When waves of sargassum - a type of seaweed - washed up on Eastern Caribbean shores seven years ago, people hoped it was a one-off. Matted piles swamped coastlines from Tobago to Anguilla.

The seaweed has inundated beaches, causing an environmental nuisance. As of June 2018, the Great Atlantic Sargassum Belt, as scientists call it, extended 8,850km (5,500 miles) and was made up of ...