INSIGHTS | P E R S P E C T I V E S
Why the Pacific is cool Natural ocean variability modulates global warming By Ben B. Booth
A
processes behind natural and external drivers of SST variability in both the Atlantic and the Pacific must be considered in projections of future climate change. The results offer an opportunity to revisit how we talk about SST variability. Fifteen years ago, Richard Kerr coined the phrase “Atlantic multidecadal oscillation” (AMO) to
aerosols and clouds (for which Steinman et al. include the “CMIP5-AIE” best estimate) and the influence of a cluster of tropospheric volcanic eruptions in the past 10 years that is not accounted for in current climate simulations (and hence lies outside Steinman et al.’s analysis). Even for those models that include aerosol-cloud interactions, ocean variability estimates are subject to uncertainty due to the way in which these interactions are modeled. Current models may also miss or inadequately represent key mechanisms. For example, recent studies have linked the cool Pacific conditions identified in (1, 2) to a strengthening of the trade winds (12) that is
fter a period of rapid global warming, the rate of global temperature rise has slowed markedly in the past 10 to 15 years. Is this “hiatus” a result of natural climate variability, or does it signify a change in the drivers of global warming? On page 988 of this issue, Steinman et al. (1) present time-series estiOcean-driven temperature variability mates of Atlantic and Pacific variability from state-of-the-art climate modeling. They show 2012 2012 that in the past 130 years, periods of natural variability both in the Atlantic and Pacific have at times enhanced or counteracted the underlying global warming 1950 1950 trend. The results support the conclusion that cool Pacific temperatures have played a key role in modulating atmospheric temperature increases in the past 10 1880 1880 years (2), only partially offset by modest warming in the Atlantic. An established literature links Natural variability. On the basis of current climate simulations and observed temperature records, Steinman et al. estimate the role observed sea surface temperathat natural ocean variability has played in modulating temperature trends over the past 130 years. ture (SST) variability—mainly in the Atlantic—to substantial decadal-scale describe the apparent 40- to 80-year oscilunprecedented in the observational record shifts in drought, precipitation, temperalation in observed North Atlantic SSTs (9). (13). Nevertheless, this wider context does ture extremes, and the frequency of tropiAt the time, the observed variability and not take away from the value of Steinman cal storms. For example, comparatively cool natural ocean variability were taken to be et al.’s study, which provides much-needed temperatures in the Atlantic during the synonymous. Since then, research has idenlonger-term context for the role that natural 1960s and 1970s were linked to prolonged tified industrial and volcanic aerosols and ocean-driven variations have played in past African drought (3, 4), whereas warmer temsolar changes as drivers of observed SST climate change. ■ peratures in the past 20 years have been asvariations (7, 8, 10). This has led to confuREFERENCES sociated with more Atlantic tropical storms sion, with papers either referring to the total 1. B. A. Steinman, M. E. Mann, S. K. Miller, Science 347, 988 (5) and a propensity for drought in the cenSST variability (both natural and externally (2015). tral United States (6) and northeast Brazil driven) as the AMO (8) or only the natural 2. Y. Kosaka, S.-P. Xie, Nature 501, 403 (2013). 3. M. J. Hoerling, J. Hurrell, J. Eischeid, J. Phillips, J. Clim. 19, (4). Steinman et al.’s study points to at what ocean component as the AMO (as done in 3989 (2006). times these observed changes can be attribSteinman et al.). There is a real need in the 4. J. R. Knight, C. K. Folland, A. A. Scaife, Geophys. Res. Lett. uted to “natural” climate variations rather community to agree on common terms to 33, L17706 (2006). 5. S. B. Goldenberg, C. W. Landsea, A. M. Mestas-Nunez, W. M. than human and volcanic activities. distinguish the two. One possibility would Gray, Science 293, 474 (2001). The analysis suggests that much of the be to use Atlantic multidecadal variability 6. G. J. McCabe, M. A. Palecki, J. L. Betancourt, Proc. Natl. Atlantic variations in the first half of the (AMV) to refer to the total temperature variAcad. Sci. U.S.A. 101, 4136 (2004). 