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1. Stickiness: A New Variable to Characterize the Temperature and Humidity Contributions toward Humid Heat

   https://doi.org/10.1175/JAS-D-23-0072.1  

   Ivanovich, Catherine C. ; Sobel, Adam H. ; Horton, Radley M. ; Raymond, Colin ( May 2024 , Journal of the Atmospheric Sciences)

   Extreme wet-bulb temperatures (T_w) are often used as indicators of heat stress. However, humid heat extremes are fundamentally compound events, and a givenT_wcan be generated by various combinations of temperature and humidity. Differentiating between extreme humid heat driven by temperature versus humidity is essential to identifying these extremes’ physical drivers and preparing for their distinct impacts. Here we explore the variety of combinations of temperature and humidity contributing to humid heat experienced across the globe. In addition to using traditional metrics, we derive a novel thermodynamic state variable named “stickiness.” Analogous to the oceanographic variable “spice” (which quantifies the relative contributions of temperature and salinity to a given water density), stickiness quantifies the relative contributions of temperature and specific humidity to a givenT_w. Consistent across metrics, we find that high magnitudes ofT_wtend to occur in the presence of anomalously high moisture, with temperature anomalies of secondary importance. This widespread humidity dependence is consistent with the nonlinear relationship between temperature and specific humidity as prescribed by the Clausius–Clapeyron relationship. Nonetheless, there is a range of stickiness observed at moderate-to-highT_wthresholds. Stickiness allows a more objective evaluation of spatial and temporal variability in the temperature versus humidity dependence of humid heat than traditional variables. In regions with high temporal variability in stickiness, predictive skill for humid heat-related impacts may improve by considering fluctuations in atmospheric humidity in addition to dry-bulb temperature.

   Extreme humid heat increases the risk of heat stress through its influence over humans’ ability to cool down by sweating. Understanding whether humid heat extremes are generated more due to elevated temperature or humidity is important for identifying factors that may increase local risk, preparing for associated impacts, and developing targeted adaptation measures. Here we explore combinations of temperature and humidity across the globe using traditional metrics and by deriving a new variable called “stickiness.” We find that extreme humid heat at dangerous thresholds occurs primarily due to elevated humidity, but that stickiness allows for thorough analysis of the drivers of humid heat at lower thresholds, including identification of regions prone to low- or high-stickiness extremes.

    

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2. The Influence of Intraseasonal Oscillations on Humid Heat in the Persian Gulf and South Asia

   https://doi.org/10.1175/JCLI-D-21-0488.1  

   Ivanovich, Catherine ; Anderson, Weston ; Horton, Radley ; Raymond, Colin ; Sobel, Adam ( July 2022 , Journal of Climate)

   Humans’ essential ability to combat heat stress through sweat-based evaporative cooling is modulated by ambient air temperature and humidity, making humid heat a critical factor for human health. In this study, we relate the occurrence of extreme humid heat in two focus regions to two related modes of intraseasonal climate variability: the Madden–Julian oscillation (MJO) and the boreal summer intraseasonal oscillation (BSISO). In the Persian Gulf and South Asia during the May–June and July–August seasons, wet-bulb temperatures of 28°C are found to be almost twice as likely during certain oscillation phases than in others. Variations in moisture are found, to varying degrees, to be an important ingredient in anomalously high wet-bulb temperatures in all three areas studied, influenced by distinct local circulation anomalies. In the Persian Gulf, weakening of climatological winds associated with the intraseasonal oscillation’s propagating center of convection allows for anomalous onshore advection of humid air. Anomalously high wet-bulb temperatures in the northwestern region of South Asia are closely aligned with positive specific humidity anomalies associated with the convectively active phase of the oscillation. On the southeastern coast of India, high wet-bulb temperatures are associated with convectively inactive phases of the intraseasonal oscillation, suggesting that they may be driven by increased surface insolation and reduced evaporative cooling during monsoon breaks. Our results aid in building a foundation for subseasonal predictions of extreme humid heat in regions where it is highly impactful.

   Understanding when and why extreme humid heat occurs is essential for informing public health efforts protecting against heat stress. This analysis works to improve our understanding of humid heat variability in two at-risk regions, the Persian Gulf and South Asia. By exploring how subseasonal oscillations affect daily extreme events, this analysis helps bridge the prediction gap between weather and climate. We find that extreme humid heat is more than twice as likely during specific phases of these oscillations than in others. Extremes depend to different extents upon combinations of above-average temperature and humidity. This new knowledge of the regional drivers of humid heat variability is important to better prepare for the increasingly widespread health and socioeconomic impacts of heat stress.

