Iron fertilization

Iron fertilization
An oceanic phytoplankton bloom in the South Atlantic Ocean, off the coast of Argentina covering an area about 300 miles (480 km) by 50 miles

Iron fertilization is the intentional introduction of iron to the upper ocean to stimulate a phytoplankton bloom. This is intended to enhance biological productivity, which can benefit the marine food chain and remove carbon dioxide from the atmosphere. Iron is a trace element necessary for photosynthesis in all plants. It is highly insoluble in sea water and is often the limiting nutrient for phytoplankton growth. Large phytoplankton blooms can be created by supplying iron to iron-deficient ocean waters. A number of ocean labs, scientists and businesses are exploring fertilization as a means to sequester atmospheric carbon dioxide in the deep ocean, and to increase marine biological productivity which is likely in decline as a result of climate change. Since 1993, thirteen international research teams have completed ocean trials demonstrating that phytoplankton blooms can be stimulated by iron addition.[1] However, controversy remains over the effectiveness of atmospheric CO2 sequestration and ecological effects.[2] The most recent open ocean trial of ocean iron fertilization, dubbed LOHAFEX, was conducted from January to March 2009 in the South Atlantic.

Fertilization also occurs naturally when upwellings bring nutrient-rich water to the surface, as occurs when ocean currents meet an ocean bank or a sea mount. This form of fertilization produces the world's largest marine habitats. Fertilization can also occur when weather carries wind blown dust long distances over the ocean, or iron-rich minerals are carried into the ocean by glaciers,[3] rivers and icebergs.[4]



Consideration of iron's importance to phytoplankton growth and photosynthesis dates back to the 1930s when English biologist Joseph Hart speculated that the ocean's great "desolate zones" (areas apparently rich in nutrients, but lacking in plankton activity or other sea life) might simply be iron deficient.[5] Little further scientific discussion of this issue was recorded until the 1980s, when oceanographer John Martin renewed controversy on the topic with his marine water nutrient analyses. His studies indicated it was indeed a scarcity of iron micronutrients that was limiting phytoplankton growth and overall productivity in these "desolate" regions, which came to be called "High Nutrient, Low Chlorophyll" (HNLC) zones.[5]

In an article in the scientific journal Nature (February 1988; 331 (6157): 570ff.), John Gribbin was the first scientist to publicly suggest that the upcoming greenhouse effect might be reduced by adding large amounts of soluble iron compounds to the oceans of the world as a fertilizer for the aquatic plants.

Martin's famous 1991 quip three years later at Woods Hole Oceanographic Institution, "Give me a half a tanker of iron and I will give you another ice age",[5][6] drove a decade of research whose findings suggested that iron deficiency was not merely impacting ocean ecosystems, it also offered a key to mitigating climate change as well.

Perhaps the most dramatic support for Martin's hypothesis was seen in the aftermath of the 1991 eruption of Mount Pinatubo in the Philippines.[citation needed] Environmental scientist Andrew Watson analyzed global data from that eruption and calculated that it deposited approximately 40,000 tons of iron dust into the oceans worldwide. This single fertilization event generated an easily observed global decline in atmospheric CO2 and a parallel pulsed increase in oxygen levels.[7]


Martin hypothesized that increasing phytoplankton photosynthesis could slow or even reverse global warming by sequestering enormous volumes of CO2 in the sea. He died shortly thereafter during preparations for Ironex I,[8] a proof of concept research voyage, which was successfully carried out near the Galapagos Islands in 1993 by his colleagues at Moss Landing Marine Laboratories.[5] Since then 9 international ocean studies have examined the fertilization effects of iron:

  • Ironex II, 1995[9]
  • SOIREE (Southern Ocean Iron Release Experiment), 1999[10]
  • EisenEx (Iron Experiment), 2000[11]
  • SEEDS (Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study), 2001[12]
  • SOFeX (Southern Ocean Iron Experiments - North & South), 2002[13][14]
  • SERIES (Subarctic Ecosystem Response to Iron Enrichment Study), 2002[15]
  • SEEDS-II, 2004[16]
  • EIFEX (European Iron Fertilization Experiment), 2004[17]
  • CROZEX (CROZet natural iron bloom and Export experiment), 2005[18]
  • LOHAFEX (Indian and German Iron Fertilization Experiment), 2009 [19][20][21]

