letters to nature Table 1 Trends in annual snow accumulation at the Mount Logan ice core site 16. Santer, B. D. et al. Statistical significance of trends and trend differences in layer-average atmospheric Time interval 1736–1850 1851–2000 1948–2000 1976–2000 temperature time series. J. Geophys. Res. Atmos. 105, 7337–7356 (2000). Trend (m per decade) 0.003 0.008 0.034 0.12 17. Jones, P. D., New, M., Parker, D. E., Martin, S. & Rigor, I. G. Surface air temperature and its changes Significance level (%) 50 95 95 99 over the past 150 years. Rev. Geophys. 37, 173–199 (1999). ............................................................................................................................................................................. 18. Houghton, J. T. et al. (eds) IPPC Third Assessment Report, Climate Change 2001: The Scientific Basis (Cambridge Univ. Press, 2001). 19. Kalnay, E. et al. The NCEP/NCAR40-year reanalysis project. Bull. Am. Meteorol. Soc. 77, 437–471 period of overlap between it and the Mount Logan time series, (1996). 20. Peixoto, J. P. & Oort, A. H. Physics of Climate (American Institute of Physics, 1992). 1736–1991, there is no statistically significant correlation between 21. Wallace, J. M., Zhang, Y. & Renwick, J. A. Dynamic contribution to hemispheric mean temperature them. trends. Science 270, 780–783 (1995). The complex relationship between the Mount Logan time series, 22. Trenberth, K. E. Recent observed interdecadal climate changes in the northern hemisphere. Bull. Am. the PNA and the PDO identified in this paper is not unique. For Meteorol. Soc. 71, 988–993 (1990). 23. Biondi, F., Gershunov, A. & Cayan, D. R. North Pacific decadal climate variability since 1661. J. Clim. example, the correlation between salmon productivity in Alaska and 14, 5–10 (2001). 24 the PDO , although robust over the past 50 years, shows changes in 24. Mantua, N. J., Hare, S. R., Zhang, Y., Wallace, J. M. & Francis, R. C. A Pacific interdecadal climate correlation and phase when 300-year proxy reconstructions of oscillation with impacts on salmon production. Bull. Am. Meteorol. Soc. 78, 1069–1079 (1997). 25. Finney, B. P., Gregory-Eaves, I., Sweetman, J., Douglas, M. S. V. & Smol, J. P. Impacts of climatic North Pacific sea surface temperature and salmon abundance are change and fishing on Pacific salmon abundance over the past 300 years. Science 290, 795–799 (2000). 25 compared . This complexity serves to highlight the need to con- 26. Mann, M. E. The value of multiple proxies. Science 297, 1481–1482 (2002). strain better the temporal and spatial variability of climatic modes, such as the PDO and PNA, using several independent proxies in Acknowledgements The Mount Logan time series update was supported by the International order to better predict their evolution and their relevance to societal Arctic Research Center, Fairbanks, the Geological Survey of Canada, the National Institute of 2,26 Polar Research, Tokyo, and the Natural Sciences and Engineering Research Council of Canada. concerns . G.H. thanks M.N. Demuth, K. Supeene, E.J. Steig and S. Rupper for assistance in acquiring and The three-century-long snow-accumulation record from the analysing the ice core. The NCEP reanalysis data was provided by the Climate Diagnostics Center Mount Logan site extends from the closing stages of the Little Ice of the US National Oceanic and Atmospheric Administration. The HADCRUTv data was Age to the warmest decade in the past millennium7. A statistically provided by the Climate Research Unit at the University of East Anglia. significant and accelerating positive trend has existed in this time Competing interests statement The authors declare that they have no competing financial series since the middle of the nineteenth century. This was preceded interests. by a period of approximately 150 years in which there was no trend. The results that we have presented suggest that this trend is Correspondence and requests for materials should be addressed to G.W.K.M. associated with a surface warming over western North America (e-mail: [email protected]). throughout the period for which we have good-quality surface data, 1870–2000. Furthermore, the Mount Logan time series is correlated with the PNA over the period for which we have good-quality information on its variability, 1925–2000. With respect to the