B24A-01
25+ Years of the Global Methane Cycle: Can We Reconcile Atmospheric Observations and Source Emissions?
Observations of atmospheric methane for the last 25 years show variability and trends that both improve and confound our understanding of the global methane cycle. Methane concentrations from the NOAA ESRL and WMO GAW networks, and recent satellite observations from SCIAMACHY, provide a crucial suite of data to understand the global methane budget including temporal and spatial distributions of terrestrial sources and their role in observed trends and interannual variations in the atmosphere. During the same period, improvements in multiple atmospheric modeling approaches, developments in modeling of wetland and wildfire dynamics, and more robust approaches for estimating anthropogenic sources provide better tools to understand and explain long- and short-term dynamics in the global methane cycle. We report on results from ~25-yr model simulation, 1980 through 2006, using reanalysis . Improvements over our previous studies include: extending the study period from 2003 to 2006, updating all source estimates to 2006; addressing problems in soil-temperature data for the wetland model, and augmenting the suite of the ground network of atmospheric methane observations with satellite retrievals.
B24A-02
Analysis of boundary layer methane, nitrous oxide, carbon dioxide and carbon monoxide measurements over California during the ARCTAS/CARB flights
High precision, 1-sec resolution, in situ measurements of methane (CH4), nitrous oxide (N2O), carbon dioxide (CO2), and carbon monoxide (CO) were made on board the NASA DC-8 aircraft during the summer 2008 Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) and California Air Resources Board (CARB) deployments. The first 4 CARB flights on June 18, 20, 22 and 24 and the ARCTAS transit flights (between Palmdale, CA and Cold Lake, Alberta on June 26 and July 13) include many hours of low-level (~1000 ft AGL) boundary layer sampling over varied geographical regions (e.g. Los Angeles and its shipping lanes, the Central Valley, and croplands near Sacramento) before and during the California wildfires. We investigate highly-correlated time series of CH4, N2O, CO2 and CO to characterize emissions from a variety of sources including urban centers, agricultural lands, oil fields, rice paddies, feed lots, wooded regions, wildfire smoke plumes, etc.
B24A-03 INVITED
Stable Isotope Constraints of Atmospheric Methane Budgets
Recent reports of methane concentrations in ice records, including the 800 kyr Antarctic EPICA ice core, confirm that the present day tropospheric methane mixing ratios of over 1.8 ppm are anomalously high for this millennial time period (PIH/LP methane mean is 0.52 ppm). Our understanding and quantification of the changes to and intensity variations of tropospheric methane sources and sinks remains imperfect. Stable carbon and hydrogen isotope ratios of atmospheric methane offer assistance to check our assessments of methane fluxes. However, due to a) the number and shifts of methane sources and sinks, b) substantial overlap in isotope signatures and c) variations on both annual and geologic time scales, the isotope mass balances are under-constrained. Our attempts to reconcile present day and past atmospheric methane mass balances continue to challenge us. Despite this limitation, isotope information is essential to elucidate and characterize the processes and pathways affecting global atmospheric methane systematics. This presentation will outline these mass balancing acts and discuss these issues surrounding the stable isotopes of methane.
B24A-04
Application of Stable Isotope Analysis to Atmospheric Methane Source Identification Using the TROICA Measurements and a Comparison with Model Results
Application of stable isotope analysis to atmospheric methane is a useful approach which can improve our understanding of sources, their distribution and variability. Most biogenic (in particular wetland) methane sources are isotopically depleted (contain less 13C and less D) relative to natural gas and biomass burning CH4. This offers a method to discriminate such source types. Here we present a comparison between modeled and measured methane concentration and 13C and D isotope data. For the model simulations we use the global transport chemistry model TM3. The air samples on which the comparison is based were collected along the Trans-Siberian railroad in summer 1999 and 2001 and spring 2004 during the TROICA campaigns (www.troica-environmental.com). The measurements of concentration were partly in situ, partly based on flask samples. These samples were also used for the isotope analyses. For the summer expeditions it was found that biogenic sources prevailed, although in a number of samples a substantial contribution of natural gas methane was detected. For the TM3 model comparison we used two scenarios. In the base scenario the contribution of the biogenic source to the observed 13C(CH4) and D(CH4) levels was underestimated and/or the contribution of the anthropogenic sources was overestimated. In the test scenario we increased the biogenic emission by 50%, which led to improved agreement between source isotopic signature (both 13C(CH4) and D(CH4)) estimated for the modeled and measured datasets. For the spring expedition the detection of the prevailing methane source appears more complicated due substantial variability of the isotopic signatures of contributing sources (mainly anthropogenic). In the model simulation the methane source signature was close to that of natural gas CH4. The work is financially supported by European Commission (Marie-Curie IIF project N 039905 - FP6-2005- Mobility-7).