Despite widespread opposition to LOHAFEX, on 26 January 2009 the German Federal Ministry of Education and Research (BMBF) gave clearance for this fertilization experiment to commence. The experiment was carried out in waters low in silicic acid which is likely to affect the efficacy of carbon sequestration.[22] A 900 square kilometers (350 sq mi) portion of the southwest Atlantic Ocean was fertilized with iron sulfate. A large phytoplankton bloom was triggered, however this bloom did not contain diatoms because the fertilized location was already depleted in silicic acid, an essential nutrient for diatom growth.[22] In the absence of diatoms, a relatively small amount of carbon was sequestered, because other phytoplankton are vulnerable to predation by zooplankton and do not sink rapidly upon death.[22] These poor sequestration results have caused some to suggest that ocean iron fertilization is not an effective carbon mitigation strategy in general. However, prior ocean fertilization experiments in high silica locations have observed much higher carbon sequestration rates because of diatom growth. LOHAFEX has just confirmed that the carbon sequestration potential depends strongly upon careful choice of location.[22]

The maximum possible result from iron fertilization, assuming the most favourable conditions and disregarding practical considerations, is 0.29W/m2 of globally averaged negative forcing,[23] which is almost sufficient to reverse the warming effect of about 1/6 of current levels of anthropogenic CO2 emissions. It is notable, however, that the addition of silicic acid or choosing the proper location could, at least mathematically, eliminate and exceed all man made CO2. Further tests and experiments are required to test this theory and observe potential negative impact.


The role of iron

About 70% of the world's surface is covered in oceans, and the upper part of these (where light can penetrate) is inhabited by algae. In some oceans, the growth and reproduction of these algae is limited by the amount of iron in the seawater. Iron is a vital micronutrient for phytoplankton growth and photosynthesis that has historically been delivered to the pelagic sea by dust storms from arid lands. This Aeolian dust contains 3–5% iron and its deposition has fallen nearly 25% in recent decades.[24]

The Redfield ratio describes the relative atomic concentrations of critical nutrients in plankton biomass and is conventionally written "106 C: 16 N: 1 P." This expresses the fact that one atom of phosphorus and 16 of nitrogen are required to "fix" 106 carbon atoms (or 106 molecules of CO2). Recent research has expanded this constant to "106 C: 16 N: 1 P: .001 Fe" signifying that in iron deficient conditions each atom of iron can fix 106,000 atoms of carbon,[25] or on a mass basis, each kilogram of iron can fix 83,000 kg of carbon dioxide. The 2004 EIFEX experiment reported a carbon dioxide to iron export ratio of nearly 3000 to 1. The atomic ratio would be approximately: "3000 C: 58,000 N: 3,600 P: 1 Fe".[26]

Therefore small amounts of iron (measured by mass parts per trillion) in "desolate" HNLC zones can trigger large phytoplankton blooms. Recent marine trials suggest that one kilogram of fine iron particles may generate well over 100,000 kilograms of plankton biomass. The size of the iron particles is critical, however, and particles of 0.5–1 micrometer or less seem to be ideal both in terms of sink rate and bioavailability. Particles this small are not only easier for cyanobacteria and other phytoplankton to incorporate, the churning of surface waters keeps them in the euphotic or sunlit biologically active depths without sinking for long periods of time.

Carbon sequestration

Air-sea exchange of CO2

Previous instances of biological carbon sequestration have triggered major climatic changes, such as the Azolla event. Plankton that generate calcium or silicon carbonate skeletons, such as diatoms, coccolithophores and foraminifera, account for most direct carbon sequestration. When these organisms die their carbonate skeletons sink relatively quickly and form a major component of the carbon-rich deep sea precipitation known as marine snow. Marine snow also includes fish fecal pellets and other organic detritus, and can be seen steadily falling thousands of meters below active plankton blooms.[27]

Of the carbon-rich biomass generated by plankton blooms, half (or more) is generally consumed by grazing organisms (zooplankton, krill, small fish, etc.) but 20 to 30% sinks below 200 meters (660 ft) into the colder water strata below the thermocline.[citation needed] Much of this fixed carbon continues falling into the abyss, but a substantial percentage is redissolved and remineralized. At this depth, however, this carbon is now suspended in deep currents and effectively isolated from the atmosphere for centuries. (The surface to benthic cycling time for the ocean is approximately 4,000 years.)

