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29th January 2008

Road traffic is by a large margin the transport sector that contributes the most to global warming. Aviation has the second largest warming effect, while shipping has a net cooling effect on the earth’s climate, according to a study by CICERO, the Oslo-based Centre for International Clmate and Environment Research and published in the Proceedings of the (U.S.) National Academy of Sciences.

The study, “Climate forcing from the transport sectors”, is described by CICERO as the first ever comprehensive analysis of the climate effect from the transport sector as a whole on a global scale. Breaking down the transport sector into road transport, aviation, rail, and shipping, five researchers at CICERO have calculated each sub-sector’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).

The study published in the Proceedings of the National Academy of Sciences (PNAS) concludes that since pre-industrial times, 15% of the RF caused by man-made CO2-emissions have come from the transport sector. The study also looks at other emissions. For ozone (O3), transport can be blamed for circa 30% of the forcing caused by man-made emissions.

The study implies that more attention needs to be put on the fast-growing road transport sector. Looking solely at CO2 emissions, road traffic alone has led to two-thirds of the warming caused by total transport emissions (using a historical perspective looking at emissions since pre-industrial times.)

Including all greenhouse gases, and at the effect today’s road emissions has on future climate, the share is even larger: the road emissions of today will constitute three quarters of the warming caused by transport over the next hundred years.

For shipping, the picture is more complicated. Until now, shipping has had a cooling effect on climate, because shipping emits large portions of the gases 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 linger long in the atmosphere. After a few decades, the longer-lived CO2 will dominate, giving shipping a warming effect in the long run.

The net cooling effect from shipping does not imply that shipping emissions don’t need to be reduced; both SO2 and NOx have other environmentally damaging impacts.

SO2 and NOx are not covered by the Kyoto Protocol; neither is black carbon (soot). The Protocol is therefore too narrow to capture the real climate effect of transport emissions, particularly for the shipping sector.

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.

While aviation contributes significantly to global warming, 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 four times bigger than that from today’s aviation emissions.

The warming effect of rail emissions is said to be almost unnoticeable 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. Current emissions from transport will be 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 ozone (O3).

(www.cicero.uio.no)

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