B23E-01 INVITED
Perspectives On The Global Budget of Methane
Early budgets of methane focused on the emissions from individual sources but the estimates had large uncertainties. These uncertainties have been reduced considerably in recent years, but we need an understanding of the trends in the sources as well as their spatial distributions if we are to use methane to control global warming. A nearly 30 year long time series of global atmospheric methane concentrations has accumulated that can provide some of the answers. One of the most dramatic findings is that the increase of methane has nearly stopped in the last decade. But the record also shows that the trend was falling ever since systematic measurements were taken, and perhaps even before that. This finding has led to some puzzles. There is a belief that the anthropogenic sources of methane are increasing but to explain the falling trend we need decreasing sources (or increasing sinks). In fact, the atmospheric measurements show only that the most probable explanation for the decreasing trend and the present near constancy of concentrations is that the global source of methane has been more or less constant over the last 30 years with many short-term ups and downs. Moreover, there is good evidence that some of the major man-made sources of methane, such as cattle, biomass burning and possibly others, have stopped increasing some time back and other sources such as rice agriculture may have decreased over the last 30 years. This allows some smaller energy based sources to have increased, consistent with expectations, and balance out the decreasing sources to keep the total more or less constant. A credible quantitative case can be made for a stable global source based on available information on the trends of the various sources and sinks of methane, but uncertainties remain. We will argue that the stability of sources and sinks is the most likely explanation of the methane concentration trends. We will use this result to re-evaluate the future of man- made methane and its role in global warming. The current IPCC scenarios project a wide range of possible anthropogenic emissions by the year 2100 from 240 Tg/y, which is 25% less than present emissions to 1070 Tg/y which is more than 3 times present emissions. The stabilization or reduction in major man-made sources at this time greatly limits the possibility of major increases in these sources in the future. We will discuss the expected trends of sources to reduce the uncertainty in projected concentrations. These results will in turn contribute to a more realistic use of methane in controlling global warming under current and pending policies or treaties to control greenhouse gas emissions. This research was supported by the Office of Science (BER), U.S. DOE grant # DE-FG02-08ER64515 and DE-FG02-04ER63913.
B23E-02
Constraining Changes in the Global CH4 Budget Based on Global Ethane Changes: Insights From 30 Years of Global Trace Gas Monitoring
UC-Irvine has directly monitored global CH4 mixing ratios for 30 years, since 1978, and it is the longest- running of the continuous global monitoring networks. Every three months 80 ground-level whole air samples are collected in the remote Pacific Basin (71 °N-46 °S) and analyzed by gas chromatography for many dozens of trace gases including hydrocarbons, halocarbons, organic nitrates and sulfur species. For example our network is unique in that, in addition to CH4, ethane has been retrieved from each air sample since 1984. Tetrachloroethene (C2Cl4) has been retrieved since 1988. Methane, ethane, and C2Cl4 are all OH-controlled species, but only CH4 and ethane share common anthropogenic sources (fossil fuel and biomass burning). Therefore CH4, ethane and C2Cl4 are a powerful combination to help us determine which source and sink variations are consistent with the observed CH4 trends. In the long-term, methane's annual growth rate has slowed from 15.2 to 18.9 ppbv yr-1 in the early-to- mid 1980s, to -3.8 to 6.6 ppbv yr-1 since 2000. For the first time, our network has simultaneously shown a long-term decline in the global ethane mixing ratio, by 150 pptv (20%) in the past two decades. In the short-term, CH4 has shown positive growth rate anomalies every 3½--4½ years since 1991, the fifth and most recent of which peaked at 6.6 ± 0.9 ppbv yr-1 in 2007. Interestingly, each short-term CH4 growth rate peak has been matched by a peak in ethane's mixing ratio. Coincident CH4 and ethane fluctuations that are not matched by C2Cl4---for example the 1998, 2002-2003, and 2007 CH4 and ethane fluctuations---point to anomalous biomass burning emissions at those times as an influencing source. Methane and ethane are emitted from biomass burning in quantitative ratios relative to each other and to CO, and our results suggest that roughly one-third of the 1998 and 2007 CH4 growth rate anomalies were attributed to biomass burning, with the remainder from anomalous wetlands emissions. By comparison, we found that, to within the uncertainties, almost all of the 2002-2003 CH4 growth rate anomaly was explained by biomass burning. We also show an apparent link between strong El Niño events and strong biomass burning emissions in the northern hemisphere during the following growing season.
B23E-03
Attribution of Recent Methane Growth and Variability
Since the 1990's, the observed growth rate of methane has decreased to nearly zero, although with periods of significant variability. The causes of the inter-annual variability, as we demonstrate, are biomass burning and the response of wetland emissions to temperature and precipitation. Of particular interest is the anomalously high growth of methane during 2007, which appears to have been driven entirely by warm, wet conditions over Arctic and Tropical wetlands. Although growth of atmospheric methane appears to have stabilized, there are reasons to expect that faster growth will once again occur due to increases in anthropogenic sources and, possibly, consistent warming over high latitude wetlands. As an example, emission inventories suggest that coal production in China has doubled since 2000. Both the growth in emissions from Chinese coal production and the 2007 high-latitude wetland anomaly present opportunities to test the ability of an assimilation/flux estimation system to recover the emissions. In this study, we evaluate the ability of a prototype ensemble Kalman smoother flux estimation technique coupled with a global atmospheric tracer transport model to recover and attribute both the 2007 anomaly, and the longer-term trend in Chinese coal emissions.