Analysis and quantification

Evaluation of the biological effects and verification of the amount of carbon actually sequestered by any particular bloom requires a variety of measurements, including a combination of ship-borne and remote sampling, submarine filtration traps, tracking buoy spectroscopy and satellite telemetry. Unpredictable ocean currents have been known to remove experimental iron patches from the pelagic zone, invalidating the experiment.

The potential of iron fertilization as a geoengineering technique to tackle global warming is illustrated by the following figures. If phytoplankton converted all the nitrate and phosphate present in the surface mixed layer across the entire Antarctic circumpolar current into organic carbon, the resulting carbon dioxide deficit could be compensated by uptake from the atmosphere amounting to about 0.8 to 1.4 gigatonnes of carbon per year.[28] This quantity is comparable in magnitude to annual anthropogenic fossil fuels combustion of approximately 6 gigatonnes. It should be noted that the Antarctic circumpolar current region is only one of several in which iron fertilization could be conducted—the Galapagos islands area being another potentially suitable location.

Dimethyl sulfide and clouds

Some species of plankton produce dimethyl sulfide (DMS), a portion of which enters the atmosphere where it is oxidized by hydroxyl radicals (OH), atomic chlorine (Cl) and bromine monoxide (BrO) to form sulfate particles and ultimately clouds. This may increase the albedo of the planet and so cause cooling. This is the basic tenet of the Gaia hypothesis and was presented in James Lovelock's first paper published on this topic.

During the Southern Ocean Iron Enrichment Experiments (SOFeX), DMS concentrations increased by a factor of four inside the fertilized patch. Widescale iron fertilization of the Southern Ocean could lead to significant sulfur-triggered cooling in addition to that due to the increased CO2 uptake and that due to the ocean's albedo increase, however the amount of cooling by this particular effect is very uncertain.[29]

Financial opportunities

Since the advent of the Kyoto Protocol, several countries and the European Union have established carbon offset markets which trade certified emission reduction credits (CERs) and other types of carbon credit instruments internationally. In 2007 CERs sell for approximately €15–20/ton CO2e and European analysts project these prices will nearly double by 2012.[30] Iron fertilization is relatively inexpensive compared to scrubbing, direct injection and other industrial approaches, and can theoretically sequester for less than €5/ton CO2, creating a substantial return.[31] In August, 2010, Russia established a minimum price of €10/ton for offsets to reduce uncertainty for offset providers.[32] Scientists have reported a minimum 6–12% decline in global plankton production since 1980,[24][33] A full-scale international plankton restoration program could regenerate approximately 3–5 billion tons of sequestration capacity worth €50-100 billion in carbon offset value. Given this potential return on investment, carbon traders and offset customers are watching the progress of this technology with interest.[34]

Multilateral reaction

The parties to the London Dumping Convention (Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter 1972) adopted a non-binding resolution in 2008 on fertilization (labeled LC-LP.1(2008)). The resolution states that ocean fertilization activities, other than legitimate scientific research, "should be considered as contrary to the aims of the Convention and Protocol and do not currently qualify for any exemption from the definition of dumping".

An Assessment Framework for Scientific Research Involving Ocean Fertilization, regulating the dumping of wastes at sea (labeled LC-LP.2(2010)) was adopted by the Contracting Parties to the Convention in October 2010 (LC 32/LP 5).

Sequestration definitions

Carbon is not considered "sequestered" unless it settles to the ocean floor where it remains for eons. Most of the carbon that sinks beneath plankton blooms is dissolved and remineralized well above the seafloor and will eventually (days to centuries) return to the atmosphere, negating the original effect.

Advocates argue that modern climate scientists and Kyoto Protocol policy makers define sequestration in much shorter time frames. For example, they recognize trees and grasslands as important carbon sinks. Forest biomass only sequesters carbon for decades, but carbon that sinks below the marine thermocline (100–200 meters) is effectively removed from the atmosphere for hundreds of years, whether it is remineralized or not. Since deep ocean currents take so long to resurface, their carbon content is effectively sequestered by the criterion in use today.


While many advocates of ocean iron fertilization see it as modern society's last best hope to slow global warming, some academics, deep greens and proponents of competing technologies cite a variety of concerns.

