Since warming air temperatures ultimately also affect ocean surface temperatures, ocean warming is one of the symptoms of global warming. About 90% of the heat trapped in our atmosphere is absorbed by the oceans, which have been slowly, but steadily, rising. Globally, both land and ocean surface temperatures have been warming much faster than ever before. According to US National Oceanic and Atmospheric Administration (NOAA) records, a decade ago (2009), these temperatures were 0.56 degrees C higher than long-term average temperatures on record. Last year (2019) land and ocean surface temperatures had risen to 0.95 degrees C higher than the long-term average.
Warmer ocean surface temperatures contribute to more intense tropical storms such as hurricanes and typhoons. Warmer water temperatures also causes seawater molecules to expand, adding to sea-level rise.
Scientists are now concerned that this warming is starting to penetrate into the deeper, colder zones of the ocean as ocean waters circulate around the globe.
A study, which was recently published in Geophysical Research Letters, has revealed a warming trend in deep oceanic waters of the souther Atlantic Ocean.
After analyzing ocean temperature data recorded over a ten year period at a series of monitoring sites in the Argentine Basin between 2009-2019, a team of international marine scientists found that all four of the sites monitored had warmed. Temperature increases varied according to depth, but ranged between 0.02-0.04 degrees Celsius higher than previously recorded 10 years earlier.
At one site, the temperature was recorded at a depth of 15,600 feet (4,757 meters) below the ocean surface, yet the researchers were surprised by how much temperatures varied at this site over a one year period.
According to the researchers, these temperature increases are consistent with warming trends observed in shallower waters that are linked to human induced climate change. However, further studies are needed to better understand the driving mechanism of the temperature increases observed in the deep ocean.
"In years past, everybody used to assume the deep ocean was quiescent. There was no motion. There were no changes," said Chris Meinen, an NOAA oceanographer and lead author of the study. "But each time we go look we find that the ocean is more complex than we thought."
The Global Ocean Observing System (GOOS) currently allows researchers to monitor the upper 6,560 feet (2,000 meters) of the ocean much more closely than was previously possible. However, since deep sea waters are much more difficult to access, it is expensive and less feasible to monitor. Consequently, temperatures in deeper waters are typically measured just once every ten years. As a result, scientists do not have as clear a picture of the changes going on in deeper waters compared to the better studied surface waters.
Now, data collected by Meinen and his colleagues during a rare long-term observation study of ocean currents that they conducted in the Argentine Basin, off the coast of Uruguay in south Atlantic Ocean, suggest things are heating up rather rapidly.
How these temperature increases will impact marine ecosystems or even weather patterns above the ocean's surface remains unknown. However, since ocean circulation and ocean temperatures play an important role in driving the world's climate system, it may not bode well.
It is crucial that we understand how things are warming deep in the ocean, as this could potentially have implications that affect us on land too. Since the oceans act as a heat sink, absorbing a significant percentage of the Earth's heat, having a clearer understanding of ocean temperature trends can also help to shed light on fluctuations in atmospheric temperature as well.
Christopher S. Meinen et al, Observed Ocean Bottom Temperature Variability at Four Sites in the Northwestern Argentine Basin: Evidence of Decadal Deep/Abyssal Warming Amidst Hourly to Interannual Variability During 2009–2019, Geophysical Research Letters (2020). DOI: 10.1029/2020GL089093