|NOAA’s global data shows an ongoing increase in CO2 forcing and increase in the AGGI. Click to enlarge.|
The National Oceanic and Atmospheric Administration (NOAA) today issued the global Annual Greenhouse Gas Index (AGGI), its 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), gases that contribute to the cause of the Antarctic ozone hole. Overall, the AGGI shows a continuing, steady rise in the amount of heat-trapping gases in the atmosphere.
|CO2 and NO2 continue to increase. Methane (CH4) and CFCs are levelling off.|
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. This is mostly attributed to an equilibrium that has been reached between sources of emission of the gas, its duration in the atmosphere and areas where it is taken out of the atmosphere. Another positive result is the fact that CFCs are continuing to decline. Along with creating the ozone hole over the Antarctic, CFCs are also powerful greenhouse gases.
Most of the increase in radiative forcing measured since 1990 is due to CO2, which now accounts for approximately 62% of the radiative forcing by all long-lived greenhouse gases. The combustion of fossil fuels for transportation and power generation is the primary source of anthropogenic CO2 emissions.
During 2005, global CO2 increased from an average of 376.8 parts per million (ppm) in 2004 to 378.9 ppm—an increase of 0.6%. The pre-industrial CO2 level was approximately 278 ppm.
The AGGI is based on the analyses of atmospheric levels of all the major and minor long-lived greenhouse gases, and factors in the relative strengths of each gas in its ability to trap heat. The gases include carbon dioxide, methane, nitrous oxide, CFCs and the current replacements for CFCs, and have been measured since 1979 by NOAA’s global sampling network.