I read an article in which studies have found that more and more cars are being consumed per family today. This in turn increases the oil consumption so it is an externality. However, oil causes air pollution, thus increasing the danger of global warming. Therefore, this is a negative externality because it harms a third party that is innocent. There are also hidden social costs that are not as visible as the other ones. For example, the production of crops diminishes because the environment is affected by the air pollution. Also, fish gets more scarce and harder to catch because they go away from air pollution, where men are. There is a number of limited factors that sum up to be social costs.
Good job analyzing that article- I understand the negative externalities and the consequences of it.
& nice graph
A good article to look at Anne-Claire. Remember negative externalities refer to third party cost not the actual cost of the gasoline which as you have explained does not reflect its true social cost.