow with floodwater layer annually (convention, Ch‐FF), and winter upland crop under drained conditions (Ch‐Wheat); ridge‐cultivation without tillage, summer rice and winter fallow with floodwater layer annually (Ch‐FFR), and winter upland crop under drained conditions (Ch‐RW), respectively. On a 6‐year average, compared to the treatments with floodwater in the winter crop season, the CH4 flux during rice‐growing period from the treatments draining floodwater and planting winter crop was reduced by 42% in plain‐cultivation and by 13% in ridge‐cultivation (P &lt; 0.05), respectively. The reduction of annual CH4 emission reached 68 and 48%, respectively. Compared to plain‐cultivation (Ch‐FF), ridge‐cultivation (Ch‐FFR) reduced annual CH4 emission by 33%, and which was mainly occurred in the winter crop season. These results indicate that draining floodwater layer for winter upland crop growth was not only able to prevent CH4 emission from permanently flooded paddy soils directly in the winter crop season, but also to reduce CH4 emission substantially during the following rice‐growing period. As an alternative to the completely drainage of floodwater layer in the winter crop season, ridge‐cultivation could also significantly mitigate CH4 emissions from permanently flooded rice fields.

Contribution of winter to the annual CH4 emission from a eutrophied boreal lake

The springtime methane (CH4) emission from a small, eutrophied boreal lake was assessed during the winter ice-cover by measurement of gas ebullition and CH4 accumulation in the water column in association with the development of oxygen depletion after ice formation. The winter CH4 production was estimated to result in a loss of 3.6-7.9 g CH4 m(-2) from the lake to the atmosphere during the short period of ice melt. This could account for 22-48% of the annual CH4 emission from the pelagic zone of the lake. The contribution of winter to the annual CH4 release can be similar or even higher in seasonally ice-covered northern aquatic ecosystems than in northern terrestrial wetlands, thus winter must be considered in any studies into the aquatic CH4 emissions. The trophic state and wintertime oxygen conditions, linked to the changes in land-use in the catchments and climate, are important factors controlling the springtime lake CH4 emissions.

Carbon gas exchange at a southern Rocky Mountain wetland, 1996–1998

Carbon dioxide (CO2) and methane (CH4) exchange between the atmosphere and a subalpine wetland located in Rocky Mountain National Park, Colorado, at 3200 m elevation were measured during 1996–1998. Respiration, net CO2 flux, and CH4 flux were measured using the closed chamber method during snow‐free periods and using gas diffusion calculations during snow‐covered periods. The ranges of measured flux were 1.2‐526 mmol CO2 m−2 d−1 (respiration), −1056−100 mmol CO2m−2 d−1 (net CO2exchange), and 0.1–36.8 mmol CH4m−2 d−1 (a positive value represents efflux to the atmosphere). Respiration and CH4 emission were significantly correlated with 5 cm soil temperature. Annual respiration and CH4 emission were modeled by applying the flux‐temperature relationships to a continuous soil temperature record during 1996–1998. Gross photosynthesis was modeled using a hyperbolic equation relating gross photosynthesis, photon flux density, and soil temperature. Modeled annual flux estimates indicate that the wetland was a net source of carbon gas to the atmosphere each of the three years: 8.9 mol C m−2 yr−1 in 1996, 9.5 mol C m−2 yr−1 in 1997, and 9.6 mol C m−2 yr−1 in 1998. This contrasts with the long‐term carbon accumulation of ∼0.7 mol m−2 yr−1 determined from 14C analyses of a peat core collected from the wetland.

