It is being hailed as a sea change in scientific understanding of the global ocean circulation system and how it will respond as the world heats up. A doomsday scenario involving the collapse of the circulation—previously portrayed in both peer-reviewed research and the climate disaster movie The Day After Tomorrow—came a lot closer in the last month. But rather than playing out in the far North Atlantic, as previously assumed, it now seems much more likely at the opposite end of the planet.
A new analysis by Australian and American researchers, using new and more detailed modeling of the oceans, predicts that the long-feared turn-off of the circulation will likely occur in the Southern Ocean, as billions of tons of ice melt on the land mass of Antarctica. And rather than being more than a century away, as models predict for the North Atlantic, it could happen within the next three decades.
Meanwhile the long-standing concern about a shutdown of the ocean circulation in the North Atlantic sometime in the 21th century appears to be subsiding. A Swiss study published this month found that, contrary to past belief, the circulation did not fail at the end of the last ice age, suggesting, the researchers say, that it was more stable than previously supposed, and less likely to collapse.
Taken together the two studies bring a dramatically new perspective to the likely impact of planetary heating on ocean circulation, which is one of the great stabilizing forces of the planet’s climate system.
The ocean circulation system, often called the global conveyor, follows a regular path through the Earth’s oceans and stirs their waters from top to bottom. It starts with water plunging from the surface and disappearing to the depths, from where it travels the world and does not surface for centuries. By capturing heat and carbon dioxide from the atmosphere, and burying both deep in the ocean, it is currently moderating global warming.
The conveyor is driven by the descent of cold, salty water to the ocean floor in just two places: in the far North Atlantic near Greenland and in the Southern Ocean around Antarctica. In both regions, the mechanism is the same. In cold polar conditions, large volumes of water freeze. The salt in the water is not incorporated into the ice. It remains in the residual liquid water, which grows ever saltier. The saltier water becomes, the denser it becomes. So the residue is heavier than surrounding water and eventually sinks to the ocean floor.
About 250 trillion tons of salty water sinks in this way around Antarctica each year, subsequently spreading north along the ocean floor into the Indian, Atlantic, and Pacific oceans. Similar volumes spread south from Greenland. The process is known as deep-water formation or ocean overturning, and it has continued largely unchanged for thousands of years.
But for how much longer? As the world warms, less ice is forming in the oceans at the ends of the Earth each year. At the same time, more ice on the nearby great ice sheets of Antarctica and Greenland is melting and releasing fresh water into the ocean.
