Humans’ ability to efficiently shed heat has enabled us to range over every continent, but a wet-bulb temperature (TW) of 35°C marks our upper physiological limit, and much lower values have serious health and productivity impacts. Climate models project the first 35°C TW occurrences by the mid-21st century. However, a comprehensive evaluation of weather station data shows that some coastal subtropical locations have already reported a TW of 35°C and that extreme humid heat overall has more than doubled in frequency since 1979. Recent exceedances of 35°C in global maximum sea surface temperature provide further support for the validity of these dangerously high TW values. We find the most extreme humid heat is highly localized in both space and time and is correspondingly substantially underestimated in reanalysis products. Our findings thus underscore the serious challenge posed by humid heat that is more intense than previously reported and increasingly severe.
Humans’ bipedal locomotion, naked skin, and sweat glands are constituents of a sophisticated cooling system (1). Despite these thermoregulatory adaptations, extreme heat remains one of the most dangerous natural hazards (2), with tens of thousands of fatalities in the deadliest events so far this century (3, 4). The additive impacts of heat and humidity extend beyond direct health outcomes to include reduced individual performance across a range of activities, as well as large-scale economic impacts (5–7). Heat-humidity effects have prompted decades of study in military, athletic, and occupational contexts (8, 9). However, consideration of wet-bulb temperature (TW) from the perspectives of climatology and meteorology began more recently (10, 11).
While some heat-humidity impacts can be avoided through acclimation and behavioral adaptation (12), there exists an upper limit for survivability under sustained exposure, even with idealized conditions of perfect health, total inactivity, full shade, absence of clothing, and unlimited drinking water (9, 10). A normal internal human body temperature of 36.8° ± 0.5°C requires skin temperatures of around 35°C to maintain a gradient directing heat outward from the core (10, 13). Once the air (dry-bulb) temperature (T) rises above this threshold, metabolic heat can only be shed via sweat-based latent cooling, and at TW exceeding about 35°C, this cooling mechanism loses its effectiveness altogether. Because the ideal physiological and behavioral assumptions are almost never met, severe mortality and morbidity impacts typically occur at much lower values—for example, regions affected by the deadly 2003 European and 2010 Russian heat waves experienced TW values no greater than 28°C (fig. S1). In the literature to date, there have been no observational reports of TW exceeding 35°C and few reports exceeding 33°C (9, 11, 14, 15). The awareness of a physiological limit has prompted modeling studies to ask how soon it may be crossed. Results suggest that, under the business-as-usual RCP8.5 emissions scenario, TW could regularly exceed 35°C in parts of South Asia and the Middle East by the third quarter of the 21st century (14–16).
Here, we use quality-assured station observations from HadISD (17, 18) and high-resolution reanalysis data from ERA-Interim (19, 20), verified against radiosondes and marine observations (see the Supplementary Materials) (21, 22), to compute TW baseline values, geographic patterns, and recent trends. Uncertainties in TW from station data due to instrumentation and procedures are on the order of 0.5° to 1.0°C in all regions considered, an important consideration for proper interpretation of the results. Our approach of using TW and sea surface temperature (SST) observations as guidance for future TW projections offers a different line of evidence from previous research that used coupled or regional models without explicitly including historical station data.
Our survey of the climate record from station data reveals many global TW exceedances of 31° and 33°C and two stations that have already reported multiple daily maximum TW values above 35°C. These conditions, nearing or beyond prolonged human physiological tolerance, have mostly occurred only for 1- to 2-hours’ duration (fig. S2). They are concentrated in South Asia, the coastal Middle East, and coastal southwest North America, in close proximity to extraordinarily high SSTs and intense continental heat that together favor the occurrence of extreme humid heat (2, 14). Along coastlines, the marine influence is manifest via anomalous onshore low-level winds during midday and afternoon hours, and these wind shifts can cause rapid dew point temperature (Td) increases in arid and semiarid coastal areas (figs. S3 to S9). Regionally coherent observational evidence supports these intense values: Of the stations along the Persian Gulf coastline with at least 50% data availability over 1979 to 2017, all have a historical 99.9th percentile of TW (the value exceeded roughly 14 times in 39 years) above 31°C (Fig. 1; see fig. S1 for the all-time maximum). In the ERA-Interim reanalysis, the highest values are similarly located over the Persian Gulf and immediately adjacent land areas, as well as parts of the Indus River Valley (fig. S10). The spatiotemporal averaging inherent in reanalysis products causes ERA-Interim to be unable to represent the short durations and small areas of critical heat stress, causing its extreme TW values to be substantially lower than those of weather stations across the tropics and subtropics (fig. S11). In the Persian Gulf and adjacent Gulf of Oman, these differences are consistently in the range of −2° to −4°C (fig. S12). Larger bias but similar consistency is present along the eastern shore of the Red Sea, presenting a basis for future studies examining the reasons for this behavior, as well as further comparisons between station and reanalysis data.
