On a cold, choppy morning off Monterey, Ashley Kidd gives the order that starts the working day: "Dive No 1 … stars in the water." Divers drop below the surface carrying plastic bags, and inside them are the stars of the mission: sunflower sea stars, predators that can grow up to 24 arms. On the seabed, the divers place each animal in a cage and leave the door open. Then the researchers wait to see whether the stars will walk out and start doing the job their species once did.

The scale of the loss explains the effort. Before 2013, according to Reuven Bank, Kidd's co-founder at the Sunflower Star Laboratory, roughly six billion sunflower stars lived between Alaska and Baja California. Then a marine heatwave, fuelled by the climate crisis, warmed the west coast and accelerated sea star wasting syndrome, a disease that literally dissolves the animal's body. More than 90% of the population died.

Without the stars to keep sea urchins in check, the kelp forests they grazed on began to collapse. The stakes extend well beyond California. Kelp can grow around two feet a day, shelters thousands of species, and, worldwide, absorbs an estimated 200m tons of carbon dioxide each year. Its loss along the Pacific coast is therefore both a local ecological disaster and a small but real setback for climate defences everywhere.

Restoration is slow, and a few caged animals cannot refill an ocean that once held billions.