If you happen to be at the beach this summer, make a mental note of any giant freighters crossing the horizon. In a few years’ time those same ships will probably be moving just a fraction slower.  If the slowdown does happen — and the likes of Citigroup Inc. say it will — then the wider consequences would be profound for both the shipping industry and its customers, given that about 90 percent of world trade moves by sea. The cause of this deceleration? A rule to combat the merchant fleet’s emissions of sulfur oxides starting in January 2020.  While such a slowdown might shave billions of dollars off shipowners’ single largest expense — fuel — it would also effectively limit the number of available vessels, risking an upward spiral in freight costs.

“It’s going to impact all of shipping: containers, tankers and in particular dry-bulk,” said John Kartsonas, New York-based managing partner at Breakwave Advisors LLC, which runs an exchange-traded fund for dry-bulk shipping. “You will have shipowners instructing their captains to slow down, and if everybody does, global supply will come down significantly.”