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 WorksTitleAuthors response to the above comment by M. Vogt et al. on “New Directions: Enhancing the natural cycle to slow global warming”Permalinkhttps://escholarship.org/uc/item/7vn8v3wsJournalAtmospheric Environment, 42(19)ISSN1352-2310AuthorsWingenter, Oliver WElliot, Scott MBlake, Donald RPublication Date2008-06-01DOI10.1016/j.atmosenv.2008.05.003Copyright 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 PRESSNew Directions / Atmospheric Environment 42 (2008) 4803–48094806Authors response to the above comment by M. Vogtet al. on ‘‘New Directions: Enhancing the naturalcycle to slow global warming’’$We appreciate the interest by our colleagues(Vogt et al., 2008) in Wingenter et al. (2007),in which we presented a possible means of loweringthe temperature over the Southern Ocean (SO)region. The best application of our proposal maybe to cool the surface temperature, which wouldreduce sea-level rise (i) by slowing sea waterexpansion; and (ii) by diminishing the breakupof Antarctic coastal ice shelves, which buttressglacial flow into the SO. Our paper had two mainintentions: first to encourage modeling studies inthis direction, and secondly to show that large-scaleiron fertilization for the purpose of carbon seques-tration could have severe negative impacts byovercooling the region. Vogt et al. raised severalconcerns about our initial calculations, which weclarify below.Our proposed iron fertilization of 2% of the SOwas estimated to ‘‘result in a 2 1C decrease intemperature over the SO region’’. Vogt et al. haveextrapolated similar results globally and annually,to suggest an impact on the order of 0.005 1C onthese larger scales. While we do not dispute thegeneral validity of this scaled up calculation, theassertion that the cooling effect of our proposedestimate is therefore a factor of 400 too large(i.e. 2C0.005) is incorrect, because we never statedthat the calculated 2 1C decrease applied to theentire planet for 12 months. Instead, the 2 1CTable 1Initial, outside-the-patch, and maximum-inside-the-patch DMS concenExperiment [DMS] (nM)Initial DMS Out Max inSOIREEa 0.5 0.5 3.4EISENEXa 1.9 1.8 3.1SOFeX Nb 1.6 7.7SOFeX Sc,d[DMS] is in nM, and the % changes are from the published iron additDMS references.aTurner et al., 2004.bWingenter et al., 2004.cWingenter, unpublished data.dDMS was observed for only 11 days after the initial iron application,in DMS was never observed.eThe maximum in chlorophyll a was observed 46 days after the ini408–414).decrease that we estimated over the SO region is afactor of three higher than the 0.6 1C determined byVogt et al. for this region.The factor of three difference that remainsbetween the two estimates mostly results fromdisagreement over the amount of DMS which willincrease over the SO (20% in Wingenter et al. vs.10% in Vogt et al., or a factor of two). Of the threepublished SO iron experiments that made DMSmeasurements, the response that we used (a 480%increase in DMS after fertilization)—from ourSOFeX North observations (Wingenter et al.,2004)—was in the middle of the three experiments(Table 1). The three published responses range from170–680%, and we agree that the uncertainty in theocean response (4807250%; a 50% uncertainty)cannot be neglected. Considering the relativelylimited surface area covered by each of theseexperiments, the much larger area in our proposalwould probably exhibit heterogeneous responses,i.e. higher than the SOFeX North response is someareas, and lower in others. It should also be notedthat the three published SO iron fertilizationexperiments listed in Table 1 indicate that the initialbackground chlorophyll a and DMS concentrationswere low. In the proposed experiment, iron wouldnot be added indiscriminately to areas already inbloom, as Vogt et al. suggest. Instead, satelliteobservations of chlorophyll a in conjunction withfast in situ biological indicators such as chlorophyllfluorescence and DMS would be used to direct thelimited fertilization and maximize the impact of theiron distributed.trations[Chlor a] (mgm3)Change (%) Initial or out Inside680 0.25 2.3170 0.5 2.8480 0.2 2.4Increasedc,d 0.2 3.7eion experiments in the SO region.after which the Revelle headed back to the North patch. The peaktial fertilization on the Melville (Coale et al., 2007, Science 304,ARTICLE IN PRESSNew Directions / Atmospheric Environment 42 (2008) 4803–4809 4807An additional area of disagreement relates to thecalculations that determine the extent of coolingfollowing DMS enhancements. Keys are (i) theextent to which total condensation nuclei (CCN)increase following a 20% increase in DMS (10% inWingenter et al. vs. 2.6% in Vogt et al.), and (ii) thenumber density of total CCN that is used in thecalculations (400CCNcm3 in Wingenter et al. vs.100CCNcm3 in Vogt et al.). The relative increasein additional CCN enhances cloud albedo by either3Wm2 (Wingenter et al.) and 0.8Wm2 (Vogtet al.), to give estimated temperature decreases of 2and 0.6 1C, respectively.We