The problem is that it is politically impossible to implement the only solution that would take them to zero emissions. It is a solution that uses existing, proven, decades-old technology, but thanks to extremely powerful anti-science lobby groups we can't use it. Think about that for a minute: just how powerful does a lobby group have to be to prevent us implementing a solution to 1/3 of global air polution?

Everyone reading this, if they think for a few minutes, will be able to figure out what the technology is (because it's marine propulsion, and how do we make that zero-emission?) and who the political groups are who oppose it, so I'm not going to actually specify them because I think the point will have more impact if people work it out for themselves. The sad thing is that half of the people who do figure it out will have an immediate reaction of, "Well of course we can't do that!", thereby dismissing a practical, safe solution to 1/3 of global air pollution based on the nothing but politics, because what does the well-being of the planet matter when implementing a workable solution means changing your political stance on anything? That's something only those looney pro-science people do, right?