B24A-05
Methane from Arctic Lakes: Observations from 50 lakes in Alaska and Siberia
Microbial methane emitted from arctic lakes (currently 15-35 Tg yr-1) is a significant portion of global atmospheric methane sources despite strong variability in the gas" origin and spatial patterns of emission. Observations from 2003-2008 of methane bubbling from 50 lakes in Alaska and Siberia are presented. In-situ flux measurements, methane isotopes and remote sensing of terrestrial permafrost changes revealed that thawing permafrost is a key driver of methane production and emission from arctic lakes. 700-950 Gt of organic carbon currently sequestered in permafrost will be subject to microbial decomposition and production of methane and CO2 when permafrost warms and thaws. Release of microbial methane from arctic lakes is expected to increase to hundreds of Tg per year as climate warming accelerates widespread thaw and degradation of permafrost beneath boreal forest and tundra lakes over the Arctic. Permafrost thaw may lead to an additional source of methane if expanding thaw bulbs beneath lakes and rivers intersect faults and unconsolidated sediments leading to the escape of geologic methane sources, such as those recently observed on the North Slope of Alaska (60-100 Kg CH4 m-2 d-1). Results presented here aim to improve understanding of microbial and geologic methane emission dynamics from various regions of the Arctic in order to better constrain current and future atmospheric methane budgets.
B24A-06
Methane emissions from a high arctic valley: findings and challenges
Wet tundra ecosystems are well-known to be a significant source of atmospheric methane. With the predicted stronger effect of global climate change on arctic terrestrial ecosystems compared to lower-latitudes, there is a special obligation to study the natural diversity and the range of possible feedback effects on global climate that could arise from Arctic tundra ecosystems. One of the prime candidates for such a feedback mechanism is a potential change in the emissions of methane. Long-term datasets on methane emissions from high arctic sites are almost non-existing but badly needed for analyses of controls on interannual and seasonal variations in emissions. To help fill this gap we initiated a measurement program in a productive high arctic fen in the Zackenberg valley, NE Greenland. Methane flux measurements have been carried out at the same location since 1997. Compared with the manual chamber measurements from the late 1990s, however, an automatic chambers system with laser off- axis integrated-cavity output spectroscopy analyzer, in place since 2005, dramatically increased the time resolution of more recent data. The latest data brought up some intriguing findings on seasonal variations in methane fluxes which will be presented and discussed.
B24A-07
Methane emission rates from the Arctic coastal tundra at Barrow are log-normally distributed: Is this a tail that wags climate?
Over the past two growing seasons, we have conducted >200 point measurements of methane emission and ecosystem respiration rates on the Arctic coastal tundra within the Barrow Environmental Observatory. These measures reveal that methane emission rates are log-normally distributed, but ecosystem respiration rates are normally distributed. The contrast in frequency distributions indicates that methane and carbon dioxide emission rates respond in a qualitatively different way to their environmental drivers: while ecosystem respiration rates rise linearly with increasing temperature and soil moisture, methane emissions increase exponentially. Thus, the long positive tail in methane emission rates does generate positive feedback on climate change that is strongly non-linear. To further evaluate this response, we examined the spatial statistics of our dataset, and conducted additional measures of carbon flux from points on the landscape that typically had the highest rates of methane emission. The spatial analysis showed that neither ecosystem respiration nor methane emission rates have spatial co-correlation beyond that predicted by macroscopic properties of vegetation (e.g., species composition, plant height) and soil (e.g., permafrost depth, temperature, water content), suggesting that our findings can be used to scale up. Our analysis of high-emission points focused on wet and flooded areas where Carex aquatilis growth was greatest. Here, we found variation in methane emission rates to be correlated with Carex aboveground biomass and rates of gross primary production, but not ecosystem respiration. Given the sensitivity of Carex's phenotype to inundation, permafrost depth and soil temperature, we anticipate that the magnitude the climate-methane feedback in the Arctic coastal plain will depend strongly on how permafrost thaw alters the ecology of Carex aquatilis.
B24A-08
Estimating Spatially Heterogeneous Contributions to Ecosystem Scale Fluxes Directly From Eddy Covariance Measurements: A Case Study in Siberian wet Polygonal Tundra on Samoylov Island, Lena River Delta
The eddy covariance method is used widely to measure the turbulent exchange fluxes of climate relevant gases such as carbon dioxide and methane. One important assumption in eddy covariance theory is homogeneity of the surface over which measurements are conducted. However, in reality the method is often applied in very heterogeneous areas and the effect of that heterogeneity on the measurement time series is not fully agreed on in the scientific community. Since the eddy covariance method relies on the time-for- space substitution concept (Taylor's frozen turbulence field assumption), spatial disturbance of the assumed homogeneity should leave similar traces in the time series data that could be detected with state-of-the-art times series statistics approaches, primarily frequency analysis. This should be easiest to detect where the observed heterogeneity is characterized by the steepest possible small-scale spatial contrast of fluxes while also exhibiting certain regularity. Thus, polygonal tundra with its regular micro-relief of very wet polygon depressions with high rates of photosynthesis and methane emission on the one hand and relatively "dry" elevated polygon rims with lower rates of photosynthesis, higher rates of respiration, and extremely low methane emissions on the other hand is considered well-suited to explore the performance of this technique of flux footprint separation. We present a case study using eddy covariance data of water vapour, carbon dioxide and methane from the Lena River Delta, Siberia, and high-resolution aerial photography to demonstrate that spatial heterogeneity correlates with deviations in eddy covariance co-spectra from the idealized co-spectra. This new method - if successful beyond a single case study - could become widely used wherever fluxes are measured over spatially heterogeneous surfaces. It would be especially helpful to move towards more accurate upscaling in areas where emission rates and processes vary greatly on small spatial scales, such as the vast and hard to access high latitude tundra ecosystems.