Ocean fertilization – A viable geoengineering option or a pipe dream?

 

As most people know atmospheric CO₂ levels are increasing, whether this is cause for concern depends upon your opinions on climate change. For the purpose of this article (and because I’m an environmental scientist) we will push forward with the knowledge that the increase of CO₂ is caused by humans. It is suggested that it might already be too late to reverse the changes high CO2 levels will have on our environment but also that there just might be some time. Prevention of high CO₂ emissions is the best option, however extremely hard to accomplish with countries having different political views and our comfort in our modern lifestyle and thevtechnological age we live in.

Geoengineering is the term encompassing all proposals to remove CO₂ from the atmosphere for long periods of time. They include irrigating the Sahara to plant trees, pumping deep nutrient rich waters to the surface ocean, iron fertilization in the ocean, pumping liquid CO₂ into rocks, putting giant reflectors into orbit and many more. Here we will focus solely on iron fertilization (addition of iron) of the ocean.This works as phytoplankton are often inhibited by iron, particularly in the Southern Ocean and some parts of the Pacific. Iron is needed for them to be able to photosynthesise, and if it is the only limiting element, then its addition should create huge phytoplankton blooms. Also needed for photosynthesis is inorganic carbon, which for phytoplankton comes from the atmosphere and dissolves in the sea. Therefore, during blooms phytoplankton are locking in atmospheric CO₂. This is all very well and nice, but what happens to the CO₂ when the blooms subside? This is the crucial part.

Phytoplankton sink from the surface to the deep ocean as dead cells which often clump together to form large aggregates of phytodetritus. If the entire phytoplankton community ended up buried on the seafloor and in the sediments, atmospheric carbon dioxide could be locked away for millennia. However, phytoplankton are the base of many marine food webs and so only a tiny fraction (1-10 %) is removed for significant timescales.

Bacteria and zooplankton are the initial utilizers of this sinking organic pool of carbon. Bacteria can attach themselves to aggregates or be free-living, but ultimately solubilize the particulate organic carbon and depending at what depth this occurs, the carbon can be remixed back to the surface and if in its inorganic form, be a source of CO₂ to the atmosphere. Zooplankton consume the sinking phytodetritus and so the carbon is converted back to CO₂ through respiration or is assimilated, egested as faeces or excreted. However zooplankton are also a contributor to the sink of organic carbon as their faecal pellets sink through the ocean with the phytodetrital aggregates. However they too can be eaten, sometimes by their producers! So you see there are lots of biological factors effecting how much carbon reaches the deep, and the addition of regional, seasonal and temporal variation shows its not a straight forward process to understand.

 

Pros and Cons

A recently published paper in Nature by Keller et al. (2014) showed model simulations for 5 different geoengineering options including iron fertilization (see figure). Rising CO2 and temperature would be slowed, but only slightly but the largest change would be in O₂ concentration, which would decrease, the largest decrease of the 5 types. However if iron fertilization ceased there wouldn’t be a dramatic increase in temperature or CO₂. So whilst this paper showed iron fertilization would decrease CO₂ and temperature, for such small amounts and short time scale is it worth the risk when we know so little about how it would work and what the side effects to the oceanic ecosystems would be? Some of the possible side effects include, changes in phytoplankton species which will have an affect on the food web, increased fish stocks, harmful algal blooms, more jellyfish, production of nitrous oxide and methane and nutrient depletion elsewhere when fertilized waters resurface.

Ignoring side effects, we STILL (after 12 experiments) do not know if it would be successful. Ship budgets and schedules mean scientists observe the increase in phytoplankton biomass after iron addition, but miss the subsequent sinking of carbon to the deep. Natural experiments where iron isn’t added but a naturally iron replete area and deplete area are compared, are good examples, such as in the Crozet experiment in 2004-2005 in the Southern Ocean. This experiment showed at times a 10 x increase in the carbon flux to the deep ocean in the iron replete area. However this occurred during Antarctic spring and who knows if such large increases would persist throughout the whole year.

Another additional problem is legality. There are strict laws on dumping anything at sea and even though these experiments have been done in international waters, there are still legal obstacles to overcome. Especially in Antarctica, which might even, become a marine protected area if politicians can work out their differences!

In my opinion ocean iron fertilization is not the answer to decreasing the atmospheric CO₂ levels. Gaining international approval, understanding the long term effects of the local and non local ecosystems and quantifying the effect it will have are all still questions scientists face after 2 decades of work and experiments. I actually do not think I will witness any successful geoengineering solution in my lifetime – and I’m only 25!