Precautionary principle

The precautionary principle (PP) states that if an action or policy has a suspected risk of causing harm, in the absence of scientific consensus, the burden of proof that it is not harmful falls on those who would take the action. The side effects of large-scale iron fertilization are not yet known. Creating phytoplankton blooms in naturally iron-poor areas of the ocean is like watering the desert: in effect it changes one type of ecosystem into another. Such large and rapid conversions have no precedent.

The argument can be applied in reverse, by considering emissions to be the action and remediation an attempt to partially offset the damage.

20th century phytoplankton decline

While advocates argue that iron addition would help to reverse a supposed decline in phytoplankton, this decline may not be real. One study reported a decline in ocean productivity comparing the 1979–1986 and 1997–2000 periods,[35] but two others found increases in phytoplankton.[36][37]

Satellite image of a large, natural, coccolithophore bloom in the Bering Sea in 1998.

Comparison to prior phytoplankton cycles

Fertilization advocates respond that similar algal blooms have occurred naturally for millions of years with no observed ill effects. The Azolla event occurred around 49 million years ago and accomplished what fertilization is intended to achieve (but on a larger scale).

Sequestration efficiency

Fertilization may sequester too little carbon per bloom, supporting the food chain rather than raining on the ocean floor, and thus require too many seeding voyages to be practical.[14][38] A 2009 Indo-German team of scientists examined the potential of the south-western Atlantic to sequester significant amounts of carbon dioxide, but found few positive results.[39]

The counter-argument to this is that the low sequestration estimates that emerged from some ocean trials are largely due to these factors:[citation needed]

  1. Data: none of the ocean trials had enough boat time to monitor their blooms for more than 27 days, confining their measurements to that period. Blooms generally last 60–90 days with the heaviest "precipitation" occurring during the last two months.
  2. Scale: most trials used less than 1,000 kilograms (2,200 lb) of iron and thus created small blooms that were quickly devoured by opportunistic zooplankton, krill, and fish that swarmed into the seeded region.

Some ocean trials reported positive results. IronEx II reported conversion of 1,000 kilograms (2,200 lb) to carbonaceous biomass equivalent to one hundred full-grown redwoods within two weeks. Eifex recorded fixation ratios of nearly 300,000 to 1.

Current estimates of the amount of iron required to restore all the lost plankton and sequester 3 gigatons/year of CO2 range widely, from approximately 2 hundred thousand tons/year to over 4 million tons/year. The latter scenario involves 16 supertanker loads of iron and a projected cost of approximately €20 billion ($27 billion).

Ecological issues

Algal blooms

A "red tide" off the coast of La Jolla, San Diego, California.

Critics are concerned that fertilization will create harmful algal blooms (HAB). The species that respond most strongly to fertilization vary by location and other factors and could possibly include species that cause red tides and other toxic phenomena. These factors affect only near-shore waters, although they show that increased phytoplankton populations are not universally benign.[citation needed]

Most species of phytoplankton are harmless or beneficial, given that they constitute the base of the marine food chain. Fertilization increases phytoplankton only in the deep oceans (far from shore) where iron deficiency is the problem. Most coastal waters are replete with iron and adding more has no useful effect.[citation needed]

A 2010 study of iron fertilization in an oceanic high-nitrate, low-chlorophyll environment, however, found that fertilized Pseudo-nitzschia diatom spp., which are generally nontoxic in the open ocean, began producing toxic levels of domoic acid. Even short-lived blooms containing such toxins could have detrimental effects on marine food webs.[40]

Deep water oxygen levels

When organic bloom detritus sinks into the abyss, a significant fraction will be devoured by bacteria, other microorganisms and deep sea animals which also consume oxygen. A large enough bloom could render certain regions of the sea deep beneath it anoxic and threaten other benthic species.[citation needed]

The largest plankton replenishment projects under consideration are less than 10% the size of most natural wind-fed blooms. In the wake of major dust storms, natural blooms have been studied since the beginning of the 20th century and no such deep water dieoffs have been reported.[citation needed]

Ecosystem effects

Depending upon the composition and timing of delivery, iron infusions could preferentially favor certain species and alter surface ecosystems to unknown effect. Population explosions of jellyfish, disturbance of the food chain impacting whale populations or fisheries are potential risks.[citation needed]

A 2010 study shows that iron enrichment stimulates toxic diatom production in high-nitrate, low-chlorophyll areas [1], which, the authors argue, raises "serious concerns over the net benefit and sustainability of large-scale iron fertilizations".

