What is ocean carbon sequestration? Ocean carbon sequestration is the direct forcing of liquefied CO2 into the ocean to store and isolate it from the atmosphere in order to reduce the impacts of climate change. The ocean naturally takes up atmospheric CO2 on an enormous scale -- approximately 7 billion metric tons of CO2 per year -- absorbing it at the surface and transferring it to the deep waters, out of contact with the atmosphere for centuries. Prior to the industrial revolution, ocean and atmosphere CO2 concentrations were in balance. But elevated atmospheric CO2 levels caused by the burning of fossil fuels not only have created global warming but also have forced the “passive” invasion of this excess CO2 into the surface waters of the ocean. This fossil fuel CO2 gas invasion into the ocean is now proceeding at about 1 million tons of CO2 per hour. Measurements of atmospheric and oceanic concentrations of CO2 taken over time show this invasion of atmospheric CO2 into the ocean.
The ocean will eventually take up about 85% of all fossil fuel CO2 emitted to the atmosphere. However, this uptake process is extremely slow and it will take more than a thousand years to reach a new equilibrium. Ocean carbon sequestration could speed up this natural process and store the excess carbon in the deep ocean, away from the surface waters where most marine organisms live. In 1977 it was first suggested by C. Marchetti (1) that society consider the direct injection of fossil fuel CO2 into the deep ocean before its release to the atmosphere, thereby avoiding the time that CO2 stays in the atmosphere, minimizing global warming, and accelerating the ocean's inevitable uptake process (2).
Why can't we simply reduce our CO2 emissions? Do we need to consider mitigation options like this? Many scientists believe that stabilizing atmospheric CO2 concentration at 550 parts per million (ppm) may avoid the worst impacts on climate. Atmospheric concentration of CO2 is currently ~380 ppm and, if no precautionary action is taken, is expected to reach 550 ppm by the middle of this century. Stabilizing CO2 at 550 ppm will be a global challenge on an unprecedented scale. According to the Intergovernmental Panel on Climate Change (IPCC), the most authoritative source for scientific assessments of climate change, this may not be achieved through emissions reductions alone but rather through a carefully crafted portfolio of actions that also includes investments to develop low-cost, low-carbon or no-carbon energy sources, improvements in energy efficiency and carbon management options. The latter includes storing carbon in the terrestrial biosphere (e.g. planting trees, limiting deforestation), or capturing the CO2 emitted from an industrial source and storing it in geological formations or in the deep ocean (3). The ocean has an enormous natural capacity to absorb and store carbon. If we were to take all the atmospheric CO2 and put it in the deep ocean, the concentration of CO2 in the deep ocean would change by less than 2% (4). While there are still many knowledge gaps that must be filled before assessing the risks and potential for ocean carbon sequestration, the vast potential of the ocean to store carbon cannot be ignored when considering mitigation options. However, recent science reviews and international regulations are clear: much more research is needed to determine if storage of CO2 in the ocean is efficient and safe, and other methods such as storage in geological formations should be considered before pursuing this option.
How would you store CO2 in the Oceans? There are three possibilities for using the ocean environment to store carbon: in geological formations under the seabed, on the seafloor, and in the water column of the deep ocean. For all three of these methods, the first step is to capture CO2 from an industrial source such as a power station and compress it into a pure liquid stream of CO2 that could be injected into a suitable storage reservoir. The liquid CO2 must then be transported to the injection site, either via pipeline or from a tanker to an injection platform. Sub-seabed Storage - Underground geological structures such as depleted oil and gas reservoirs or deep saline reservoirs may be used to store liquid CO2. Thousands of oil and gas fields will be depleted in the near future, providing a storage capacity that could store more than 50% of global CO2 emissions for the next 50 years. Some of these depleted reservoirs are offshore, under the seabed. In Norway, the offshore is a large-scale experiment where over 1 million tons of CO2 per year have been injected into a saline aquifer located 1000 meters below the seabed. This project includes a research and monitoring program to examine long-term storage, migration and leakage.
Statoil's Sleipner Project in the Norwegian Sector of the North Sea. CO2 is separated from the natural gas production stream and re-injected back into a deep saline reservoir in the seabed. This reservoir has low permeability and leakage of CO2 is minimal. Graphics courtesy of Statoil. See also "", by the International Energy Agency's Greenhouse Gas R&D Programme. (5) It is estimated that if storage sites and injection procedures are carefully chosen and implemented, the potential for leakage into the ocean represents a smaller danger to marine life than the CO2 that is absorbed naturally at the surface ocean (6). As documented in the recent IPCC Special Report on Carbon Dioxide Capture and Storage (7), more projects like these are required in a range of geological, geographical, and economic settings to improve understanding of processes and long-term impacts. Several other on-going or planned offshore storage projects are being investigated by the Netherlands, Denmark, Germany, Australia and the UK.
