Since there are no currently active contests, we have switched Climate CoLab to read-only mode.
Learn more at
Skip navigation
Share via:


The ocean has unlimited space for seaweed forests, the only profitable method to remove 2 trillion tons of CO2 from air and water.



Seaweed forests can remove more than 2 trillion tons of CO2 from air and water, reversing climate change ocean acidification.  These “forests” do this by (1) replacing fossil fuels, (2) removing CO2 to safe storage, and (3) increasing the diversity of life in and around them. Ocean seaweed forests do not require fresh water and grow in nutrient desert areas much larger than potential terrestrial areas.

Our studies for our entry for the Paul G. Allen Ocean Challenge suggest ocean forests can remove 20 to 40 billion tons/yr of CO2 while paying for themselves selling food, energy, and other products.  The largest forests will mimic the free-floating and biodiverse Sargasso Sea.  Unlike the Sargasso Sea, our managed plant nutrient cycles will allow higher primary productivity and biomass density.

Seaweed forests employ photosynthesis to mitigate both acidification and the accelerated denitrification, which has occurred in past warming periods.  The key is recycling the nitrogen and other nutrients to ensure sustainability.

Some of our team members have examined seaweed forests and anaerobic digestion since the 1980’s.  One member’s efforts confirmed a pressurized seawater macroalgal digester operating at typical ocean temperature will produce 90% bio-methane in acceptable time frames.  Only now is this concept becoming economically viable due to continually improving materials science (particularly geosynthetics and nanotechnology), artificial intelligence (autonomous and remotely operated sensors and systems), microbiology, chemistry, and ocean engineering.  The new science allows relatively large and inexpensive submerged process containers, harvesting processes, nutrient treating equipment, nutrient dispersal systems, and energy conversions for transporting product energy.

Full description of Ocean Afforestation in the peer reviewed "Negative carbon via Ocean Afforestation".

Seaweed forests are a subset of Marine Agronomy.

What actions do you propose?

The Ocean Foresters have integrated knowledge from many disciplines of science, engineering, business, and politics to prepare a preliminary design for sustainably managed seaweed forest ecosystems.  Our primary proposed action is to development our managed ecosystems to completely replace the fossil fuel industry.  The following activities explain the proposed ocean seaweed forest ecosystem.

1.  Grow seaweed: Sunlight powers seaweed to grow about 18 ash-free tons per hectare per year with typical nutrient supplies.  Our managed ocean-gyre forests may be free-floating, like the Sargasso Sea, but with higher nutrient density.

Our sheltered -- or shallow-water -- seaweed forests may be anchored.  We propose to start economically-viable forests in sheltered water, such as Fiji’s bays, and then build toward ocean gyres.  Each seaweed forest becomes an instant local haven of higher pH.   

2. Harvest seaweed: Biodiverse forests producing energy and raising pH would be harvested differently than humans harvest seaweed farms.  Our digestion step handles any array of organic matter.  Diverse forest ecology can be more profitable than mono-culture farming, and our low-energy harvest can happen at the pace of biology, wind, and wave.  For example, allow a few days to contract an open-bottom floating curtain net around the harvested area and another day or two to winch the contracted "tea bag" of biomass over and into the digestion container.  Spot harvesting high density areas in the forest center may be augmented by rounding up elusive patches breaking away from the forest due to weather anomalies. 

3. Digest seaweed: The plant nutrients are separated from the plant energy (carbon and hydrogen from the nitrogen, phosphorous, iron, etc.) by anaerobic digestion or other optional processes that improve economics or reduce the 9% of world ocean surface needed to replace 100% of global fossil fuel demand. 

4. Recover the separated bio-CO2 and bio-CH4:  Dr. Brune demonstrated direct production of 90% biomethane (differential dissolution) from seaweed at 7 atmospheres pressure in the 1980s.  The higher concentrations of dissolved CO2 improved process pH stability.  Our LCA is based on differential dissolution producing two gas streams: 1) 90:10 CH4:CO2 gas at the pressure of the digester depth (likely 10 to 50 atmospheres); and 2) 10:90 CH4:CO2 dissolved inside the digester.  Today, more technologies are available to separate the CH4 from the CO2, including: pressure swing absorption, cryogenics, gas-liquid separation nozzles, supersonic desublimation, mimicking carbonic anhydrase enzyme, aminosilicones, etc.) are available to separate the gases of anaerobic digestion. 

