Longer-term externally forced trends in global mean surface temperatures (GMSTs) are embedded in the background noise of internally generated multidecadal variability1. A key mode of internal variability is the Interdecadal Pacific Oscillation (IPO), which contributed to a reduced GMST trend during the early 2000s1,2,3. We use a novel, physical phenomenon-based approach to quantify the contribution from a source of internally generated multidecadal variability—the IPO—to multidecadal GMST trends. Here we show that the largest IPO contributions occurred in its positive phase during the rapid warming periods from 1910–1941 and 1971–1995, with the IPO contributing 71% and 75%, respectively, to the difference between the median values of the externally forced trends and observed trends. The IPO transition from positive to negative in the late-1990s contributed 27% of the discrepancy between model median estimates of the forced part of the GMST trend and the observed trend from 1995 to 2013, with additional contributions that are probably due to internal variability outside of the Pacific4 and an externally forced response from small volcanic eruptions5. Understanding and quantifying the contribution of a specific source of internally generated variability—the IPO—to GMST trends is necessary to improve decadal climate prediction skill.
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The authors thank C. Tebaldi for her contributions to formulating the GMST trend distributions and for stimulating discussions on how to quantify the relative roles of internal variability and externally forced response, and G. Jones for providing the masked CMIP5 model data. Portions of this study were supported by the Regional and Global Climate Modeling Program (RGCM) of the US Department of Energy’s Office of Biological & Environmental Research (BER) Cooperative Agreement # DE-FC02-97ER62402 and the National Science Foundation. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups for producing and making available their model output. For CMIP, the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
The authors declare no competing financial interests.
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Meehl, G., Hu, A., Santer, B. et al. Contribution of the Interdecadal Pacific Oscillation to twentieth-century global surface temperature trends. Nature Clim Change 6, 1005–1008 (2016). https://doi.org/10.1038/nclimate3107
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