IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group III: Mitigation of Climate Change

Emission trends

In 2004, the contribution of transport to total energy-related GHG emissions was about 23%, with emissions of CO2 and N2O amounting to about 6.3-6.4 GtCO2-eq. Transport sector CO2 emissions (6.2 GtCO2-eq. in 2004) have increased by around 27% since 1990 and its growth rate is the highest among the end-user sectors. Road transport currently accounts for 74% of total transport CO2 emissions. The share of non-OECD countries is 36% now and will increase rapidly to 46% by 2030 if current trends continue (high agreement, medium evidence) [5.2.2].

The transport sector also contributes small amounts of CH4 and N2O emissions from fuel combustion and F-gases from vehicle air-conditioning. CH4 emissions are between 0.1–0.3% of total transport GHG emissions, N2O between 2.0 and 2.8% (all figures based on US, Japan and EU data only). Emissions of F gases (CFC-12 + HFC-134a + HCFC-22) worldwide in 2003 were 4.9% of total transport CO2 emissions (medium agreement, limited evidence) [5.2.1].

Figure TS.15

Figure TS.15: Historical and projected CO2 emissions from transport [Figure 5.4].

Estimates of CO2 emissions from global aviation increased by a factor of about 1.5, from 330 MtCO2/yr in 1990 to 480 MtCO2/yr in 2000, and accounted for about 2% of total anthropogenic CO2 emissions. Aviation CO2 emissions are projected to continue to grow strongly. In the absence of additional measures, projected annual improvements in aircraft fuel efficiency of the order of 1–2% will be largely surpassed by traffic growth of around 5% each year, leading to a projected increase in emissions of 3–4% per year (high agreement, medium evidence). Moreover, the overall climate impact of aviation is much greater than the impact of CO2 alone. As well as emitting CO2, aircraft contribute to climate change through the emission of nitrogen oxides (NOx), which are particularly effective in forming the GHG ozone when emitted at cruise altitudes. Aircraft also trigger the formation of condensation trails, or contrails, which are suspected of enhancing the formation of cirrus clouds, which add to the overall global warming effect. These effects are estimated to be about two to four times greater than those of aviation’s CO2 alone, even without considering the potential impact of cirrus cloud enhancement. The environmental effectiveness of future mitigation policies for aviation will depend on the extent to which these non-CO2 effects are also addressed (high agreement, medium evidence) [5.2.1; 5.2.2].

All of the projections discussed above assume that world oil supplies will be more than adequate to support the expected growth in transport activity. There is ongoing debate, however, about whether the world is nearing a peak in conventional oil production that would require a significant and rapid transition to alternative energy sources. There is no shortage of alternative energy sources, including oil sands and oil shales, coal-to-liquids, biofuels, electricity and hydrogen. Among these alternatives, unconventional fossil carbon resources would produce the least expensive fuels most compatible with the existing transportation infrastructure. Unfortunately, tapping into these fossil resources to power transportation would increase upstream carbon emissions and greatly increase the input of carbon into the atmosphere [5.2.2; 5.3].