A 2008 article by Fuglestvedt et al estimated the amount of ‘climate forcing’ by transport sectors – road, aviation, shipping and rail. The climate forcing is expressed as ‘radiative forcing’ (RF) which is the warming applied to the earth (measured as milliwatts per square metre).
The article may be found on the PNAS web site and considers the emissions of each type of gas or particulate from a range of transport sectors. While there are diagrams in the article showing this, there is a more informative table in a supporting document. The table is reproduced here.
It should be noted that numbers refer to a conventional type of RF calculation. It estimates the warming in one year – 2000 – due to emissions historical emissions up to 2000. There are other methods which look at impacts in the future from past emissions. These are discussed in the article and different methods give rather different results.
Best estimates (mW/m2) with uncertainty ranges (1 SD) for RF since preindustrail times:
It should be noted that there are wide error limit on most of these values. However, the conclusions are clear. Road transport is the biggest contributor by a large margin. Aviation comes second, way ahead of the other sectors. This is largely because of the impact of non-CO2 emissions. Shipping is odd in that its RF is negative. This is due to mainly to suphates, derived from sulphur impurities in the fuel, more than offsetting the carbon emissions.