Briefs: NOAA Greenhouse Gas Index

Researchers at the US National Oceanic and Atmospheric Administration (NOAA) developed what they call a "scientifically unambiguous" index, that they say will allow them to track changes in the gases that contribute the most to the warming greenhouse effect. To create the Annual Greenhouse Gas Index (AGGI), NOAA scientists used measurements of greenhouse gas concentrations, recorded around the world since 1979. They accounted for only the main "long-lived" greenhouse gases that remain in the atmosphere and are uniformly mixed throughout the global atmosphere; those gases include carbon dioxide, methane, nitrous oxide, CFC-11, CFC-12 and a set of 10 minor long-lived halogen gases including CFC-113, HCFC-22, HFC-141b, HCFC-142b and HFC-134a. For the year 2007, the total AGGI was 1.24 (an increase in total radiative forcing of 24% since 1990). As expected, CO2 (63.4%) dominates the total radiative forcing with methane. CFC-11 and CFC-12 (8.6% together) are relatively smaller contributors thanks to the CFC phase-out in application of the Montreal Protocol. The 10 minor halogen gases represent 3.2% of the total forcing with HFC-134a representing 0.26%. NOAA concludes that "the slowdown in the methane growth rate and the decline in the CFCs has tempered the increase in the net radiative forcing considerably".