Within the EU area, the only sector to see an increase in emissions of greenhouse gases is the transport sector: Emissions increased a full 20 percent in the period 1990-1999. Transportation emissions also increased in Norway – by 14 percent – although the greatest increase was registered in the petroleum sector (almost 50 percent). (Source: Miljøstatus i Norge).
Not just CO2
In addition to emissions of CO2 from the combustion of fossil fuels, mobile sources also emit a number of gases that live only a relatively short time in atmosphere (up to a few months) but can have a significant radiative forcing. Combustion engines emit nitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbons, which are chemically transformed in the atmosphere, creating other gases such as ozone. Ozone is a greenhouse gas and poses a regional air pollution problem damaging human health and agricultural crops. Sulfuric fuels, particularly heavy oil used aboard ships, lead to the creation of sulfate particles that directly and indirectly increase the reflection of sunlight and thus have a cooling effect. Diesel engines emit considerable amounts of small soot particles that absorb sunlight and thus lead to a warming of the climate.
Climate Impact of Transport System (CITS), a joint project between CICERO and the University of Oslo, has analyzed the total climate forcing from the transport sector for all relevant emissions. Figure 1 shows the radiative forcing for the emissions within the transport sector and by gasses/particles from pre-industrial times to the present. The sub-category “Other” includes mainly vehicles used off road, such as construction machinery, tractors, and so on. The estimate for soot does not include the so-called semi-direct effect of soot – that is, the evaporation of clouds at low altitudes that provides additional warming.
Road traffic is a main contributor
Road traffic clearly provides the largest net contribution to warming through its large emissions of CO2 and significant emissions of ozone and soot. Total warming from road traffic is estimated to be about 0.19 Watts per square meter (W/m2), or about seven percent of the total climate forcing, because of the increase in the concentrations of ozone, soot, and greenhouse gases included in the Kyoto Protocol. This surprisingly low percentage results from road traffic having a shorter history than other emissions sectors, and thus having less responsibility for the accumulated concentrations of long-lived greenhouse gases. This share will increase in the future.
Ships and planes not included in Kyoto
In a climate context, emissions from ships and planes are in a special category. Not only are they not covered by the Kyoto Protocol, but – more fundamentally – these emissions contain components with short lifetimes that have specific local effects. With respect to ships, two particular conditions stand out: the use of sulfuric heavy oil that causes the formation of sulfate particles, and emissions of NOx in areas with little other pollution. Ship emissions of NOx in unpolluted areas have a particularly large effect on ozone formation compared to, for example, emissions from road traffic or land-based industry. The analysis from CITS shows that if the indirect effect of sulfate particles in clouds is included, then emissions from ships up to the present have had a net cooling effect on the climate. This picture will nevertheless change in the future because the cooling sulfate particles have a short lifetime in the atmosphere, while the contribution of CO2 increases slowly but surely.
Air traffic is the sub-category within the transport sector that shows the most rapid increase in emissions (with a temporary pause after 11 September 2001). As is the case with ship emissions, air traffic emits NOx in areas that are relatively clean, which has an especially large effect on ozone formation. More recent research in the period after the 1999 report from the Intergovernmental Panel on Climate Change (IPCC) on air traffic suggests that the occurrence of ice clouds (cirrus) at flying altitudes is increasing in areas with heavy air traffic because the contrails, under the right meteorological conditions, can expand. Cirrus clouds at altitudes of 8-12 kilometers have a warming effect on the climate because their greenhouse effect is stronger than their cooling effect through the reflection of light. This is because the temperature at this height is very low.
The preliminary results from the project show that there are large differences between the various forms of transportation, and that for the transport sector it is particularly important to expand the perspective from the traditional greenhouse gases (Kyoto gases) to also include components with short lifetimes. The project will go on to look at climate effects of various emissions scenarios for the transport sector, and consider the effects in the light of the actual transportation work performed (in the form of, e.g., number of passenger kilometers).