Adding iron to the Southern Ocean to promote phytoplankton blooms, like the one pictured above, has been proposed as a way to pull carbon dioxide from the atmosphere and mitigate climate change. New research shows that this method, called ocean iron fertilization, might not be as effective as previously thought. Credit: NASA

By Alexandra Branscombe

WASHINGTON, DC – A plan to reduce carbon from the atmosphere by adding large amounts of iron to the Southern Ocean around Antarctica may not be as effective as previously thought, according to new research.

A modeling study shows that 66 percent of the carbon initially stored deep in the Southern Ocean, through a process known as ocean iron fertilization, would resurface in less than 40 years.

The new study published in April in Geophysical Research Letters, a journal of the American Geophysical Union, shows that carbon stored by ocean iron fertilization in the Southern Ocean would resurface much sooner than previously thought due to strong currents and ocean upwelling, a process that pushes cold, deep water to the surface.

Climate change experts have said that the sequestered carbon must remain trapped in the deep ocean for at least 100 years if it is to help reduce global warming.

“In terms of biochemistry, the Southern Ocean appears to be prime location for iron fertilization”, said Josie Robinson, a doctoral student in physical oceanography at the National Oceanography Centre in Southampton, United Kingdom and lead author of the new paper. “But when you consider the entire system, particularly its physics, then you begin to see the Southern Ocean is not as good as it first appears.”

Because the Southern Ocean has naturally low iron, which is a nutrient consumed by plant-like microorganisms that absorb carbon dioxide from the atmosphere during photosynthesis, scientists have suggested adding iron to the water. This would cause blooms of the microorganisms, called phytoplankton, which could potentially take up large amounts of carbon. Some of this carbon would remain trapped as organic material when the phytoplankton die and sink into the depths of the ocean. This way ocean iron fertilization seems to be a means to geoengineer against climate change, said the researchers.

The study’s authors set out to test the assumption that carbon sequestered by iron fertilization would remain underwater in the Southern Ocean for up to a century if the carbon sinks below 1,000 meters (3,281 feet). Scientists generally assume that, at this depth, the carbon should remain in the deep, cold water, and not circulate up to the surface or contact the atmosphere for centuries, Robinson said.

The Intergovernmental Panel on Climate Change implied in its 2007 assessment report that sequestered carbon must remain below 1,000 meters for at least 100 years if it is to make a dent in the current global warming trend, according to the paper.

The study’s authors modeled how nearly 25,000 particles, representing the geoengineered carbon, would move through the deep ocean over a 100-year cycle. The model simulates the full circulation of the ocean, shallow and deep, including large circular currents in the Antarctic Ocean called gyres.

The researchers discovered that the majority of the particles – 66 percent – were re-exposed to the atmosphere after only 37.8 years. Only 34 percent of the carbon particles remained in the deep ocean for the full duration of the 100-year experiment.

Additional simulations that positioned the carbon particles deeper in the ocean, below 2,000 meters (6,561 feet), saw improvements in long-term sequestration; only 29 percent of the particles returned to the surface and these particles remained in the deep ocean for 58.3 years. The study offers this as a possible new depth criterion for any future ocean iron fertilization proposals.

At both depths, the models show that particles resurfaced partly because of the Antarctic Circumpolar current, a large-scale circular current around Antarctica. This strong current mixes the cold, deep water and warmer surface waters. The carbon particles within the current are transported to the surface. Particles are also returned to the surface through ocean upwelling, a wind-driven motion that draws up deep, nutrient-rich water to the surface.

The longer the resurrected carbon remains at the surface layer of the ocean, the more likely it is to be released back into the atmosphere, the researchers note in their paper. Carbon reaching the surface would have to be quickly re-submerged into deeper water to ensure that it does not re-enter the atmosphere, they wrote.

Robinson said that a key finding of the study is that only looking at biogeochemical processes, such as the ability of phytoplankton to take up and store carbon, does not paint the entire picture of iron fertilization in the Southern Ocean. This new study shows that ocean currents and upwelling, which play an important role in determining how long it takes for the captured carbon to come to the surface, can be as important as the biogeochemistry, said the researchers.

The new paper “is verification that keeping carbon sequestered is much more difficult than previously thought,” said Anand Gnanadesikan, an associate professor in Earth and Planetary Sciences at Johns Hopkins University in Baltimore, Maryland, who was not involved in the new study.

He also noted that iron can last up to 100 years in the ocean, so if it returns to the surface with the carbon, it could restart the fertilization and sequestration process, possibly making geoengineering more effective. The authors of the new study state in their paper that they recognized this potential for re-fertilization, but left it out of their model because the resurfaced iron would not always be in a form that phytoplankton can use.

 – Alexandra Branscombe is a science writing intern in AGU’s Public Information department