B41D-01 08:05h
Climate / Greenhouse Gas Feedbacks in Northern Peatlands
Wetlands are relatively unique ecosystems in relation to greenhouse gas exchange. They often are net sinks for CO$_{2}$, and at the same time can be small to large sources of CH$_{4}$. In considering the effect of climate change on wetlands, the magnitude and direction of change in flux of both CO$_{2}$ and CH$_{4}$ must be considered. However, assessment of the strength of the climate feedback is not straight forward since CO$_{2}$ and CH$_{4}$ have different atmospheric life times and radiative strengths. We examine this problem by coupling a simple model of CO$_{2}$ and CH$_{4}$ perturbations to the atmosphere with a simple model that calculates carbon accumulation and methane emission for northern peatlands and is driven by mean climate data. We examine two cases of climate perturbation impacts and feedbacks for an eastern North American peatland: (1) a bog that has reached quasi-equilibrium ($\sim$15 kyr, representing a possible climax state), with net CO$_{2}$ flux about zero and a very small CH$_{4}$ flux; and (2) a bog that has not reached equilibrium ($\sim$8.5 kyr, representing a possible contemporary state), with net CO$_{2}$ uptake of about 20 g C m$^{-2}$ yr$^{-1}$ and CH$_{4}$ emission around 0 to 5 g m$^{-2}$ yr$^{-1}$. For the climate perturbation, we changed mean annual precipitation by $\pm$5%, $\pm$10%, and $\pm$20%. The results show that while the exchanges of CO$_{2}$ and CH$_{4}$ are affected by climate perturbation, they return to the conditions prior to the perturbation in several hundred years. These changed fluxes affect atmospheric burdens, and feedbacks to climate occur through changes in radiative forcing by CO$_{2}$ and CH$_{4}$.
B41D-02 INVITED 08:20h
Variability in Methane Emissions from a Temperate Poor Fen
Methane is the most abundant organic compound in Earth's atmosphere, where it is an efficient infrared radiation absorber. Methane influences the global climate by contributing approximately 25% to the radiative forcing of greenhouse gases. Total annual emissions of methane range from 500 to 600 Tg. Methane emissions from wetlands are the most important natural source but also one that is highly uncertain. Recent estimates vary by a factor of ca. 2.5. There is much uncertainty in our present understanding of methane dynamics. Without such knowledge, we cannot understand how the natural contribution to the global methane balance is likely to change under different climate scenarios in the future. Long term datasets are important in evaluating the responses of natural systems to climatic forcing. Methane exchange has been measured in Sallie's Fen in New Hampshire USA since 1989 using the same static chamber methodology at the same sites linked to consistent analytical standards from CMDL. Individual surface fluxes are highly variable on a daily to seasonal temporal scale indicating they are controlled by weather. The 16 year dataset shows that annual fluxes have varied by a factor of 3 (ca. 40-120 gC-CH$_{4}$ m$^{-2}$) with differing seasonal distributions of the flux. On average most of the flux is in the summer and about 4% in the winter with the spring and fall emissions dependent on weather. CH$_{4}$ emissions have been lower (ca. 40 g C m$^{-2}$) since 2000 compared to the previous decade (ca. 75 g C m$^{-2}$) probably because of a shift in nutrient status as peat accumulates and the fen grows to be more similar to a nutrient poor bog. Interpolation of a C balance model suggest that Sallie's Fen has been a sink of atmospheric CO2 from the atmosphere in with most (52 - 91 gC- CO$_{2}$ m$^{-2}$) fixed in the summer and a consistent loss of 12-19 gC- CO$_{2}$ m$^{-2}$ in the winter. However, when CH$_{4}$ emissions (48-122 gC-CH$_{4}$ m$^{-2}$) and DOC export (ca. 3 gC m$^{-2}$) are added to the total, Sallie's Fen lost C in 1994 and was only a weak sink of C in 1995-1997 demonstrating that CH$_{4}$ emissions can have a significant impact on the C balance of a natural wetland. Longer term decadal patterns in flux are controlled by the growth of the peatland and the surface hydrology or moisture regime which will be determined by climate.
B41D-03 08:45h
Factors Affecting Methane and Nitrous Oxide Emissions From Rice Fields
Methane and nitrous oxide are important non-CO2 greenhouse gases. Agriculture in general, and growing rice in particular, are important anthropogenic sources of both. It is recognized that agricultural practices greatly affect methane emissions from rice fields, while less is known about nitrous oxide. The most significant factors are water management and fertilizer applications. Continuous inundation of the fields during the growing season and organic manure applications lead to high fluxes of methane while intermittent flooding and application of nitrogen based fertilizers lead to low emissions. The latter practice is becoming increasingly prevalent in China and possibly elsewhere. It leads to low methane emissions but higher nitrous oxide emissions. We have conducted detailed field experiments at three locations in China -Tu Zu and Jin Sa in Sichuan Province and Quin Yuan in Guangdong Province. In these experiments we have used established fields and taken our measurements under the prevailing agricultural practices of the local farmers. In the later studies we asked the farmers for some changes in practices to investigate and compare the emissions under the various prevailing water management and fertilization practices. We will discuss the effects of these two variables on the fluxes of methane and nitrous oxide from rice fields as determined in the field.
