WASHINGTON - The U.S. National Oceanic and Atmospheric Administration (NOAA) has issued its Annual Greenhouse Gas Index (AGGI), a benchmark measurement of gases in the atmosphere that affect the earth's climate. This year's AGGI reflects an increase in carbon dioxide (CO2) and nitrous oxide (N2O), but a leveling off of methane (CH4), and a decline in two chlorofluorocarbons (CFCs). Overall, the AGGI shows a continuing, steady rise in the amount of heat-trapping gases in the atmosphere.

"We have a better understanding of the dynamics of earth's climate through our extensive, high quality and sustained observations," said retired Navy Vice Admiral Conrad C. Lautenbacher, Ph.D. undersecretary of commerce for oceans and atmosphere and NOAA administrator. "NOAA adds operational value to climate research by observing and quantifying the changes that are occurring around us, and reporting their effects."

The AGGI is referenced to a baseline value of 1.00 for the greenhouse gas levels that were present in the atmosphere in 1990. The value of the AGGI for 2005 is 1.215. This reflects a continuing upward trend in the accumulation of greenhouse gases, as well as the change in the amount of radiative forcing. Radiative forcing indicates the balance between radiation coming into the atmosphere and radiation going out. Positive radiative forcing tends on average to warm the surface of the earth, and negative forcing tends on average to cool the surface. Radiative forcing, as measured by the index, is calculated from the atmospheric concentration of each contributing gas and the per-molecule climate forcing of each gas.

The constant or declining growth rates of methane and CFCs have slightly slowed the overall growth rate of the AGGI. Methane concentrations have been holding relatively steady since 1990. Another positive result is the fact that CFCs are continuing to decline.

Most of the increase in radiative forcing measured since 1990 is due to CO2, which now accounts for approximately 62 percent of the radiative forcing by all long-lived greenhouse gases. During 2005, global CO2 increased from an average of 376.8 parts per million (ppm) in 2004 to 378.9 ppm. This increase of 2.1 ppm means that for every one million air molecules there were slightly more than two new CO2 molecules in the atmosphere. The pre-industrial CO2 level was approximately 278 ppm.

Publication date: 05/15/2006