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Wetlands are one of the most important sources of atmospheric methane (CH4), but the strength of this source is still highly uncertain. To improve estimates of CH 4 emission at the regional and global scales and predict future variation requires a process-based model integrating the controls of climatic and edaphic factors and complex biological processes over CH 4 flux rates. This study used a methane emission model based on the hypothesis that plant primary production and soil organic matter decomposition act to control the supply of substrate needed by methanogens; the rate of substrate supply and environmental factors, in turn, control the rate of CH 4 production, and the balance between CH 4 production and methanotrophic oxidation determines the rate of CH 4 emission into the atmosphere. Coupled to data sets for climate, vegetation, soil, and wetland distribution, the model was used to calculate spatial and seasonal distributions of CH 4 emissions at a resolution of 1 o latitude x 1 o longitude. The calculated net primary production (NPP) of weftands ranged from 45 g C m -2 yr -• for northern bogs to 820 g C m -2 yr -1 for tropical swamps. CH 4 emission rates from individual gridcells ranged from 0.0 to 661 mg CH 4 m -2 d -1, with a mean of 40 mg CH 4 m -2 d -• for northern wetland, 150 mg CH 4 m -2 d -1 for temperate wetland, and 199 mg CH 4 m -2 d -1 for tropical wetland. Total CH 4 emission was 92 Tg yr -•. Sensitivity analysis showed that the response of CH 4 emission to climate change depends upon the combined effects of soil carbon storage, rate of decomposition, soil moisture and activity of methanogens. CH4; Seiler and Conrad [1987] reported that total emission was 47 Tg yr '•, of which 38 Tg yr -• was released from tropical wetlands. However, field measurements in the 1980s revealed that northern tundra and peatlands also made a major contribution to the global source strength. Matthews and Fung [1987] generated a global distribution of wetlands from wetland vegetation types, pond soils, and land inundation data. Using this data set and CH 4 flux rates measured in the early 1980s, these authors estimated that the annual emission of CH 4 from natural wetlands was 111.1 Tg, and that northern wetlands contributed 60% of this total emission. Aselmann and Crutzen [1989] developed a different data set for wetland distribution and CH 4 flux rates. Their estimate of annual CH 4 emission was in the range 40 to 160 Tg. Bartlett and Harris [1993] recently reviewed the published measurements of CH 4 flux rates from wetlands globally, and using averaged CH 4 fluxes for various wetland types, they calculated the total CH 4 emission to be 109 Tg yr -•. Considerable uncertainty exists in the estimates of CH4 emissions at both the regional and global scales since, although there has been a large increase in the number of CH 4 flux rate measurements, their spatial coverage is still poor, and most of the measurements were made over less than 1 m 2 and over periods of only a few months. Few studies have records spannin...

Evaluating the role of terrestrial ecosystems in the global carbon cycle requires a detailed understanding of carbon exchange between vegetation, soil, and the atmosphere. Global climatic change may modify the net carbon balance of terrestrial ecosystems, causing feedbacks on atmospheric CO2 and climate. We describe a model for investigating terrestrial carbon exchange and its response to climatic variation based on the processes of plant photosynthesis, carbon allocation, litter production, and soil organic carbon decomposition. The model is used to produce geographical patterns of net primary production (NPP), carbon stocks in vegetation and soils, and the seasonal variations in net ecosystem production (NEP) under both contemporary and future climates. For contemporary climate, the estimated global NPP is 57.0 Gt C y–1, carbon stocks in vegetation and soils are 640 Gt C and 1358 Gt C, respectively, and NEP varies from –0.5 Gt C in October to 1.6 Gt C in July. For a doubled atmospheric CO2 concentration and the corresponding climate, we predict that global NPP will rise to 69.6 Gt C y–1, carbon stocks in vegetation and soils will increase by, respectively, 133 Gt C and 160 Gt C, and the seasonal amplitude of NEP will increase by 76%. A doubling of atmospheric CO2 without climate change may enhance NPP by 25% and result in a substantial increase in carbon stocks in vegetation and soils. Climate change without CO2 elevation will reduce the global NPP and soil carbon stocks, but leads to an increase in vegetation carbon because of a forest extension and NPP enhancement in the north. By combining the effects of CO2 doubling, climate change, and the consequent redistribution of vegetation, we predict a strong enhancement in NPP and carbon stocks of terrestrial ecosystems. This study simulates the possible variation in the carbon exchange at equilibrium state. We anticipate to investigate the dynamic responses in the carbon exchange to atmospheric CO2 elevation and climate change in the past and future.

The interest in national terrestrial ecosystem carbon budgets has been increasing because the Kyoto Protocol has included some terrestrial carbon sinks in a legally binding framework for controlling greenhouse gases emissions. Accurate quantification of the terrestrial carbon sink must account the interannual variations associated with climate variability and change. This study used a process-based biogeochemical model and a remote sensing-based production efficiency model to estimate the variations in net primary production (NPP), soil heterotrophic respiration (HR), and net ecosystem production (NEP) caused by climate variability and atmospheric CO 2 increases in China during the period 1981±2000. The results show that China's terrestrial NPP varied between 2.86 and 3.37 Gt C yr 21 with a growth rate of 0.32% year 21 and HR varied between 2.89 and 3.21 Gt C yr 21 with a growth rate of 0.40% year 21 in the period 1981±1998. Whereas the increases in HR were related mainly to warming, the increases in NPP were attributed to increases in precipitation and atmospheric CO 2 . Net ecosystem production (NEP) varied between 20.32 and 0.25 Gt C yr 21 with a mean value of 0.07 Gt C yr 21, leading to carbon accumulation of 0.79 Gt in vegetation and 0.43 Gt in soils during the period. To the interannual variations in NEP changes in NPP contributed more than HR in arid northern China but less in moist southern China. NEP had no a statistically significant trend, but the mean annual NEP for the 1990s was lower than for the 1980s as the increases in NEP in southern China were offset by the decreases in northern China. These estimates indicate that China's terrestrial ecosystems were taking up carbon but the capacity was undermined by the ongoing climate change. The estimated NEP related to climate variation and atmospheric CO 2 increases may account for from 40 to 80% to the total terrestrial carbon sink in China.