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Human Causes Only Plausible Source of Warming in Southern Ocean

Synthesis of observational data and models shows trend not due to natural variability

Argo floats deployed from R/V Palmer during NSF-funded SOCCOM cruise to Southern Ocean. Photo: Greta Shum

The Southern Ocean that surrounds Antarctica has been getting warmer and less salty since 1950 at a rate nearly twice that of global oceans overall.

Now, for the first time, researchers using data from a global network of data-gathering ocean floats known as Argo and pre-Argo records, along with a climate model, have attributed these trends to human-caused climate change including greenhouse gas emissions and depletion of the ozone layer. Sarah Gille, a physical oceanographer at Scripps Institution of Oceanography at the University of California San Diego and co-author of a new study, said the research validates how scientists have been interpreting long-term trends.

It also shows that a climate model validated against historic ship-based observations is able to provide realistic representations of long-term trends, which will help scientists understand what could happen in the Southern Ocean in the future.

“The Southern Ocean is far from most ports and it has high winds and high waves, so few observations were collected before the start of the Argo program in 2004,” said Gille.  “In this study, we were able to sample the climate model at the same dates and locations where ships sampled the ocean in the 1950s or 1960s. We found that the sparsely sampled models were consistent with the full model results, indicating that our sparse historic observations are able to show us realistic trends.”

The Southern Ocean is a key player in global climate in that it absorbs a disproportionate amount of excess heat and carbon created by human activities. Trends in the Southern Ocean are consistent with warming in the world’s polar regions that is well in excess of global averages.

“Our study shows that observed Southern Ocean warming and freshening is due to human-caused increases in greenhouses gases and stratospheric ozone depletion,” said lead author Neil Swart of the Canadian Centre for Climate Modelling and Analysis, Environment and Climate Change Canada. “It is vital to understand changes in the Southern Ocean because it is a key region of global heat and carbon uptake, and it underlies ice sheets with many meters of sea-level rise potential. As far as we know, this is the first time that the influence of stratospheric ozone depletion has been detected in the ocean.”

The study is among the first to identify the human fingerprints of warming in this specific ocean region. More than 20 years ago, other Scripps Oceanography researchers were able to conclude that there was no plausible explanation for warming except anthropogenic influence but their analysis considered much larger scales with less resolution. In this study the researchers also singled out greenhouse gas emissions and ozone depletion as key factors. Another potential human source of ocean change – aerosol pollution – did not appear to be a factor in Southern Ocean trends.

“While the influence of ozone depletion and greenhouse gas increases on the Southern Ocean have been suggested for some time, our research provides the evidence that links the observed changes to these mechanisms, and defines their relative importance,” said Swart. “Moving forward, the ozone layer is recovering while greenhouse gases are continuing to increase, leading to opposing trends. Our work suggests that greenhouse gases are more important than ozone in determining Southern Ocean trends.”

The study, “Recent Southern Ocean warming and freshening driven by greenhouse gas emissions and ozone depletion,” appears Sept. 24 in the journal Nature Geoscience.  The National Science Foundation supported Gille’s portion of the research.


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