If not storing CO2, separation is not essential.  CH4:CO2 ratios as low as 1:1 may be combusted as-is to produce electricity or converted to liquid fuels.  Higher than about 95% CH4 purity may be liquefied or piped as natural gas or converted to synthetic diesel, methanol, jet fuel, etc.

5. Recycle the nutrients:  Seaweeds concentrate the nutrients that were in the water.  Digesting the seaweed further concentrates the nutrients in two forms: dissolved in seawater and trapped in partially digested solids.  The nutrient distribution will be carefully managed to prevent ammonia toxicity and maximize macroalgal forest sustainability without microalgae blooms. 

Our LCA includes energy and materials to recycle 100% of nutrients all three ways: 1) dispersing the water with dissolved nutrients and some suspended solids; 2) floating “teabags” of “solids”; and 3) creating an artificial upwelling from 200 meters depth to return falling nutrients not collected during harvesting. 

Where there are excess nutrients (deadzones), we can interrupt the nutrient cycle by returning nutrients to terrestrial agriculture.  See our related concept: “Seaweed raises ocean pH while remediating ocean dead zones.”   

6. Capture and compress the bio-CO2:  When anaerobic digestion is completed in a submerged digester, we are left with the liquid and solid nutrients plus dissolved gas, which is 90% bio-CO2 and 10% bio-CH4. As we lift the liquid to the ocean surface, the gases bubble out of solution and are captured at 1-atm pressure. 

All the captured (previously dissolved) bio-CO2, bio-CH4, H2S, N2O, etc. is compressed to 50-bar and cooled as it is moved to the 500-meter depth.    The compressed bio-CO2 condenses to a liquid.  Other gases will either be recovered with most of the bio-CH4 or will remain dissolved in the liquid bio-CO2.  Researchers of exhaust gas carbon capture and storage continue to improve on this step.  

7. Store the bio-CO2: Several carbon storage technologies are available, including: deep earth hot “geologic” storage; as rock via reaction with olivine and other minerals; in conjunction with H2 production via electrolysis with silicate minerals on electrodes, or as a contained hydrate.  See the concept: “Raise ocean pH by storing CO2 captured from seaweed forests as contained hydrate.”  Our LCA predicts US$16 per ton of CO2 to produce hydrate in a container, and maintain the hydrate storage structures for millennia.

 8. Additional mitigation and adaptation with other products: Replacing fossil fuels requires so much seaweed forest that the production of more than 0.5 kg of fish and sea vegetables per person per day for 10 billion people could be almost an “incidental” by-product. 

Economic Sustainability

Analysis derived from our LCA projects economic viability, even with no price on carbon.  However, a price on carbon more quickly mitigates ocean acidification with a two-step funding strategy.  First use excess income from the bio-CO2 storage to reduce the price of bio-CH4 in order to underprice fossil fuels.  After fossil fuel use ceases, we can keep storing the legacy carbon by increasing the price of bio-CH4.  The atrophied fossil fuel producers would not revive because they would know we could drop our price by pausing the bio-CO2 storage.

Specifically, our LCA suggests a carbon fee of US$50 per metric ton of CO2 allows a bio-CH4 price of US$0.10 per m3 at the digester ($0.28/therm, or $2.80/MMBtu).  When there is no more fossil fuel use, charging US$0.16 per m3 ($0.47/therm) would subsidize legacy capture and permanent storage of the bio-CO2 and the captured CO2 from the powerplants.

No Other Alternative Can Meet the Scale Required

McLaren (2011 and 2012) has analyzed the cost, readiness and potential capacity of dozens of other carbon removal technologies, including forestry.  He concludes that terrestrial forest restoration, creation and enhanced management could only sequester 1.5 – 3 Gt/yr of CO2, less than 10% of the total current emissions of 32 Gt/yr, and less than 5% of the requirement to reverse global warming, which, as this chartshows, can only be done by Ocean Afforestation.

Who will take these actions?

Most of the continually increasing 20 scientists, engineers, and business people on the Ocean Foresters team are listed in Mark Capron’s profile.

The initial action to develop ocean forests will be taken by a combination of Ocean Foresters, governments, non-profits funding universities, crowd investors, individual philanthropists, and individual and corporate investors.

The activities listed above will be performed by Ocean Foresters.

Our related proposals explain how to start replacing fossil fuels with the bio-methane economy.  Once any of several types of managed seaweed forests become self-supporting, business investment will dominate product development, but more regulation may be necessary to ensure the ecosystems remain sustainable.

Where will these actions be taken?

Seaweed forests over 9% of the world’s oceans would reverse ocean acidification by producing and storing over 30 billion tons of bio-CO2 and captured bio-CH4 combustion-CO2 per year. 

