Early emergence of anthropogenically forced heat waves in the western United States and Great Lakes

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Climate projections for the twenty-first century suggest an increase in the occurrence of heat waves. However, the time at which externally forced signals of anthropogenic climate change (ACC) emerge against background natural variability (time of emergence (ToE)) has been challenging to quantify, which makes future heat-wave projections uncertain. Here we combine observations and model simulations under present and future forcing to assess how internal variability and ACC modulate US heat waves. We show that ACC dominates heat-wave occurrence over the western United States and Great Lakes regions, with ToE that occurred as early as the 2020s and 2030s, respectively. In contrast, internal variability governs heat waves in the northern and southern Great Plains, where ToE occurs in the 2050s and 2070s; this later ToE is believed to be a result of a projected increase in circulation variability, namely the Great Plain low-level jet. Thus, greater mitigation and adaptation efforts are needed in the Great Lakes and western United States regions.

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We acknowledge D. Enfield, G. Foltz (NOAA/AOML), E. Johns (NOAA/AOML) and G. Derr (NOAA/AOML) for their comments and suggestions that greatly improved the manuscript. This research was carried out in part under the auspices of the Cooperative Institute for Marine and Atmospheric Studies, a cooperative institute of the University of Miami and the National Oceanic and Atmospheric Administration (NOAA), cooperative agreement NA10OAR4320143. This work was funded by NOAA’s Atlantic Oceanographic and Meteorological Laboratory and by the Climate Observations Division of the NOAA Climate Program Office. S.D. acknowledge funding by NASA grant NNH13AW33I. H.L. acknowledges funding from NOAA Climate Program Office CVP program (GC16-208). S.-K.L. acknowledges funding from NOAA Climate Program Office CVP program (GC16-207). B.P.K. acknowledges funding from the US National Science Foundation (OCE1419569).

Author information


  1. Cooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, FL, USA

    • Hosmay Lopez
    •  & Ben Kirtman
  2. Atlantic Oceanographic and Meteorological Laboratory, NOAA, Miami, FL, USA

    • Hosmay Lopez
    • , Shenfu Dong
    • , Gustavo Goni
    • , Sang-Ki Lee
    •  & Robert Atlas
  3. Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL, USA

    • Robert West


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H.L. conceived the study, designed and performed the statistical analysis and wrote the initial draft of the paper. H.L., R.W., S.D., S.-K.L., G.G., B.K. and R.A. contributed to the discussion and interpretation of the results as well as to the writing of the final version of the paper.

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The authors declare no competing interests.

Corresponding author

Correspondence to Hosmay Lopez.

Supplementary information

  1. Supplementary Information

    Supplementary Table 1 and Supplementary Figures 1–14