The ocean is a carbon toilet. Marine heat waves are clogging it.

The planet would be a whole lot hotter if it weren’t for fecal pellets. Across the world’s oceans, tiny organisms known as phytoplankton harvest the sun’s energy, gobbling up carbon dioxide and releasing oxygen. They’re eaten by little animals called zooplankton, which poop out pellets that sink to the seafloor. What is essentially a giant toilet, then, flushes carbon at the surface into the depths, where it stays locked away from the atmosphere, thus keeping the amount of CO2 up there in check.

But as humans pump ever more carbon into the sky, relentlessly raising ocean temperatures, worrying signals are flashing that this commode could be changing in profound ways. Consider the northeastern Pacific, off the coast of Alaska, where two major heat waves took hold of the sea, one from 2013