An ominous heating event is unfolding in the oceans

To call what’s happening in the oceans right now an anomaly is a bit of an understatement. Since March, average sea surface temperatures have been climbing to record highs, as shown in the dark line in the graph below.

Since this record-keeping began in the early 1980s—the other squiggly lines are previous years—the global average for the world’s ocean surfaces has oscillated seasonally between 19.7° and 21° Celsius (67.5° and 69.8° Fahrenheit). Toward the end of March, the average shot above the 21° mark and stayed there for a month. (The most recent reading, for April 26, was just a hair under 21°.) This temperature spike is not just unprecedented, but extreme.

“It’s surprising to me that we’re this far off the trajectory,” says Robert Rohde, lead scientist at Berkeley Earth, a nonprofit that gathers climate data. “Usually when you have a particular warming event, we’re beating the previous record by a little bit. Right now we’re sitting well above the past records for this time of year, for a considerable period of time.”

Rohde points out that temperatures this week were just under two-tenths of a degree warmer than the previous record. “Two-tenths doesn’t sound like a lot—but in ocean terms two-tenths is actually a lot because it doesn’t warm as quickly as the land,” he says.

As you can see from the chart’s record of past years, March is normally when average sea surface temperatures start declining. That’s because the Southern Hemisphere has transitioned from summer to autumn—and that hemisphere has more ocean covering it than the Northern Hemisphere, which has more bulky land masses. As southern oceans cool, they bring down the average global sea surface temperature.

But at the moment, temperature anomalies are widespread around the world’s oceans. (That near-real-time data comes from a network of satellites, buoys, and other ocean instruments.) “It’s above-average temperatures nearly everywhere,” says Rohde. “And there’s a significant heat wave in the North Pacific, which has been going on for many months.”