It's controversial because we already have a decent solution and the competing ones suck for a lot of reasons that can't be developed away.

It would be as if someone started doing research on vacuum tube transistor technology today if we were having a global compute crisis and we also had limited amount of sand with which to make glass and sand was an essential resource in all sorts of other things like the food supply chain.

But that's not the situation we're in. We can't plant enough trees without destroying a lot of cities. There are in fact better places to place trees than other places, besides the fact that we can't plant enough and they don't do enough fast enough. The issue is that we currently don't have a full solution. We're close, but not all the way there.

If sinking trees is not a good enough solution, developing and improving a solution that is currently immeasurably worse and might someday be a hundred times worse is lunacy.

What impact on cities? Cities take up almost no space and are irrelevant when talking about planting forests and then sinking them in the ocean. At worst there will be more freight train noise.