PERSPECTIVES

Such ion gels have shown synthesis of almost any desired architecture, superior performance as gate Self-assembly. Block polymers with almost unlimited choice of monomers, can self-assemble into (top) dielectrics in organic thin-film such that tailored multiblock polymers can be hexagonally packed cylinders transistors (5). For many organic or (bottom) the double gyroid. readily produced. The outstanding challenges semiconductors, device perfor- The ionic liquid is confined to are to design block polymer–ionic liquid com- mance is constrained by the num- the white channels, whereas posite materials with desired combinations of ber of charge carriers rather than the red matrix consists of the mechanical integrity, controlled nanostruc- their mobility; the high capaci- insoluble block. ture, and ionic liquid properties. This class of tance of the ion gel boosts the materials is only beginning to be explored and carrier density in the semicon- remains difficult to achieve many design rules are yet to be mapped out in ductor channel. Further, the high macroscopic orientation and detail, but rapid progress can be expected ionic mobility enables switching perfection of the resulting because related polymer-based technologies speeds that are orders of magni- membrane, which is impor- are well established. tude higher than with conven- tant for some applications. tional polymer electrolytes (6). Alternatively, a network stru- References Similar ion gels could form cture such as the double 1. T. Ueki, M. Watanabe, Macromolecules 41, 3739 (2008). 2. M. A. B. H. Susan, T. Kaneko, A. Noda, M. Watanabe, J. the basis of electromechanical gyroid is isotropic, obviating Am. Chem. Soc. 127, 4976 (2005). actuators (7, 8); differential ion the need for orientation (see 3. Y. He, P. G. Boswell, P. Bühlmann, T. P. Lodge, J. Phys. migration in response to an applied electric the second figure, bottom panel). Unfor- Chem. B 111, 4645 (2007). 4. Y. He, T. P. Lodge, Macromolecules 41, 167 (2008). field leads to differential gel swelling and tunately, this structure can only be achieved 5. J. Lee, M. J. Panzer, Y. He, T. P. Lodge, C. D. Frisbie, J. Am. thus to bending. A possible route to accentu- under limited combinations of copolymer Chem. Soc. 129, 4532 (2007). ate this effect would be to polymerize the compositions, molar masses, and processing 6. J. H. Cho et al., Adv. Mater. 20, 686 (2008). 7. W. Lu et al., Science 297, 983 (2002). cations into the B blocks, thus immobilizing conditions. Use of multiblock polymers, such 8. J. Ding et al., Chem. Mater. 15, 2392 (2003).

some or all of that charge (9, 10); the much as ABC terpolymers, allows network materi- 9. J. E. Bara et al., Ind. Eng. Chem. 46, 5397 (2007). on July 3, 2008 more mobile anions could then generate a als to be prepared over much wider ranges of 10. J. Tang et al., Chem. Commun. 2005, 3325 (2005). highly asymmetric swelling. molecular variables and with greatly en- 11. P. M. Simone, T. P. Lodge, Macromolecules 41, 1753 (2008). The same molecular architecture also hanced mechanical strength (12). 12. T. H. Epps III et al., Macromolecules 37, 7085 (2004). holds promise for gas separation. Ionic liquids Recent progress in the development of

strongly prefer to dissolve CO2 and SO2 over, controlled polymerization has enabled the 10.1126/science.1159652 for example, N2 and CH4. Because transport through an ionic liquid is so facile, it is possi- ble to achieve combinations of selectivity and

OCEANS www.sciencemag.org throughput comparable to those of the best materials currently available. However, a functional gas separation membrane must withstand a substantial pressure drop. The Emissions and Acidification ionic liquid could be literally blown out of the Richard E. Zeebe1*, James C. Zachos,2 Ken Caldeira,3 Toby Tyrrell4 simple ion gel. A polymerized ion gel should not suffer from this drawback, because the Avoiding environmental damage from requires reductions in

attraction between ions would far outweigh emissions regardless of climate change. Downloaded from the external pressure. Direct polymerization

of organic cations has recently been achieved uch of the scientific and public atmospheric CO2 considerably, thus alleviat- (9, 10). By incorporating an appropriate focus on anthropogenic carbon ing climate change caused by anthropogenic difunctional monomer, Bara et al. have pre- dioxide (CO ) emissions has been greenhouse gas emissions. But it also alters M 2 pared and evaluated cross-linked films for the on climate impacts. Emission targets have ocean chemistry, with potentially serious con-

