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Global warming: Oceans’ coldest water is vanishing

Oceanographers measure dramatic changes around Antarctica

The deepest part of the Southern Ocean is changing fast. PHOTO BY BOB BERWYN.

By Summit Voice

SUMMIT COUNTY — A vast pool of very cold water at the bottom of the Southern Ocean is shrinking at a surprisingly fast rate, according to oceanographers with NOAA and the University of Washington, who studied records going back several decades to track the changes.

By analyzing thousands of measurements, the researchers determined that the so-called Antarctic Bottom Water is disappearing at an average rate of about eight million metric tons per second over the past few decades, equal to about fifty times the average flow of the Mississippi River or about a quarter of the flow of the Gulf Stream in the Florida Straits.

The bottom water forms in a few specific locations around Antarctica, where seawater is cooled by the atmosphere and made saltier by ice formation. The dense water then sinks to the sea floor and spreads northward.

It fills most of the deep ocean around the world as it slowly mixes with warmer waters above it, driving global currents and heat exchange at different latitudes in ways that aren’t fully understood.

“In every oceanographic survey repeated around the Southern Ocean since about the 1980s, Antarctic Bottom Water has been shrinking at a similar mean rate, giving us confidence that this surprisingly large contraction is robust,” said lead author Sarah Purkey, graduate student at the School of Oceanography at the University of Washington in Seattle, Wash.

“Because of its high density, Antarctic Bottom Water fills most of the deep ocean basins around the world, but we found that the amount of this water has been decreasing at a surprisingly fast rate over the last few decades,” Purkey said.

While previous studies have shown that the bottom water has been warming and freshening over the past few decades, these new results suggest that significantly less of this bottom water has been formed during that time than in previous decades.

“We are not sure if the rate of bottom water reduction we have found is part of a long-term trend or a cycle,” said co-author Gregory C. Johnson, Ph.D., an oceanographer at NOAA’s Pacific Marine Environmental Laboratory in Seattle. “We need to continue to measure the full depth of the oceans, including these deep ocean waters, to assess the role and significance that these reported changes and others like them play in the Earth’s climate.”

Changes in the temperature, salinity, dissolved oxygen, and dissolved carbon dioxide of this prominent water mass have important ramifications for Earth’s climate, including contributions to sea level rise and the rate of Earth’s heat uptake.

“People often focus on fluctuations of currents in the North Atlantic Ocean as an indicator of climate change, but the Southern Ocean has undergone some very large changes over the past few decades and also plays a large role in shaping our climate,” said Johnson.

The data used in this study are highly accurate temperature data repeated at roughly 10-year intervals by an international program of repeated ship-based oceanographic surveys (www.goship.org). Within the U.S., the collection of these data has been a collaborative effort of governmental laboratory and university scientists, funded primarily by NOAA and the National Science Foundation. However, much of the data used in this study were measured by international colleagues.

“Collection of these data involves 12-hour days, seven days a week, of painstaking, repetitive work at sea, often for weeks on end with no sight of land. We are grateful for the hard work of all those who helped in this effort,” said Purkey.

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2 Responses

  1. [...] Ocean around Antarctica, a reservoir that helps drive ocean currents worldwide — in effect, the globe’s coldest water is vanishing, which could weaken a major pump in global ocean [...]

  2. […] The new work can also help explain a scientific mystery. It has recently been discovered that Antarctic Bottom Water, which fills the deepest layer of the world ocean, has been shrinking over the last few decades. […]

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