Blake, DR

Elliot, SM

Wingenter, OW

English

Wingenter, Oliver W

Elliot, Scott M

Blake, Donald R

eScholarship - University of California

UC IrvineUC Irvine Previously Published WorksTitleNew Directions: Enhancing the natural sulfur cycle to slow global warmingPermalinkhttps://escholarship.org/uc/item/3t2791f1JournalAtmospheric Environment, 41(34)ISSN1352-2310AuthorsWingenter, Oliver WElliot, Scott MBlake, Donald RPublication Date2007-11-01DOI10.1016/j.atmosenv.2007.07.021Copyright InformationThis work is made available under the terms of a Creative Commons Attribution License, availalbe at https://creativecommons.org/licenses/by/4.0/ Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaARTICLE IN PRESS1352-2310/$ - sedoi:10.1016/j.at$Somethingfor other topicsor to contribucontact new.die044/apex/newAtmospheric Environment 41 (2007) 7373–7375www.elsevier.com/locate/atmosenvNew Directions: Enhancing the natural sulfur cycleto slow global warming$Full scale ocean iron fertilization of the SouthernOcean (SO) has been proposed previously as ameans to help mitigate rising CO2 in the atmosphere(Martin et al., 1990, Nature 345, 156–158). Here wedescribe a different, more leveraged approach topartially regulate climate using limited iron en-hancement to stimulate the natural sulfur cycle,resulting in increased cloud reflectivity that couldcool large regions of our planet. Some regions of theEarth’s oceans are high in nutrients but low inprimary productivity. The largest such region is theSO followed by the equatorial Pacific. Severalmesoscale (102 km2) experiments have shown thatthe limiting nutrient to productivity is iron. Yet, theeffectiveness of iron fertilization for sequesteringsignificant amounts of atmospheric CO2 is still inquestion. However, marine microorganisms notonly consume inorganic carbon but also produceand consume many climate relevant organic gases.The greatest climate effect of iron fertilization maybe in enhancing dimethyl sulfide (DMS) production,leading to changes in the optical properties of theatmosphere and cooling of the region.DMS is believed to play a major role inmaintaining Earth’s climate in the range necessaryfor life. This presumption is called the CLAWhypothesis after the authors Charlson, Lovelock,Andreae and Warren (Charlson et al., 1987, Nature326, 655–661). Initially increasing atmospheric CO2concentration warms the planet, which inducesadditional productivity of marine phytoplanktonthat results in a greater production of DMS.e front matter r 2007 Elsevier Ltd. All rights reservedmosenv.2007.07.021to say? Comments on this article, or suggestions, are welcome. To write a response to this article,te your own article to New Directions, pleaserections@uea.ac.uk, or go to www.uea.ac.uk/dir2.html for further details.The CLAW hypothesis further states that greaterDMS production would result in additional flux tothe atmosphere, more cloud condensation nuclei(CCN) and greater cloud reflectivity by decreasingcloud droplet size. Thus, increased DMS wouldcontribute to the homeostasis of the planet bycountering warming from increasing CO2. A cor-ollary to the CLAW hypothesis is that elevated CO2itself increases DMS production which has beenobserved during a mesocosm scale CO2 enrichmentexperiment (Wingenter et al., 2007, GeophysicalResearch Letters 34, L05710). The CLAW hypoth-esis relies on the assumption that DMS sea-to-airflux leads to new particles and not just the growth ofexisting particles. If the CLAW hypothesis iscorrect, the danger is that enormous anthropogenicinputs of CO2 may overcome Earth’s natural abilityto self-regulate. Here we present initial calculationsand hypotheses intended to spark future, morecomprehensive research in amplification of thenatural sulfur cycle, aerosol growth and formation,and its possible outcomes.During the Southern Ocean Iron EnrichmentExperiments (SOFeX), we observed seawater con-centrations of DMS about five-fold higher inside theiron fertilized patch (7.7 nM; 7.7 109moles l1)versus outside (1.6 nM), 28 days after the initialfertilization (Wingenter et al., 2004, Proceedings ofthe National Academy of Science 101, 8537–8541),while undergoing an approximate three-fold dilu-tion in geochemical signal as reported by Coale etal. (2004, Science 304, 408–414). During similar SOiron fertilization projects Turner et al. (2004,Geophysical Research Letters 31, L14307), ob-served comparable enhancements of DMS.If approximately two percent of the SO werefertilized to about 1 nM of iron three times over the.ARTICLE IN PRESSNew Directions / Atmospheric Environment 41 (2007) 7373–73757374course of a week, in well-spaced streaks on the orderof 2 km wide, a three-fold dilution in iron similar tothat observed during SOFeX would result in an areaof about five percent of the SO. The resulting areawould be enhanced in DMS concentrations similarto those observed previously (Wingenter et al.,2004), leading to a 20 percent increase in DMSproduction integrated over the entire SO. If thefertilized strips were orientated roughly North-to-South, orthogonal to the prevailing winds, theenhanced DMS flux may lead to 20 percent moreCCN derived from DMS oxidation.Approximately 200CCNcm3 arise from DMSoxidation and 200CCN cm3 from sea-salt in theSO marine boundary layer (Watson and Liss, 1998,Philosophical Transactions of the Royal Society B353, 41–51). Therefore, the total CCNcm3 wouldincrease from 400 to 440CCNcm3, assuming