Heat and Humidity Extremes (covering years 2023-26)

State of the Science Blog by Colin Raymond (UCLA)

The last few years have seen considerable progress toward identifying the atmospheric processes that control the most intense humid-heat episodes^1,2 and their modulation by large-scale modes of variability. In some cases, detailed timelines are starting to be sketched out³; however, regional dynamics outside the typical focus regions of South and East Asia, the Persian/Arabian Gulf, and eastern North America largely remain unexplored. Oceanic contributions to humid heat are gaining attention for marine heat waves⁴ and the global annual mean⁵, but ocean and sea-breeze dynamics remain a notable blind spot. Debate also continues about where and when evapotranspiration amplifies (or dampens) humid heat, seemingly partly due to differences in analysis resolution^6-10.

The physical properties of water vapor contribute to several other forms of compounding with a burst of recent research energy, including ‘day-night’ humid-heat events¹¹ and correlations with heavy precipitation^12-14, severe storms¹⁵, and poor air quality¹⁶. In some cases, despite strong suppositions, unambiguously demonstrated linkages with impacts remain elusive. Similarly, we seem little closer to resolving the split-view understanding of the additive importance of humidity from physiological versus epidemiological perspectives as we were when the problem was so sharply articulated by ref. ¹⁷.

One of the newer and more sociophysically integrated developments has been the conception of low-latitude humid heat as a chronic phenomenon, one for which many physical- and social-science framings are ineffective (ref. ¹⁸ and references therein). Humid heat’s labor effects have been studied rather extensively in past years and recently¹⁹, but still largely with models that leave uncertainties around individual-level exposures, sociocultural differences, and adaptive practices of the sort often noted in media coverage.

A growing body of evidence has refined the temperature-humidity combinations at which deadly humid heat occurs²⁰, typically lowering inferred mortality and morbidity thresholds^21-23; several papers have applied these revised thresholds to observations and model projections^24,25, but the far-reaching implications for adaptation, mitigation, equity, and physical-process research have only begun to be explored²⁶. Translating certain tantalizing research findings, such as the high predictability of humid heat from particular initial conditions^27,28, into operational forecasts and early warnings represents another major opportunity for rapid near-term advance.

                  The impending incorporation of humidity into widely used AI weather and climate models offers significant possibilities for spurring model assessment and improvement, and for enabling ever-higher-resolution examinations of humid-heat patterns and processes. While microscale modeling of humid heat has been done for select cities²⁹ with global work underway, widespread validation at such scales remains elusive^30,31. In general, intercomparison among observed and model datasets for humid heat remains an important frontier given large discrepancies in the historical period for both extreme values³² and trends³³.

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