Even after nearly three months of winter, the oceans of the Northern Hemisphere are disturbingly warm. Last summer’s unprecedented temperatures—remember the “hot tub” waters off the coast of Florida?—have simmered down to a sea-surface average around 68 degrees Fahrenheit in the North Atlantic, but even that is unprecedented for this time of year. The alarming trend stretches around the world: 41 percent of the global ocean experienced heat waves in January. The temperatures are also part of a decades-long hot streak in the oceans. “What we used to consider extreme is no longer an extreme today,” Dillon Amaya, a research scientist at the National Oceanic and Atmospheric Administration’s Physical Sciences Laboratory, told me. The situation is expected to worsen. Research suggests that by the end of the century, much of the ocean could be in a permanent heat wave relative to historical thresholds, depending on the quantity of greenhouse gases that humans emit. Many other changes will unfold alongside those hot ocean temperatures: stronger hurricanes, rising sea levels, unmanageable conditions for marine life. Our seas, …