Ocean Carbon Sequestration: Solution to Climate Change or Policy Distraction?

Geoengineering, including ocean carbon sequestration, is often promoted as a potential solution to climate change. But is ocean sequestration feasible? A review of the research indicates that all available techniques would either be expensive, ineffective, or both.

Growing Calls for Abatement of CO₂ Emissions

The United Nations Intergovernmental Panel on Climate Change (IPCC) estimates that between 1750 and 2011, humans emitted a total of 545 petagrams of carbon¹ (PgC, or 109 metric tons), with current anthropogenic emissions levels at about 8.5 to 13.1 PgC per year.² Since the beginning of the industrial era, this rise has led to an increase in atmospheric concentration from 278 to 390 parts per million.³

Significant temperature increases have been projected in both the near and long term, and global sea levels are projected to rise by 0.18 to 0.59 meters by the turn of the next century.⁴ Understandably, governments, multilateral organizations, and the private sector have become increasingly focused on ways to mitigate and reverse the destabilizing potential effects of climate change. Well-known policy prescriptions include mechanisms designed to limit emissions, such as carbon taxes, the Clean Development Mechanism (the UN’s carbon offset program under the Kyoto Protocol), and market-based cap-and-trade arrangements, which have been implemented in jurisdictions from the European Union to California to Australia.

Such policies, designed to limit and eventually reduce CO₂ emissions, can be supplemented with “geoengineering” programs designed to actively sequester atmospheric carbon. The oceans, with their enormous volume, are seen as a potentially vast source of “storage” space for greenhouse gases.

The Promise of Geoengineering

“Geoengineering” refers to human manipulation of the environment on a global scale to avert climate change. The process takes advantage of “carbon sinks,” or naturally occurring systems that trap and absorb large quantities of carbon otherwise present in the atmosphere.

Rain forests, farmed crops, algae, and other forms of plant life constantly breathe in CO₂, retaining it within their structures. Dead matter, detritus, and other forms of biomass (along with the carbon they contain) are then gradually incorporated (or “sequestered”) into soil and water through a regular process of cyclical decay and growth.

Through land management techniques, afforestation programs (planting trees in previously unforested areas), production of biochar (charcoal used to trap carbon in soil), and other methods, climate scientists have developed a diverse set of potential tools for increasing the total carbon tonnage removed from the atmosphere.

Where is the Carbon Stored?

While much of the academic literature and policy discussion on geoengineering focuses on soil sequestration, soil in fact contains a tiny minority of total sequestered carbon. Of all terrestrial carbon, most is contained deep within the earth, and not relevant to human efforts to increase sequestration. But even on the earth’s surface, soil carbon represents only about eight percent of the total, while ocean carbon constitutes most of the remainder (see Figure 1).

Figure 1.

Distribution of Earth’s Surface Carbon Sequestration by Area

Data from Stockmann et al. (2013)⁵

Oceans as a Solution

The earth’s oceans are estimated to contain approximately 38,000 PgC, nearly fifty times more than the atmosphere. Of this amount, most is dissolved within intermediate and deep waters. Surface waters contain less than 1000 PgC in dissolved form, and surface water biomass contains only about 3 PgC (by contrast, land biomass contains several hundred PgC).⁶

The IPCC, various governmental agencies, and independent researchers confirm that in terms of absolute quantity, the oceans have a far greater capacity to absorb carbon than all other sinks combined. Although this specialized field of climate research is still in its infancy, policymakers have expressed interest in the possibility of artificially increasing the ocean’s ability to absorb atmospheric CO₂. Scientists have explored possible methods for years, but only recently has the environmental policy community begun to look at the wider effectiveness, safety issues, and economic feasibility of such programs.

Ocean Sequestration Methods

Options for increasing the amount of carbon...