The world’s nations are nowhere near to meeting the global Paris Agreement’s goals on climate change of holding global temperature increases to 2 degrees Celsius compared to 19th-century averages, much less its more aspirational goal of holding temperatures to a 1.5°C rise.
The most recent Emissions Gap Report from the United Nations Environment Program notes “global greenhouse gas emissions show no signs of peaking.” According to another study, the chance that humans can limit warming to no more than 2°C by 2100 is no more than 5 percent, and it’s likely that temperatures will rise somewhere between 2.6°-3.7°C by the end of the century.
These foreboding trends have led to an increasing focus on ways to remove carbon dioxide from the atmosphere. Among the methods being explored is the use of the ocean to absorb and/or store carbon by adding crushed rocks or other sources of alkalinity to react with CO2 in seawater, ultimately consuming atmospheric CO2.
Could this type of large-scale carbon dioxide removal work? A closer look illustrates the potential environmental trade-offs of deploying marine carbon dioxide removal and the complex technical, economic and international governance issues it raises.
Land versus ocean carbon capture and storage
We and other researchers see the ocean as a logical place to look for additional carbon dioxide removal opportunities since it currently passively absorbs about 10 gigatons (10,000,000,000 tons) of CO2 per year or about one-quarter of the world’s annual emissions. In addition, the oceans contain vastly more carbon than the atmosphere, soils, plants and animals combined, and may have the potential to store trillions of tons more.
The latest report from the Intergovernmental Panel on Climate Change focused heavily on land-based methods for carbon capture and storage. One prominent technique is called bioenergy with carbon capture and storage, BECCS, where plant biomass would be burned to produce usable energy and the resulting CO2 is pumped underground.
However, there are a number of concerns about the potential negative impacts of large-scale deployment of BECCS and other land-plant-based methods, notably the worry that huge amounts of agricultural land would be diverted to grow dedicated crops. This could reduce access of low-income populations to food, place demands on water and have serious negative impacts on biodiversity due to ecosystem disruption.
Speeding up geochemistry
Perhaps the best-known – and at times, controversial – method for marine carbon dioxide removal is stimulating photosynthesis to increase CO2 absorption. For example, in regions where marine plant growth is limited by iron, this element can be added to enhance CO2 uptake and carbon storage where at least some of the biomass carbon formed eventually sinks to and is buried in the ocean floor. Other approaches include restoring, adding or culturing marine plants or microbes, such as Blue Carbon.
Another technique being considered is to try to accelerate the chemical reaction of CO2 with common rock minerals, a natural process known as mineral weathering. When rain reacts with alkaline rocks and CO2, there’s a chemical reaction, which can be catalyzed by biological activity in soils, that converts the CO2 to dissolved mineral bicarbonate and carbonate ions which then typically run off into the ocean. Mineral weathering plays a major role in removing excess atmospheric CO2, but only on geologic time scales – 100,000 years or more.
Greg Rau, CC BY
Various ways to accelerate mineral weathering and ocean carbon storage that have been proposed include adding to surface waters finely ground alkaline minerals or adding common, industrially produced alkaline chemicals, such as quicklime (CaO), calcium hydroxide (Ca(OH)2), and lye or caustic soda (NaOH). Once added to the ocean, these compounds react with excess CO2 in seawater and air, principally forming stable, dissolved mineral bicarbonate, thus removing and sequestering CO2.
Such ocean alkalization could be achieved via distribution from shore or by ships. Another proposal is to manufacture alkalinity at sea using local marine energy sources: for example, employing electricity derived from the ocean’s very significant vertical temperature gradient. Reacting waste CO2 with minerals on shore and then pumping the resulting dissolved alkaline material into the ocean is also an option. All of the preceding would simply add to the already vast bicarbonate and carbonate reservoir in the ocean.