separation of CO2 from CH4 or N2 (9), with been suggested based primarily on arguments sequences for marine life (1). promising results. for preventing climate from shifting signifi- Oceanic uptake of anthropogenic CO2 Applications to other technologies such as cantly from its preindustrial state. However, leads to a decrease in seawater pH and thus fuel cell membranes and lithium battery sepa- recent studies underline a second major lowers the saturation state for carbonate min-

rators often require much greater mechanical impact of carbon emissions: ocean acidifica- erals such as calcite and aragonite (CaCO3). rigidity and high-temperature stability while tion. Over the past 200 years, the oceans have This process, termed ocean acidification, is

retaining high ionic mobility along a given taken up ~40% of the anthropogenic CO2 expected to have detrimental consequences axis. Here, the ability of block polymers to emissions. This uptake slows the rise in for a variety of marine organisms (2, 3). For self-assemble into well-defined nanostruc- example, a decline in carbonate saturation tures with long-range order holds the key (11). state will affect the stability and likely produc- 1Department of Oceanography, University of Hawaii, For example, macroscopic orientation of Honolulu, HI 96822, USA. 2Earth and Planetary Sciences tion rates of CaCO3 minerals, which are the block polymer cylinders has been achieved by Department, University of California at Santa Cruz, Santa building blocks of coral reefs and form the various strategies, including application of Cruz, CA 95060, USA. 3Department of Global Ecology, shells and skeletons of other marine calcify- flow fields and electric fields and by prepara- Carnegie Institution, Stanford, CA 94305, USA. 4National ing species. Independent of climatic consider- Oceanography Centre, Southampton University, South- tion of suitably treated underlying substrates ampton, SO14 3ZH, UK. *To whom correspondence should ations, carbon emissions must be reduced to

CREDIT: ADAPTED FROM TIMOTHY P. LODGE ADAPTED FROM TIMOTHY P. CREDIT: (see the second figure, top panel) . However, it be addressed. E-mail: [email protected] avoid these consequences.

www.sciencemag.org SCIENCE VOL 321 4 JULY 2008 51 Published by AAAS PERSPECTIVES

The range of tolerable pH 1000 data and field observations. Most changes (where “tolerable” means 0.15 studies indicate reduced calcifica- 900 0.2 0.25 − “without substantial detriment tion rates at high CO2 (2 8, 11), but to organism fitness”) is as yet 0.3 some find little change or increasing 800 0.35 unknown for many marine organ- 0.4 cellular calcification rates (9, 15). isms. In laboratory and mesocosm 700 0.45 The key is to understand the re- studies, a decrease of 0.2 to 0.3 sponse of the functional groups that 0.5 units in seawater pH inhibits or 600 drive marine biogeochemical cycles. slows calcification in many marine 0.55 For instance, very few studies have organisms, including corals (4), 500 0.6 examined foraminifera (5), which Release time (years) foraminifera (5), and some calcare- 0.65 are major contributors to the produc- ous plankton (6, 7). A drop of 0.3 400 tion and deposition of calcium car- pH units corresponds to a doubling 0.7 bonate in the ocean. Also, long-term of the hydrogen ion concentration 300 experiments are necessary with dif- 0.75 [H+], because pH is expressed on ferent calcifying and non-calcifying 200 a logarithmic scale as pH = 600 1000 2000 3000 4000 5000 groups to test their ability to adapt to − + log([H ]). emissions (Pg) a high–CO2 world. Compared with preindustrial To monitor and quantify future Surface ocean pH decline. The white contour lines illustrate the expected levels, average surface ocean pH maximum pH decrease of average surface ocean waters in the future (in pH changes in ocean chemistry and has already decreased by ~0.1 units) as a function of total anthropogenic CO emissions (in petagrams of biogeochemical fluxes, intensified 2 units (2). If future increases in sea- carbon, 1 Pg = 1015 g) and release time (in years, see supporting online global-ocean carbon dioxide surveys water acidity affect calcification in material). For example, if humans release a total of 1200 Pg C over 1000 in combination with carbon-cycle coral reefs, such that erosion out- years, surface ocean pH will drop by about 0.2 units (arrow). modeling will be necessary. Aware-

weighs accretion, then the reefs ness must be raised among the public on July 3, 2008 could lose structural stability, with further 13). Fortunately, and in contrast to climate and policy-makers of the effects of ocean acid- negative implications for reef communities model predictions, such future ocean chem- ification and the steps required to control it. and for shore protection (3). Reduced calcifi- istry projections are largely model-independ- Ocean chemistry changes, and not only climate cation in shellfish such as oysters and mus- ent on a time scale of a few centuries, mainly effects, should be taken into consideration

sels would impact worldwide commercial because the chemistry of CO2 in seawater is when determining CO2 emission targets; such aquaculture production (8). Effects of ocean well known (14) and changes in surface ocean consideration is likely to weigh in favor of acidification on noncalcifying organisms carbonate chemistry closely track changes in lower emission targets.

