The Annual Greenhouse Gas Index (AGGI) is a simple way of measuring the direct climate effect due to changes in greenhouse gases in the atmosphere. The purpose of the index is to provide non-technical information about climate forcing for policy makers and the public. Values for 2010, released on 9 November 2011, indicate that since 1990, the baseline year of the Kyoto protocol, the index has risen by 29%. The AGGI has increased every year since at least 1979, when the observational network expanded substantially.
The Annual Greenhouse Gas Index
The AGGI is actually a measure of the change in the greenhouse effect, measured in Watts per square metre. In 1979, the value was 1.71 and has increased to 2.81 in 2010. The increase is relative to the pre-industrial age, set for this purpose at the values in the atmosphere in 1750.
The AGGI is computed using annual, global average values of the major greenhouse gases, including in order of importance CO2, CH4, N2O, CFCs and 15 other minor greenhouse contributors. It is important to use annual and global averages, as CO2, for example, has a substantial annual variation due to the respiration and growth of the biosphere.
Otherwise the gases are well mixed in the lower atmosphere, and are measured by analysing bottles of air collected from the NOAA global air sampling network. About 80 clean air sites have been chosen around the world for these samples. After filling with a sample of the local air, the bottles are returned to the laboratory in Boulder, Colorado for detailed analysis.
In 1750, concentrations of CO2, CH4 and N2O were about 280 parts per million (ppm), 700 parts per billion (ppb) and 270 ppb respectively. The values for 2010 were approximately 389 ppm, 1799 ppb and 324 ppb: increases of 39%, 157% and 20%.
The AGGI formulae
The AGGI uses formulae determined by the Intergovernmental Panel on Climate Change (IPCC, 2001). The formulae take into account the interaction between the radiation bands in the different molecules. To put it simply, different molecules compete for the same photons.
CFCs affect the AGGI proportionally to their concentration whereas the CO2 effect is a logarithmic function, which increases more slowly with concentration. So a doubling of CO2, expected towards the end of the century, increases the greenhouse effect of CO2 by 70% not 100%. That is small comfort.
After the destruction of ozone due to CFCs was recognised, the Montreal Protocol and its amendments have effectively eliminated the emissions of CFCs into the atmosphere. Not only did this set up the conditions for the recovery of the ozone layer, but the regulations also helped to reduce the greenhouse effect. It is calculated (Velders et al., 2007) that without the CFC regulations, the radiative forcing would have been 0.3 Watts per square metre greater, about half the sum of the other greenhouse gases.
Climate Change
The AGGI does not measure climate change. Rather, it measures the direct effect of the greenhouse gases. In addition, what are known as indirect effects also contribute to climate change. These include changes in reflectivity of the ice in the polar regions and any changes in clouds.
Other aspects of the climate system such as changes in airborne particles are not included. Together, these additional effects mean that climate models are needed to determine actual climate change. However, the current greenhouse forcing of 2.81 corresponds approximately to a global average temperature change of 1C (1.8F), although there is a further committed warming before the atmosphere-ocean system reaches balance with the new climate forcing.
Future Greenhouse Gas Index
In the future, the index will be monitored using the same network of observations. But at this time, despite international agreements in place limiting their emissions into the atmosphere, no progress has apparently been made in reducing the trend in greenhouse gas concentrations.
References
NOAA greenhouse gas index continues climbing, NOAA News, published online, November 9, 2011.
Butler, J.H., The NOAA annual greenhouse gas index (AGGI), Updated fall 2011.
IPCC (2001), Climate Change 2001: The Scientific Basis. Cambridge Univ. Press, Cambridge UK and New York, NY USA.
Velders, G. J. M., S. O. Andersen, J. S. Daniel, D. W. Fahey, and M. McFarland, (2007), The importance of the Montreal Protocol in protecting climate, Proc. Nat. Acad. Sciences 104, 4814-4819.
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