thesame sulfate to CCN conversion efficiency for theextra DMS air–sea flux. The cloud albedo (reflec-tivity) over the SO in the summer is about 46percent (Watson and Liss, 1998). If the number ofparticles were to increase over the SO by anadditional 40CCNcm3 (from the additionalDMS), the albedo would increase by 0.8 percent(employing an analysis similar to that in Watsonand Liss, 1998 and in Twomey, 1991, AtmosphericEnvironment 25, 2435–2442). The amount of solarenergy reaching the top of the SO atmosphereduring the summer is about 400Wm2. Thus, anadditional 3Wm2 would be reflected back tospace. Using a global estimate that each Wm2corresponds to 0.75 1C (Hansen et al., 2005, Science308, 1431–1435) the initial fertilization of twopercent of the SO would result in a 2 1C decreasein temperature over the SO region, and perhaps afew tenths over the entire Southern Hemisphere.It is difficult to predict the full repercussions of alarge, 2 1C, cooling over the SO without directobservations or at least full scale coupled ocean–atmosphere–ice sheet model simulations. Coolersurface temperature could lead to some slowing ofsea-level rise by (i) slowing sea water expansion; and(ii) cooler temperatures may result in diminishedbreak up of ice near the Antarctic coast and inhibitlarge ice masses, such as the Western Antarctic IceSheet (WAIS), from raising sea level rapidly. Meltingof the WAIS would raise the sea level by about 6m.To fertilize two percent of the SO south of 501latitude, about 22 kton of FeSO4 are needed,assuming a 50m mixed layer depth, which is withinthe capacity of the smallest class of tanker ships.However, to accomplish this at the beginning of thegrowing season over 30 days, about 30 smaller shipswould be needed. Changes in albedo and chlor-ophyll would be monitored and validated bysatellite measurements. Depending on the numberof satellite and shipboard measurements andanalyses made, the yearly cost of such an operationwould be on the order of US $10 million to $100million. Initially, a smaller (perhaps 2 percent of106–107 km2) test area could be enhanced todemonstrate proof of the concept.An important caveat to this plan is that uncertaintiesstill exist in the understanding of particle conversion ofDMS oxidation products. They can either add toexisting particles or create new ones depending onmany factors, including the atmospheric temperatureand the type and amount of oxidant present (e.g. vonGlasow and Crutzen, 2004, Atmospheric Chemistryand Physics 4, 589–608). Whether new aerosols form orexisting ones grow is a critical factor concerning theviability of enhancing of DMS production to cool theclimate. A large-scale experiment, as described here,should greatly add to our understanding of aerosolgrowth and formation in the SO region.Even limited iron enhancement over several yearsmay result in significant changes in phytoplanktonecosystems, which could have unknown, or dama-ging, effects to climate or fishing. For example, theSO is known to be a sink of the stratospheric ozonedepleting substance methyl bromide. During SOFeX,we observed that net methyl bromide consumptionceased inside the iron fertilized North Patch.Another possible risk is the buildup of iron belowthe mixed layer; but because of rapid vertical mixingsouth of the Antarctic Front this seems unlikely. Thismixing could be perturbed if excessive cooling of,perhaps, several degrees resulted in increased sea-icearound Antarctica which would inhibit air–sea gasexchange. Furthermore, decreased buoyancy fluxwould also suppress upwelling (Watson and NavieraGarabato, 2006, Tellus 58B, 73–87). Therefore, ironand turbulence should be monitored.The estimates presented here are of first order andpresent the need for fully coupled ocean–atmo-sphere–cryosphere modeling studies. This should befollowed by a pilot experiment over 1–2 years. Iflimited iron enhancements, on the order of twopercent of the entire SO, are carried out, they shouldbe viewed as temporary measures to slow one ormore symptoms of climate change until a perma-nent solution for controlling greenhouse gas emis-sions can be implemented.ARTICLE IN PRESSNew Directions / Atmospheric Environment 41 (2007) 7373–7375 7375End Note. As opposed to our plan, full scale ironfertilization of the SO for the purpose of sequester-ing CO2 seems unfeasible from the perspective ofsimply overcooling the SO region by several degreesand the likely inhibition of agriculture on the landmasses bordering the SO, not to mention otherunintended and unknown consequences of massiveiron fertilization.OWW and DRB were supported by the US NSFChemical Oceanography program. SME was sup-ported by the US DOE OBER Acidic project. Thismanuscript benefited greatly from the comments oftwo anonymous reviewers, William Sturges, andBarkley C. Sive.Oliver W. WingenterDepartment of Chemistry and Geophysical ResearchCenter, New Mexico Institute of Mining andTechnology, Socorro, NM 87801, USAE-mail address: oliver@nmt.eduScott M. ElliotClimate Ocean Sea Ice Modeling Project,Los Alamos National Laboratory,Los Alamos, NM 87545, USADonald R. BlakeDepartment of Chemistry,University of California, Irvine, CA 92697, USA

New Directions: Enhancing the natural sulfur cycle to slow global warming

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New Directions: Enhancing the natural sulfur cycle to slow global warming

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