We favor starting in Fiji and the University of the South Pacific member countries with support from developed countries.  Then grow to the Small Island Developing States (SIDS) and larger countries.

As our “how to” knowledge improves we would expand to: lakes, lagoons, and bays (many lakes in Africa with excess or invasive water plants); any sheltered water, any country; Hawaii, or California, or Mexico, but also start in India, China, Chile, or Japan.  Hawaii has the advantage of proximity to both many small islands, the equator, and the center of the North Pacific gyre.  (We could remove plastic debris incidental to our energy, food, and biodiversity production operations.)

How much will emissions be reduced or sequestered vs. business as usual levels?

Emissions reductions are not enough.  Humanity needs economically and environmentally sustainable solutions which can scale to eliminate emissions while removing CO2 from the atmosphere.

20 years – Net zero emissions.

30 more years – Time to store 1 trillion tons of CO2 in our “best reasonable scenario” of 36 billion tons per year.  Including oceans off-gassing, about 1 trillion tons of CO2 would need to be removed in order to reduce atmospheric CO2 by 100 ppm.  (Say for example from 450 back to 350 ppm).

200 years – Time to store 2 trillion tons of CO2 in our “bad case scenario.”  The bad case is needing to mitigate ocean acidification by reducing atmospheric concentrations from 550 ppm to 350 ppm, no BECCS (because there is no fossil fuel use paying for BECCS), and the market for seaweed forest energy limited to about 300 quads (less than about 5% of ocean surface), but the income from seaweed forest energy is covering the cost of storing 10 billion tons of bio-CO2 per year.

What are other key benefits?

Food, negative carbon, biodiversity.  We can increase incidental but sustainable fish production to potentially provide 200 kg/yr/person for 10 billion people.

Ocean Macroalgal Afforestation (OMA or seaweed forests) is a holistic ecosystem sustainable at scale in every sense of the word:

  1. Environmental sustainability addresses issues beyond solar radiation or carbon capture such as: species biodiversity, food, and energy.
  2. Climate sustainability is a robust process unaffected by increasing CO2 nor a cause of droughts, floods, heat, cold, changing wind patterns, and ocean acidification.
  3. Political sustainability improves the quality of life and opportunities in all countries.
  4. Social sustainability counters the water, food, jobs, and natural disaster stresses of climate change.
  5. Energy sustainability stores more carbon permanently with less than 10% of the energy generated.
  6. Economic sustainability via multiple products from the seaweed forest ecosystem.

What are the proposal’s costs?

Expect an economically self-sustaining operation, net profits, not costs.

Expect US$1 million to the first economically self-sustaining sheltered-water operation, likely in Fiji per cash-flow worksheet.

Expect US$100 million to the first economically self-sustaining open-water operation per two worksheets: Building out Fiji operations in Phases 1 and 2; Developing and building out open ocean operations in Phases 3 and 4.

Expect total ecosystem income to exceed costs by US $3 trillion per year as early as 2045.

Expect the bio-CO2 capture and artificial geologic seafloor container hydrate storage operations to cost less than US$16/ton of CO2.

The above is supported by our life cycle assessment (LCA).  View a table of the global scale calculations showing these important numbers:

600 Quadrillion Btu/year (176 million GWh) – The U.S. Energy Information Agency predicts this much fossil fuel use in 2035.  600 quads also corresponds to the energy produced by seaweed forests covering 9% of ocean area.  The extent of the forests, their energy production, and their CO2 storage is limited by the demand for bio-methane.

0 – Remaining fossil fuel energy in scenario year.

19 billion metric tons of CO2 per year – Mass of CO2 removed from air and water in the form of stored bio-CO2 when managing seaweed forests covering 9% of the ocean surface.  Stored bio-CO2 is incidental to energy and food production.

17 billion metric tons of CO2 per year – We can also use BECCS (bio-energy carbon capture and storage) on the combustion-CO2 that is made when the bio-CH4 combusts.  17 billion tons represents half the combustion exhaust.  We estimated about half the combustion exhaust would be from residences or vehicles and not be captured.

36 billion metric tons of CO2 per year – This is the net CO2 removed from air and water with above assumptions.

Time line

Expect 2-3 years to economically self-sustaining sheltered water operations, likely one 2,000+ ha forest in a Fiji lagoon.  

