The oceans contain more carbon than soils, plants, animals and the atmosphere combined. Every cubic meter of seawater contains about 120 grams of negatively charged bicarbonate ions, which are balanced with positive ions such as calcium and magnesium. This carbon pool was created naturally over millions of years by mineral weathering. A recent review article published in Reviews of Geophysics explores the possibility of accelerating weathering processes to increase bicarbonate ions in the ocean, and thus prevent climate change and potentially ameliorate ocean acidification. The editors asked one of the authors some questions about the scientific basis for this idea and how it might work in practice.
How can the oceans help prevent climate change?
About a quarter of our current carbon dioxide (CO2) emissions in the atmosphere are absorbed by the oceans. This has caused a slight decrease in pH levels, a process known as “ocean acidification.” If we ceased emissions today, the oceans would eventually absorb most of the emitted CO2 but the acidification would be neutralized by the dissolution of carbonate sediments. This would take thousands of years.
Small perturbations in the chemistry of the global oceans is thought to be responsible for some of the climate variability of the past. That also means that it may be possible for us now to artificially alter ocean chemistry to reduce the impacts of climate change. An increase in ocean alkalinity could be achieved by dissolving rocks and minerals either directly in the open ocean or through engineered systems. This would lead to a build-up of calcium, magnesium, or sodium ions in seawater and thus an uptake of CO2 to form bicarbonate ions. This long-term storage of carbon in order to mitigate climate change is known as “carbon sequestration.”
What is the capacity of the oceans to sequester carbon?
We emit approximately 40 billion metric tons of CO2 every year, and our cumulative emissions over the next 100 years may be in the order of a trillion metric tons. Global carbon cycle modelling suggests that the ocean has the capacity to store carbon on this scale with minimal global environmental impact. This storage capacity is similar to underground injection of CO2, a practice which is already taking place in some countries. It may be possible to sequester additional carbon, but the environmental impact may become unacceptable.
What are the advantages of carbon sequestration in the ocean?
The idea of dissolving minerals to sequester CO2 has been around since the early 1990s. The initial work focused on dissolving silicate rocks in high temperature/pressure reactors to form mineral carbonates. For every mole of calcium/magnesium that precipitates as a carbonate mineral, one mole of CO2 is sequestered. By increasing alkalinity, every mole of calcium/magnesium is balanced by nearly 2 moles of bicarbonate ions. This means that the engineering requirements may be substantially less per net ton of CO2 sequestered. It is also possible to dissolve carbonate minerals to form alkaline, bicarbonate-rich, solutions. The rates of carbonate mineral dissolution are considerably more favorable than other minerals such as silicates, which again has positive implications for the engineering requirements of the technology.
How might higher carbon levels in the ocean affect ecosystems and marine life?
Globally, the environmental impact of increasing ocean alkalinity depends on what concentration of CO2 is in the atmosphere. If CO2 is high, then alkalinity addition may help ameliorate the impacts of ocean acidification. Increasing ocean alkalinity also increases the saturation state of carbonate minerals, which, if too low, will negatively impact carbonate-producing organisms in the ocean such as shellfish and coral. The local and regional impact of increasing ocean alkalinity is potentially more acute, and specific to the technology used. For some, this may impact coastal ecosystems, while others are concerned specifically with the open ocean.
What are the techniques for increasing carbon storage in the ocean?
There are three techniques that may be used to increase ocean alkalinity, and conclusions from early research suggest the technoeconomic cost is comparable to other methods of climate change prevention.
One method is enhanced weathering which is the application of rock powder to terrestrial, coastal, and open ocean environments. For instance, some have proposed distribution of crushed silicate rocks to tropical cropland. The wet and warm climate of these environments would be conducive to dissolution, the products of which would be transported via rivers to the ocean. Agricultural liming could also be viewed as enhanced weathering, however given that the emissions from lime production are currently unabated, the overall process remains CO2 positive.
Another method is accelerated weathering of limestone, which involves promoting limestone dissolution in a reactor with seawater and CO2 rich gas (e.g., greater than 5000 parts per million by volume). This creates a slight increase in the pH and alkalinity of the seawater in the reactor, which is returned to the ocean. An alternative method is to promote mineral dissolution through electrochemistry, where the acidic conditions are created around the anode.
Where are additional research efforts needed in this field?
More research is needed to address the impact of elevated ocean alkalinity on marine ecosystems. Most of the research in this area has focused, quite rightly, on acidifying oceans. Within this question, we need to find out more about the stability of bicarbonate ions in the ocean. The current residence time is 100,000 years but if we increase ocean alkalinity, this may decrease resulting in a (partial) reversal of carbon storage. Thus far there has been little work beyond small scale pilot experiments and energy balance calculations on the feasibility of techniques to increase ocean alkalinity.
—Phil Renforth, School of Earth and Ocean Sciences, Cardiff University, UK; email: [email protected]