7. B. B. Booth, N. J. Dunstone, P. R. Halloran, T. Andrews, N. 20th century arose from ocean variability. In ability and AMO for the natural component. Bellouin, Nature 484, 228 (2012). contrast, during the second half of the cenSteinman et al. provide insight into the 8. M. F. Knudsen, B. H. Jacobsen, M. S. Seidenkrantz, J. Olsen, tury, more of the observed Atlantic decadal natural ocean contributions—but only if curNat. Commun. 5, 3323 (2014). 9. R. A. Kerr, Science 288, 1984 (2000). variations could be explained as responses to rent climate models correctly represent both 10. T. Wang, O. H. Otterå, Y. Gao, H. Wang, Clim. Dyn. 39, 2917 volcanic and industrial aerosols (7, 8), with the external drivers of past climate and the (2012). a smaller contribution from ocean-driven climate responses to them. There are rea11. G. A. Schmidt, D. T. Shindell, K. Tsigaridis, Nat. Geosci. 7, 158 (2014). changes. The analysis makes it clear that the sons for being cautious on both fronts. For 12. M. Watanabe et al., Nat. Clim. Change 4, 893 (2014). example, Schmidt et al. (11) identify two cli13. M. H. England et al., Nat. Clim. Change 4, 222 (2014). mate-forcing factors missing from many curMet Ofce Hadley Centre, Exeter EX1 3PB, UK. E-mail: [email protected] 10.1126/science.aaa6113 rent climate simulations: the interactions of 952
sciencemag.org SCIENCE
27 FEBRUARY 2015 • VOL 347 ISSUE 6225
Published by AAAS
Downloaded from www.sciencemag.org on February 26, 2015
CLIMATE CHANGE
Why the Pacific is cool Ben B. Booth Science 347, 952 (2015); DOI: 10.1126/science.aaa4840
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Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2015 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
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Why the Pacific is cool Natural ocean variability modulates global warming By Ben B. Booth
A
processes behind natural and external drivers of SST variability in both the Atlantic and the Pacific must be considered in projections of future climate change. The results offer an opportunity to revisit how we talk about SST variability. Fifteen years ago, Richard Kerr coined the phrase “Atlantic multidecadal oscillation” (AMO) to
aerosols and clouds (for which Steinman et al. include the “CMIP5-AIE” best estimate) and the influence of a cluster of tropospheric volcanic eruptions in the past 10 years that is not accounted for in current climate simulations (and hence lies outside Steinman et al.’s analysis). Even for those models that include aerosol-cloud interactions, ocean variability estimates are subject to uncertainty due to the way in which these interactions are modeled. Current models may also miss or inadequately represent key mechanisms. For example, recent studies have linked the cool Pacific conditions identified in (1, 2) to a strengthening of the trade winds (12) that is
fter a period of rapid global warming, the rate of global temperature rise has slowed markedly in the past 10 to 15 years. Is this “hiatus” a result of natural climate variability, or does it signify a change in the drivers of global warming? On page 988 of this issue, Steinman et al. (1) present time-series estiOcean-driven temperature variability mates of Atlantic and Pacific variability from state-of-the-art climate modeling. They show 2012 2012 that in the past 130 years, periods of natural variability both in the Atlantic and Pacific have at times enhanced or counteracted the underlying global warming 1950 1950 trend. The results support the conclusion that cool Pacific temperatures have played a key role in modulating atmospheric temperature increases in the past 10 1880 1880 years (2), only partially offset by modest warming in the Atlantic. An established literature links Natural variability. On the basis of current climate simulations and observed temperature records, Steinman et al. estimate the role observed sea surface temperathat natural ocean variability has played in modulating temperature trends over the past 130 years. ture (SST) variability—mainly in the Atlantic—to substantial decadal-scale describe the apparent 40- to 80-year oscilunprecedented in the observational record shifts in drought, precipitation, temperalation in observed North Atlantic SSTs (9). (13). Nevertheless, this