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sary amount of sulfur injection would need GEOENGINEERING to be estimated according to comprehensive forecast models, requiring extensive mod- eling capabilities. The total cost of SAM A cirrus dial? would also need to include compensation for potential side effects and would thus be seeding may help to reduce climate warming, much higher than the delivery costs (16). Currently, a single person, company, or but large uncertainties remain state may be able to deploy SAM without in- depth assessments of the risks, potentially By Ulrike Lohmann and Blaž Gasparini greenhouse gases. Their long-wave warm- causing global impacts that could rapidly ing (greenhouse) effect prevails over their lead to confl ict. As such, it is essential that limate engineering is a potential reflected solar radiation (cooling) effect, international agreements are reached to means to offset the climate warming causing a net-positive radiative effect of 5 regulate whether and how SAM should be caused by anthropogenic greenhouse to 6 W m–2 in the present-day climate (2, 3). implemented (3). A liability regime would gases. Suggested methods broadly fall However, the long-wave radiative effect of rapidly become essential to resolve confl icts, into two categories. Methods in the cirrus depends on their optical depth especially because existing international li- first category aim to remove carbon and on the at which they form. Two ability rules do not provide equitable and dioxide (CO ) from the atmosphere, whereas cirrus clouds of the same optical thickness C 2 ef ective compensation for potential SAM those in the second aim to alter ’s radia- formed at different will have differ- damage (17). Such complexities will require tion balance. The most prominent and best ent radiative effects. The higher and there- the establishment of international gover- researched approach in fore colder one will have a larger warming Downloaded from nance of climate intervention, overseeing re- the second category is the injection of atmo- effect on climate because of the larger tem- search with frequent assessments of benefi ts spheric aerosol particles or their precursor perature difference to the mid- and side ef ects. gases into the (1), where these from where cloud-free radiation is emitted Climate intervention should only be seen particles reflect solar radiation back to space. to space. In a warmer climate, cirrus clouds as a supplement and not a replacement for Climate engineering through cirrus cloud will form at higher altitudes, increasing their

greenhouse gas mitigation and decarboniza- thinning, in contrast, mainly targets the long- warming effect (4). http://science.sciencemag.org/ tion ef orts because the necessary level and wave radiation that is emitted from Earth. The effect of would application time of SAM would continuously Wispy, thin, and often hardly visible to be opposite in sign to that of in grow with the need for more cooling to coun- the human , cirrus clouds do not reflect present-day climate. Because of increases in teract increasing greenhouse gas concentra- a lot of solar radiation back to space. Be- air traffic, contrails and the associated in- tions. A sudden disruption of SAM would cause they form at high altitudes and cold crease in cirrus caused a positive cause an extremely fast increase in global temperatures, cirrus clouds emit less long- (climate warming) of 0.05 temperature. Also, SAM does not ameliorate wave radiation to space than does a cloud- W m–2 between 1750 and 2011. This forcing is

major consequences of the CO2 increase in free atmosphere. The climate impact of negligible in magnitude compared with the the atmosphere, such as ocean acidifi cation, cirrus clouds is therefore similar to that of radiative forcing of 2.83 W m–2 from green- j

which would continue to worsen. on July 22, 2017

REFERENCES AND NOTES 1. R. Joeri et al., Nature 534, 631639 (2016). How seeded cirrus clouds could cool the climate 2. K. Anderson, G. Peters, Science 354, 182 (2016). Cirrus clouds reflect some and absorb long-wave radiation; on balance, they warm the climate. 3. J. Pasztor, Science 357, 231 (2017). Cirrus cloud thinning aims to change the radiative properties of cirrus clouds by reducing their lifetime a nd 4. P. J. Crutzen, Clim. Change 77, 211 (2006). the altitude at which they form. 5. U. Lohmann, B. Gasparini, Science 357, 248 (2017). 6. A. Robock, Earth’s Future 4, 644 (2016). 7. U. Niemeier et al., J. Geophys. Res. 118, 11905 (2013). Unperturbed cirrus cloud Ice Cirrus cloud thinning Large crystals form 8. M. J. Mills et al., J. Geophys. Res. 121, 2332 (2016). on ice-nucleating 9. U. Niemeier, C. Timmreck, Atmos. Chem. Phys. 15, 9129 Solar particles (2015). radiation 10. R. Moriyama et al., Mitig. Adapt. Strat. Global Change 21, 1 (2016). 11. D. Visioni, G. Pitari, V. Aquila, Atmos. Chem. Phys. 17, 3879 (2017). 12. S. Tilmes, R. Müller, R. Salawitch, Science 320, 1201 (2008). 13. D. Keith, D. K. Weisenstein, J. A. Dykemaa, F. N. Keutsch, Homogeneous Ice-nucleating Heterogeneous Proc. Natl. Acad. Sci. U.S.A. 113, 14910 (2016). cirrus cloud particles cirrus cloud 14. J. Dykema, D. Keith, J. G. Anderson, D. Weisenstein, Philos. Trans. R. Soc. A 372, 20140059 (2014). 15. S. Tilmes, B. M. Sanderson, B. C. O’Neill, Geophys. Res. Long-wave Lett. 43, 8222 (2016). radiation 16. J. Reynolds, A. Parker, P. Irvine, Earth’s Future 4, 562 (2016). 17. B. Saxler et al., Law Innov. Technol. 7, 112 (2015). Temperature C IE NCE ACKNOWLEDGMENTS S We thank B. Sanderson, Y. Richter, and H. Schmidt for very valu- Ó N / able comments and A. Jones and C. Kleinschmidt for providing U L GR data for the figure. . A P H I C : G

