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Projections of tropical heat stress constrained by atmospheric dynamics


Extreme heat under global warming is a concerning issue for the growing tropical population. However, model projections of extreme temperatures, a widely used metric for extreme heat, are uncertain on regional scales. In addition, humidity needs to be taken into account to estimate the health impact of extreme heat. Here we show that an integrated temperature–humidity metric for the health impact of heat, namely, the extreme wet-bulb temperature (TW), is controlled by established atmospheric dynamics and thus can be robustly projected on regional scales. For each 1 °C of tropical mean warming, global climate models project extreme TW (the annual maximum of daily mean or 3-hourly values) to increase roughly uniformly between 20° S and 20° N latitude by about 1 °C. This projection is consistent with theoretical expectation based on tropical atmospheric dynamics, and observations over the past 40 years, which gives confidence to the model projection. For a 1.5 °C warmer world, the probable (66% confidence interval) increase of regional extreme TW is projected to be 1.33–1.49 °C, whereas the uncertainty of projected extreme temperatures is 3.7 times as large. These results suggest that limiting global warming to 1.5 °C will prevent most of the tropics from reaching a TW of 35 °C, the limit of human adaptation.

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Fig. 1: \({{\rm{TW}}}_{\max }\) and \({T}_{\max }\) trends in climate models under RCP 8.5.
Fig. 2: Model agreement on regional \({{\rm{TW}}}_{\max }\) projections.
Fig. 3: \({{\rm{TW}}}_{\max }\) in observations and reanalysis data.
Fig. 4: Uncertainty of \({T}_{\max }\) and \({{\rm{TW}}}_{\max }\) projection in a 1.5 °C warmer world (land between 20° S and 20° N).

Data availability

CMIP5 model data provided by the World Climate Research Programme’s Working Group on Coupled Modelling, and climate modelling groups can be accessed at ERA-Interim data provided by European Centre for Medium-Range Weather Forecast (ECMWF) can be accessed at HadISD global sub-daily station dataset (v3.0.1.201909p) provided by Met Office Hadley Centre can be accessed at HadISST data provided by the Met Office Hadley Centre can be accessed at

Code availability

The computer code used in this paper is available from the corresponding author.


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Y.Z. thanks J. Lin and G. Vecchi for suggestions on the manuscript. Y.Z. acknowledges support under award NA18OAR4320123 from the National Oceanic and Atmospheric Administration, US Department of Commerce (the statements, findings, conclusions, and recommendations are those of the author and do not necessarily reflect the views of the National Oceanic and Atmospheric Administration or the US Department of Commerce). S.F. acknowledges support from National Science Foundation under award AGS-1733818.

Author information




Y.Z. conceived the theory, performed the data analysis and wrote the manuscript. I.H. suggested the examination of observations/reanalysis. S.F. interpreted the widening of \(\small {{\rm{TW}}}_{\max }\) trend distribution in reanalysis (Fig. 3c). All authors discussed the results and edited the manuscript.

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Correspondence to Yi Zhang.

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The authors declare no competing interests.

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Peer review informationNature Geoscience thanks the anonymous reviewers for their contribution to the peer review of this work. Primary Handling Editors: Tamara Goldin, Heike Langenberg, Tom Richardson.

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Supplementary Figs. 1–8 and Table 1.

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Zhang, Y., Held, I. & Fueglistaler, S. Projections of tropical heat stress constrained by atmospheric dynamics. Nat. Geosci. 14, 133–137 (2021).

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