wider context does ture extremes, and the frequency of tropiAt the time, the observed variability and not take away from the value of Steinman cal storms. For example, comparatively cool natural ocean variability were taken to be et al.’s study, which provides much-needed temperatures in the Atlantic during the synonymous. Since then, research has idenlonger-term context for the role that natural 1960s and 1970s were linked to prolonged tified industrial and volcanic aerosols and ocean-driven variations have played in past African drought (3, 4), whereas warmer temsolar changes as drivers of observed SST climate change. ■ peratures in the past 20 years have been asvariations (7, 8, 10). This has led to confuREFERENCES sociated with more Atlantic tropical storms sion, with papers either referring to the total 1. B. A. Steinman, M. E. Mann, S. K. Miller, Science 347, 988 (5) and a propensity for drought in the cenSST variability (both natural and externally (2015). tral United States (6) and northeast Brazil driven) as the AMO (8) or only the natural 2. Y. Kosaka, S.-P. Xie, Nature 501, 403 (2013). 3. M. J. Hoerling, J. Hurrell, J. Eischeid, J. Phillips, J. Clim. 19, (4). Steinman et al.’s study points to at what ocean component as the AMO (as done in 3989 (2006). times these observed changes can be attribSteinman et al.). There is a real need in the 4. J. R. Knight, C. K. Folland, A. A. Scaife, Geophys. Res. Lett. uted to “natural” climate variations rather community to agree on common terms to 33, L17706 (2006). 5. S. B. Goldenberg, C. W. Landsea, A. M. Mestas-Nunez, W. M. than human and volcanic activities. distinguish the two. One possibility would Gray, Science 293, 474 (2001). The analysis suggests that much of the be to use Atlantic multidecadal variability 6. G. J. McCabe, M. A. Palecki, J. L. Betancourt, Proc. Natl. Atlantic variations in the first half of the (AMV) to refer to the total temperature variAcad. Sci. U.S.A. 101, 4136 (2004). 7. B. B. Booth, N. J. Dunstone, P. R. Halloran, T. Andrews, N. 20th century arose from ocean variability. In ability and AMO for the natural component. Bellouin, Nature 484, 228 (2012). contrast, during the second half of the cenSteinman et al. provide insight into the 8. M. F. Knudsen, B. H. Jacobsen, M. S. Seidenkrantz, J. Olsen, tury, more of the observed Atlantic decadal natural ocean contributions—but only if curNat. Commun. 5, 3323 (2014). 9. R. A. Kerr, Science 288, 1984 (2000). variations could be explained as responses to rent climate models correctly represent both 10. T. Wang, O. H. Otterå, Y. Gao, H. Wang, Clim. Dyn. 39, 2917 volcanic and industrial aerosols (7, 8), with the external drivers of past climate and the (2012). a smaller contribution from ocean-driven climate responses to them. There are rea11. G. A. Schmidt, D. T. Shindell, K. Tsigaridis, Nat. Geosci. 7, 158 (2014). changes. The analysis makes it clear that the sons for being cautious on both fronts. For 12. M. Watanabe et al., Nat. Clim. Change 4, 893 (2014). example, Schmidt et al. (11) identify two cli13. M. H. England et al., Nat. Clim. Change 4, 222 (2014). mate-forcing factors missing from many curMet Ofce Hadley Centre, Exeter EX1 3PB, UK. E-mail: [email protected] 10.1126/science.aaa6113 rent climate simulations: the interactions of 952
sciencemag.org SCIENCE
27 FEBRUARY 2015 • VOL 347 ISSUE 6225
Published by AAAS
Downloaded from www.sciencemag.org on February 26, 2015
CLIMATE CHANGE
Why the Pacific is cool Ben B. Booth Science 347, 952 (2015); DOI: 10.1126/science.aaa4840
This copy is for your personal, non-commercial use only.
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.
The following resources related to this article are available online at www.sciencemag.org (this information is current as of February 26, 2015 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/347/6225/952.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/347/6225/952.full.html#related This article cites 13 articles, 4 of which can be accessed free: http://www.sciencemag.org/content/347/6225/952.full.html#ref-list-1 This article appears in the following subject collections: Anthropology http://www.sciencemag.org/cgi/collection/anthro
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2015 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on February 26, 2015
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here.
