When explaining causes of greenhouse-related climate changes to decision makers and the public, it is difficult to simply, but quantitatively, describe trends in the composite greenhouse gas content of the air. A simple Greenhouse Gas Index (GGI) is proposed to summarize the global level of well-mixed tropospheric greenhouse gases (carbon dioxide, methane, nitrous oxide, and halocarbons) based on paleodata, observations, or a scenario. The GGI is defined as a percentage of the preindustrial level (GGI=100) of these gases, linearly weighted by their relative molecular forcing. The presentation will discuss why the GGI includes only these gases and not other radiative forcing factors, why the index is defined as a quantity of gases rather than in terms of radiative forcing, and how this index differs from the common modeler usage of "equivalent carbon dioxide."

Based on published data, the GGI reached 142.2 in 2001 (greenhouse gases were 42.2 percent above the "natural" preindustrial level), with an average growth rate of 0.42 percent per year from 1990-2001. With the "Business as Usual" scenario, the GGI may reach 200 (doubled greenhouse gases) around 2058. The concentration scenarios published in Climate Change 2001: The Scientific Basis produce GGI values ranging from 177.3 to 361.7 in 2100. The percentage increase from the preindustrial average to 2001 is almost identical to the rise from the Last Glacial Maximum to preindustrial times.

The GGI should not change investigation objectives and procedures, or be used to drive a climate model. GGI trends should primarily be presented along with study results, when observed or projected climate or environmental changes are being explained to the public. The GGI is a communication tool to help place such changes into the context of the rate and accumulated amount of human-caused changes in the global atmospheric greenhouse gas content.