PDO, .............................................................. there exists a statistically significant correlation only after the middle of the twentieth century. It therefore appears that the secular Altered performance of forest trend in the Mount Logan snow accumulation time series is associated with a long-term intensification of the PNA. Our analysis pests under atmospheres suggests that the PDO has undergone a similar intensification, but only since the middle of the twentieth century. Over the past 50 enriched by CO2 and O3 years, this relationship between the PNA and the PDO may have contributed to the observed rapid increase in snow accumulation at Kevin E. Percy*, Caroline S. Awmack†, Richard L. Lindroth†, the Mount Logan site and the acceleration in the warming over Mark E. Kubiske‡, Brian J. Kopper†, J. G. Isebrands§, Kurt S. Pregitzerk, northwestern North America. A George R. Hendrey{, Richard E. Dickson‡, Donald R. Zak#, Elina Oksanenq, Jaak Soberk, Richard Harrington** Received 3 April; accepted 22 October 2002; doi:10.1038/nature01229. & David F. Karnoskyk 1. Wunsch, C. The interpretation of short climate records, with comments on the North Atlantic and Southern Oscillations. Bull. Am. Meteorol. Soc. 80, 245–255 (1999). * Natural Resources Canada, Canadian Forest Service-Atlantic Forestry Centre, 2. Alverson, K. et al. A global paleoclimate observing system. Science 293, 47–48 (2001). Fredericton, New Brunswick, E3B 5P7, Canada 3. Kistler, R. et al. The NCEP-NCAR50-year reanalysis: Monthly means CD-ROM and documentation. † Department of Entomology, University of Wisconsin-Madison, Madison, Bull. Am. Meteorol. Soc. 82, 247–267 (2001). Wisconsin 53706, USA 4. Wallace, J. M. & Gutzler, D. S. Teleconnections in the geopotential height field during the northern ‡ North Central Research Station, US Forest Service, Rhinelander, Wisconsin hemisphere winter. Mon. Weath. Rev. 109, 784–812 (1981). 5. Barnston, A. G. & Livezey, R. E. Classification, seasonality and persistence of low-frequency 54501, USA atmospheric circulation patterns. Mon. Weath. Rev. 115, 1083–1126 (1987). § E7323 Highway 54, P.O. Box 54, New London, Wisconsin 54961, USA 6. Zhang, Y., Wallace, J. M. & Battisti, D. S. ENSO-like interdecadal variability: 1900-93. J. Clim. 10, k School of Forest Resources and Environmental Science, Michigan Technological 1004–1020 (1997). University, Houghton, Michigan 49931, USA 7. Esper, J., Cook, E. R. & Schweingruber, F. H. Low-frequency signals in long tree-ring chronologies for { Brookhaven National Laboratory, 1 South Technology Street, Upton, New York reconstructing past temperature variability. Science 295, 2250–2253 (2002). 11973, USA 8. Blackmon, M. L. Climatological spectral study of 500 Mb geopotential height of northern hemisphere. # School of Natural Resources & Environment, The University of Michigan, 430 J. Atmos. Sci. 33, 1607–1623 (1976). 9. Smirnov, V. 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Ice-core sulfate from 3 northern-hemisphere sites—Source and temperature forcing implications. Atmos. Environ. Part A 27, 2915–2919 (1993). Human activity causes increasing background concentrations of 1 13. Moore, G. W. K., Holdsworth, G. & Alverson, K. Extra-tropical response to ENSO as expressed in an the greenhouse gases CO2 and O3 . Increased levels of CO2 can be ice core from the Saint Elias Mountain range. Geophys. Res. Lett. 28, 3457–3460 (2001). 2 found in all terrestrial ecosystems . Damaging O3 concentrations 14. Moore, G. W. K., Alverson, K. & Holdsworth, G. Variability in the climate of the Pacific Ocean and currently occur over 29% of the world’s temperate and subpolar North America as expressed in an ice core from Mount Logan. Ann. Glaciol. (2002) (in the press). 15. Mann, M. E. & Lees, J. Robust estimation of background noise and signal detection in climatic time- forests but are predicted to affect fully 60% by 2100 (ref. 3). series. Clim. Change 33, 409–445 (1996). Although individual effects of CO2 and O3 on vegetation have NATURE | VOL 420 | 28 NOVEMBER 2002 | www.nature.com/nature © 2002 Nature Publishing Group 403 letters to nature been widely investigated, very little is known about their inter- carbon dioxide enrichment (FACE) technology11. action, and long-term studies on mature trees and higher trophic Here we report on the first free-air