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The deep sea is slowly warming up

The deep sea is slowly warming up

A new study finds that temperatures in deep water fluctuate more than scientists previously thought. Credit: Doug White.

A new study reveals that temperatures in deep waters fluctuate more than scientists have previously thought, and the warming trend is already being detected at the bottom of the ocean.

In a new study in the journal AGU Geophysical research letters, researchers analyzed decades-old temperature records of moorings anchored at four depths in the Argentine Atlantic basin off the coast of Uruguay. Depths range from an average ocean depth of 3,682 meters (1

2,080 feet), with the shallowest at 1,360 meters (4,460 feet) and the deepest at 4,757 meters (15,600 feet).

They found that all sites showed a warming trend of 0.02 to 0.04 degrees Celsius per decade between 2009 and 2019 – a significant warming trend in the deep seas, where temperature fluctuations are usually measured in thousands of degrees. According to the authors of the study, this increase is in line with the warming trends in the shallow ocean associated with anthropogenic climate change, but more research is needed to understand what drives rising temperatures in the deep ocean.

“In years past, everyone thought the deep ocean was at rest. There was no movement. There was no change,” said Chris Mainen, an oceanographer at NOAA’s Atlantic Oceanographic and Meteorological Laboratory and lead author of the new study. “But every time we go to look, we find that the ocean is more complex than we thought.”

The challenge is to measure depth

Today, researchers are observing the best 2,000 meters (6,560 feet) of ocean more closely than ever, thanks in large part to an international program called the Global Ocean Observing System. Devices called Argo floats, which sink and rise in the upper ocean, moving in ocean currents, provide a rich set of continuous data on temperature and salinity.

However, the deep sea is notoriously difficult to access and expensive to explore. Scientists usually measure its temperature with the help of ships that lower an instrument to the seabed only once every ten years. This means that scientists’ understanding of the daily changes in the lower half of the ocean lags far behind their knowledge of the surface.

Mainen is part of a NOAA team conducting a rare long-term study of the ocean floor, but until recently, the team believed that the four devices that docked at the bottom of the Argentine basin were simply gathering information about ocean currents. Mainen then saw a study from the University of Rhode Island showing a feature of the device that he was not fully aware of. The instrument’s pressure sensor has a built-in temperature sensor used to study currents and has randomly collected temperature data for their entire study. All they had to do was analyze the data they already had.

“So we went back and calibrated all our hourly data from these instruments and collected what was essentially a continuous 10-year hourly record of the temperature one meter from the seabed,” Mainen said.

Dynamic depths

The researchers found at the two shallower depths of 1360 and 3535 meters (4,460 feet and 11,600 feet), temperatures fluctuate approximately monthly by up to degrees Celsius. At depths below 4,500 meters (14,760 feet), temperature fluctuations were smaller, but the changes followed an annual pattern, showing that the seasons still have a measurable impact far below the ocean surface. The average temperature in the four places has risen over the decade, but the increase of about 0.02 degrees Celsius per decade has been statistically significant only at depths above 4,500 meters.

According to the authors, these results show that scientists need to measure the temperature in the deep ocean at least once a year to account for these fluctuations and take into account significant long-term trends. Meanwhile, other people around the world who have used the same ports to study deep-sea ocean currents can analyze their own data and compare temperature trends in other ocean basins.

“There are a number of studies around the world that have collected such data, but they have never been considered,” Mainen said. “I hope this will lead to a re-analysis of a number of these historical datasets to try to see what we can say about the deep variability of ocean temperatures.”

A better understanding of temperature in deep waters can have consequences that extend beyond the ocean. Because the world’s oceans absorb so much of the world’s heat, learning about ocean temperature trends can help researchers better understand temperature fluctuations in the atmosphere as well.

“We are trying to build a better Global Ocean Observing System so that we can make better weather forecasts in the future,” Mainen said. “At the moment we can’t give really accurate seasonal forecasts, but we hope that by getting better forecasting opportunities, we will be able to tell farmers in the Midwest that it will be a wet spring and you may want to plant crops accordingly. . ”

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More information:
Christopher S. Mainen et al., Observed ocean floor variability at four sites in the northwestern Argentine Basin: Evidence of decimal deep / abyssal warming against hourly to interannual variability in 2009–2019, Geophysical research letters (2020). DOI: 10.1029 / 2020GL089093

Provided by the American Geophysical Union

Quote: The Deep Sea is slowly warming up (2020, 13 October), extracted on 13 October 2020 from https://phys.org/news/2020-10-deep-sea-slowly.html

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