From "Negative carbon via Ocean Afforestation". Authors: Antoine de Ramon N'Yeurt, David P. Chynoweth, Mark E. Capron, Jim R. Stewart, Mohammed A. Hasan - Elsevier, 2012
Ocean Afforestation, more precisely Ocean Macroalgal Afforestation (OMA), has the potential to reduce atmospheric carbon dioxide concentrations through expanding natural populations of macroalgae, which absorb carbon dioxide, then are harvested to produce biomethane and biocarbon dioxide via anaerobic digestion. The plant nutrients remaining after digestion are recycled to expand the algal forest and increase fish populations. A mass balance has been calculated from known data and applied to produce a life cycle assessment and economic analysis. This analysis shows the potential of Ocean Afforestation to produce 12 billion tons per year of biomethane while storing 19 billion tons of CO2 per year directly from biogas production, plus up to 34 billion tons per year from carbon capture of the biomethane combustion exhaust. These rates are based on macro-algae forests covering 9% of the world’s ocean surface, which could produce sufficient biomethane to replace all of today’s needs in fossil fuel energy, while removing 53 billion tons of CO2 per year from the atmosphere, restoring pre-industrial levels. This amount of biomass could also increase sustainable fish production to potentially provide 200 kg/yr/person for 10 billion people. Additional benefits are reduction in ocean acidification and increased ocean primary productivity and biodiversity.
Note that the OMA renewable energy output of 176 million GWh/year (600 quadrillion Btu/year) is equivalent to all the energy projected to be needed from fossil fuels in 2035. We assume additional energy needed in future years would come from other renewables, such as wind, solar power, geothermal, and other ocean energy technologies.
Getting Ocean Forests to completely replace fossil fuels and reverse climate change requires many disciplines, many geographies, hundreds of technologies, some of which are mentioned below. Ocean Forests won the “Best New Concept” Award in the American Society of Civil Engineers (ASCE) 2016 Innovation Contest for Big Picture Resilience via Ocean Forests, which presents a possible schedule of goals:
2030 Integrate coastal defense with food-water-energy producing ecosystems
2040 Recover 90% of “waste” resources
2050 Grow more food with less fresh water than 10 billion people need
2060 Replace 100% of global oil and gas demand
2100 Eliminate excess ocean acidity
2200 Reduce atmospheric CO2 concentration below 350 ppm