B41D-04 INVITED 09:10h
A Simulation Model of Carbon Cycling and Methane Emissions in Amazon Wetlands
An integrative carbon study is investigating the hypothesis that measured fluxes of methane from wetlands in the Amazon region can be predicted accurately using a combination of process modeling of ecosystem carbon cycles and remote sensing of regional floodplain dynamics. A new simulation model has been build using the NASA-CASA concept for predicting methane production and emission fluxes in Amazon river and floodplain ecosystems. Numerous innovations area being made to model Amazon wetland ecosystems, including: (1) prediction of wetland net primary production (NPP) as the source for plant litter decomposition and accumulation of sediment organic matter in two major vegetation classes -- flooded forests (varzea or igapo) and floating macrophytes, (2) representation of controls on carbon processing and methane evasion at the diffusive boundary layer, through the lake water column, and in wetland sediments as a function of changes in floodplain water level, (3) inclusion of surface emissions controls on wetland methane fluxes, including variations in daily surface temperature and of hydrostatic pressure linked to water level fluctuations. A model design overview and early simulation results are presented.
http://geo.arc.nasa.gov/sge/casa/
B41D-05 09:35h
Land Use Effect on Methane Oxidation and Its Kinetics in Tropical Upland Soils
Upland soil represents the important biological sink for atmospheric methane. In the tropics, one of the most remarkable changes in land use in the past decades is deforestation. It is estimated that deforestation rates in the tropic are as rapid as 2% yr$^{-1}$. Such land use change may lead to loss of methane oxidation and thus may have implicated the global methane budgets. However, little information is available on magnitudes and variations of methane oxidation upon changing land use in the tropics. Here we report methane oxidation in three land use types in Thailand; a natural forest (SK soil), a reforested site (AC soil), and an agricultural field (CF soil). Monthly methane oxidation fluxes were measured with the closed chamber method during January to December 2003. Soil samples were also taken for kinetic study in the laboratory. Results reveal that methane oxidation occurred in all land use types but oxidation rate varied according to season, land use types, and sampling spots. Both SK and AC soils showed the oxidation rates comparable to that found in temperate forests. High rate of methane oxidation was found during the summer months. In raining season, net methane emission was occasionally observed, indicating the importance of soil moisture as the controlling factor. On one-year average basis, soil at both SK and AC forests were the net methane sinks (1.06 and 1.26 mg CH$_{4}$ m$^{-2}$ day$^{-1}$, respectively). On the other hand, high methane emission during raining season made soil at CF site became a net methane source on annual average basis. In SK and AC soils a clear zonation for active methane oxidation layer was detected along the soil depth. The most active oxidation layers in SK and AC soils lied between 15 cm and 40 cm while in CF soil no clear active layer was observed. Stratification of active oxidation zones coincides with the trends of inorganic nitrogen content profile. In SK and AC soils, high concentration of inorganic nitrogen compounds (usually $<$100 mg NO$_{3}$$^{-}$ or NH$_{4}$$^{+}$ kg soil$^{-1}$) was detected in the top 15-cm soil while there was no clear distribution trend found in CF soil. It was assumed that such high concentration of inorganic nitrogen in the topsoil inhibited the activity of methane oxidizing bacteria thus only in the subsoil that methanotrophs were active. Examining kinetic coefficients of these active layers revealed that soil at SK site had high affinity for methane (Km of 52 ppmv) but rather low methanotrophic capacity (Vmax of 0.82 nmol soil$^{-1}$ h$^{-1}$). Soil at AC and CF sites, on the other hand, showed low affinity for methane (Km of 724 ppmv and 1454-2362 ppmv, respectively). However, soils at these two sites were capable of oxidizing high concentration of methane (Vmax about 10 nmol soil$^{-1}$ h$^{-1}$). These results indicate that land use type significantly affects rates, depth distribution and kinetics of methane oxidation in tropical soils.
B41D-06 09:50h
Arctic Methane and the 8.2 kyr BP Event: Past Warning of Future Danger?
Methane mixing ratio is closely linked to temperature in the record. This implies strong feedbacks, but which factor drives and which responds remains controversial. Understanding near-past examples may tell us much about the possible future. The 8.2 kyr BP climate event (Spahni et al., 2003) is generally attributed to 8.45 kyr BP drainage of Glacial Lake Agassiz. However, the 8.2 kyr BP Storegga slide (Bondevik et al., 2003) and associated mega-earthquakes (with 5-15m offsets -Arvidsson, 1996) may have also contributed. Ice from the slide and surface freshwater introduced by floods from simultaneous lake ruptures may have temporarily collected in the northern Norwegian and Barents Seas. Earthquake uplift may have reduced Atlantic inflow over the then-upwarped entry to the Barents Sea. Permafrost and hydrate carried to depth by the slide would have introduced freshwater and gas to the deep Norwegian basin, perhaps briefly weakening Atlantic meridional overturning circulation. All these factors may have extended the time of winter ice cover in the Barents, Kara, and Norwegian seas, cooling boreal wetlands and reducing methane emissions. Conversely, hydrate in Storegga debris would have emitted methane for a century, perhaps hastening return of normality. If the 8.2 kyr BP event was simply the product of special events geologically unlikely today, that briefly shut down boreal wetland methane emissions, then there is little concern. But the 8.2 kyr BP event's lesson may lie as much in the rapid recovery as in the cooling:- that on a quasi-modern Earth, methane can increase sharply, by up to 100 ppb in a few years, just as it did in the glacial terminations (Nisbet, 2002). Future boreal warming, if moisture is adequate, could sharply increase methane and VOC emissions from boreal wetland and forest. Disseminated hydrate decay as well as local Arctic methane pockmark blowouts may occur in the mid-late 21st century if the Barents and Kara seafloors warm markedly. Slumps and earthquakes may occur in the high Arctic as ice melts. Though perhaps not large compared to other anthropogenic forcing effects, these short-term inputs to an already warming global climate system could perhaps trigger northward shift of northern summer jet streams, with significant consequences. Arvidsson, R. (1996) Science 274: 744-746 Bondevik. S et al.. (2003) EOS 84, 289, 293 Nisbet, E.G. (2002) Phil. Trans. R. Soc. Lond. A 360, 581-607 Spahni, R., et al. (2003) G.R.L., 30, 25-1-25-4.