CIESIN Thematic Guides

Global Warming Potential of Chlorofluorocarbons and Their Replacements

The role of chlorofluorocarbons (CFCs) in ozone depletion is well recognized. CFCs are also known to be greenhouse gases that may contribute to global warming, and increases in atmospheric CFC levels may exacerbate this environmental impact. Carbon dioxide (CO2) is also a well-documented greenhouse gas that contributes to the greenhouse effect. In fact, the effectiveness of other compounds in contributing to global warming on a molecule-by-molecule basis is typically measured relative to CO2 and is referred to as the compound's Global Warming Potential (GWP). This parameter depends primarily on the infrared radiation absorption properties of the gas and its atmospheric lifetime. The Alternative Fluorocarbons Environmental Acceptability Study's (AFEAS) publication "Relative Contributions of Greenhouse Gas Emissions to Climate Forcing" offers a brief description of the factors involved in evaluating GWPs (1992d).

Of future concern in global warming scenarios are not only the CFCs, but also the hydrofluorocarbons (HFCs) and other compounds slated to replace CFCs. These alternative substances have been rigorously evaluated for their ozone depletion potential (ODP) but now are also characterized by their GWP. Values of GWP for CFCs, HCFCs, and other greenhouse gases, and the limitations inherent in this parameter, are described in the chapter "Radiative Forcing of Climate" from Climate Change 1992: The Supplementary Report to the IPCC Scientific Assessment (Isaken et al. 1992). Ko et al. (1993) provide a more mathematical treatment of GWP evaluation in "Global Warming from Chlorofluorocarbons and Their Alternatives."

More recently, Fisher and McFarland (1992) recognized that impact assessment of CFC alternatives on global warming must not only include direct-emission effects based on GWP, but indirect, energy-related CO2 emissions as well in "Alternatives to CFCs and Global Warming." The authors contend that if using a particular CFC alternative requires higher fuel consumption, any benefit due to decreased GWP could be offset by increased CO2 emissions. The sum of the direct and indirect emissions associated with a particular gas has been defined as its Total Equivalent Warming Impact (TEWI). The AFEAS report Energy and Global Warming Impacts of CFC Alternative Technologies describes the methodology for determining TEWI and its application in several commercial, industrial, and household case studies (Fischer et al. 1991).