do not agree that fertilizing 2% of the SO willresult in only to a 2.6% increase in total CCN. Ouroriginal estimate predicts a 20% increase in DMSwould lead to a 10% increase in CCN when applyingan analysis similar to that of Watson and Liss (1998).Wingenter et al. assessed the impact of additionalDMS flux on albedo by following the example ofWatson and Liss and assuming that initially200CCNcm3 came from sea salt and another200 cm3 originated from non-sea salt sulfate aero-sols derived from DMS. A 20% increase in DMSshould result in a 20% more CCN derived fromDMS oxidation (Wingenter et al.), or a 10% increasein the total CCN (from 400 to 440CCNcm3). Vogtet al. suggest another way to treat the impact ofadditional DMS on CCN concentration using thelinear relationship found in Ayers et al. (1997),where the monthly mean DMS mixing ratiosand monthly mean CCN levels are described by[CCN]normalized ¼ 0.52[DMS]normalized+0.48. It is im-portant to realize that by this correlation, a 20%increase in DMS leads to a 10% increase in totalCCN, which is equivalent to a 20% increase in DMS-derived CCN if half of the CCN were derived fromDMS. Fortuitously, this is the same increase in CCNderived in Wingenter et al., using the treatment ofWatson and Liss. However, Vogt et al. using thesame equation came up with only 2.6% increase inCCN for a 2% fertilization of the SO. Half of thiscan be explained by the factor-of-two difference inDMS response to iron fertilization; but it appearsthey applied both Ayers’ equation and Watson andLiss’ treatment. However, Ayers et al. treatmentincludes all CCN, not just non-sea salt sulfateparticles.Using the equation in Ayers et al., instead of atreatment similar to that in Watson and Liss, makesthe initial number and type of CCN irrelevantbecause the cloud susceptibility or thickness isadjusted to the observed cloud albedo (Twomey,1991). However, we continue to examine theconcentration of CCN over the SO because therelationship between sulfate particles, sea-salt CCNand new particle growth may be important in futurestudies. During the First Aerosol CharacterizationExperiment (ACE-1), which used land, ship andaircraft measurements near Tasmania and over theSO from 401S to 551S, new particle formation wasnot observed in the marine boundary layer. Rather,newly nucleated sulfate particles were observed as aresult of cloud processing in regions with low aerosolsurface area, with the source of the sulfate beingDMS (Bates et al., 1998; Clarke et al., 1998). Duringthe experiment, Bates et al. observed about 300 totalCCNcm3on average under background conditions,of which about 270CCN particles were from non-sea salt sulfate and 30 particles were from sea salt.Hudson et al. (1998) also measured total CCN(saturation of 1.2%) of about 220CCNcm3 duringACE-1 south of Tasmania under clean marineconditions. At Cape Grim the average total CCNconcentration under clean marine conditions in thesummer is about 300CCNcm3 (Ayers et al., 1997).However, we note that most of these CCN arenon-sea salt sulfate particles, and none of theseestimates include ultrafine or new nucleated parti-cles, which are also derived from DMS. If we assume300CCNcm3 initially, a 20% increase in DMSflux, and we apply Ayers’ equation, then the finalCCN number would increase from 300 to330CCNcm3. This would result in an albedoincrease from 46%, using the treatment, to 46.8%,or an extra 3 Wm2 reflected off to space—the sameresult as determined in Wingenter et al.It should be noted that there are severaluncertainties in new particle production frommolecular precursors, cloud formation, albedo andcloud microphysics. These are areas of activeresearch that have been ongoing for many years.As noted in Wingenter et al., a large-scale experi-ment should greatly add to our understanding ofaerosol growth and formation in the SO region.Careful, controlled application of our proposal maybe an important test of the CLAW hypothesis.Other major questions related to this proposal arehow long would DMS production continue andhow much additional DMS would be released intothe atmosphere? Does the additional DMS come asa pulse over a few days after iron fertilization orwould the additional DMS flux be spread out over afew weeks? For example, chlorophyll a persistedARTICLE IN PRESSNew Directions / Atmospheric Environment 42 (2008) 4803–48094808during SOIREE for at least 40 days and bothSOFeX North and South iron-enriched patches alsopersisted about 40 days but would DMS productionlast this long?It is difficult to estimate the individual uncertain-ties of each process in our proposal. If each step inour analysis introduces a factor of two uncertainty,then the propagated error would be about a factor offour. Thus, even though the difference in tempera-ture cooling between our estimate (2 1C) vs. that ofVogt et al.’s (0.6 1C) Vogt et al.’s is quantifiable, thefactor of three difference is within the estimateduncertainty. After full-scale modeling and initial fieldexperiments are performed, the amount