In hot water: here’s why ocean temperatures are the hottest on record

Large swathes of the world’s oceans are warm. Unusually warm. The heat this year is likely to break records. Since mid-March, the global average sea surface temperature is over 21℃—the highest since satellite records began.

What’s going on? Climate change is the big picture—nine-tenths of all heat trapped by greenhouse gases goes into the oceans. But there’s an immediate cause too: the rare triple-dip La Niña is over. During this cycle, cooler water from deep in the ocean upwells to the surface. It’s like the Pacific Ocean’s air conditioner is running. But now the air conditioner is turned off. It’s likely we’re set for an El Niño.