10.1126/science.aan3317 GR

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DA_0721Perspectives.indd 248 7/19/17 11:24 AM house gases (5) but is of the same sign. Cirrus alistically, the upper limit of cirrus seeding if so, whether this enhances or dampens the cloud thinning tries to achieve the opposite is given by the difference in radiation be- effect of cirrus thinning (13). by reducing the coverage and optical thick- tween a climate model simulation in which If the time and place of seeding is selected ness of cirrus clouds. all cirrus clouds freeze homogeneously and with care, the climate effect of cirrus thin- Cirrus clouds frequently form through one in which all cirrus clouds form through ning can be enhanced. For that, only the homogeneous nucleation of liquid aerosol heterogeneous nucleation. This difference is long-wave warming effect of cirrus clouds particles such as sulfuric or nitric acid. Al- on the order of 2 to 3 W m–2 (8, 9), which cor- should be targeted, and their solar effect

ternatively, they can form through hetero- responds to 50 to 80% of a doubling of CO2 should be avoided. This can be achieved if geneous nucleation with the help of solid (5). Yet, even this is an overestimate, given seeding is limited to high-latitude winters aerosol particles such as desert , pollen, that already in the present-day climate, cir- (11) or to nighttime seeding (13). Contrary to or other biological particles, which act as ice- rus clouds form both through homogeneous solar radiation management methods, cirrus nucleating particles (INPs). The cirrus cloud and heterogeneous nucleation (10). seeding is more effective at high than at low thinning concept is based on the assumption One problem with cirrus seeding is over- latitudes. A small-scale deployment of cirrus that most cirrus clouds in the present climate seeding, which occurs if too many INPs are seeding could therefore be envisioned—for nucleate homogeneously. Homogeneously injected. In overseeding, the cirrus clouds instance, in the to avoid further melt- formed cirrus clouds occur in regions that become optically thicker, leading to warm- ing of Arctic sea ice. Governance of such are cold and/or have high relative humidities. ing. Current models give different critical local climate engineering might be easier If suitable INPs are injected into cirrus lev- INP concentrations (INP*) at which over- to achieve than for solar radiation manage- els, cirrus clouds would form via nucleation seeding starts (2, 11, 12). INP* depends on ment, especially if substantial climate effects on these INPs at lower relative humidity and the fraction of cirrus clouds that nucleates outside the targeted region could be avoided. higher temperature, preventing the forma- The climatic side effects of cirrus thinning Downloaded from tion of cirrus clouds through homogeneous have yet to be explored in larger detail. Inten- (6). Paradoxically, seeding cirrus sification of tropical and changes clouds in this way decreases their radiative “If cirrus thinning works, in the tropical circulation seem likely if cir- impact and their lifetime, therefore reducing it should be preferred over rus thinning is applied globally. Ideal target the overall warming effect compared with regions, seeding frequency, and transport

that of “natural” cirrus clouds. methods that target changes mechanisms of seeded INPs also remain http://science.sciencemag.org/ The cooling ef ect of seeded cirrus clouds in solar radiation...” to be identified. Only after these questions has three contributions. First, the cirrus are addressed could one move further to ex- clouds form at lower relative humidities that plore the costs and feasibility of cirrus cloud occur at lower altitudes in the atmosphere heterogeneously in the present climate (11) thinning. It is also important to remember (see the fi gure), where they have a smaller as well as on the chemical composition (12) that, like solar radiation management, cirrus

