The role of carbon dioxide in climate forcing from 1979 to 2004: introduction of the Annual Greenhouse Gas Index

Abstract

High-precision measurements of CO₂, CH₄, N_2O, 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.