Deep Ocean Revisions Do Not Spell Global Warming Rethink

A new paper suggests that our model for the circulation of the deep ocean may be wrong. But claims that all global warming science should therefore be thrown overboard just don't hold water.

The Ocean Is a Heat Mover

The ocean is a lot of things -- a thing of great mystery and beauty, a perfect beach spot, home to all sorts of marine life. It's also a huge reservoir of -- and engine for -- heat.

In tandem with the atmosphere, the ocean helps distribute warm temperatures from the globe's tropical regions and cool temps from its polar regions to far-off areas. Without the oceanic and atmospheric mechanism, the temperature difference between the poles and the equator would be an astounding 200 degrees Fahrenheit rather than the 55-degree difference that exists today.

The ocean part of this feat is accomplished by an overturning of the waters, with surface waters sinking and deep ocean waters upwelling. Sinking occurs primarily in three bodies of water: the Greenland, Norwegian and Labrador Seas.

Cold, dense waters in these marginal seas sink to great depths (because cold water is denser than warm water), then flow into the North Atlantic Ocean, making it a key location to measure the overturning circulation.

In the past, the reason why the ocean waters overturn here has been attributed to the strong winter cooling of surface waters (see graphic depicting ocean conveyor belt concept), but today, oceanographers are also exploring the role of wind-forcing in the overturning. Though all the whys behind the forcing of this overturning are still being worked out, measurements through the years have made it abundantly clear that the waters are overturning.

Conveyor Belt Keeps Things Moving

The water sinking in the North Atlantic has to go somewhere, and the thinking for quite a few decades has been that it forms a narrow tongue of deep ocean water that flows southward along the ocean's western boundary. Eventually this water reaches the Southern Hemisphere and other parts of the globe's oceans, where it rises back to the surface and begins moving northward to the tropics and begins the path again. The sum total of this overturning is often referred to as a conveyor belt as it takes water in one direction along the surface and in the other direction in the deep ocean.

The conveyor belt model is an important component of our understanding of the long-term workings of the climate. It controls how the ocean works in concert with the atmosphere to move excess heat from the tropics to the poles and also how heat and gases like carbon dioxide (CO2) are transferred from the atmosphere into the ocean.

New Paper Complicates the Bottom Part of Conveyor Belt

Last week my colleague at Duke, Susan Lozier, her graduate student, Stefan Gary, and colleagues from the Woods Hole Oceanographic Institute and the Leibniz-Institüt für Meereswissenschaften published a paper in Nature that threatens to overturn the conveyor belt model. They used an array of floats combined with model simulations to track the motion of subsurface water as it moved southward from the Labrador Sea.

Their findings confirmed what was found in earlier, preliminary studies (see here and here): the deep portion of the North Atlantic's southward flow does not entail a narrow tongue of water but rather a more diffuse, less organized flow occurring in the interior instead of the western boundary. In the parlance of a conveyor belt, the work of Lozier and her colleagues shreds that belt into lots of little strands as it moves along the lower limb of its path.

There are important implications to this study. For example, a shredded conveyor belt is going to make it harder to follow and document the movement of anthropogenic CO2 in the deep ocean. We had thought that this could be done by monitoring that narrow tongue of ocean bottom water flowing along the western North Atlantic boundary. The new paper suggests that it's not that simple.

Keep Time in Mind

But the ocean conveyor belt moves very slowly. It takes on the order of 1,000 years for water to run the entire course. For this reason, the study's findings are not terribly important to understanding and predicting the decadal-to-century response of the climate to greenhouse warming. Far more relevant is the
fact that the ocean circulation occurs, that it causes the ocean to
overturn on itself, and more specifically that the North Atlantic's
surface water sinks and thus carries heat and excess CO2 into the deep
ocean. The Nature paper does not call into question the validity of any of these facts.

Not So Fast, Mr. and Ms. Skeptic

The work by Bower, Lozier and their team has been received with glee in the skeptic blogosphere, with declarations that their paper undermines global warming science, the role of human activities, and future climate predictions. (See some skeptics' reactions here, here, and here.)

Wrong. The Nature paper has nothing to do with whether or not climate change is due to human activities, and it does not significantly undermine climate predictions, at least not for the 21st century. But don't take my word for it. Read what Susan Lozier has to say.

"It has been quite surprising (and dismaying) that some global warming skeptics have argued that our research 'means that all the current climate prediction models are significantly wrong.'" [See an example here.]

"This statement is clearly nonsense. Apparently, by stating that the conceptual model of the lower limb of the overturning circulation as a conveyor belt is broken, the skeptics took this to mean that there is no overturning circulation. From there, they concluded that all climate prediction models are wrong, an astoundingly erroneous interpretation of our work!"

Dr. Bill Chameides is the dean of Duke's Nicholas School of the Environment and a member of the National Academy of Sciences. He blogs regularly at theGreenGrok.com.

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