However, CO2-induced surface water heating and rising carbonic acidity are already shifting population distributions for phytoplankton, zooplankton and many other creatures. Optimal fertilization could potentially help restore lost/threatened ecosystem services.[citation needed]

Conclusion and further research

Critics and advocates generally agree that most questions on the impact, safety and efficacy of ocean iron fertilization can only be answered by much larger studies.

A statement published in Science in 2008 maintained that it would be

premature to sell carbon offsets from the first generation of commercial-scale OIF experiments unless there is better demonstration that OIF effectively removes CO2, retains that carbon in the ocean for a quantifiable amount of time, and has acceptable and predictable environmental impacts.[41]

One pilot project planned by Planktos, a U.S. company, was cancelled in 2008 for lack of funding.[42] The company blamed environmental organizations for the failure.[43][44]

See also


  1. ^ Boyd, P.W., et al.; Jickells, T; Law, CS; Blain, S; Boyle, EA; Buesseler, KO; Coale, KH; Cullen, JJ et al. (2007). "Mesoscale Iron Enrichment Experiments 1993-2005: Synthesis and Future Directions". Science 315 (5812): 612–7. Bibcode 2007Sci...315..612B. doi:10.1126/science.1131669. PMID 17272712. 
  2. ^ Buesseler, K.O., et al.; Doney, SC; Karl, DM; Boyd, PW; Caldeira, K; Chai, F; Coale, KH; De Baar, HJ et al. (2008). "ENVIRONMENT: Ocean Iron Fertilization—Moving Forward in a Sea of Uncertainty". Science 319 (5860): 162. doi:10.1126/science.1154305. PMID 18187642. 
  3. ^
  4. ^,1518,599213,00.html#ref=rss
  5. ^ a b c d Weier, John. "John Martin (1935-1993)". On the Shoulders of Giants. NASA Earth Observatory. Retrieved 2007-03-31. 
  6. ^ "Ocean Iron Fertilization – Why Dump Iron into the Ocean". Café Thorium. Woods Hole Oceanographic Institution. Archived from the original on 2007-02-10. Retrieved 2007-03-31. 
  7. ^ Watson, A.J. (1997-02-13). "Volcanic iron, CO2, ocean productivity and climate" (PDF). Nature 385 (6617): 587–588. Bibcode 1997Natur.385R.587W. doi:10.1038/385587b0. 
  8. ^ Ironex (Iron Experiment) I
  9. ^ Ironex II, 1995
  10. ^ SOIREE (Southern Ocean Iron Release Experiment), 1999
  11. ^ EisenEx (Iron Experiment), 2000
  12. ^ SEEDS (Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study), 2001
  13. ^ SOFeX (Southern Ocean Iron Experiments - North & South), 2002
  14. ^ a b "Effects of Ocean Fertilization with Iron To Remove Carbon Dioxide from the Atmosphere Reported" (Press release). Retrieved 2007-03-31. 
  15. ^ SERIES (Subarctic Ecosystem Response to Iron Enrichment Study), 2002
  16. ^ SEEDS-II, 2004
  17. ^ EIFEX (European Iron Fertilization Experiment), 2004
  18. ^ CROZEX (CROZet natural iron bloom and Export experiment), 2005
  19. ^
  20. ^ Bhattacharya, Amit (2009-01-06). "Tossing iron powder into ocean to fight global warming". The Times Of India. 
  21. ^
  22. ^ a b c d
  23. ^ Lenton, T. M., Vaughan, N. E. (2009). "The radiative forcing potential of different climate geoengineering options". Atmos. Chem. Phys. Discuss. 9: 2559–2608. doi:10.5194/acpd-9-2559-2009. 
  24. ^ a b Ocean Plant Life Slows Down and Absorbs less Carbon NASA Earth Observatory
  25. ^ Sunda, W. G., and S. A. Huntsman (1995). "Iron uptake and growth limitation in oceanic and coastal phytoplankton". Mar. Chem. 50: 189–206. doi:10.1016/0304-4203(95)00035-P. 
