Air Pollution as a Climate Forcing: A Workshop
Day 1 Presentations
Measurement Constraints on the Global CH4 Budget
NOAA Climate Monitoring and Diagnostics Laboratory, Boulder, CO, U.S.A.
Atmospheric methane is an important greenhouse gas and affects the chemistry of the troposphere and stratosphere, yet details of its budget, and how that budget is changing with time, are not well known. A quantitative understanding of CH4 sources and sinks is necessary to formulate reasonable strategies to mitigate its potential influence on climate and air quality.
NOAA CMDL has made high-precision measurements of atmospheric methane from a globally-distributed network of air sampling sites since 1983. Air samples are collected approximately weekly and analyzed in Boulder, Colorado against an internally consistent standard gas scale.
Above is a plot of zonally-averaged atmospheric CH4 at earth's surface for 1992-2001. The plot is based on measurements from about 50 sites, which have been smoothed spatially and temporally. Though the plot is not very quantitative by itself, it qualitatively demonstrates the main features in the global distribution of atmospheric CH4. These features include the long-term increase (because CH4 sources and sinks are not in balance), seasonal variations (because of the photochemical sink, which is strongest during summer), and the strong latitude gradient in CH4 (because ~2/3 of emissions are in the northern hemisphere).
From the matrix of CH4 values used to define this plot, the following quantitative constraints on the global CH4 budget are obtained:
Atmospheric methane burden: the globally, annually averaged CH4 abundance for 2001 is 1751 nmol/mol, corresponding to a burden of 4840 Tg (where 1 Tg = 1012g) CH4.
Source/sink imbalance: the globally averaged CH4 growth rate decreased from ~14 nmol/mol·yr in 1984 (source-sink ~40 Tg CH4) to near zero in 2000 and 2001. Relative uncertainties in the burden and source/sink imbalance are small, likely less than 5%.
Annual global emissions: with an estimate of the global CH4 lifetime, global emissions can be calculated: Emissions = Increase + Burden/Lifetime. Using a lifetime of 9 years, constant over the period of measurements, global emissions averaged ~540 Tg/yr, with no significant trend. Uncertainty in total global emissions (possibly ~15%) is larger than that in the burden and source/sink imbalance and driven by the uncertainty in CH4 atmospheric lifetime.
Interannual variations in emission rates from individual sources: in cases where large, observed changes in CH4 growth rate or spatial gradients can be linked to an "event", the measurements can be used to quantify variations in emissions from specific sources. Strong growth rate anomalies in 1992 (negative) and 1998 (positive) have been linked to interannual variations in temperature and soil moisture content in wetland regions, thereby affecting CH4 emissions, and emphasizing the strong link between wetland CH4 emissions and climate. For the 1998 anomaly, observations suggested that global emissions were greater than average by 24 Tg CH4; a process based model, which included soil-temperature and precipitation anomalies, was used to calculate CH4 emission anomalies from wetlands of +24.6 Tg CH4, split nearly equally between high-northern latitudes and the southern tropics. The excellent agreement between process model and observations is likely somewhat fortuitous, since global process models of wetland emissions are in early stages of development. But the result emphasizes the usefulness of the measurements in constraining models of CH4 emission processes.
Long-term changes in emission rates from individual sources: a comparison of the difference in CH4 abundance between northern and southern polar latitudes from a 3-D chemical transport model with that from observations suggests a step decrease in emissions from a high northern latitude source in 1992. We speculate that this step change resulted from decreased emissions on order of 10 Tg CH4 from the fossil fuel sector in the former Soviet Union.
In summary, the measurements provide important constraints on the global CH4 budget. Uncertainties associated with these constraints cover a wide range, from the global burden and trend, with relatively small uncertainties, to speculative scenarios of changes in specific sources that can only be tested with models. Improvements in using measurements to constrain the global CH4 budget will come by expansion of the current network of low-frequency sampling, addition of high-frequency measurements downwind of strong CH4 source regions, and improvements to models.