warming ef ect. Second, because the number and the size of the injected INPs (13). In ad- thinning cannot prevent the CO2 increase concentration of INPs is much lower than dition, seeding needs to be avoided in cloud- in the atmosphere and the resulting ocean that of solution droplets, heterogeneously free regions with high relative humidities acidification. For the time being, cirrus cloud formed cirrus clouds contain fewer ice crys- where no cirrus clouds form. Here, seeding thinning should be viewed as a thought ex-

tals. These can grow to larger sizes with INPs could lead to cirrus clouds that periment that is helping to understand cir- on July 22, 2017 and sediment more readily from cirrus levels, cause a warming effect on the climate, same rus cloud–formation mechanisms. j reducing the lifetime and optical thickness of as that from contrails. cirrus clouds and hence their warming poten- Unintended cirrus formation is especially REFERENCES AND NOTES 1. U. Niemeier, S. Tilmes, Science 357, 246 (2017). tial. Third, sedimenting ice crystals remove pronounced if the seeded INPs start to nu- 2. B. Gasparini, U. Lohmann, J. Geophys. Res. 121, 4877 , the most important natural cleate ice at very low relative humidities (2). (2016). 3. Y. L. Hong, G. S. Liu, J. L. F. Li, J. Climate 29, 7651 (2016). greenhouse gas, from the upper troposphere. Because of the competition of these various 4. O. Boucher et al., in 2013: The Physical If cirrus thinning works, it should be pre- factors, the radiative forcing of cirrus cloud Science Basis. Contribution of Working Group I to the Fifth ferred over methods that target changes in seeding varies between –1.8 and +2.1 W m–2. Assessment Report of the Intergovernmental Panel on Climate Change, T. F. Stocker et al., Eds. (Cambridge Univ. solar radiation, such as stratospheric aero- Thus, if cirrus seeding is not done carefully, Press, 2013), pp. 571–657. sol injections, because cirrus thinning would the effect could be additional warming rather 5. T. F. Stocker et al., in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth counteract greenhouse gas warming more di- than the intended cooling. If done carefully, Assessment Report of the Intergovernmental Panel on rectly. Solar radiation management methods the negative radiative effect from cirrus seed- Climate Change, T. F. Stocker et al., Eds. (Cambridge Univ. cannot simultaneously restore temperature ing should be stronger in a warmer climate, Press, 2013), pp. 33–115. 6. D. L. Mitchell, W. Finnegan, Env. Res. Lett. 4, 045102 and at present-day levels but in which the overall radiative effect of cirrus (2009). lead to a reduction in global mean precipita- clouds will be larger. 7. J. E. Kristjansson, H. Muri, H. Schmidt, Geophys. Res. Lett. 42, 10807 (2015). tion because of the decreased solar radiation The results from model studies of cirrus 8. U. Lohmann, P. Spichtinger, S. Jess, T. Peter, H. Smit, Env. at the surface. This adverse ef ect on precipi- thinning suggest that the perfect seeding Res. Lett. 3, 10.1088/1748 (2008). tation is minimized for cirrus seeding (7) be- INPs should be large and that seeding could 9. T. Storelvmo et al., Geophys. Res. Lett. 40, 178 (2013). 10. D. J. Cziczo et al., Science 340, 1320 (2013). cause of the smaller change in solar radiation. be geographically or temporally limited. Bis- 11. T. Storelvmo, N. Herger, J. Geophys. Res. 119, 2375 (2014). The maximum cirrus seeding potential muth triiodide (BiI ) has been suggested as a 12. J. E. Penner, C. Zhou, X. Liu, Geophys. Res. Lett. 42, 8775 3 (2015). would be achieved by removing all cirrus nontoxic and affordable substance for cirrus 13. B. Gasparini, S. Münch, L. Poncet, M. Feldmann, clouds. The resulting radiative forcing of 5 to seeding (6); other substances such as min- U. Lohmann, Atmos. Chem. Phys. 17, 4871 (2017). 6 W m–2 (2, 3) would more than offset the eral dust should work as well. However, fur- radiative forcing of a CO doubling. More re- ther research is needed to investigate which ACKNOWLEDGMENTS 2 We thank S. Münch and the anonymous reviewers for useful particles would be good seeding agents. It is comments and discussions. ETH Zurich, Institute of Atmospheric and Climate Science, also important to determine whether these Switzerland. Email: [email protected] INPs also influence lower-lying clouds, and 10.1126/science.aan3325

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DA_0721Perspectives.indd 249 7/19/17 11:24 AM A cirrus cloud climate dial? Ulrike Lohmann and Blaz Gasparini

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