Greenhouse gas variations observed from the surface over the past 25 years: groundtruthing future satellite instruments.

The perturbation to radiative climate forcing which has the largest magnitude and the least scientific uncertainty is the forcing related to changes in those greenhouse gases that are long-lived and well mixed in the atmosphere, in particular carbon dioxide (CO_2), methane (CH_4), nitrous oxide (N_2O) and the halocarbons (mainly CFCs). All of these long-lived greenhouse gases have been monitored around the world since the 1970's mainly by NOAA's Climate Monitoring and Diagnostics Laboratory (CMDL), in Boulder, Colorado, and its forerunner, the Geophysical Monitoring for Climatic Change (GMCC) program. CMDL operates four fully instrumented baseline climate observatories at Pt. Barrow, Alaska; Mauna Loa, Hawaii; American Samoa; and South Pole Station, Antarctica, where the concentrations of the greenhouse gases are measured continuously as well as by flask sample. In a cooperative program CMDL collects air samples in flasks from over 50 global sites. All flask air samples are analyzed for carbon gas concentrations and isotopic ratios in Boulder. The global cooperative sites are in clean, unpolluted regions such as on seacoasts where the measurements are generally representative of large-scale maritime regions. These data will be presented here and analyzed in terms of their changes and the changes in radiative forcing during the 25-year period encompassing 1978 through 2003. The most notable change in the past several years is that because the forcing by CFCs is declining and that due to methane has stopped increasing, the fraction related to carbon dioxide has increased from about 59% to 62%. The measurements reported here will form the basis for ground-truthing future efforts to detect carbon dioxide from space with new satellite instruments.