Even the deepest parts of the ocean are slowly beginning to warm

Global warming is beginning to penetrate even the deepest parts of our oceans.  

While the surfaces of these vast bodies of water have absorbed the vast majority of human-induced warming, as sea water circulates, the worrisome changes are slowly making their way downward.

Scientists at the US National Oceanic and Atmospheric Administration (NOAA) have now detected a warming trend in some of the deepest parts of the southern Atlantic ocean.

Analysing ten years of temperature recordings in the Argentine Basin from 2009 to 2019, researchers found all four sites warmed somewhere between 0.02 to 0.04 degrees Celsius, depending on the depth.

One of the recording devices was located 4,757 metres (15,600 feet) below the waves, and it experienced a surprising amount of variability over the course of a year.

“In years past, everybody used to assume the deep ocean was quiescent. There was no motion. There were no changes,” explains oceanographer Chris Meinen, who works for NOAA.

“But each time we go look we find that the ocean is more complex than we thought.” 

Part of that mystery has to do with location. The deep ocean is, well, deep – which means obtaining real world data is quite the challenge, especially long-term data.

However, recent estimates based on some measurements and climate models have predicted that some parts of the deep ocean are growing warmer as the ocean circulates and turns over the water. 

The Southern Ocean, which is relatively well-ventilated, was found to experience these changes even more quickly than the Northern Ocean. Now, data from the Argentine Basin located off the coast of Uruguay suggest things are certainly moving quickly down south.

Four devices, moored to the bottom of the basin, have revealed hour-by-hour, year-by-year data on what’s actually going on one metre off the seafloor.

At the shallower depths of 1,360 and 3,535 metres (4,460 feet and 11,600 feet), temperatures fluctuated far more than scientists were expecting, and while these changes were somewhat smaller at even deeper levels, their overall pattern matches what is going on at the surface.

How that will ultimately impact ecosystems or weather above the waves is still unknown, but given how important ocean circulation and temperature is for our global climate system, it’s not exactly good news.

Still, there are some positives to this study. Submerged devices, moored on the deep ocean floor, are usually used to collect data on deep ocean currents, but Meinen and his colleagues recently realised they were also equipped with temperature sensors.

The sensors had been recording the temperature this whole time, and it’s allowed the team to gain an unprecedented insight into the super-deep, even if only for a decade of warming.

“There are a number of studies around the globe where this kind of data has been collected, but it’s never been looked at,” says Meinen.

“I’m hoping that this is going to lead to a reanalysis of a number of these historical datasets to try and see what we can say about deep ocean temperature variability.”

The study was published in Geophysical Research Letters.

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