Feature | June 13, 2017


NASA-MIT study evaluates efficiency of oceans as heat sink, atmospheric gases sponge

Credit: NASA’s Goddard Space Flight Center.

By Ellen Gray,
NASA’s Goddard Space Flight Center

The world’s oceans are like brakes slowing down the full effects of greenhouse gas warming of the atmosphere. Over the last ten years, one-fourth of human-emissions of carbon dioxide as well as 90 percent of additional warming due to the greenhouse effect have been absorbed by the oceans. Acting like a massive sponge, the oceans pull from the atmosphere heat, carbon dioxide and other gases, such as chlorofluorocarbons, oxygen and nitrogen and store them in their depths for decades to centuries and millennia.

New NASA research is one of the first studies to estimate how much and how quickly the ocean absorbs atmospheric gases and contrast it with the efficiency of heat absorption. Using two computer models that simulate the ocean, NASA and MIT scientists found that gases are more easily absorbed over time than heat energy. In addition, they found that in scenarios where the ocean current slows down due to the addition of heat, the ocean absorbs less of both atmospheric gases and heat, though its ability to absorb heat is more greatly reduced. The results were published in Geophysical Research Letters, a journal of the American Geophysical Union.

“As the ocean slows down, it will keep uptaking gases like carbon dioxide more efficiently, much more than it will keep uptaking heat. It will have a different behavior for chemistry than it has for temperature,” said Anastasia Romanou, lead author and climate scientist at NASA’s Goddard Institute for Space Studies and Columbia University in New York City.

Credit: NASA’s Goddard Space Flight Center.

She and colleagues at the Massachusetts Institute of Technology in Cambridge, Massachusetts used the NASA GISS ocean model and the MIT General Circulation Model to simulate one of the Atlantic’s major current systems that delivers absorbed heat and gases to the depths.

In the Atlantic Ocean, the Gulf Stream is part of what’s called the Atlantic Meridional Overturning Circulation, a conveyor belt of ocean water that carries warm water from Florida to Greenland where it cools and sinks to 1000 meters (about 3281 feet) or more before traveling back down the coast to the tropics. On its northward journey, the water at the surface absorbs gases like carbon dioxide and chlorofluorocarbons (CFCs) – the latter are, to a large extent, the gases responsible for the ozone hole over Antarctica – as well as excess heat from the atmosphere. When it sinks near Greenland, those dissolved gases and heat energy are effectively buried in the ocean for years to decades and longer. Removed from the atmosphere by the ocean, the impact of their warming on the climate has been dramatically reduced.

To understand and quantify the ocean’s sponge-like capabilities, the researchers used the two independent models of Atlantic Ocean currents together with shipboard observations of chlorofluorocarbons as a starting point. Chlorofluorocarbons are what’s called a passive tracer.

AMOC/Gulf Stream map

The red line on the map shows the Gulf Stream current, the surface portion of the Atlantic Meridional Overturning Circulation. Shown in shades of blue are the concentrations of CFCs at depth in the ocean. Nearer to the equator, the CFCs only occur at the surface. As the Gulf Stream current moves north, they begin to be drawn down to depth with the downward pull of the conveyor belt. Image credit: NASA/Jenny Hottle.

“I think of it as a colored dye,” said co-author John Marshall, a professor of oceanography at the Massachusetts Institute of Technology. “If I have a bucket of water and just stir it around and put some food coloring in it, the dye goes down into the water, and it doesn’t influence the circulation of the water.”

In the real world as well as in the model, this allows scientists to “see” how much of the gas is absorbed from the…