The oceans have a chronic fever

The number of heat wave days in the oceans each year have gone up by 54% since 1925.
The number of heat wave days in the oceans each year have gone up by 54% since 1925.
Image: Unsplash/Jakob Owens
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For humans, climate change is currently most felt in the warming air temperature over land. The global average temperature has risen by nearly 1°C (about 1.7°F) since preindustrial times, and that’s contributing to phenomena like worsening droughts, desertification, and changing seasons, to name a few.

But that only represents a fraction of the warming the Earth is actually undergoing. We live on a watery planet. Oceans cover 70% of earth’s surface. As we’ve pointed out before, since the 1970s, more than 93% of excess heat captured by greenhouse gases has been absorbed by the oceans. For perspective, right now the average temperature on land is  59°F; if oceans stopped absorbing heat from climate change, that would shoot up to 122°F.. Neither us nor much of anything else could live here.

All that excess heat in the water is causing heat waves. According to a paper published this week in the journal Nature Communications, the frequency of those heat waves is going way up.

The researchers defined a “marine heat wave” as when the average ocean temperature in a particular region exceeded a seasonal maximum range for longer than five days. They found that between 1925 and 2016, the number of annual marine heat wave events went up by 34%, and lasted on average 17% longer. When they compared the periods of 1925-1954 and 1987-2016, they found that the number of heat waves days per year went up by 54% between the two periods.

That means the heat waves are increasing in number, and the rate of the increase is accelerating. The oceans’ chronic fever is getting more severe.

“These trends can largely be explained by increases in mean ocean temperatures, suggesting that we can expect further increases in marine heat wave days under continued global warming,” the researchers wrote.

Heat waves do obvious and immediate damage to oceans, like bleaching vast tracts of coral reefs. One recent UN report warned that the world’s most significant coral reefs could die out completely by the end of the century, if not sooner.

But as E&E News points out, a study published in Science Advances last year found the oceans have been storing more and more heat since the 1980s, and that the warming has penetrated to ever-deeper layers of the ocean. The damage that causes is more gradual, but just as worrying. Just like warming on land is altering myriad ecological systems and affecting species in innumerable ways, it’s doing the same in the oceans.

It could also be affecting the way oceans circulate. The seas—which, remember, are all connected—are governed by a global system of circulation currents that work a bit like conveyor belts. The Atlantic meridional overturning circulation, or AMOC, is a major one, and s key to marine habitat over a vast swath of the planet. The AMOC moves warm water from the equator into the far northern reaches of the Atlantic Ocean, circulating nutrients and distributing temperature. It is also partly responsible for the weather in several regions, including the temperate climate of Western Europe.

According to another paper published in Nature this week, that circulation is slowing down as oceans warm and ice is discharged from the poles. The AMOC, the authors write, “appears to have reached a new record low,” declining in strength by around 15% since the mid-20th century. As a result, the AMOC is pushing 3 million fewer cubic meters of water per second than it was before. (Or three “Sverdrups,” if you want to sound like an oceanographer.)

The researchers note that the weakening of the current could already be affecting Europe’s weather. A weaker AMOC can mean colder water in the north Atlantic ocean during northern hemisphere summers, which in turn can cause temperatures in Western Europe to rise higher than usual.

Model simulations suggest it could also be “potentially leading to increased storminess in Europe,” the researchers write. “AMOC weakening has also been connected to above-average sea-level rise at the US east coast and increasing drought in the Sahel.”

The AMOC is considered a “tipping element” when it comes to climate change—meaning that once it reaches a certain severity of slow down, the physical process will worsen by itself in a runaway effect. In other words, much like the collapse of an ice sheet, there would be no going back. The AMOC study says that “the proximity of the Atlantic to this threshold is still poorly known.” Maybe we are still far away from runaway ocean current changes. Or maybe we’re not. But if global warming is the driver here—as it appears to be—it’s one more reason humanity has to curb emissions that lead to rising temperatures as fast as possible.