It is one of those headlines to get the attention without going very far into the statistic.
I do not know if the 50,000,000 number is true but they do make a lot of pollution.
The biggest part of the problem is the fuel they use, it is not very refined, it is basically the leftover dirty stuff after they make the plane fuel and the fuel for cars. The shipping companies get it really cheap but it pollutes much more than the better stuff.