Hofmann, D. J., J. H. Butler, E. J. Dlugokencky, J. W. Elkins, K. A. Masarie, S. A. Montzka and P. P. Tans, (2006), The role of carbon dioxide in climate forcing from 1979 to 2004: introduction of the Annual Greenhouse Gas Index, Tellus Series B: Chemical and Physical Meteorology, 58, 5, 614-619, doi:10.1111/j.1600-0889.2006.00201.x
High-precision measurements of CO2, CH4, N2O, CFC-12, CFC-11 (major greenhouse gases) and 10 minor halogenated gases from a globally distributed network of air sampling sites are used to calculate changes in radiative climate forcing since the pre-industrial era (1750) for the period of measurement, 1979–2004. The five major greenhouse gases account for about 97% of the direct radiative forcing by long-lived gases. The fraction of the sum of radiative forcings by all long-lived gases that is due to CO2 has grown from 60% to 63% over this time. Though the long-term increase in this sum is due primarily to increased anthropogenic emissions of these radiatively important gases, interannual variations in the growth rate of radiative forcing due to CO2 are large and likely related to natural phenomena such as volcanic eruptions and ENSO events. The annual value of the total global radiative forcing of the long-lived gases is used to define an Annual Greenhouse Gas Index (AGGI). The AGGI is normalized to 1990, the Kyoto Protocol baseline year.