Road Emissions Dominate Global Transport Emissions

Today’s global road emissions have a strong and long-lasting effect on climate. After 100 years these emissions will lead to a temperature increase that is six times greater than the temperature increase from today’s air transport, according to a new CICERO study. The study includes the effects of all climate-relevant components of the emissions, not only CO₂.

“Among the transport sectors, road transport has a strongly dominating temperature effect, both on short and longer terms,” says Jan Fuglestvedt, researcher at CICERO in Oslo, Norway.

Total global emissions

“It is important to underline that the study looks at the effect of the total global emissions, not emissions per passenger kilometer,” says Terje Berntsen, the other researcher behind the study.

The researchers will calculate climate impacts per passenger kilometer in a later study.

The study has investigated how the global emissions from different transport sectors (road, rail, shipping, and aviation) in year 2000 affect the future temperature. While air transport has some strong climate effects that decrease relatively quickly over time, emissions from road transport have a strong and long-lasting effect on climate. The reason is the much larger total fuel consumption and thus higher CO₂ emissions from road traffic, while for aviation there is a strong short-term warming by aviation induced contrails and cirrus clouds.

Air transport effects short lasting

“In contrast to road transport, air transport has several strong, but short lasting, effects on the global temperature,” says Fuglestvedt. “But there are large uncertainties in our understanding of these effects. It is important to work towards reducing this uncertainty.”

Current shipping emissions differ from emissions from the road and aviation sectors by having a cooling effect on climate that lasts 30-70 years. This cooling effect results from the very high emissions of SO₂ and NO_x. However, the warming effect will dominate in the long term because shipping also emits significant amounts of CO₂.

Neither international shipping nor international aviation is covered by the Kyoto Protocol today. Ongoing climate negotiations are debating whether one these emissions should be included in a post-Kyoto agreement.

Choice of method important

Understanding the climate impact of transportation requires not only taking into account the total quantity of emissions, but also how emissions of various components interact with one another and the climate. Transportation emits a broad mix of components with very different characteristics with respect to climate impacts. They operate on different timescales and cause both warming and cooling. Aviation emits between 2 and 3 percent of the total human-produced CO₂ emissions, but that does not tell the full story. Effects down the cause-effect chain must be considered when we assess the climate impacts of this sector.

“When we quantify and compare the climate impacts of the different transport sectors, the conclusions will vary strongly depending on which method and climate indicator is used and the adopted time perspectives,” says Fuglestvedt. “In this work we have looked at the emissions’ effect on global mean temperature. This is a significant step forward compared to earlier work. In our previous study we quantified the climate impacts in terms of accumulated radiative forcing, which is similar to the Global Warming Potential (GWP) method used in the Kyoto Protocol.”

Strong memory

Integrated radiative forcing and GWPs give equal weight to effects over time up to the chosen time horizon.

“After 100 years, this method still gives full weight to impacts on climate that happened 99 years ago. It therefore has a ‘strong memory,’” Fuglestvedt explains. “When used on short-lived gasses and particles with strong climate effect, this characteristic of GWP can give results that are quite different from what we get when we use temperature as indicator.”