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.

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

—Jan Fuglestvedt, co-author

Current shipping emissions differ from emissions from the road and aviation sectors by having a cooling effect—resulting from the very high emissions of SO₂ and NO_x—on climate that lasts 30-70 years. 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.

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, the authors say. 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. Although aviation emits between 2-3% of the total human-produced CO₂ emissions, effects down the cause-effect chain must be considered when assessing 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. 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.

—Jan Fuglestvedt

Integrated radiative forcing and GWPs give equal weight to effects over time up to the chosen time horizon. When used on short-lived gasses and particles with strong climate effect, this characteristic of GWP can give results that are quite different from those obtained when using temperature as indicator, Fuglestvedt said.