1 ship producing the same pollution as 50 million cars? I find this very, very difficult to believe. Let's look at some numbers: The biggest ship engine in the world has around 25,000 litres displacement, weighs 2,300,000 kg, produces 109,000 hp and has a fuel efficiency of 0.26 lbs/hp/hour.

Using very conservative equivalent estimates for a car of 1 litre, 100 kg, 75 hp and 0.6 - 50 million cars would displace 50 million litres (2000 times more than ship engine), weigh around 2175 times as much, produce 34,000 times as much power and use 79,000 times as much fuel.

Keeping in mind the ships engine is very fuel and thermal efficient, 1 ship producing equivalent pollution to 50 million cars just does not seem plausible, in the slightest.