  26. ^ de Baar H . J. W., Gerringa, L. J. A., Laan, P., Timmermans, K. R (2008). "Efficiency of carbon removal per added iron in ocean iron fertilization". Mar Ecol Prog Ser. 364: 269–282. doi:10.3354/meps07548. 
  27. ^ Video of extremely heavy amounts of "marine snow" in the Charlie Gibbs Fracture Zone in the Mid-Atlantic Ridge. Michael Vecchione, NOAA Fisheries Systematics Lab. Published at Census of Marine Life website
  28. ^ Schiermeier Q (January 2003). "Climate change: The oresmen". Nature 421 (6919): 109–10. doi:10.1038/421109a. PMID 12520274. 
  29. ^ Wingenter, Oliver W.; Karl B. Haase, Peter Strutton, Gernot Friederich, Simone Meinardi, Donald R. Blake and F. Sherwood Rowland (2004-06-08). "Changing concentrations of CO, CH4, C5H8, CH3Br, CH3I, and dimethyl sulfide during the Southern Ocean Iron Enrichment Experiments". Proceedings of the National Academy of Sciences (National Academy of Sciences) 101 (23): 8537–8541. Bibcode 2004PNAS..101.8537W. doi:10.1073/pnas.0402744101. PMC 423229. PMID 15173582. Retrieved 2006-11-27. 
  30. ^ Feb 2007 Carbon Update, CO2 Australia
  31. ^ the Ocean, Scienceline
  32. ^ Russia sets minimum carbon offset price
  33. ^ Plankton Found to Absorb Less Carbon Dioxide BBC, 8/30/06
  34. ^ Recruiting Plankton to Fight Global Warming, New York Times, Business Section, page 1, 5/1/07
  35. ^ Gregg WW, Conkright ME, O'Reilly JE, et al. (March 2002). "NOAA-NASA Coastal Zone Color Scanner reanalysis effort". Appl Opt 41 (9): 1615–28. Bibcode 2002ApOpt..41.1615G. doi:10.1364/AO.41.001615. PMID 11921788. 
  36. ^ (Antoine et al.., 2005)
  37. ^ Gregg et al.. 2005
  38. ^ Basgall, Monte (2004-02-13). "Goal of ocean 'iron fertilization' said still unproved". Retrieved 2006-11-27. 
  39. ^
  40. ^ Tricka, Charles G., Brian D. Bill, William P. Cochlan, Mark L. Wells, Vera L. Trainer, and Lisa D. Pickell (2010). "Iron enrichment stimulates toxic diatom production in high-nitrate, low-chlorophyll areas". PNAS 107 (13): 5887–5892. Bibcode 2010PNAS..107.5887T. doi:10.1073/pnas.0910579107. PMC 2851856. PMID 20231473. 
  41. ^ Buesseler, KO; Doney, SC; Karl, DM; Boyd, PW; Caldeira, K; Chai, F; Coale, KH; de Baar, HJ; Falkowski, PG; Johnson, KS; Lampitt, RS; Michaels, AF; Naqvi, SW; Smetacek, V; Takeda, S; and Watson, AJ (11 January 2008) "Ocean Iron Fertilization--Moving Forward in a Sea of Uncertainty." Science, 319(5860):162. DOI:10.1126/science.1154305
  42. ^ Scientists to fight global warming with plankton 2007-05-21
  43. ^ Planktos kills iron fertilization project due to environmental opposition 2008-02-19
  44. ^ Venture to Use Sea to Fight Warming Runs Out of Cash New York Times 2008-02-14

Changing ocean processes

Micronutrient iron and ocean productivity

Ocean biomass carbon sequestration

Ocean carbon cycle modeling

  • Andrew Watson, James Orr (2003). "5. Carbon Dioxide Fluxes in the Global Ocean". In Fasham, M. J. R.. Ocean Biogeochemistry. Berlin: Springer. ISBN 3-540-42398-2. 
  • J.L. Sarmiento and J.C. Orr (December 1991). "Three-Dimensional Simulations of the Impact of Southern Ocean Nutrient Depletion on Atmospheric CO2 and Ocean Chemistry". Limnology and Oceanography 36 (8): 1928–50. doi:10.4319/lo.1991.36.8.1928. JSTOR 2837725. 

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