I wonder how much slack there is in steel consumption. 2/3 of US steel production is already from recycled material, I can see the volume of recycled material increasing with higher scrap prices (so steel production would be able to stay at 65+ percent of current levels), and low value producers would be the first ones to shut down (so probably consumer products like low quality chain or fencing, things that people can pretty well do without). I think it's an easy strategic decision to build a second larger lock, but the probability of catastrophe seems overblown.
https://en.wikipedia.org/wiki/Environmental_impact_of_shippi... indicates that up to 4% of climate change gasses come from ships.
https://en.wikipedia.org/wiki/Environmental_impact_of_transp... has a breakdown of CO2 per ton-mile. It shows that ships are the most efficient followed by zeppelins and trucking is the source of 20% of the UK's carbon emissions.
In fact I recently saw an article [1] talking about the Soo Locks in the Great Lakes and how devastating it would be if they shut down because there would be no way that much cargo is getting sent over land by trucks or trains. So much that the US government is afraid the national economy would crash if these "no one has ever heard of them" locks stopped for any amount of time.
Sure ships burn nasty shit and spew a ton of pollutants, but compared to other forms of transportation, they's so vastly superior in almost every way.
[1] http://www.freep.com/story/news/politics/2016/03/03/us-michi...