One of the most egregious of those earlier changes was the die-off of kelp forests along thousands of miles of coast, from Baja California to as far north as British Columbia. Kelp complexes are some of the most dynamic and biologically productive ecosystems on Earth, the “underwater rainforests” of temperate and Arctic coastal oceans. They offer food, nursery grounds, and shelter from predators and rough currents to hundreds of marine species. Kelp canopies, which consist of rubbery floating fronds, are home to sea otters, seabirds, and juvenile fish, while abalone, sea urchins, lobsters, and octopus live on or near the seafloor.

“The Pacific kelp forest has lasted for millions of years,” said Melissa Carter, a biological oceanographer with UC San Diego’s Scripps Institution of Oceanography, thanks to upwellings of cold water along the coast. “But it has a specific temperature threshold where it falls apart and it can’t survive. And everything that lives on the coast depends on that ecosystem.”