The reason for the difference between diesel and gasoline is in the composition of their emissions. We know from before that passenger cars with diesel engines emit, depending on their condition, about 20 percent less of the greenhouse gas carbon dioxide (CO2) per kilometer than the same car type with gasoline engines. However, besides CO2, the exhaust contains a number of other components such as nitrogen oxide (NOx), carbon monoxide (CO), and particles. With the exception of CO, emissions of these components are generally higher from diesel cars, and lead to lower air quality and negative health effects.
But the particles CO and NOx also have important climate effects. The emission of NOx not only contributes to the formation of ozone that has a climate warming effect, but it also reduces the amount of the greenhouse gas methane, which results in a cooling effect. Soot particles from diesel engines absorb solar radiation and cause warming.
The climate effects of the various components work on very different time scales. While CO2 gives a long-term warming effect over several centuries, soot particles and NOx give an intense but short-term warming effect over circa one decade. Warming from diesel, with its higher emissions of the latter components, will therefore be intense right away. On the other hand, with a time perspective of more than 10-15 years where warming from CO2 dominates, gasoline causes greater warming.
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