# **1. Introduction**

Measuring and managing the “greenhouse gas (GHG) footprint” (or in the following, “carbon footprint”) of air travel involving individual passengers poses challenges at several levels. A major policy challenge arises in the international arena, since emissions from international travel for the most part do not physically take place in any one given country. While various allocation methods to national emissions volumes could in principle be employed, all are in the end ad hoc.<sup>2</sup> Another, related, policy issue is the very mobility of the aviation sector. Any attempt by a particular country to tax fuel used in international aviation which is not matched by similar taxes or charges in other countries could face problems, inducing tax avoidance as airlines may seek to tank up in lower-tax jurisdictions. In practice, moreover, international treaties constrain countries in applying carbon prices or physical emissions quotas to international aviation, though the need for changing this restriction has been the subject of some debate.<sup>3</sup> Problems of measuring the carbon footprint of aviation arise from several factors, among which we here focus on two.<sup>4</sup> The first is the multiplicity of GHGs resulting from airplane emissions, and the difficulty of actually computing the global warming (or “climate forcing”) potential of GHG emissions in the upper troposphere and lower stratosphere (at altitudes of about 9-12 kms, relevant for most sub-sonic air transport). These factors include high-level water vapor release, and catalytic interactions with other GHGs such as ozone and methane. This has resulted in great uncertainty, and controversy, over the total climate forcing effect of aviation activity, which is not yet resolved. A seminal report by the IPCC (1999) raised the issue; for more recent scientific work see Jardine (2005), Kollmuss and Crimmins (2009), Kollmuss and Lane (2009), Lee (2009), Peeters and Williams (2009), and Lee et al (2009). Further discussion is found in Keen and Strand (2007). A widely accepted “radiative forcing index” (RFI; by which regular carbon emissions must be multiplied to arrive at a correct “climate imprint”) is 2.7, but with a high range of uncertainty.<sup>5</sup> Several factors are at work, some of which imply positive and others

> 2 For example, the emissions could be associated with the nationality of the air carrier, or the nationality of the traveler. Other options could be, for an international flight from country A to country B, to ascribe the entire resulting emissions either to country A or to country B; or perhaps better, ascribe half of the emissions to country A, and half to country B. The first of these alternatives would not, in today’s climate policy situation, effectively account for travelers from non-Annex B countries, or air carriers headquartered there. Under the second set of options, a similar problem would arise with flight departure sites and/or destinations being in non-Annex B countries.

> 3 For further discussion see Keen and Strand (2007); and Keen, Parry and Strand (2012).

> 4 The notion of carbon footprint is here taken to comprise the overall climate impact, or footprint, of aviation, due to increased net emissions also for several other climate gases including nitrogen oxides, ozone, methane, and water vapor; as well as non-conventional effects of carbon emissions such as those accruing specifically with aviation, at high altitudes. See IPCC (1999) for a more detailed discussion of relevant climate gases and their expected impacts.

> 5 In particular, Jardine (2005) cites a radiative forcing index (RFI), relative to that of CO2 alone, of 2-4 as “reasonable”, but settles on a (conservative) consensus factor of 1.9. World Bank (2010, p 17) states: “Both the WRI and the EPA are reviewing this issue and may decide to integrate RFI into air travel emissions calculations. If international consensus is reached on the appropriate application of RFI, the WBG will revisit this issue.” Note however that caution must be shown in embedding an RFI in such calculations. No technically complete, formal, analysis seems to yet exist in the literature dealing with the issue. It is, in particular, conceivable that the true RFI is

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