Geoengineering and Iron

This is a topic I’ve increasingly become involved with over the past few years. Geoengineering as popularly defined are attempts to do large regional, or even planetary scale efforts to mitigate the impacts of global warming due to the build-up of anthropogenically produced greenhouse gases (GHG) in the atmosphere. These range the gamut from injecting aerosols into the atmosphere to block sunlight from reaching Earth’s surface, to putting giant mirrors out in space to reflect some sunlight from reaching Earth, to a plethora of ways to try to remove GHG’s, principally carbon dioxide from the atmosphere.

I’ve taught a couple of Geoengineering courses at Colby College, and have several research projects that are directly or tangentially associated with aspects of GHG removal, only one of them fits in the Geoengineering category. This is ocean iron fertilization or OIF. Over the last 40 years oceanographers have learned that at least a third of world’s open ocean is limited for iron. This means that the phytoplankton that are the base of marine food web don’t have enough iron to grow, since iron is an essential micro-nutrient and a key component of many of an organisms enzymes and oxygen transporting molecules. So, add iron and you get more phytoplankton growth. Because phytoplankton are photosynthetic autotrophs (i.e. plants) they convert carbon dioxide (CO2) to biomass or organic carbon.  This ‘new’ carbon trickles up the food web and results in more zooplankton that eat phytoplankton, things that eat zooplankton, and things that eat the things that eat zooplankton, all the way up to the odd whale. What’s important from the geoengineering side is what trickles down into the deep ocean, because much more of that plankton organic carbon trickles down than trickles up. If this ‘new’ carbon reaches a depth of 500 meters or more (average open ocean water depth is around 4000 meters) then, even if that organic carbon is consumed by microbes, and turned back into CO2, it can be tens, hundreds, and even thousands of years before that CO2 is returned to the atmosphere.

These are some basic facts that drive the idea of OIF being a possible way to capture carbon dioxide from the atmosphere. Add more iron to ocean regions that are iron limited and you will get more export of ‘new’ organic carbon into the deep ocean, since that carbon was originally CO2 that came from the atmosphere you are essentially capturing atmospheric CO2 and sinking it into the deep ocean. Simple, and it gets even better. Iron is a micronutrient, meaning you need very little to stimulate a substantial amount of phytoplankton growth and organic carbon production from CO2. Adding a ton of iron to the ocean surface could result in capturing over a thousand tons or more  of CO2. Iron as a commodity is also cheap, you can buy a ton of iron oxide from China for less than $1,000. Some colleagues and I recently did a cost estimate for OIF and estimated the cost of removing a ton of carbon dioxide from the atmosphere could be as little as a few dollars, to as much as over five hundred dollars per ton of captured CO2. Why such a big range? That’s where all the ‘IF’s’ come in. The low dollar figure assumes an even distribution of iron to the ocean surface, efficient uptake of the iron by the phytoplankton leading to stimulation of a large bloom, and then efficient export of at least 10 or 15% of that carbon to the deep ocean. The export part of this equation is the biggest unknown and, of course, is also the most important.

Oceanographers have ways of measuring how much carbon makes it from the surface of the ocean to the depths, and the development of autonomous methods that don’t require immense amounts of time at sea to make these measurements are getting better and better. Still it’s challenging to make accurate measurements. And then there’s another problem, the ocean moves!

As a non-oceanographer, the most remarkable thing I have learned from working at an institution that studies the world’s oceans is how much structure there is to the ocean. Having grown up on the water, I was very familiar with waves and tides, but had never realized that were you to swim from the surface to ocean floor off Bermuda, a mere 12,000 ft jaunt, you would pass through essentially separate bodies of water that originated in the Pacific Ocean, near Antarctica, as well as in the North and South Atlantic. These are like giant planetary rivers moving beneath the ocean surface in different directions, at different depths, and remaining largely separate from one another. These major deep currents are the conveyors that move ocean nutrients and heat around the planet. To further complicate things there are also surface gyres, or eddies, often hundreds of miles across that sweep across the oceans as well. Practically what this means is that were we to fertilize an area of the ocean around the size of New England (and this is the scale needed to even begin to have any meaningful CO2 capture), the captured CO2 might be equivalent distance of Kansas or Florida or Northern Quebec are from New England, by the time it reached a depth where you could say it was ‘captured’, so your measurements would need to account for this movement.

The even bigger problem is that phytoplankton capturing CO2 in the water are not directly removing it from the atmosphere. They are using CO2 in the form of bicarbonate (baking soda) that is dissolved in the ocean, bicarbonate is replenished by exchange with CO2 gas in the atmosphere, so in theory, if CO2 bicarbonate in the ocean is removed, it is be replaced by CO2 coming from the atmosphere, thereby reducing the atmosphere’s CO2 concentration. The dilemma is that we don’t have ways of accurately measuring this air-sea gas exchange, so even if we can demonstrate OIF is exporting substantial amounts of carbon and equivalent CO2 to the deep ocean, how will we know how much CO2 is actually being drawn down from the atmosphere? The theory behind this is all pretty well understood, and I would even say we have a reasonable general understanding of the mechanistic processes involved, although I wouldn’t be at all surprised if more surprises about a fundamental lack of understanding of these processes are in store as we delve further into these mechanisms. Those surprises could be good or bad regarding OIF’s potential as a way to capture CO2 and remove it from the atmosphere.

Having taught about all the different approaches people are considering for GHG removal, and being actively involved in researching a couple of them, my intuition is that OIF has as good a potential as any. It won’t be an answer in and of itself, no CDR method is. OIF is easily scalable, low cost, perhaps too low cost for market-driven tastes, has natural analogs that don’t have proven negative environmental consequences, there’s paleo-climatic evidence that OIF plays an important role in planetary climate control through atmospheric CO2 drawdown, and it can easily be halted. True, it’s hard to verify and will need to be done at large scale (minimally 100,000’s km2) to have an impact, and at its most efficient, it probably wouldn’t account for more than 10 – 20% of the CO2 removal that’s required to keep Earth’s temperature rise below 2 or 2.5°C by 2100 (assuming a coterminous decline in GHG emissions). An accelerated research program could provide more answers and/or clarification of what OIF strategies will or will not work, at the least develop boundaries for the challenging issues facing OIF. If those boundaries are acceptable to a scientific consensus and can survive the social barriers, then OIF is worth a shot.