Rock formations with high potential for CO2 Storage. Several of these potential sites are offshore. Seafloor Storage - At depths greater than 3000 meters, liquid CO2 is denser than the surrounding sea water, and forms a dense plume that sinks to the seafloor, accumulating as a pool of liquid CO2 and hydrate, an ice-like solid formed when water molecules surround the CO2 molecule. Once created, CO2 lakes such as this would dissolve slowly into the surrounding seawater, eventually returning to the atmosphere, making this a "time-delayed release" of CO2 rather than a permanent sequestration method. Depending on the dynamics and depth of the environment, however, some lakes may have lifetimes on the order of thousands of years. Ocean scientists have skillfully devised many small-scale experiments to test many of the physical and chemical concepts associated with deep ocean injection, such as:
The impacts of such lakes on deep-sea ecosystems, however, is largely unknown, and much research is required before we can critically assess the potential and risks for storage of CO2 in the deep ocean. Deep sea organisms have evolved in this region of the ocean where environmental changes are very small, making it unlikely that these organisms would be able to adapt to rapid changes. Seawater with high levels of dissolved CO2 is highly acidic. Depending on the volumes injected, measurable changes in ocean chemistry could be expected and marine organisms near the injection site may be harmed. Few controlled experiments have been performed in the deep ocean and it is not yet possible to determine critical thresholds or adaptation / avoidance mechanisms of the organisms (6) (see also "How will ecosystems be affected?"). Water Column Injection - Liquid CO2 released at depths of less than 2500 meters is less dense that the surrounding seawater and tends to rise towards the surface. As it rises, it dissolves into the surrounding seawater where it will remain out of contact with the atmosphere over timescales of centuries before returning to the atmosphere. One concept to minimize the local environmental impacts of injection is to inject the liquid CO2 from a moving ship towing an injection pipe. Such a system could produce a seawater plume with relatively low initial CO2 concentrations (7). As with seafloor storage, the impact on ocean ecosystems is the largest unknown. While water column injection would be expected to have a smaller impact than seafloor storage, much more research is required to study the spatial and temporal evolution of such an injection plume and its immediate and long-term effects on local organisms.
Illustration of some ocean storage strategies.
What is the status of ocean carbon sequestration? Is this already being carried out? There are several sub-seabed storage projects currently operational or being planned. The 1996 Protocol to the "Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter", which prohibits the dumping of wastes at sea, was recently amended to exempt CO2 streams from the list of banned substances. Storage of CO2 under the seabed has been allowed since February 2007, and new guidelines on how to safely store CO2 in sub-seabed geological formations in a manner that meets the requirements of the Protocol will be developed in November 2007. Amendments to this decision legally rule out placement of CO2 in the water-column of the sea and on the seabed because of potential negative effects (8). For seafloor and ocean injection, only laboratory and small-scale field investigations have been carried out. A considerable amount of research is needed for this method to be deemed safe or efficient, and to determine whether the mitigation effect is worth the potential impacts to deep-sea biology. Research could include:
What about fertilizing phytoplankton in the surface ocean to absorb CO2? In the same way that planting trees can reduce atmospheric CO2 through photosynthesis, small plants in the surface ocean called phytoplankton can be fertilized with nutrients to stimulate their growth and take up additional CO2 from the atmosphere. When these organisms die, they sink to the deep ocean, carrying the carbon with them. In many regions, phytoplankton growth is limited by lack of an essential micro-nutrient, iron. Over the past decade, eight small-scale experiments have shown that introducing iron to iron-poor regions can stimulate phytoplankton growth to 20 to 30 times the natural rate. However, all available research indicates that iron fertilization of the surface waters would be a very inefficient method for sequestering atmospheric CO2, both from the viewpoint of the limited amount of carbon that could be sequestered by this method and the likelihood that, even if iron limitations were eliminated, other nutrients and environmental factors would eventually limit growth. The bacterial remineralization (or decay) of this excess plant material uses oxygen in the water, and most models predict that any large-scale iron fertilizations would drive most of the underlying water column to be undersaturated in oxygen, which would have substantial impacts on mid-water and deep-sea organisms. It is also likely that iron fertilization may lead to increases in production of N2O, another greenhouse gas, reducing the overall efficiency of using iron fertilization to reduce climate impacts. Several international and intergovernmental scientific groups have developed position statements warning against this practice (see the Resource Library Fact Sheets). In its review of carbon capture and storage, the Intergovernmental Panel on Climate Change called ocean fertilization "highly speculative" and noted that it was premature to review ocean iron fertilization as a potential sequestration method. Unfortunately, this proposed form of sequestration has captured the attention of several commercial companies, which promote the large-scale implementation of this technique as a means of offsetting CO2 emissions without performing the necessary environmental impact assessments before selling carbon credits on the voluntary markets. In the UN system, the Scientific Advisors to the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter International Maritime Organization recently advised caution, stating "Knowledge about the effectiveness and potential environmental impact of iron fertilization is currently insufficient to justify large-scale operations." (10)
References used in this section 1. Marchetti, C. (1977). On Geoengineering and the CO2 Problem. Climate Change 1(1), 59-68. 2. Brewer, P.G. (2000). Contemplating Action: Storing Carbon Dioxide in the Ocean. Roger Revelle Commemorative Lecture. Oceanography, v. 13, 84-92. 3. . 4. (pdf) 5. . (pdf) 6. . 7. (2005) 8. . 9. . 10. See also Resources. Ocean Acidification Network editors for this section: Dr. Maria Hood - Intergovernmental Oceanographic Commission - UNESCO
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