Radiative Forcing Index

The term 'radiative forcing' describes the influence of a particular factor on changing the balance of incoming and outgoing radiation within the atmosphere. The effect of greenhouse gases on atmospheric warming can be described as a radiative forcing.

There is some uncertainty over the environmental impact of aircraft emissions which arises from the complexity of atmospheric chemistry. In particular, the emission of non-CO2 products (e.g. water vapour, contrails, NOx) at high altitudes is argued to exert a greater global warming effect compared with similar emissions at ground level. As such, the impact on net irradiance (the radiative forcing) of emissions associated with aviation is debated.

Aircraft-associated greenhouse gas emissions are most directly estimated on the basis of the quantity of fuel burned together with data on the properties of the fuel such its carbon content. In order to account for the discrepancy between these absolute emissions and the effective radiative forcing (which is potentially greater), emissions can adjusted using a multiplicative factor called the Radiative Forcing Index (RFI). The RFI represents the ratio of all radiative forcing (CO2 + non-CO2 emissions) to that arising from CO2 emissions only. Applying the RFI inflates the emissions estimates in the proportion considered to more accurately represent their true radiative forcing.

Estimates for RFI range between 1 and 4 with a value of 2.7 recommended by the IPCC in 1999. No specific recommended value was quoted in the most recent IPCC report (2007), and the current best estimate for RFI is 1.9 (Sausen et al. (2005)).