Over the past century, atmospheric methane (CH4) rose dramatically before leveling off in the late 1990s. The processes controlling this trend are poorly understood, limiting confidence in projections of future CH4. The MOZART-2 global tropospheric chemistry model qualitatively captures the observed CH4 trend (increasing in the early 1990s and then leveling off) with constant emissions. From 1991–1995 to 2000–2004, the CH4 lifetime versus tropospheric OH decreases by 1.6%, reflecting increases in OH and temperature. The rise in OH stems from an increase in lightning NOx as parameterized in the model. A simulation including annually varying anthropogenic and wetland CH4 emissions, as well as the changes in meteorology, best reproduces the observed CH4 distribution, trend, and seasonal cycles. Projections of future CH4 abundances should consider climate-driven changes in CH4 sources and sinks.
In the ALE/GAGE/AGAGE global network program, continuous high frequency gas chromatographic measurements of two biogenic/anthropogenic gases (methane, CH4; nitrous oxide, N2O) and six anthropogenic gases (chlorofluorocarbons CFCl3, CF2Cl2, and CF2ClCFCl2; methyl chloroform, CH3CCl3; chloroform, CHCl3; and carbon tetrachloride, CCl4) have been carried out at five globally distributed sites for over 20 years. Data are currently available for all of the species, although chloroform data are only available for recent years. Additional important species have been added at select sites during the course of the program. These include: hydrogen (H2), carbon monoxide (CO), methyl chloride (CH3Cl), methyl bromide (CH3Br), HFC-134a, HFC-152a, HCFC-141b, HCFC-142b, HCFC-22, Halons 1211 and 1301, sulfur hexafluoride (SF6), methylene chloride (CH2C12), trichloroethylene (C2HC13), and tetrachloroethylene (C2C14). The last 3 short-lived gases arise from solvents; AGAGE is attempting to assess their source terms, trends, and seasonal influences that are related to their destruction by the hydroxyl (OH) radical. Units are dry air mole fractions in parts per 10 12 (picomoles/mole) for all halocarbons and SF6, and parts per 10 9 (nanomoles/mole) for nitrous oxide, methane, carbon monoxide and hydrogen.
The current station locations are Cape Grim, Tasmania (41° S, 145° E), Cape Matatula, American Samoa (14° S, 171° E), Ragged Point, Barbados (13° N, 59° W), Mace Head, Ireland (53° N, 10° W), and Trinidad Head, California (41° N, 124° W)(Pictured below). Stations also previously existed at Cape Meares, Oregon (45° N, 124° W), and Adrigole, Ireland (52° N, 10° W). The Mace Head station came on line in January, 1987 as a replacement for the Adrigole station which ceased operations at the end of December, 1983; the Trinidad Head station began operations during AGAGE in October 1995, as an essential replacement for the Cape Meares station from which GAGE data were not available after June, 1989.
The 13th WMO/IAEA Meeting of Experts on Carbon Dioxide Concentration and Related Tracer Measurement Techniques will be held in Boulder, Colorado, USA, 19-22 September 2005, and will be hosted by NOAA CMDL, CIRES/University of Colorado, and NCAR. The meeting is co-sponsored by the World Meteorological Organization (WMO) and the International Atomic Energy Agency (IAEA). For information regarding topics, participation, and accommodation, etc., please visit the Web site (http://www.cmdl.noaa.gov/ccgg/co2experts/).