NOAA ISSUES GREENHOUSE GAS INDEX
May 1, 2006 — NOAA issued the Annual Greenhouse Gas Index today, 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. (Click NOAA illustration for larger view. Click here for high resolution version. Please credit “NOAA.”)
"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. 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 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.
NOAA's AGGI, produced by The Global Monitoring Division of the Earth System Research Laboratory in Boulder, Colo., is a recently developed index that provides an easily understood and scientifically unambiguous point of comparison for tracking annual changes in levels of atmospheric greenhouse gases. The NOAA AGGI will be included in the annual Greenhouse Gas Bulletin issued by the World Meteorological Organization (WMO) in November.
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.
Atmospheric greenhouse gas levels change from year to year depending on natural and human-influenced processes. The largest annual increase in the AGGI, 2.8 percent, occurred between 1987 and 1988. The smallest was .81 percent from 1992 to 1993. While the index has increased in every year since NOAA's global measurements began in 1979, the increase during 2005 was 1.25 percent, which is relatively low.
NOAA's network of five global baseline observatories and about 100 global cooperative sampling sites extends from the high Arctic to the South Pole. Samples also are taken at five-degree latitude intervals from three oceanic ship routes. A Baltic ferry line collects samples as it makes its daily crossing. All samples are sent to Boulder for analysis and comparison with NOAA's world standards for the gases.
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