The ocean surface around Antarctica has become less salty over the past several decades, creating a barrier that prevents it from mixing with warmer waters below and trapping the heat in the depths of the ocean, according to a recent study published in Nature Climate Change.
The researchers found that higher CO2 levels are likely to have increased the fall of rain and snow onto the ocean around Antarctica over the past three decades, freshening the surface layer.
Photo courtesy of Eric Galbraith
The study, co-authored by CIFAR Fellow Eric Galbraith (McGill University), analyzed historical oceanographic data and compared them to global climate model simulations. The researchers found that higher CO2 levels are likely to have increased the fall of rain and snow onto the ocean around Antarctica over the past three decades, freshening the surface layer.
“If you make the surface fresh it caps it off, so it isolates the deep ocean from the surface,” says Galbraith, a member of the Earth System Evolution program.
That skin of fresh water prevents the surface from dipping down and mixing with the warmer water below. Galbraith says the results were surprising, because it helped to explain a phenomenon that researchers didn’t completely understand — a huge hole in the Antarctic ice during the winters of 1974, 1975 and 1976, that has been absent ever since.
The hole in the ice, called the Weddell polynya, marked the last time the surface of the Southern Ocean surrounding Antarctica mixed directly with the deep ocean, which released stores of heat contained in the deep water.
“Everybody was expecting it to come back again,” Galbraith said. “They were just waiting and waiting, and it never came back.”
Galbraith and his colleagues found that the polynya’s continued absence could be explained by the freshening effect of global warming on the Southern Ocean.
“It’s a response that you would not predict from simple warming itself, and this is why it’s come as a surprise,” Galbraith says.
CIFAR helped fund the research, and Galbraith says the transdisciplinary approach of the Earth System Evolution program gave him a clue that piqued his interest in studying this phenomenon.
“In the geological record, I could see hints that something like this had happened at the end of the last Ice Age, during the last big, natural global warming. I was very curious and this led me to look more into this,” Galbraith said.
“So really it was the Earth System Evolution perspective that gave a clue that there was something here that might be relevant for the future as well.”
The researchers say global warming’s effect on ocean circulation has also affected Antarctic bottom water, the cold, oxygen-rich layer of water that lines the bottom of all of the world’s oceans.
“Antarctic bottom water has been steadily shrinking, and nobody has known why,” Galbraith says.
The study suggests that deep mixing of ocean waters filled up Antarctic bottom water, but as circulation slowed down, it began to disappear.
“We’ve actually slowed the production of Antarctic bottom water by capping the surface with fresh water.”
While rapid changes to the ocean’s processes affect marine life, Galbraith says it’s unclear what other effects they could have on the planet.
“The climate is a very complex and dangerous system. And when we change the climate, unpredictable things can happen because we don’t understand it completely,” he says.