In the first ever analysis of the effect of the transport sector on climate, it has been established that road traffic contributes the most to global warming, aviation sector has the second largest warming effect, and shipping has a net cooling effect on the Earth's climate.
According to a report in ENN (Environmental News Network), for the analysis, five researchers from CICERO (Center for International Climate and Environmental Research) broke down the transport sector to four subsectors: road transport, aviation, rail, and shipping.
The research team then calculated each subsector's contribution to global warming by looking at the radiative forcing (RF) caused by transport emissions.
The study concludes that since pre-industrial times, 15 percent of the RF caused by man-made CO2-emissions has come from the transport sector.
It also looks at other emissions. For ozone (O3), transport can be blamed for 30 percent 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.
When the researchers looked at the effect today's road emissions has on future climate, they found that 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.
For shipping, the picture is more complicated.
Until now, 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.
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.
Following road transport, aviation is the second largest transport contributor to global warming.
However, the historical contribution from aviation emissions to global warming is more than doubled by the contribution from road emissions. In fact, 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 report said.
In general, the transport sector's contribution to global warming will be continuously high in the future.