Over the last weeks, commercial efforts have been launched to manipulate a portion of the Pacific Ocean to increase the uptake of atmospheric carbon dioxide by artificially enhancing phytoplankton activity. A research vessel is currently sailing to the Galapagos Sea to seed an area larger than Puerto Rico with tonnes of iron, to stimulate the CO2 sequestration into the deep ocean. However, such iron fertilization is also a way of generating carbon offsets, whereby CO2 polluters can buy “ecosystem restoration credits” and shrink their carbon footprint.
This experiment is based on the fact that in about one-third of the surface ocean, the growth of phytoplankton is limited by the lack of iron; a well researched phenomenon. However, a valuable question to raise is to what extent artificial iron fertilization represents a real option for CO2 reduction. Indeed, a group of EUR-OCEANS scientists using the Kerguelen Plateau as a site study for natural iron fertilization of the Southern Ocean, recently showed that artificial iron fertilization of the surface ocean is a mechanism 10 to 100 times less efficient than the naturally occurring processes, in increasing CO2 capture through the biological pump.
While the scientific community, governments, private sectors and society are coming to a consensus regarding the anthropogenic causes of global warming, the need to consider concrete actions and to develop strategies to reduce atmospheric CO2 concentrations and the greenhouse effect is becoming urgent. It is then crucial to examine to what extent oceans could be use to store our excess atmospheric CO2.
The CO2 Problem
Today, concentrations of atmospheric carbon dioxide far exceed the natural range over the last 650,000 years and it is expected to at least double over the next century. A 30% increase has occurred in the last 200 years, which has been primarily attributed to worldwide use of the fossil fuels. Natural processes responsible of CO2 regulation are not enough to absorb the excess of this greenhouse gas. The recent IPCC report states that warming of the climate system is unequivocal and scientists point out numerous long-term changes which may have dramatic environmental and socio-economic consequences.
The best solution to control this increase of carbon dioxide would be to dramatically cut fossil fuel use and strongly reduce our CO2 emissions. However, this challenge raised by the Kyoto protocol seems today hardly attainable in the short term. In the meantime it becomes almost unavoidable to consider alternatives options, such as developing technologies allowing the capture and storage of atmospheric CO2.
The world’s oceans cover 75% of the Planet’s surface and play a key role in moderating the climate. They naturally take up about one-third of our CO2 emissions, and with an average depth of 3800 m, it also offers a potentially huge storage capacity.
Ocean’s CO2 Uptake
The oceans take up carbon dioxide from the atmosphere through the so-called physical pump and biological pump.
In the physical pump, carbon dioxide actively dissolves into the ocean at high latitudes and is then carried down into the ocean interior by sinking currents - where it stays for hundreds of years- and is eventually brought back to the surface.
The effect of the biological pump is due to phytoplankton activity (photosynthesis) that converts CO2 into organic matter and oxygen. Animals eat the micro algae contributing to the oceanic food web. When the algae and animals die a small fraction of the organic material sinks to the sea floor where it is eventually buried and stored in sediments for millions of years.
Over the last decade a lot of research has been conducted to investigate to which extent, the biological and physical potential of the ocean could be used to damp the CO2 problem.
A Biological Option: Seeding the Oceans with Iron
“Give me a half tanker of iron, and I will give you an ice age”. This was the inflammatory words from the American oceanographer John Martin who, for the first time in the early 90’s, establishing the iron hypothesis and proposed a way to artificially increase the oceans absorption of the carbon dioxide released by the burning of fossil fuels. Martin noticed that large areas of the ocean were almost devoid of phytoplankton despite the presence of large amount of nutrients required for their growth. He further determined that those “paradoxical zones” were indeed lacking iron, a micro-nutrient essential for life. Martin’s theory was that iron fertilization of the surface ocean will stimulate phytoplankton growth and take in so much carbon from the atmosphere that it could reverse the greenhouse effect and cool the Earth.
Twelve oceanographic expeditions were carried out between 1993 and 2005 is the North Pacific, the Equatorial Pacific and the Southern Ocean to test the iron fertilization hypothesis. However, as pointed out by Hein de Baar, from the Netherlands Institut voor Onderzoek der Zee (NIOZ, Texel, The Netherlands), today after all those experiments we know that they are many losses and that this manipulation is not a real efficient way to capture and store CO2 into the deep ocean. Much more iron is needed that had originally been suggested for algae to bring down a certain amount of CO2 from the atmosphere.