B23E-04 INVITED
Sensing methane from space - Results from SCIAMACHY onboard ENVISAT
Methane is the second most important anthropogenic greenhouse gas. Although the global budget is relatively well constrained, partitioning among sources remains highly uncertain limiting our ability to make reliable future projections of climate change. SCIAMACHY from its vantage point in space offers the unique opportunity to measure methane concentrations globally with high sensitivity towards the surface. Previous investigations of SCIAMACHY-observed methane received much attention since they pointed to grossly underestimated emissions in the tropics. Until then, tropical methane emissions were speculated to be very important but were weakly constrained in absence of suitable measurements. We will present an overview of the findings so far and report new results from a revised retrieval version using updated spectroscopic parameters. Especially over tropical regions, the new retrieval version is systematically lower than previously and we will discuss the origin of the errors and the impact on tropical emission estimates. We will present how satellite measurements can be used to invert sources by means of a four-dimensional variational (4D-Var) data assimilation system. Further, we will give a brief general overview of the current and future potential of satellite measurements with respect to methane source estimations.
B23E-05
Validation of Tropical CH4 Emission Inventories Using SCIAMACHY Measurements
Methane is an important greenhouse gas in the Earth's atmosphere, with an estimated contribution of 18% to the present-day radiative forcing caused by long-lived greenhouse gases. The future contribution of methane to climate change is very difficult to predict, as illustrated by the fact that the current level of methane is already significantly outside the envelope of scenarios that were formulated about a decade ago. Methane emissions in the tropics are an important term in the global uncertainty budget, and therefore key to improving our understanding of the global concentration evolution of methane. The SCIAMACHY satellite instrument has provided us a new view on atmospheric methane, greatly expanding the measurement coverage particularly in the tropics. It provided interesting new pieces of the puzzle of the global methane budget, however, the question remains how they fit together. In this study, several bottom-up inventories have been compiled of methane emissions from rice paddies and natural wetlands, which together account for a large fraction of the tropical emission uncertainty. Atmospheric transport model simulations have been carried out to study the concentration differences that arise from the differences between these emission inventories. The results of the simulations have been compared with SCIAMACHY CH4 measurements. We will discuss which inventories are most consistent with the satellite data and the extent to which the satellite measurements can discriminate between them given measurement and transport model uncertainties. Further, we will address the question which future CH4 monitoring system would be most effective in further reducing the uncertainty of tropical methane emissions, and what be can expected from planned network expansions and the upcoming GOSAT satellite mission.
B23E-06
Toward an Adequate Quantification of Summer CH4 Emissions From Asia Using Satellite Observation by AIRS and Atmospheric Transport Model
The emission of CH4 from rice paddies is one of the major CH4 emission sources in Asia, and its largest emission occur during the rice growing season from the summer to the early autumn. Satellite observations of methane (CH4) using the Atmospheric Infrared Sounder (AIRS) on the EOS/Aqua platform from 2003 to present demonstrate a strong, plume-like enhancement of CH4 in the middle to upper troposphere over the South Asia during July, August and September, and its maximum occurs in early September. Simulations using the global tracer model version 3 (TM3) also show similar seasonal enhancement of CH4 in the same region. The model results also suggest that this enhancement is associated with transport process and local surface emissions. While most "top-down" approaches in estimating CH4 sources only utilize the ground-based measurement of CH4 concentration in the troposphere as the constraint to the models, the observation to CH4 in the middle to upper troposphere during the monsoon season may provide additional information to constrain the models for a better estimation of Asian CH4 sources. Comparison between AIRS observations and the model simulations indicate that the CH4 emission of 60 Tg/yr from rice paddies is possibly overestimated in the model. Further study using model simulation and long-term satellite observations by NOAA using AIRS, IASI and future operational sounders like CrIS in the next 20+ years may provide an adequate quantification of CH4 emissions from Asia for climate change study.
B23E-07 INVITED
Potential contributions of process modeling to understanding and constraining the global methane budget
The global methane budget is fairly well-constrained in aggregate, but the partitioning of global emissions into the various natural and anthropogenic sources is not. Direct measurements of emissions are essential for quantifying the various terms, but alone are not up to the task as some important source terms are widely distributed across the planet and often remote, and emissions can be highly variable in space and time. The various components of global methane emissions can also be estimated by inverse modeling, which can be constrained by the network of atmospheric concentration and isotopic measurements, but has coarse spatial resolution, and is not so useful for prognostic modeling. Process-based modeling can also help to constrain and evaluate the global methane budget. Using the DNDC model as an example, we review both strengths (e.g., spatial and temporal extrapolation), weaknesses (e.g., incomplete representations; generalization), and challenges (e.g., adequate ancillary data) of process models as a scientific tool for understanding the global methane budget.