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\begin{document}

\title{Exceptional Heat-Humidity Combinations in the Observational Record}



\author[1]{Colin Raymond}%
\author[2]{Tom Matthews}%
\author[3]{Radley Horton}%
\affil[1]{Columbia University of New York City}%
\affil[2]{Loughborough University}%
\affil[3]{Columbia University}%


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  \date{May 24, 2024}


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\begin{abstract}
Large increases in joint heat-humidity extremes are a robust feature of
climate projections, more so than increases in either variable
independently. The wet-bulb temperature threshold of 35\selectlanguage{ngerman}°C is considered
particularly important, corresponding to the level at which it is
impossible for the human body to maintain homeostasis. Our analysis of
more than 7500 global weather stations from HadISD reveals that
instances of 35°C have already been briefly but reliably recorded in
some cities of the coastal Middle East. Additionally, South Asia and
coastal areas throughout the subtropics regularly experience values
perilously close to this threshold. Station and ERA-Interim reanalysis
data both indicate that the most extreme heat-humidity combinations have
increased significantly over the 1979-2017 period. We develop and employ
a generalized-extreme-value model to estimate that the 30-year return
value of global-maximum wet-bulb temperature at ERA-Interim resolution
will exceed 35°C when global-mean temperature has risen between 1.5°C
and 2.0°C above the pre-industrial, masking even higher values at
smaller spatial scales. The occurrence in the coming decades of such
severe heat and humidity over large populated regions represents a
situation never before experienced. We also present observational
evidence that these wet-bulb temperature extremes are fostered by
locally high SSTs as well as modulated by regional dynamics such as
monsoons. Large-scale climate modes of variability, such as ENSO, lead
to highly correlated interannual variability between the number of
global exceedances of different wet-bulb temperature thresholds.
Overall, our results show that the wet-bulb temperature `safety margin'
between currently reported values and 35°C is both smaller than
previously understood and rapidly shrinking, presenting a serious
challenge to human survival in the hottest and most humid places on
Earth.%
\end{abstract}\selectlanguage{ngerman}%



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