Color symbols represent the 99.9th percentile of observed daily maximum TW for 1979–2017 for HadISD stations with at least 50% data availability over this period. Marker size is inversely proportional to station density.
Other >31°C hotspots in the weather station record emerge through surveying the globally highest 99.9th TW percentiles: eastern coastal India, Pakistan and northwestern India, and the shores of the Red Sea, Gulf of California, and southern Gulf of Mexico (Fig. 1). All are situated in the subtropics, along coastlines (typically of a semienclosed gulf or bay of shallow depth, limiting ocean circulation and promoting high SSTs), and in proximity to sources of continental heat, which together with the maritime air comprise the necessary combination for the most exceptional TW (11). That subtropical coastlines are hotspots for heat stress has been noted previously (23, 24); our analysis makes clear the broad geographic scope but also the large intraregional variations (Fig. 1). Western South Asia stands as the main exception to this coastline rule, likely due to the efficient inland transport of humid air by the summer monsoon together with large-scale irrigation (15, 25). Tropical forest and oceanic areas generally experience TW no higher than 31° to 32°C, perhaps a consequence of the high evapotranspiration potential and cloud cover, along with the greater instability of the tropical atmosphere. However, more research is needed on the thermodynamic mechanisms that prevent these areas from attaining higher values.
Steep and statistically significant upward trends in extreme TW frequency (exceedances of 27°, 29°, 31°, and 33°C) and magnitude are present across weather stations globally (Fig. 2). Each frequency trend represents more than a doubling of occurrences of the corresponding threshold between 1979 and 2017. Trends in ERA-Interim are strongly correlated with those of HadISD but are smaller for the highest values (Fig. 2), consistent with ERA-Interim’s underestimation of extreme TW that is largest for the most extreme conditions (fig. S11). We also find a sharp peak in the number of global TW = 27°C and TW = 29°C extremes during the strong El Niño events of 1998 and 2016. Linearly detrending this global-TW-extremes time series reveals that the El Niño–Southern Oscillation (ENSO) correlation is largest for TW values that are high but not unusual (~27° to 28°C) across the tropics and subtropics (fig. S13). Further work is necessary to test to what extent this relationship may be related to the effect of ENSO on hydrological extremes at the global scale, on tropospheric-mean temperatures, or on SSTs in particular basins, and the implications of these effects for TW predictability (26, 27). Overall, TW extremes in the tropics largely correspond on an interannual basis to mean TW (fig. S14), indicating that climate forcings and modes of internal variability resulting in mean temperature shifts can be expected to modulate tropical TW extremes. This is the case in the subtropics as well, although to a somewhat lesser extent.
(A to D) Annual global counts of TW exceedances above the thresholds labeled on the respective panel, from HadISD (black, right axes, with units of station days) and ERA-Interim grid points (gray, left axes, with units of grid-point days). We consider only HadISD stations with at least 50% data availability over 1979–2017. Correlations between the series are annotated in the top left of each panel, and dotted lines highlight linear trends. (E) Annual global maximum TW in ERA-Interim. (F) The line plot shows global mean annual temperature anomalies (relative to 1850–1879) according to HadCRUT4 (40), which we use to approximate each year’s observed warming since preindustrial; circles indicate HadISD station occurrences of TW exceeding 35°C, with radius linearly proportional to global annual count, measured in station days.
We also observe modulation on a seasonal scale, by considering as an illustrative example the South Asian monsoon region. There, the timing of peak TW varies with the advance of the summer monsoon (15). Splitting South Asia into “early monsoon” and “late monsoon” subregions, we find that the number of TW extremes is largest around the time of the local climatological monsoon onset date (Fig. 3). Although equivalent extreme values of TW are possible before, during, and after the monsoon rains in any given year, they are of a different character; especially in the northern and western parts of the subcontinent, they become continually moister and have lower dry-bulb temperatures as summer progresses. Across the globe, such temperature and humidity variations occur within a well-defined bivariate space (fig. S15). That these variations are systematically associated with the summer monsoon in South Asia emphasizes the important role of moisture, and of weather systems on synoptic to subseasonal time scales, in controlling extreme TW (15, 28). Our findings underscore the diversity of conditions that can lead to extreme humid heat in the same location at different times, suggesting that impacts adaptation strategies may benefit from taking this recognition into account. Such intraseasonal variability in TW also matters for physiological acclimation, which requires several-day time scales to develop (29); TW character is especially relevant when considering effects on human systems that vary in their sensitivity to humidity and temperature—for example, thermoregulation and energy demand for artificial cooling are strongly affected by TW, whereas the materials that make up the built environment are principally affected by temperature alone (13, 30).