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Iceberg in the Southern Ocean, where deep ocean waters are seeing rising temperatures and a trend toward lower salinity values. Photo: Freedive Antarctica/Barcroft Media via Getty Images

The Southern Ocean is the planet's main heat and greenhouse gas sink, and without it, global warming would be a lot worse. It's also one of the least-observed and understood parts of the global ocean, due largely to its remoteness and competing influences there, from Antarctic sea ice to changing weather patterns and the slow healing of the stratospheric ozone layer.

The context: We've long known that on the ocean surface, parts of the Southern Ocean have been cooling over time, while deeper waters have been warming and freshening. But why these disparate trends have occurred has eluded scientists.

In a new study, published this week in the journal Nature Geoscience, scientists compiled the most comprehensive set of Southern Ocean observations available from 1950-2014. The study compares observations collected by ships and floats with model results.

They then used a "detection and attribution" approach using computer modeling combined with statistical techniques and observations to determine what causes of climate change best align with observed changes.

What they found: A combination of increasing amounts of greenhouse gases in the atmosphere and the gradual healing of the human-caused ozone hole over Antarctica are the main two drivers of change in a large part of the Southern Ocean, with greenhouse gases playing the dominant role.

  • This suggests that as we emit more greenhouse gases, deep ocean warming and freshening will continue, potentially altering regional or even global ocean circulation.
  • The Southern Ocean is the region where deep water interacts with the atmosphere, acting as "Kind of a gateway between the atmosphere and the ocean" study lead author Neil Swart of the Canadian Center for Climate Modeling and Analysis, told Axios.

Study co-author Sarah Gille, a scientist at the Scripps Institution of Oceanography, told Axios that it's critical that scientists better understand changes underway in the Southern Ocean, otherwise we won't be able to project the future climate with any accuracy.

"The Southern Ocean is the major connector between the Atlantic, Indian, and Pacific Oceans. The current system of the Southern Ocean, the Antarctic Circumpolar Current, carries more water than any other current on the planet," Gille told Axios.

"We want to understand this system, because we'd like to know how much heat and carbon dioxide it will continue to absorb, and what might make it stop taking up heat and carbon dioxide."
— Sarah Gille, Scripps Institution of Oceanography

In an accompanying news & views piece, Nathaniel Bindoff, who was not involved in the study, says it's the first to apply a rigorous approach to estimate the relative importance of each factor causing Southern Ocean changes.

Go deeper

"Atmospheric river" to whiplash Northern California from drought to flood

A map depicting 24-hour preciptation forecast (inches) ending Monday at 5a.m. local time. Photo: NOAA

A series of powerful "atmospheric river" storms are set dump historic amounts of rainfall across parts of drought-stricken California and the Pacific Northwest from this weekend, forecasters warn.

Why it matters: A strong atmospheric river, packing large amounts of moisture, is predicted to whiplash Northern California from drought to flood.

10,000 trees near giant sequoia groves to be removed after fires

A firefighter looks up at a giant sequoia tree after fire burned through the Sequoia National Forest near California Hot Springs, California, on Sept. 23. Photo: Patrick T. Fallon/AFP via Getty Images

"Upwards of" 10,000 trees near giant sequoia groves have been "weakened by drought, disease, age, and/or fire" and must be removed in the wake of California's wildfires, the Sequoia and Kings Canyon National Parks announced.

Why it matters: The damage to these trees, considered "national treasures," and work to remove them means a nearby key highway must remain closed to visitors as they have "the potential to strike people, cars, other structures, or create barriers to emergency response services," per a statement from the national parks.

Obama stumps for McAuliffe, urges Virginians not "to go back to the chaos"

Photo: Win McNamee/Getty Images

Former President Barack Obama framed a Nov. 2 gubernatorial race as a bellwether for the Democratic Party and the country, telling a crowd at a campaign event for Terry McAuliffe on Saturday that "I believe you, right here in Virginia, are going to show the rest of the country and the world that we're not going to indulge in our worst instincts."

Why it matters: With just over a week to go before Election Day in the Commonwealth, McAuliffe is bringing out the big guns. The 44th president appeared on the campus of Virginia Commonwealth University to urge supporters to get to the polls.