The Southern Ocean around Antarctica is warming at an alarming rate—twice that of the rest of the world's oceans. Now, researchers have developed more powerful evidence pointing to the human causes.
Though warming had been observed in the past, there was little historical data to allow scientists to pinpoint the causes with much certainty.
In a new study, researchers used climate models, the past observations that did exist and data flowing in from new ocean-going sensors to show how greenhouse gas emissions and the depletion of ozone in the atmosphere have led to both a warming of the Southern Ocean and an increase in its freshwater content. The findings also rule out natural variability as a major source of those changes.
"The observed warming is due to human influence," said oceanographer Neil Swart, a research scientist with Environment and Climate Change Canada who led the study, published Monday in the journal Nature Geoscience. "That may have been suspected or proposed before, but this is the evidence that really proves it."
Ocean-Going Floats and Climate Models
The Southern Ocean is notoriously inhospitable, clogged with ice and home to rough seas and weather. As a result, there weren't many measurements in the past.
In 2004, a partnership of 30 countries across the world launched the Argo program to improve what's known about the world's oceans, and now there are close to 4,000 programmable floats collecting data in the oceans worldwide. That is helping improve what's known, but the lack of complete data going back decades has, in the past, left researchers wondering if their conclusions were robust.
For this study, Swart and his colleagues used data collected in the Southern Ocean from 1950 onward, which showed the pattern of warming and freshening. Then they turned to computer models to try to replicate the cause.
"We sampled the model just the way the ships sampled the ocean so we could compare them," explained Sarah Gille, a physical oceanographer at Scripps Institution of Oceanography and a co-author of the paper.
So if a ship had taken a sample at a specific latitude and longitude on a certain date, the researchers could sample the model at the same location during the same month and year. Within the model, they were then able to turn off and on different influences, like natural variability (from solar radiation and volcanoes), the emissions of aerosols, greenhouse gas emissions and ozone depletion.
When they looked at natural variability and aerosol emissions, neither affected warming or freshening enough to explain what had been observed. But greenhouse gas emissions and ozone depletion did.
Because of global action under the Montreal Protocol, through which the world in 1987 committed to stop producing ozone-depleting substances that had been used in such products as refrigerants and aerosol sprays, it's expected that the role of ozone depletion will shrink. "That action—the Montreal Protocol—is often put forward as an example of, "Hey, as a society we can tackle these problems as they come up," Swart said.
The role that ozone depletion is playing in warming the Southern Ocean is far less than the role of greenhouse gas emissions, though, and so far, those emissions have been a stickier problem to fix.
"That signal of greenhouse gas emissions is just going to continue on intensifying as long as we keep emitting more and more," Swart said.
What's Causing the Freshwater Increase?
The study also suggested that most of the increasing freshwater content in the oceans is coming from precipitation, rather than melting ice from Antarctica.
Cecilia Bitz, a sea ice and climate scientists at the University of Washington who was not involved in the study, said she found the study's observations about the increased freshening particularly interesting. "According to their study, an increase in runoff of meltwater from Antarctica is not needed to explain the decrease in salinity in the upper ocean when considering the linear changes over the last 70 years," she said.
In the future, Gille said, the role that glacial melt is playing in the freshening of the Southern Ocean is "one of the enormous questions that the research community would like to address."
What Warmer Water Could Mean for the Ice
Another study published last week in Nature offers more insight into the future of East Antarctica's massive ice sheet.
David Wilson, a geochemist at Imperial College London, and his co-authors found that even 2 degree Celsius of warming above pre-industrial times had been enough to melt a significant part of the East Antarctic Ice Sheet in the past.
During the Pleistocene era, roughly 125,000 years ago, sea levels were as much as 20 to 30 feet higher than they are now. Wilson and his co-authors focused on the Wilkes Subglacial Basin and how it responded to warming then. Unlike the land-based glaciers, the Wilkes Subglacial Basin is built up from the ocean floor, which can make it particularly susceptible to warming waters. Their findings suggested that 2°C warming now, if sustained over a couple of millennia, could begin melting that location.
"If we wish to avoid the worst consequences—and you can think, this can have effects on coastal regions, agriculture, island nations—clearly we need to make efforts to change our behavior patterns and decarbonize the economy," Wilson said. "This is clearly what it's pointing towards."