Methane emission from paddy fields in Taiwan

In order to investigate the effect of environmental conditions on CH4 emission from paddy fields in Taiwan, four locations, two cropping seasons and two irrigation systems were studied. CH4 emission was high at the active tillering and the booting stages in the first cropping season, whereas it was low at the transplanting and the ripening stages with an intermittent irrigation system. CH4 emission was high at the transplanting stage in the second cropping season, and decreased gradually during rice cultivation. Daily temperature and light intensity increased gradually during rice growth in the first cropping season (February–June), while it was reversed in the second cropping season (August–December). The seasonal CH4 emission from paddy fields ranged from 1.73 to 11.70 g m–2, and from 10.54 to 39.50 g m–2 in the first and second cropping seasons, respectively. The seasonal CH4 emission in the second cropping season was higher than that in the first cropping season in all test fields. The seasonal CH4 emission was 32.65 mg m–2 in the first cropping season of the National Taiwan University paddy field with continuous flooding, and it was 28.85 mg m–2 in the second cropping season. The annual CH4 emission ranged from 12.3 to 49.3 g m–2 with an intermittent irrigation system, and the value was 61.5 g m–2 with a continuous flooding treatment. The annual CH4 emission from paddy fields was estimated to be 0.034 Tg in 1997 from 364,212 ha of paddy fields with an intermittent irrigation system, which was less than the 0.241 Tg calculated by the IPCC method with a continuous flooding treatment

Human impacts on methane emission from mangrove ecosystems in India

This study deals with the emission of methane in relation to changing environmental conditions and human impact, in three mangrove ecosystems of south India. Time-varying fluxes of methane adopting the close chamber technique were used to estimate CH4 emission from an unpolluted site (Pichavaram mangroves) and two polluted sites viz. (1) Ennore Creek mangroves (affected by fertilizer effluents and crude oil discharges) and (2) Adyar estuary mangroves (affected by the discharges of organic and industrial wastes), covering monthly and seasonal variations. The results indicate annual average CH4 emissions of 7.4, 5.02 and 15.4 mg m−2 h−1 from the sediment–water interface of the Pichavaram, Ennore Creek and Adyar estuary respectively. Emission characteristics obtained at Pichavaram mangroves represent a natural variability with changing physico-chemical factors, whereas the emission characteristics at Ennore Creek and Adyar estuary mangroves show anthropogenic influence. Several environmental factors such as oxygen availability, organic matter, soil physical and chemical properties, in addition to human-mediated interventions have been identified as influencing emission rates in the mangrove ecosystems. Preliminary CH4 emission estimates for the mangrove ecosystems along the Indian sub- continent and the tropical and subtropical coastline of the world by linear extrapolation based on surface area range from 0.05 to 0.37 and 2.8 to 19.25 Tg CH4 year−1 respectively. Our results also highlight the impact of human activities on future emission of methane from the mangrove ecosystems.

Evidence for a link between climate and northern wetland methane emissions

Wetlands are an important source of atmospheric methane (CH4), but the strength of this source and its sensitivity to potential changes in climate are still uncertain. In this study, continuous measurements from 1990 to 1998 of atmospheric CH4 from the Canadian observational sites at Fraserdale (49°53′N 81°34′W) and Alert (82°27′N 62°31′W) are used to estimate CH4 emissions from the Hudson Bay Lowland (HBL), a 320,000 km2 semicontinuous wetland region in central Canada. The HBL comprises ∼10% of the total area of northern wetlands. A conceptually simple approach was used to calculate the methane emission flux using the CH4 concentration difference between Alert and Fraserdale, the residence time of the air mass over the HBL, and the mixing height of the convective boundary layer. Emission rates estimated using this approach for 1990 compare well with empirical aircraft and tower flux measurements made within the HBL during the same time period, thus indicating that the methodology used is reasonable. Annual CH4 emission rates range from 0.23 to 0.50 Tg CH4 yr−1 and are much lower than many empirical flux measurements observed at other northern wetland sites. A seasonal temperature sensitivity with a Q10 of about 4 was found. Moreover, the observed interannual variations in emissions are well correlated to variations in annual air temperatures corresponding to a sensitivity of Q10 ≈ 7. That is, a 10°C change in annual temperature would result in a sevenfold change in wetland emissions which is much larger than Q10 values used in current global CH4 models (typically Q10 ≈ 1.5). Our findings suggest that northern wetland emissions are probably overestimated to date but may increase significantly due to predicted global warming.