According to the IPCC's 4th Assessment Report, civil aviation is one of the world’s fastest-growing sectors of significant greenhouse gas emissions. Analysis shows that air traffic “is currently growing at 5.9% per year [and] forecasts predict a global average annual passenger traffic growth of around 5% – passenger traffic doubling in 15 years.”[i] Under current projections, carbon emissions from aircraft might increase from 0.2 billion tons per year to 1.2 billion tons annually over the next fifty years.[ii]
In addition to emitting CO2, airplanes increase “radiative forcing” through emissions of other greenhouse gases and by creating contrails and cirrus clouds, thus changing atmospheric conditions. Although the science remains uncertain, it appears that these contributions to global warming may be “2 to 4 times larger than the forcing by aircraft carbon dioxide alone.”[iii]
Let’s assume, conservatively, that the other effects of aviation add up to twice the impact of CO2 emissions. By my calculations, preventing half this increase would give us half a wedge from carbon alone (0.5 billion tons less carbon emissions annually, fifty years from now) and one and a half wedges overall, while holding total flights at current levels would supply a full wedge from carbon and three wedges overall.
Such reductions could be achieved by increasing the cost of air travel by taxing it; such a proposal was recently put before the parliament of the European Union.[iv]
Alternatively, countries might decide that GCC is important enough to demand sacrifices from all their citizens, even rich ones, and strictly limit the number of allowable discretionary flights per person. The United States rationed gasoline use during World War II, perhaps GCC demands an equally strenuous and across-the-board response.
The 4th Assessment Report did not consider such demand-reduction alternatives, nor do most governments or policy analysts. But they should. A recent study titled “International Aviation Emissions to 2025: Can Emissions Be Stabilized without Restricting Demand?” answered with a resounding “no.” With efficiency improving three times slower than the rate of increased demand and with no transformative technologies on the horizon, the air transport sector cannot make a sufficient contribution to mitigating GCC without limiting demand for air travel.[v] Meanwhile total emissions may need to decrease 60% to 80% in the next fifty years in order to avert catastrophic GCC and clearly this cannot happen while major economic sectors increase their emissions.
For my most recent attempt to calculate the possible carbon mitigation benefits of reducing air traffic, see the attached (draft) paper "Reducing Consumption to Avert Catastrophic Global Climate Change: the Case of Aviation." This article is a follow-up to Macintosh and Wallace's article "Can Emissions Be Stabilized Without Restricting Demand?" (http://law.anu.edu.au/cclp/WP1_2008%20_International_aviation_emissions_to_2025.pdf) It goes into some detail regarding the various policy options for reducing aviation consumption.
Unfortunately, I think publishing this article may be difficult! The reviewers for the journal Energy Policy were particularly critical of my suggestion that policymakers might distinguish necessary from luxury air traffic and set strict limits to the latter (see their comments, also attached below). And yet such an approach becomes an obvious alternative, once one takes the notion of limits seriously and combines this with a continued commitment to fairness.
The reviewers did point to real problems with my attempt to distinguish necessary from luxury consumption. Yet I cannot help wondering whether they would be any happier with a more convincing account of this distinction. Isn't it convenient not to have one? In any case, readers' suggestions for clarifying this distinction, in aviation or more generally, would be appreciated.
[i] IPCC, Climate Change 2007: Mitigation, p. 334.
[ii] IPCC, Aviation and the Global Atmosphere (1999).
[iii] IPCC, Climate Change 2007: Mitigation, “Summary for Policymakers,” section 4.8.
[iv] Commission of the European Communities, “Proposal for a Directive of the European Parliament and of the Council amending Directive 2003/87/EC so as to include aviation activities in the scheme for greenhouse gas trading within the community” (Brussels), COM (2006) 818 final, 2006/0304 (COD).
[v] Andrew Macintosh and Lailey Wallace, “International Aviation Emissions to 2025: Can Emissions Be Stabilized Without Restricting Demand?” Energy Policy, 37 (2009), 264-273.