The planet has been warming and it has been the surface of our oceans that has absorbed much of the excess temperature in the atmosphere. Yet it isn’t just the water’s surface that has been warming, say scientists at the National Oceanic and Atmospheric Administration (NOAA) in the United States.

Heat from the atmosphere, they explain, appears to have been radiating downwards into deeper layers of the oceans all the way to the seafloor in some areas of the Atlantic Ocean. In other words, global warming can now be detected even at the bottom of the ocean.

The scientists reached this conclusion by analyzing a decade’s worth of temperature recordings from devices anchored at four depths in the Argentine Basin off the coast of Uruguay in the Atlantic. The depths averaged 3,682 meter with the shallowest at 1,360 meters and the deepest at 4,757 meters.

Everywhere they looked the scientists observed a warming trend of 0.02 to 0.04 degrees Celsius over a decade between 2009 and 2019, which they note is “a significant warming trend in the deep sea where temperature fluctuations are typically measured in thousandths of a degree.”

What drives this warming trend deep underwater is still little understood, however.

“In years past, everybody used to assume the deep ocean was quiescent. There was no motion. There were no changes,” Chris Meinen, an oceanographer at NOAA, said in a statement on the findings. “But each time we go look we find that the ocean is more complex than we thought.”

Scientists worldwide have been monitoring the top layer of the oceans, down to a depth of 2,000 meters, very closely for fluctuations in temperature in the international program called the Global Ocean Observing System.

“Devices called Argo floats that sink and rise in the upper ocean, bobbing along in ocean currents, provide a rich trove of continuous data on temperature and salinity. The deep sea, however, is notoriously difficult to access and expensive to study,” the NOAA scientists note.

Because of the challenges involved, scientists take temperature readings from near the seafloor by lowering an instrument from a ship only once every 10 years, which limits their understanding of the daily changes in the bottom half of the ocean.

It was almost by accident that the team at NOAA realized the fluctuations in temperatures in the depth of the Atlantic. The team has been working on a long-term study on ocean currents at the seafloor with the help of four devices moored at the bottom of the Argentine Basin.

Recently the scientists realized that a temperature sensor built into the instruments’ pressure sensors to study currents have also been collecting hourly temperature data, which they then set out to analyze. “So we went back and we calibrated all of our hourly data from these instruments and put together what is essentially a continuous 10-year-long hourly record of temperature one meter off the seafloor,” Meinen says.

At the two shallower depths of 1,360 and 3,535 meters where changes in temperature were measured, they discovered monthly fluctuations by as much as 1 Celsius. At depths below 4,500 meters, temperature fluctuations were less pronounced, but even here changes followed an annual pattern according to the changing seasons above water.

“The average temperature at all four locations went up over the course of the decade, but the increase of about 0.02 degrees Celsius per decade was only statistically significant at depths of over 4,500 meters,” the scientists note. The task is now to better understand how temperature changes in the deep sea could impact surface temperatures, seeing as the oceans absorb much of the excess heat in the atmosphere, they add.

“We’re trying to build a better Global Ocean Observing System so that in the future we’re able to do better weather predictions,” Meinen explains. “At the moment we can’t give really accurate seasonal forecasts, but hopefully as we get better predictive capabilities, we’ll be able to say to farmers in the Midwest that it’s going to be a wet spring and you may want to plant your crops accordingly.”

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