    

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3. Greatly enhanced risk to humans as a consequence of empirically determined lower moist heat stress tolerance

   https://doi.org/10.1073/pnas.2305427120  

   Vecellio, Daniel J. ; Kong, Qinqin ; Kenney, W. Larry ; Huber, Matthew ( October 2023 , Proceedings of the National Academy of Sciences)

   Kerry Emanuel (Ed.)

   As heatwaves become more frequent, intense, and longer-lasting due to climate change, the question of breaching thermal limits becomes pressing. A wet-bulb temperature (T_w) of 35 °C has been proposed as a theoretical upper limit on human abilities to biologically thermoregulate. But, recent—empirical—research using human subjects found a significantly lower maximum T_wat which thermoregulation is possible even with minimal metabolic activity. Projecting future exposure to this empirical critical environmental limit has not been done. Here, using this more accurate threshold and the latest coupled climate model results, we quantify exposure to dangerous, potentially lethal heat for future climates at various global warming levels. We find that humanity is more vulnerable to moist heat stress than previously proposed because of these lower thermal limits. Still, limiting warming to under 2 °C nearly eliminates exposure and risk of widespread uncompensable moist heatwaves as a sharp rise in exposure occurs at 3 °C of warming. Parts of the Middle East and the Indus River Valley experience brief exceedances with only 1.5 °C warming. More widespread, but brief, dangerous heat stress occurs in a +2 °C climate, including in eastern China and sub-Saharan Africa, while the US Midwest emerges as a moist heat stress hotspot in a +3 °C climate. In the future, moist heat extremes will lie outside the bounds of past human experience and beyond current heat mitigation strategies for billions of people. While some physiological adaptation from the thresholds described here is possible, additional behavioral, cultural, and technical adaptation will be required to maintain healthy lifestyles.

    

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4. Nonlinear increases in extreme temperatures paradoxically dampen increases in extreme humid-heat

   https://doi.org/10.1088/1748-9326/ab28b7  

   Coffel, Ethan D. ; Horton, Radley M. ; Winter, Jonathan M. ; Mankin, Justin S. ( July 2019 , Environmental Research Letters)

   Nonlinear increases in warm season temperatures are projected for many regions, a phenomenon we show to be associated with relative surface drying. However, negative human health impacts are physiologically linked to combinations of high temperatures and high humidity. Since the amplified warming and drying are concurrent, the net effect on humid-heat, as measured by the wet bulb temperature (T_W), is uncertain. We demonstrate that globally, on the hottest days of the year, the positive effect of amplified warming onT_Wis counterbalanced by a larger negative effect resulting from drying. As a result, the largest increases inT_WandT_xdo not occur on the same days. Compared to a world with linear temperature change, the drying associated with nonlinear warming dampens mid-latitudeT_Wincreases by up to 0.5 °C, and also dampens the rise in frequency of dangerous humid-heat (T_W > 27 °C) by up to 5 d per year in parts of North America and Europe. Our results highlight the opposing interactions among temperature and humidity changes and their effects onT_W, and point to the importance of constraining uncertainty in hydrological and warm season humidity changes to best position the management of future humid-heat risks.

    

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5. Short Warm Distribution Tails Accelerate the Increase of Humid‐Heat Extremes Under Global Warming

   https://doi.org/10.1029/2022GL102164  

   Bekris, Yianna ; Loikith, Paul C. ; Neelin, J. David ( May 2023 , Geophysical Research Letters)

   Humid‐heat extremes threaten human health and are increasing in frequency with global warming, so elucidating factors affecting their rate of change is critical. We investigate the role of wet‐bulb temperature (T_W) frequency distribution tail shape on the rate of increase in extremeT_Wthreshold exceedances under 2°C global warming. Results indicate that non‐GaussianT_Wdistribution tails are common worldwide across extensive, spatially coherent regions. More rapid increases in the number of days exceeding the historical 95th percentile are projected in locations with shorter‐than‐Gaussian warm side tails. Asymmetry in the specific humidity distribution, one component ofT_W, is more closely correlated withT_Wtail shape than temperature, suggesting that humidity climatology strongly influences the rate of future changes inT_Wextremes. Short non‐GaussianT_Wwarm tails have notable implications for dangerous humid‐heat in regions where current‐climateT_Wextremes approach human safety limits.

    

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