Does heat-stress risk always peak in the afternoon?
Raymond, C., Matthews, T., and Tuholske, C. (2024). Evening humid-heat maxima near the southern Persian/Arabian Gulf. Communications Earth & Environment. https://doi.org/10.1038/s43247-024-01763-3.
Extreme humid heat is a major climate hazard for the coastal Arabian Peninsula. However, many of its characteristics, including diurnal and spatial variations, remain incompletely explored. Here we present evidence from multiple reanalysis and in situ datasets that evening or nighttime daily maxima in extreme wet-bulb temperature and heat index are widespread along the southern Persian/Arabian Gulf coastline and adjacent inland desert, driven principally by sea-breeze-related movements of moist maritime air. This timing runs counter to the general expectation of more intense heat and greater heat-stress risk during daytime hours. While wet-bulb temperature is one of many metrics relevant for understanding heat hazards, it has featured prominently in recent literature and its values are closer to uncompensable-heat limits in coastal Arabia than anywhere else. Deviations from an afternoon-peak assumption about heat risks are thus of critical importance and heighten the value of improved understanding of extreme-humid-heat meteorology, in this region and in others subject to similar physical processes.
Do summertime atmospheric rivers meaningfully promote humid heat?
Raymond, C., Shreevastava, A., Slinskey, E., and Waliser, D. (2024). Linkages between atmospheric rivers and humid heat across the United States. Natural Hazards and Earth System Sciences. https://doi.org/10.5194/nhess-24-791-2024.
The global increase in atmospheric water vapor due to climate change tends to heighten the dangers associated with both humid heat and heavy precipitation. Process-linked connections between these two extremes, particularly those which cause them to occur close together in space or time, are of special concern for impacts. Here we investigate how atmospheric rivers relate to the risk of summertime humid heat in the US. We find that the hazards of atmospheric rivers and humid heat often occur in close proximity, most notably across the northern third of the US. In this region, high levels of water vapor — resulting from the spatially organized horizontal moisture plumes that characterize atmospheric rivers — act to amplify humid heat, generally during the transition from dry high-pressure ridge conditions to wet low-pressure trough conditions. In contrast, the US Southeast, Southwest, and Northwest tend to experience atmospheric rivers and humid heat separately, representing an important negative correlation of joint risk.
How are key compound risks changing in a warming and moistening world?
Raymond, C., Suarez-Gutierrez, L., Kornhuber, K., Pascolini-Campbell, M., Sillmann, J., and Waliser, D. (2022). Increasing spatiotemporal proximity of heat and precipitation extremes in a warming world quantified by a large model ensemble. Environmental Research Letters. https://doi.org/10.1088/1748-9326/ac5712.
Many devastating weather and climate impacts— affecting human health, agriculture, water resources, natural ecosystems, and other sectors — occur when multiple hazards are compounded. How might the frequency and geography of these extreme stressors shift? Based on empirically observed relationships linking hazards to impacts, we consider four types of situations: heat waves, consecutive heavy-precipitation events, spatially concurrent maize-breadbasket failures triggered by heat/drought, and interannual wet/dry oscillations promoting wildfire and landslide risk. For the MPI-Grand Ensemble under the RCP4.5 scenario, each type increases moderately to dramatically over much of the globe. Relative changes in the frequency of extreme heat waves and pluvials are especially large in the tropics, while drought-pluvial volatility may increase up to 50% in some subtropical areas. None of the events see any meaningful areas of decrease, although more heat- and drought-tolerant maize varietals could substantially reduce the probability of multiple-breadbasket failure. Our results highlight the importance of accounting for compound climate extremes when looking at possible tipping points of socio-environmental systems.
Does humid heat increase at a relatively faster or slower rate in high elevations?
Raymond, C., Waliser, D., Guan, B., Lee, H., Loikith, P., Massoud, E., Sengupta, A., Singh, D., and Wootten, A. (2022). Regional and elevational patterns of extreme heat stress change in the US. Environmental Research Letters. https://doi.org/10.1088/1748-9326/ac7343.
We analyzed the elevation dependence of a multivariate-heat metric (the environmental stress index, ESI) to assess whether, and to what extent, temperature, moisture, and radiation changes tend to offset each other at higher versus lower elevations. Using reanalysis and downscaled CMIP5 models, we found that multivariate heat does tend to increase more rapidly in mountain areas, dominated by amplified temperatures (e.g. increases at 2000 m are 4.0-4.5 C, versus 3.5-4.0 C at sea level). However, in the humid southern and eastern US, the elevation profile of moisture changes accounts for nearly half of the corresponding ESI-change profile, and the low-level cooling effect of the Pacific is also notable both historically and in the future on the West Coast. The climatological rate of decrease of ESI is much greater in the more humid regions too, which we hypothesize stems from the larger chance of precipitation which removes atmospheric moisture. Looking at the historical top 1% of summer days, we found that these are projected to become up to 50 times more common in the late 21st century under RCP8.5 than they were in the late 20th century. This study aids in furthering knowledge about drivers and characteristics that shape current and future multivariate heat, at scales difficult to capture with standard-resolution Earth-system models.