The idea of seeding iron on the oceans to cool down the Earth on large scale may not be operationally possible. But some scientists think that locally this strategy could be feasible.(”More arguments have to be evaluated about artificial iron fertilizations of the ocean before final conclusions can be drawn”, says Prof. Ulrich Bathmann from the Alfred Wegener Institute for Polar and Marine Research (AWI, Bremerhaven, Germnany). Hence, research continues and further studies are required to be able help better understand the processes of the production and export of organic matter related to these iron fertilization experiments as well as their consequences on the food web.
A Physical Option: CO2 Injection into the Deep Ocean
The increase in atmospheric CO2 concentrations due to anthropogenic emissions has resulted in the oceans taking up about 7 Gt of CO2 per year. The ocean has a far greater potential to store carbon dioxide. Various technologies based on direct injections of CO2 into the ocean have been envisioned to increase the CO2 storage by the ocean. After capture and liquefaction CO2 transported by pipelines or ship, can be injected at great depths where it would remain isolated from the atmosphere for centuries.
James Orr from the CEA (now in IAEA, Monaco) has been using numerical models to evaluate this option. He simulated the injection of CO2 into the deep ocean at 7 different sites and followed its propagation over time. His results indicate that after a century 90% of the injected CO2 was still in the ocean and after 500 years half of it escaped back to the atmosphere. But, after all through this process we would only be “buying time”!
Also it must not be forgotten that increasing CO2 concentration in the ocean leads to seawater acidification and the long term consequences of such changes on marine ecosystems are still poorly known.
A Chemical Solution
It has however been proposed that the increased sea water acidity resulting from CO2 addition could be largely neutralized by the slow natural dissolution of carbonate minerals. This neutralization would allow the ocean to absorb more CO2 from the atmosphere while inducing less change in ocean pH. But limestone and other alkaline compounds that would readily dissolve in surface sea water have not been found in sufficient quantities to store carbon in the ocean on scales comparable to fossil CO2 emissions.
Geological Storage: A more Promising Solution?
A solution that might currently provide the best compromise is underground CO2 storage using depleted oil and gas fields; coal seams and deep saline reservoirs. Many of the reservoirs that are being considered have already stored gases and liquids for thousands of years. Oil and gas fields are known to be effective stores for hydrocarbons and natural gas. Similarly, methane has been trapped in coal seams since the coal was formed and deep saline reservoirs in sedimentary basins have held water for many thousands of years. These examples give confidence that CO2 can be stored safely for thousands of years. Once it is stored, slow releases of CO2 from geological reservoirs, especially those under the ocean, are unlikely to give rise to safety concerns. The risk of a large-scale sudden release of CO2 can be avoided by careful selection of the storage reservoirs. An advantage is also that the technology required is already available because it has been developed at industrial scales by oil companies.
A promising project has been implemented at the Sleipner West gas field off the Norwegian where CO2 is being injected into an offshore deep saline reservoir since 1996. To date over 2 million tonnes of CO2 have been injected underground instead to be emitted into the atmosphere.
Despite the great natural potential of the oceans to uptake atmospheric CO2, our current knowledge indicates that seeding iron on the oceans or dissolving more carbon dioxide into the depths do not seem conceivable solutions to solve our CO2 problem. However, given the current situation, taking no action may lead to even greater risks. We need to decrease our emissions of carbon dioxide and will probably have to adopt multiple strategies.
About EUR-OCEANS and the Network of Aquaria
EUR-OCEANS (European Network of Excellence for Ocean Ecosystems Analysis) is a network of excellence co-funded by the Sixth Framework Programme for Research and Technological Development of the European Communities (FP6). The network gathers more than 60 research institutes and universities from 25 countries. Its activities started in January 2005, running for 4 years until December 2008.
The EUR-OCEANS network of Aquaria is the network public outreach arm and has developed a number of tools to facilitate communication and the exchange of information between scientists and the general public including; an interactive web site (http://www.eur-oceans.info), online conferences and an educational programme that links students directly with scientists in the field.
Materials provided by Alfred Wegener Institute for Polar and Marine Research. Note: Content may be edited for style and length.
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