Team members are starting related projects.  Prof. Charles Yarish has been establishing businesses along the U.S. New England coast which grow seaweed and shell fish while remediating an excess of nutrients.  Several reseachers at Scuola Superiore Sant'Anna, Italy and the University of the South Pacific are addressing converting an excess of seaweed into energy and terrestrial fertilizer.  At the same time, USP member islands need terrestrial fertilizers and use imported diesel fuel to make electricity.  The fuel cost alone for diesel-electricity exceeds US$0.15 per kWh.  These (and other) projects can be replicated in bays throughout the world.

Expect at least a decade to economically self-sustaining open-ocean operations, likely a cluster of ten 10,000+ ha forests in the North Pacific Gyre.  Development time depends on rate of expenditure.  A decade implies learning lessons expensively with large-scale trials.  One could spend less but take longer by taking many small steps in sequence.

Learning to operate seaweed forests may take more time and be more difficult than we expect.  But our learning time pales next to the centuries humans must take to mitigate ocean acidification by any other means (See edited chart from McLaren, D., 2012. A comparative global assessment of potential negative emissions technologies.).  Even if humans can stop using coal, oil, and natural gas, we have to contend with methane hydrates.  The U.S. Department of Energy DE-FOA-0000891 states: “… A frequently quoted estimate of the global methane hydrate resource is 20,000 trillion cubic meters …”  Should that volume of methane escape or combust, it would become 40 trillion tons of CO2.  The oceans need mitigation ecosystems worthy of the scale and duration of humanity’s addiction to fossil fuels plus our inadvertent release of methane hydrates.

Related proposals

Finalist - Scaling renewables: “Fiji, then Small Island Ocean Afforestation Initiative, then Indian Ocean” shows steps toward ocean afforestation.

Finalist - Replacing Diesel: “Replace the diesel, reuse the engines, waste & seaweed biogas” aids transition from diesel to biogas fuel. 

Finalist - Hydraulic fracturing: “Methane-sniffing drones with distributed mobile sensors!” reduces methane leakage.

Electric power sector: “Replace coal and oil with renewable natural gas (biomethane)” – Ocean afforestation provides sustainable biomethane.

Geoengineering: Withdrew to avoid Negative carbon via Ocean Afforestation being linked with adverse perceptions of geoengineering.

Shifting Cultures: “Rapid Planet Change” needs ocean afforestation holistic global ecosystem.

Electric power sector and Fossil fuel sector: “Save the methane!” suggests a carbon fee slows oil drilling while ocean afforestation builds the biomethane economy.


N’Yeurt, A., Chynoweth, D., Capron, M.E., Stewart, J., Hasan, M. 2012. Negative carbon via Ocean Afforestation.  Process Safety and Environmental Protection 90, 467-474.

The above paper lists 32 references for the calculations involving the Ocean Macroalgal Afforestation ecosystem.  There are seven supplemental information documents, some with more references:

OMA-Macroalgae Production & Density Calcs 2012 – 42 references

OMA-Process Concepts 2012 – 2 references

OMA-Global Calculations 2012 for Table 2 – 2 references

OMA-Algal Yields Calcs & Refs 2012– 45 references

OMA-N2O-Emissions Calcs 2012 – 2 references

OMA-Artificial Geologic Seafloor Storage Of CO2, 2012 – 38 references

OMA-Ammonia Concentrating To Fertilizer 2012

McLaren, D., 2011. First, Stop Digging: An assessment of the potential for negative emission techniques to contribute safely and fairly to meeting carbon budgets in the 21st Century, (

McLaren, D., 2012. A comparative global assessment of potential negative emissions technologies. Process Safety and Environment Protection, Vol. 90, p. 489-500.

Migliore, G., Alisi, C., Sprocati, A.R., Massi, E., Ciccoli, R., Lenzi, M., Wang, A., Cremisini, C., 2012 Anaerobic digestion of macroalgal biomass and sediments sourced from the Orbetello lagoon, Italy, Biomass and BioEnergy 42, 69-77

Latimer, J.S., M. Tedesco, R.L. Swanson, C. Yarish, P. Stacey and C. Garza. 2013.  Long Island Sound: Prospects for the Urban Sea. Springer Publishers, NY

Yarish, C., A.E. Sperr, R. Wilkes, X.G. Fei, A.C. Mathieson, and I. Levine. "Developing a Commercially Viable Seaweed Industry in New England." for Nori Research Project (1996-1998)

Yarish, C. 2010. Macroalgae for CO2 Capture and Renewable Energy--A Pilot Project. Gas Technology Institute Project Number 20960, Department of Energy Netl Program

CBS news, June 20, 2013. Long Island Sound Cash Crop Makes Its Way To Manhattan Tables.