What are the patterns and processes controlling the most intense humid heat?
Raymond, C., Matthews, T., Horton, R., Fischer, E., Fueglistaler, S., Ivanovich, C., Suarez-Gutierrez, L., and Zhang, Y. (2021). On the controlling factors for globally extreme humid heat. Geophysical Research Letters. https://doi.org/10.1029/2021gl096082
We analyzed the geographical and meteorological factors that shape the location and magnitude of the most severe humid heat on Earth. Using ERA5 reanalysis, supplemented with several other datasets, we focused on four regions and found that during extreme humid heat events they share two key characteristics: an absence of deep convection and a presence of abundant low-level moisture. These broadly explain both latitudinal patterns as well as specific longitudinal hotspots, and proxies for them can reconstruct with a good degree of accuracy the actual spatial pattern of humid heat (lower right figure). However, across regions there are important differences in how humid heat extremes are achieved -- for example, whether moisture primarily originates from the ocean or land surface, whether topography aids in producing a stable shallow boundary layer, and whether the most severe events are relatively dry or humid. Such wrinkles motivate future work. Our study provides a first global-scale theory for understanding where extreme humid heat is most severe, and a sampling of the regionally distinctive combinations of processes through which it preferentially occurs.
How might extreme climate events be connected, and why does it matter?
Raymond, C., Horton, R., Zscheischler, J., Martius, O., AghaKouchak, A., Balch, J., Bowen, S., Camargo, S., Hess, J., Kornhuber, K., Oppenheimer, M., Ruane, A., Wahl, T., and White, K. (2020). Understanding and managing connected extreme events. Nature Climate Change. https://doi.org/10.1038/s41558-020-0790-4
We described how the largest impacts from extreme climate events can often only be understood through consideration of the interactions of the events with human decision-making. Decisions and political/legal/cultural structures can unintentionally amplify impacts, such as when the response to one event hampers the response to another, or when multiple geographically distributed components of a system (e.g. the global food system) are struck simultaneously in a way that could have been anticipated. Through detailed description of such scenarios and the state of knowledge and practice surrounding them for five key sectors, we summarized current approaches and desired areas for improvement. Basing our proposals on a clear-eyed picture of data limitations and of the operational constraints faced by practitioners, we also recommended principles for improving the coordination between climate scientists and specialists in various other disciplines that is essential to developing appropriate preparations for the more-severe connected events of the future, including those outside the bounds of historical experience.
How close is intense humid heat to physiological limits?
Raymond, C., Matthews, T., and Horton, R. (2020). The emergence of heat and humidity too severe for human tolerance. Science Advances. https://doi.org/10.1126/sciadv.aaw1838
We conducted the first comprehensive survey of humid heat using weather-station data (as opposed to reanalysis datasets or historical climate-model runs). A large portion of our efforts was devoted to extensive quality control, as reflected in the final dataset, available upon request. We found that at subdaily timescales global maxima are significantly higher than was previously known, and have in a few cases briefly exceeded the human survivability limit of about a 35C wet-bulb temperature. These exceptional values are concentrated along subtropical coastlines (the Gulf of California, Red Sea, and Persian Gulf in particular), as well as the Indo-Gangetic Plain of South Asia. Globally, there are also strong trends, with extreme humid heat overall more than doubling in frequency just since 1979.
Do Eastern-US coastal waters really cool nearby land, and do models show this?
Raymond, C., and Mankin, J. (2019). Assessing present and future coastal moderation of extreme heat in the eastern United States. Environmental Research Letters. https://doi.org/10.1088/1748-9326/ab495d
We investigated the sometimes-subtle temperature difference between coastal and inland areas of the Eastern U.S. We found that in summer months, this 'coastal cooling' averages about 2C (over a band ~50 km wide) in most regions, but more like 4C in Texas and in Northern New England. These differences decrease from May to September, especially in the north. Downscaled climate models, even those with spatial resolutions comparable to the observational data that we used, underestimate this cooling by 50-75%. Looking forward, these model errors cause a relative overestimate of future population exposure to extreme heat within about 10 km of the coast, where the cooling effect is (and will likely remain) strongest. Population densities are high there, especially in Florida, making even small sea-breeze decreases in extreme heat matter.
What are the fundamental variations in humid heat across the US?
Raymond, C., Singh, D., and Horton, R. (2017). Spatiotemporal patterns and synoptics of extreme wet-bulb temperature in the contiguous United States. Journal of Geophysical Research: Atmospheres. https://doi.org/10.1002/2017jd027140
We produced the first comprehensive regional analysis of the patterns and meteorology of humid-heat extremes in the U.S. Through development of a new methodology to decompose temperature and moisture contributions, we found that these are driven primarily by temperature anomalies in the cooler and wetter parts of the country, and primarily by moisture anomalies in the hotter and drier parts of the country. We also found significant potential predictability contained in preceding sea-surface temperatures and wave-activity fluxes. Contrary to initial hypotheses, for most U.S. regions SST anomalies in the central Pacific are most closely related with humid-heat extremes, much more so than SSTs just offshore.