How rain and snow are speeding up global warming

A small change in Antarctica is affecting the entire globe.

A study recently published in Nature Climate Change explains how the convection cycle in the ocean is slowing down, which will likely exacerbate global warming.

The study was conducted by professor Irina Marinov and postdoctoral researcher Raffaele Bernadardello in collaboration with researchers from McGill University.

Open-ocean convection stores anthropogenic, or human-produced, heat and carbon dioxide from the atmosphere in dense water which sinks to the bottom of the ocean. This dense water forms when surface water freezes in the winter, leaving behind all the salt for much denser water which sinks to the bottom of the ocean, known as Antarctic Bottom Water.

With climate change and global warming, there has been more rain and snow near the poles over the past few decades, including at the Southern Ocean. This increase in precipitation makes water fresher at the surface, making it more difficult for open-ocean convection to take place, which then takes less heat and carbon dioxide out of the atmosphere. Currently, more than half of the extra heat and carbon dioxide in the atmosphere is stored under the Southern Ocean.

One sign that open-ocean convection is slowing down is the disappearance of polynyas, ice-free zones in the polar regions of the ocean. One example is the Weddell Sea Polynya, which disappeared in the 1970s. Marinov thinks people should be more aware of the disappearance of polynyas.

“When the polynya happened in the ’70s , people paid attention for a while but hasn’t been mentioned for more than twenty years,” Marinov said. She expects to resuscitate the interest through the recent findings.

One possible implication of the slowing down of open-ocean convection is that less anthropogenic heat and carbon dioxide from the atmosphere will be stored in the ocean, warming the planet further and increasing global warming in a “positive feedback” loop.

The decrease in open-ocean convection might also affect ocean ecosystems. Nutrients and oxygen that are needed by life in the ocean sink down with Antarctic Bottom Water, reaching not just oceans around Antarctica but around the world.

“This is one place in the ocean that is connected to all parts of the world,” Marinov said.

Marinov, Bernardello, and their collaborators started the research from a global climate model they were running which projected changes in Antarctic Bottom Water over time. 

Similar mechanisms emerged from an analysis of real data combined with a massive analysis of the 35 latest generation climate models. “We thought that the results from these models were coming out around the world and wanted to see if they were consistent,” Bernardello said.

They are now working on follow-up research focusing on the global implications of change in South Ocean open-sea convection.

“We anticipate a slowdown of Antarctic Bottom Water will affect both the uptake of heat and anthropogenic carbon into the ocean,” Marinov said.

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