The Deepest, Darkest, Most Frigid Depths of the Ocean Are Warming

Thermometers anchored to the seafloor revealed that even the deep sea is not impervious to rising global temperatures

A hydrothermal vent in the deep ocean produces a dark, cloud of hot air against a dark blue background. The vent in the foreground is rocky, like a conglomerate of shells and rocks piled on top of each other.
To get a glimpse of what's happening in the deep blue, scientists deployed instruments to measure changes on the sea floor. This hydrothermal vent exists at 3,300 meters deep. MARUM – Cen­ter for Mar­ine En­vir­on­mental Sci­ences via Wikimedia Commons under CC BY 4.0

Scientists have mountains of data that show just how dramatically temperatures are warming on land and at the ocean's surface, but what's happening in the darkest, nearly unreachable depths of the ocean has been shrouded in mystery. A new study suggests that even temperatures at the seafloor are rising, reports Maria Temming for Science News.

To get a glimpse of what's happening in the deep blue, a team of scientists deployed thick, glass spheres anchored by barbell plates in four spots at the bottom of the Argentine Basin, off the coast of Uruguay. The instruments continuously collected data on the seafloor by logging measurements every hour from 2009 to 2019.

Since studying the deep ocean is often too expensive and challenging to access, these types of measurements are usually only taken every ten years by research vessels, so scientists only have sparse snapshots of data—but this team captured even the most minute changes over time, reports Emily Holden for The Guardian.

Their study, published last month in the journal Geophysical Research Letters, revealed that at 4,757 meters underwater—nearly three miles down—temperatures increased from 0.232 degrees Celsius to 0.248 degrees Celsius. In comparison, land and ocean surface temperatures in 2019 were 0.95 degrees Celsius higher than the long-term average.

"In years past, everybody used to assume the deep ocean was quiescent. There was no motion. There were no changes," says Chris Meinen, lead author on the study and an oceanographer at the U.S. National Oceanic and Atmospheric Administration (NOAA), in a press release. "But each time we go look, we find that the ocean is more complex than we thought."

The ocean's temperature changes may seem slight, but they're definitely not, Meinen says.

“If you think about how large the deep ocean is, it’s an enormous amount of heat,” he tells The Guardian. Plus, warm water rises, so it would take some serious heat to trigger even the slightest increase in temperature, Meinen tells Science News.

“We didn’t expect that you would see hour-to-hour and day-to-day variations down that deep,” he tells The Guardian. “There are processes in the deep ocean that are making things change rapidly, and we don’t really know what those processes are yet.”

The ocean plays a crucial role in regulating earth's climate since it absorbs an estimated 90 percent of the planet's heat. But as it absorbs more heat and gradually warms, the water molecules expand, leading to rising sea levels and more intense hurricanes.

Meinen, who speaks for himself and not NOAA, tells The Guardian that these changes are consistent with human-caused climate change, but more data is needed to confirm that statement. To do so, Meinen says measurements need to be taken every year—not every decade—to better understand the long-term trends.

Editor’s Note, October 27, 2020: A previous version of this article incorrectly stated that ocean temperatures are increasing "4,757 feet underwater," when, in fact ocean temperatures are increasing "4,757 meters underwater." The story has been edited to correct that fact.

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