Dive In: Geoengineering the Southern Ocean

This is the fourth post in the Dive Into Science series.  Here I'll be explaining results from recent scientific papers.  Dive Into Science gives you a glimpse of current research in an easy to read format that anyone can understand.  To read more, just use the Dive Into Science tag.

Today's article is "How deep is deep enough? Ocean iron fertilization and carbon sequestration in the Southern Ocean" by Josie Robinson, et al, published in Geophysical Research Letters, 2014.

One of the main factors influencing climate change is the amount of greenhouse gases, specifically carbon dioxide, in the atmosphere.  I'm not going to go into how greenhouse gases increase global temperatures, or what the sources are.  Instead, let's jump ahead to: what can we do about it?

One major option is that we can store a lot of it in the deep ocean.  This carbon sequestration keeps the carbon dioxide in the ocean for maybe hundreds of years, giving us more time to cope with climate change.  How do we increase the amount of carbon sequestration?  By geo-engineering the ocean.

When phytoplankton bloom, they take up sunlight and carbon dioxide.  Some of the phytoplankton are eaten, and the nutrients and carbon dioxide they take up are eventually returned to the surface ocean.  Some of the phytoplankton sink out of the surface ocean and are exported to deep waters, maybe even to the bottom, storing carbon dioxide and nutrients deep in the ocean for long periods of time.

To geo-engineer the ocean to increase carbon sequestration, we would need to stimulate phytoplankton to bloom more, over longer time periods, and ensure a large portion of that bloom sinks.  The Intergovernmental Panel on Climate Change (IPCC) has guidelines on carbon sequestration.  They suggest, that to be truly sequestered, the carbon needs to stay at least 1000 meters deep in the ocean for 100 years.

The prime location to geo-engineer is the Southern Ocean.  A large portion of it is iron-limited, which means we just need to add iron (called iron fertilization) to get more phytoplankton to bloom.  It also has certain locations where dense water sinks to the bottom of the ocean and stays there for hundreds of years.  Sounds promising, right?

This premise is what the authors of this paper tested.  Suppose we geo-engineer the Southern Ocean.  We add enough iron to the Southern Ocean at the right time, stimulating a massive bloom, which sinks to 1000 meters.  Everything goes right.  What happens to the carbon next?  Does it stay below the 1000 meter limit for 100 years?  Or do we need new guidelines for carbon sequestration?

Using an ocean model, the authors of the paper tested this theory.  They put in almost 25,000 tracer particles at 1000 meters all throughout the Southern Ocean.  Then they ran the model for 100 years to see where the particles ended up.  Particles that make it above the Mixed Layer Depth (MLD), or the depth to which surface forcing, such as wind, can mix the water, are considered to be exposed to the atmosphere and aren't sequestered.

Figure from Robinson et al.  Panel a shows the starting location of all the particles that made it to the surface, and the color indicates how long it took them to upwell.   Panel b shows the percentage of particles in each block that stayed sequested for the entire 100 year simulation.

They find that after 100 years, 66% of the particles have been exposed to the atmosphere.  That means less than half of them that met the original standard actually stayed sequestered for 100 years.  On average, it took a particle 37.8 years to make it back to the surface.  However, if the particle got out of the Southern Ocean, it tended to stay in the deep ocean.  Of the ones that reached the surface, 97% were still in the Southern Ocean.

So what does this mean for carbon sequestration and iron fertilization?  Even if we manage to get all the geo-engineering aspects of iron fertilization right, it doesn't mean that the carbon will stay down long enough for it to be useful.

From Robinson et al. Shows the percentage of particles that stayed sequestered at different time points over the model run, based on whether they started at 1000 or 2000 meters deep.

The authors also re-ran the same simulation, but this time set all the particles at 2000 meters, instead of 1000.  By requiring a deeper depth in order to be "sequestered", they found that only 29% made it back to the surface ocean over 100 years, a large improvement.


Overall, this paper demonstrates the issues with geo-engineering the Southern Ocean.  It suggests that new guidelines are needed to define carbon sequestration - it must initially sink to a depth of 2000 meters instead of 1000 meters.  It also demonstrates that the Southern Ocean might not be the best place for geo-engineering, as the dynamics of the water cause a large portion of particles to rise to the surface, rather than keep sinking.