Study: emissions from road transport to make up 75% of all man-made traffic emissions over the next century

According to researchers from the Oslo-based Center for International Climate and Environmental Research (CICERO), road traffic is by large the transport sector that contributes most to global warming. Aviation has the second largest warming effect, while shipping has a net cooling effect on the earth’s climate, they write in an open access study published recently in the Proceedings of the National Academy of Sciences (PNAS).

Since pre-industrial times, road traffic has contributed around 15% of all man-made CO2 emissions and takes a two-third share of all emissions from transport. But looking at the effects of today’s road emissions on future climate, the researchers predict the share to grow to 75% of the warming caused by transport over the next hundred years. Road transport should therefor be a clear priority for the implementation of green policies that reduce emissions, the researchers say.

The analysis gives some credence to the European Commission's rationale for its 10% target for biofuels: there is no alternative to oil for road transport, as (bio-)electric or (bio-)hydrogen transport systems are not available yet. Biofuels offer the only way to tackle emissions from transport today. Obviously, as much effort as possible should be invested into stimulating the use of low carbon public transport and into transporting goods via rail and waterway. However, this is not likely to make as major a difference as introducing biofuels in the much larger sector of personal, individual road transport.

The new study, “Climate forcing from the transport sectors”, is the first comprehensive analysis of the climate effect from the transport sector as a whole on a global scale. Breaking down the transport sector to four subsectors: road transport, aviation, rail, and shipping, five researchers at CICERO have calculated each subsector’s contribution to global warming. The researchers have looked at the radiative forcing (RF) caused by transport emissions. The RF describes the warming effect in the unit Watt per square meter (W/m2).

Road transport
The study concludes that since preindustrial times, 15% of the RF caused by man-made CO2-emissions have come from the transport sector (graph, click to enlarge). The study also looks at other emissions. For ozon (O3), transport can be blamed for ca 30% of the forcing caused by man-made emissions.

The study implies that more attention needs to be put on the fast growing road sector. Looking solely at CO2 emissions, road traffic alone has led to two-thirds of the warming caused by total transport emissions (this is using a historical perspective looking at emissions since pre-industrial times.) Including all gasses, not just CO2, and looking at the effect today’s road emissions has on future climate, the share is even larger: the road emissions of today will constitute three-fourth of the warming caused by transport over the next hundred years.

Shipping
For shipping, the picture is more complicated. Until today, shipping has had a cooling effect on climate. This is because shipping emits large portions of the gasses SO2 and NOx, which both have cooling effects. However, although these two gases, until now, have given the shipping industry a cooling effect, this effect will diminish after a while, as the gases don’t live long in the atmosphere. After a few decades, the long-lived CO2 will dominate, giving shipping a warming effect in the long run:
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The net cooling effect from shipping does not imply that shipping emissions don’t need to be cut back on. Both SO2 and NOx have other impacts that damage the environment.

A remark can be made here saying that SO2 and NOx are not covered by the Kyoto Protocol; neither is black carbon (soot). Therefore, the Protocol is too narrow to capture the real climate effect of transport emissions, particularly for the shipping sector.

Aviation
Following road transport, aviation is the second largest transport contributor to global warming. The reason that road transport tops the list is mainly the amount of vehicles on the roads and the smaller cooling effect from their emissions. The researchers have not yet looked at emissions per kilometre or per person at a certain distance using different transport modes.

Also, aviation has a strong contribution to global warming. However, the historical contribution from aviation emissions to global warming is more than doubled by the contribution from road emissions. Over the next 100 years, today’s road emissions will have a climate effect that is four times higher than the climate effect from today’s aviation emissions.

Rail
The warming effect by rail emissions is very small, almost not noticable at all, compared to the effects from road transport and aviation.

In general, the transport sector’s contribution to global warming will be continously high in the future. The current emissions from transport are responsible for approximately 16% of the net radiative forcing over the next 100 years. The dominating contributor to this warming is CO2, followed by tropospheric O3.

Graph: Historical development in emissions and radiative forcing for CO2 from the transport sector. (A) Development in CO2 emissions from the various transport subsectors and the fraction (right axis) of total man-made CO2 emissions (excluding land use changes). (B) Development in RF due to CO2 from these sectors.

References:
Jan Fuglestvedt, Terje Berntsen, Gunnar Myhre, Kristin Rypdal, and Ragnhild Bieltvedt Skeie. "Climate Forcing from the Transport Sectors" [open access], PNAS 10.1073/pnas.0702958104, 7 January 2008.

CICERO: Cars Warm Up, Ships Cool Down - January 24, 2008.