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Quantifying underestimates of long-term upper-ocean warming

Abstract

The global ocean stores more than 90% of the heat associated with observed greenhouse-gas-attributed global warming1,2,3,4. Using satellite altimetry observations and a large suite of climate models, we conclude that observed estimates of 0–700 dbar global ocean warming since 1970 are likely biased low. This underestimation is attributed to poor sampling of the Southern Hemisphere, and limitations of the analysis methods that conservatively estimate temperature changes in data-sparse regions5,6,7. We find that the partitioning of northern and southern hemispheric simulated sea surface height changes are consistent with precise altimeter observations, whereas the hemispheric partitioning of simulated upper-ocean warming is inconsistent with observed in-situ-based ocean heat content estimates. Relying on the close correspondence between hemispheric-scale ocean heat content and steric changes, we adjust the poorly constrained Southern Hemisphere observed warming estimates so that hemispheric ratios are consistent with the broad range of modelled results. These adjustments yield large increases (2.2–7.1 × 1022 J 35 yr−1) to current global upper-ocean heat content change estimates, and have important implications for sea level, the planetary energy budget and climate sensitivity assessments.

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Figure 1: Upper-ocean (0–700 dbar) heat content trends for 1970–2004.
Figure 2: Global and hemispheric OHC compared to total steric anomalies simulated by models for 1970–2004.
Figure 3: Southern Hemisphere fractional contributions to global upper-OHC or global average SSH anomaly for varying trend lengths (1–35 years).
Figure 4: Histogram of the observed and simulated Southern Hemisphere contribution to global OHC trends for 1970–2004.
Figure 5: Observed and simulated hemispheric and global upper-ocean (0–700 dbar) heat content change for 1970–2004.

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Acknowledgements

The work of P.J.D., P.J.G. and K.E.T. from Lawrence Livermore National Laboratory is a contribution to the US Department of Energy, Office of Science, Climate and Environmental Sciences Division, Regional and Global Climate Modeling Program under contract DE-AC52-07NA27344. The work of F.W.L. was performed at the Jet Propulsion Laboratory, California Institute of Technology and is supported by NASA ROSES Physical Oceanography grant NNN13D462T and the NASA Sea Level Change Team (NSLCT). We thank numerous colleagues from the Program for Climate Model Diagnosis and Intercomparison (PCMDI) for valuable feedback and input into this project. We also thank J. Durack of the University of California, San Francisco (USA), M. V. Durack of educAID (Australia), T. P. Boyer from the National Oceanographic Data Center, Silver Spring (USA), C. M. Domingues from the Antarctic Climate and Ecosystems CRC, Hobart (Australia) and J. A. Church from the Centre for Australian Weather and Climate Research, Hobart (Australia). We acknowledge the sources of observed data used in this study: D. Smith and J. Murphy (Smi07), C. M. Domingues (Dom08), M. Ishii and M. Kimoto (Ish09), S. Levitus and T. Boyer (Lev12) and the International Argo Program and the national programs that contribute to it. 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 (listed in Supplementary Tables 1 and 2) 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 DW10 data presented in this study can be downloaded from the CSIRO Ocean Change website at www.cmar.csiro.au/oceanchange. LLNL Release #: LLNL-JRNL-651841.

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P.J.D. completed the OHC analysis, P.J.G. assisted in the OHC analysis and F.W.L. completed the SSH analysis. All authors assisted with interpretation and shared responsibility for writing the manuscript.

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Correspondence to Paul J. Durack.

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The authors declare no competing financial interests.

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Durack, P., Gleckler, P., Landerer, F. et al. Quantifying underestimates of long-term upper-ocean warming. Nature Clim Change 4, 999–1005 (2014). https://doi.org/10.1038/nclimate2389

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