of areafertilized can be adjusted to achieve the desiredresult. The estimate by Vogt et al. suggests that wewould need to fertilize 6% of the SO to achieve a2 1C temperature decrease over the region. However,we suggest a more conservative approach, starting atthe low end of the range. Furthermore, a 2 1Cdecrease in temperature may be too severe. Thisamount was proposed merely for illustrative pur-poses, a starting point for discussion, whereas a0.5 1C cooling may accomplish the intended effectwith fewer unintended perturbations.Once these questions are better understood andour goals are more clearly recognized, we will knowhow frequently to fertilize with iron and in whatareas. Stabilizing Antarctic ice shelves may be thebest application of limited iron fertilization and mayrequire less iron over a shorter time each season andperhaps in a smaller area closer to Antarctica. Useof fully coupled models would aid in the planningand would precede such a task. We reiterate thatfull-scale iron fertilization for the purpose of carbonsequestration will (i) overcool the region and(ii) would introduce about 20 times more iron tothe region. Any plan to fertilize the SO must firstaddress the issue of overcooling. Because the scalesand goals of the full and limited iron fertilizationproposals are very different, each proposal must beweighed on its own merits and weaknesses. At thispoint we are only advocating modeling studies,which we hope scientific organizations will endorse.Climate change is a serious problem. The enduringsolution will ultimately come when greenhouse gasemissions are controlled. However, to date CO2concentration increases are accelerating. Therefore,we need to explore and evaluate possible ClimateEngineering solutions, which could counter, for theshort term (a few decades), some effects of globalwarming such as sea level rise.AcknowledgmentsWe appreciate the opportunity to clear up somemisunderstandings and the thoughtful commentsand editing by Isobel Simpson. O.W. Wingenter andD.R. Blake were supported by the US NSFChemical Oceanography. S.M. Elliot was supportedby the US DOE OBER Acidic project.ReferencesAyers, G.P., Cainey, J.M., Gillett, R.W., Ivey, J.P., Smith,M.H., Lenton, T., 1997. Atmospheric sulfur and cloudcondensation nuclei in marine air in the southern hemisphere.Philosophical Transactions: Biological Sciences 352 (1350),203–211.Bates, T.S., Kapustin, V.N., Quinn, P.K., Covert, D.S., Coffman,D.J., Mari, C., Durkee, P.A., De Bruyn, W.J., Saltzman, E.S.,1998. Processes controlling the distribution of aerosolparticles in the lower marine boundary layer during the FirstAerosol Characterization Experiment (ACE 1). Journal ofGeophysical Research 103, 16,369–16,383.Clarke, A.D., Varner, J.L., Eisele, F., Mauldin, R.L., Tanner, D.,Litchy, M., 1998. Particle production in the remote marineatmosphere: cloud outflow and subsidence during ACE 1.Journal of Geophysical Research 103, 16,397–16,409.Coale, et al., 2004. Southern Ocean Iron Enrichment Experiment:carbon cycling in high- and low-Si waters. Science 304,408–414.Turner, S.M., Harvey, M.J., Law, C.S., Nightingale, P.D., Liss,P.S., 2004. Iron induced changes in oceanic biogeochemistry.Geophysical Research Letters 31, L14307.Twomey, S., 1991. Aerosols, clouds and radiation. AtmosphericEnvironment 25, 2435–2442.Vogt, M., Vallina, S., von Glasow, R., 2008. A comment toWingenter et al. New Directions: Enhancing the natural cycleto slow global warming, Atmospheric Environment, in press,doi:10.1016/j.atmosenv.2008.04.005.Watson, A.J., Liss, P.S., 1998. Interactions between the marine biotaand climate via the carbon and sulfur biogeochemical cycles.Philosophical Transactions of the Royal Society of B 353,41–51.Wingenter, O.W., Haase, K.B., Strutton, P., Friedrich, G.,Meinardi, S., Blake, D.R., Rowland, F.S., 2004. Changingconcentrations of CO, CH4, C5H8, CH3Br, CH3I, anddimethyl sulfide during the Southern Ocean Iron EnrichmentExperiments. Proceedings of the National Academy ofSciences 101, 8537–8541.Wingenter, O.W., Elliot, S.M., Blake, DR., 2007. New Direc-tions: enhancing the natural sulfur cycle to slow globalwarming. Atmospheric Environment 41, 7373–7375.Oliver W. WingenterDepartment of Chemistry and Geophysical ResearchCenter, New Mexico Institute of Mining andTechnology, Socorro, NM 87801, USAE-mail address: oliver@nmt.eduARTICLE IN PRESSNew Directions / Atmospheric Environment 42 (2008) 4803–4809 4809Scott 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, USAdoi:10.1016/j.atmosenv.2008.05.003

Authors response to the above comment by M. Vogt et al. on “New Directions: Enhancing the natural cycle to slow global warming”

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Authors response to the above comment by M. Vogt et al. on "New Directions: Enhancing the natural cycle to slow global warming"

Authors response to the above comment by M. Vogt et al. on "New Directions: Enhancing the natural cycle to slow global warming"

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