A vast belt of seaweed is spreading across the Atlantic as Sargassum blooms overwhelm coastlines, threaten tourism and livelihoods, and reveal the alarming speed of environmental change.

An invader is attacking the beaches of the Caribbean and the Gulf of Mexico. It had been around for many years, but began a stepped-up and concerted assault in 2011 that is now affecting the livelihoods of millions. It is showing no mercy. The threat it poses is steadily and exponentially growing. Governments are alarmed and urgently seeking answers as to how they might fight this enemy. It is called Sargassum, a naturally occurring seaweed that has, in recent years, become a crisis. For millennia, sargassum lived out its life at sea, dying off and sinking to the floor, with a small amount reaching shore, but now it is invading coastlines in previously unimaginable volumes.