such as viruses and bacteria are largely atmospheric CO2. Predicted changes in sur- unknown, as are potential consequences for face ocean pH for given total emissions and References www.sciencemag.org marine food webs. release time of anthropogenic carbon are thus 1. K. Caldeira, M. E. Wickett, Nature 425, 365 (2003). 2. J. Raven et al., Ocean Acidification Due to Increasing Thus, although the response of different similar among different types of models over Atmospheric Carbon Dioxide (The Royal Society, Policy organisms is expected to be inhomogeneous the next few centuries [see supporting online Document, London, UK, 2005). (9), current evidence suggests that large and material and (1, 11−13)]. 3. O. Hoegh-Guldberg et al., Science 318, 1737 (2007). 4. J. A. Kleypas et al., Science 284, 118 (1999). rapid changes in ocean pH will have adverse Projected changes in ocean carbonate 5. J. Bijma, H. J. Spero, D. W. Lea, Use of Proxies in Paleo- effects on a number of marine organisms. Yet, chemistry should serve as a guideline for pol- ceanography: Examples from the South Atlantic, G. Fischer,

G. Wefer, Eds. (Springer, Berlin, 1999), pp. 489–512. Downloaded from environmental standards for tolerable pH icy protocols that identify CO2 emission tar- changes have not been updated in decades. For gets to reduce the effects of human-made 6. D. A. Wolf-Gladrow, U. Riebesell, S. Burkhardt, J. Bijma, Tellus B 51, 461 (1999). example, the seawater quality criteria of the ocean acidification. For example, to avoid a 7. U. Riebesell et al., Nature 407, 364 (2000). U.S. Environmental Protection Agency date surface ocean pH decline by more than 0.2 8. F. Gazeau, C. Quiblier, J. M. Jansen, J.-P. Gatusso, back to 1976 and state that for marine aquatic units (10), total emission targets would have to Geophys. Res. Lett. 34, L07603 (2007). 9. G. Langer et al., Geochem. Geophys. Geosyst. 7, Q09006 life, pH should not be changed by more than 0.2 range from ~700 Pg C over 200 years to ~1200 (2006). units outside of the normally occurring range Pg C over 1000 years (see the figure). Such 10. U.S. Environmental Protection Agency, Quality Criteria (10). These standards must be reevaluated scenarios would be difficult to achieve, how- for Water (Washington, DC, 1976). based on the latest research on pH effects on ever, because they require immediate reduc- 11. J. C. Orr et al., Nature 437, 681 (2005). 12. A. Montenegro, V. Brovkin, M. Eby, D. Archer, A. J. marine organisms. Once new ranges of tolera- tions in global emissions. If emissions can be Weaver, Geophys. Res. Lett. 34, L19707 (2007). ble pH are adopted, CO2 emission targets must reduced after the year 2050 and capped at 1500 13. J. C. Zachos, G. R. Dickens, R. E. Zeebe, Nature 451, 279 be established to meet those requirements in Pg C, surface ocean pH would decline by (2008). 14. R. E. Zeebe, D. A. Wolf-Gladrow, CO in Seawater: terms of future seawater chemistry changes. ~0.35 units relative to preindustrial levels. The 2 Equilibrium, Kinetics, Isotopes (Elsevier Oceanography The key variables that will determine aragonite saturation state in the warm surface Series, Amsterdam, 2001). the extent of future seawater chemistry ocean would drop from ~3.5 to ~2.1 under this 15. M. D. Iglesias-Rodriguez et al., Science 320, 336 (2008). changes—and therefore the impact on marine scenario (see supporting online material). 16. This work was supported by NSF grant NSFOCE07-51959 life—is the magnitude and time scale of the Substantial reductions in coral calcification to R.E.Z. − anthropogenic carbon release. For specific have been reported over this range (2 4). Supporting Online Material CO2 emission scenarios, changes in atmo- Predictions of changes in calcification www.sciencemag.org/cgi/content/full/321/5885/51/DC1 Figs. S1 and S2 spheric CO2, surface ocean pH, and carbonate rates and other physiological responses in mineral saturation state have been calculated marine organisms to ocean acidification are References with different models (1, 11− hampered by a lack of adequate experimental 10.1126/science.1159124

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