Memories shape who we are—and who we will become. Our past helps us figure out what to do in increasingly stressful or confusing situations, and the more experience we have the better equipped we are to make the next move.
But people and other living things aren’t the only things with memory. Oceans, in a way, remember too. Ocean memory, a relatively new term, has hit the headlines recently as new research out this month in Science has demonstrated how the ocean is “losing its memory.” How that loss, which occurs thanks to human caused climate change, will alter the future is still up in the air.
Here’s what you need to know about the concept, and what its loss could mean for the future of the oceans and the planet.
What is ocean memory?
While ocean memory might have a nice ring to it, a more technical term would be the “persistence of ocean temperature,” says Daisy Hui Shi, a postdoctoral researcher at California’s Farallon Institute and an author of the new report. Because oceans have a tremendous ability to absorb heat, temperatures change slower than the air or the atmosphere. You may have experienced that personally if you’ve dived into the ocean on a hot day only to be surprised by brutally chilly waters.
A property called “heat capacity” is the reason this exists. Basically, the ocean has a much larger heat capacity than land, which means it takes a whole lot more work to get the oceans to heat up. You can see how ocean temps trail terrestrial ones seasonally as well as over longer periods of time.
In fact, in 2019 researchers found that the deep Pacific Ocean still lags centuries behind the rest of the ocean in terms of temperature—for example, the deepest parts of the ocean were still reacting to the entry into the Little Ice Age, which happened several hundred years ago. Basically, while some parts of the ocean are seeing changes due to the warming of the planet, the deepest corners still “remember” a much cooler era and could even potentially be cooling still.
This memory, or the persistence of the temperature in the ocean, is crucial because it acts as a source of predictability for the entire climate system, including oceans, land, and the atmosphere. The memory of the ocean, the researchers find, is largely controlled by the ocean’s top layer. This layer is where the ocean interfaces with the atmosphere. Because the seawater is so close to the air, the wind is able to mix it up, creating roughly uniform salinity and temperature for tens of meters deep (sometimes reaching over 500 meters deep in the winter in subpolar regions).
The deeper mixed layers have a high heat content, which means they have a higher level of “thermal inertia.” There, the water even more slowly changes to match the temperature of its surroundings. This acts as a cushion—protecting the deepest, most “memory” intensive parts of the ocean from changes that might disrupt it.
But with climate change, this cushion-like level could be shrinking. And without it, storing those long-term climatic memories is harder and less predictable.
What could happen to ocean memory with climate change?
As the planet heats up, this mixed level is disappearing. “The layer is becoming shallower and shallower as a response to the warming because the ocean becomes more stable,” Shi says. When the mixed layer becomes less mixed, it’s an indication that the zone has become shallower. A shallower mixed zone leads to overall reduction of the memory globally, she adds.
[Related: The world now has a fifth ocean.]
Shi and the other authors looked at models to determine year-to-year changes in ocean memory. What they found was that the ocean appeared to have some sort of “amnesia.” The persistence of the ocean temperature year by year, in essence, is becoming more unpredictable. By the end of the 21st century, the authors predict that ocean memory will decrease throughout most of the world, and even completely vanish in some areas. The most pronounced changes are expected around the Indian Ocean, South China Sea, and waters near Southeast Asia, according to the paper.
Of course, other factors may be at play with reduced memory, like “ocean currents and changes in the energy exchange between the atmosphere and ocean,” co-author Robert Jnglin Wills, a research scientist at University of Washington in Seattle said in a release. Still, the “shoaling of the mixed layer depth and resulting memory decline happens in all regions of the globe, and this makes it an important factor to consider for future climate predictions,” he adds.
What reduced memory could mean is increasingly unpredictable events, like ocean heat waves. While marine heat waves can sometimes be predicted up to a year ahead of the event actually happening, a shrunken memory can reduce researchers’ ability to predict what the ocean is going to look like. This could also have implications for fisheries, since so much of the industry relies on predicting the future status of the ocean. And, of course, the ocean affects weather, temperature, and precipitation on land, which could make for even more unpredictability.
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