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Researchers at the National Oceanic and Atmospheric Administration’s Climate Monitoring and Diagnostics Laboratory in Boulder, Colo., have developed an index that provides a simple means of tracking the annual increase in greenhouse gases in the atmosphere.
The Annual Greenhouse Gas Index is based on an analysis of the atmospheric levels of all the major and minor, long-lived greenhouse gases, as measured since 1979 by NOAA/CMDL’s global sampling network. These include carbon dioxide), methane, nitrous oxide and chlorofluorocarbons and the replacements for CFCs. NOAA plans to update the index in April of each year.
The newly developed index provides an easily understood and scientifically unambiguous point of comparison for tracking annual changes in levels of atmospheric gases that contribute to the so-called “greenhouse effect,” in which the radiation emitted by Earth’s surface is re-radiated by the atmosphere back to the surface.
“This index provides us with a valuable benchmark for tracking the composition of the atmosphere as we seek to better understand the dynamics of Earth’s climate,” said retired Navy Vice Admiral Conrad C. Lautenbacher, Jr., Ph.D., under secretary of commerce for oceans and atmosphere and NOAA administrator.
NOAA’s five-year strategic plan commits the agency to understanding climate variability and change in order to enhance society’s ability to plan and respond.
“The AGGI will serve as a gauge of success or failure of future efforts to curb carbon dioxide and other greenhouse gas increases in the atmosphere both by natural and human-engineered processes,” said David Hofmann, CMDL director.
The index relates the total radiative forcing since pre-industrial times (defined as the year 1750) from all the gases sampled in a given year to the corresponding measurements taken in 1990. The 1990 baseline was chosen because greenhouse gas emissions targeted by the international Kyoto Protocol are also indexed to 1990.
Radiative forcing is the change in the balance between solar radiation coming into the atmosphere and Earth’s radiation going out. 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.
For every million air molecules in samples analyzed by NOAA/CMDL, about 375 of them are carbon dioxide, about two are methane and less than one is a nitrous oxide molecule. The CFC’s make up less than one molecule in a billion in the atmosphere but play a role in regulating Earth’s climate and are a key factor in the depletion of the protective ozone layer.
Most of the increase in radiative forcing measured since 1990 is due to CO2, which now accounts for about 62 percent of the radiative forcing by all long-lived greenhouse gases. Hofmann noted that the AGGI value for 2004 was 1.20, representing a 20 percent increase in radiative forcing since 1990. The annual increase in the index from 2003 to 2004 was 1.12 percent.
The largest annual increase, 2.8 percent, occurred between 1987 and 1988, the smallest, 0.81 percent, from 1992 to 1993. While the index has increased in every year since NOAA’s global measurements began in 1979, the increase during 2004 was on the low side.
Atmospheric greenhouse gas levels change from year to year depending on human-influenced processes as well as natural processes. The record high annual increase is believed to be related to the increased growth rate of CO2 following the 1987-1988 El Niño and the record low annual increase is related to the decreased growth rate of CO2 following the Pinatubo volcanic eruption in 1991.
NOAA’s network of five global Baseline Observatories and approximately 100 global cooperative sampling sites extends from the high Arctic to the South Pole. NOAA/CMDL also takes samples at five-degree latitude intervals from three oceanic ship routes. All the air samples are analyzed in the Boulder laboratory for the major greenhouse gases by comparing the samples to NOAA’s highly accurate world standards for these gases.
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