In transportation, road traffic contributes the most to global warming, aviation is second, railways are negligible and shipping actually has a net cooling effect on the earth’s climate, according to a study published recently.
But we can't simply switch to shipping and cure global warming. Shipping emits large portions of SO2 and NOx, which both have cooling effects, but this effect will diminish 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.
Both SO2 and NOx also have other impacts that damage the environment and since neither of those gases were covered under the Kyoto protocol (nor was black carbon - soot) it adds further evidence that the Kyoto targets were too flawed to do much good.
The study 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 the Centre for International Climate and Environmental Research (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).
The study concludes that, since preindustrial 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 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 gases, 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.
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 kilometer or per person at a certain distance using different transport modes.
Aviation has a strong contribution to global warming but the historical contribution to global warming from aviation 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.
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.
Article: Jan Fuglestvedt, Terje Berntsen, Gunnar Myhre, Kristin Rypdal, and Ragnhild Bieltvedt Skeie. "Climate Forcing from the Transport Sectors", PNAS 10.1073/pnas.0702958104, 7 January 2008.