Growing threats from heat, drought and whiplash catalyzed by global warming from past to future

Abstract

Global warming would escalate the risks of natural hazards by magnifying their frequency and severity, which might consequently lead growing threats to human health, infrastructure and ecosystem. According to our study, heat-related hazards, including heatwave, flash drought and compound weather whiplash, all exhibit significant trends toward higher frequency and enhanced strength in both historical and future contexts over Asia where large population resides. Specifically, for heatwave with extremely high temperature persisting for days, we investigate its spatiotemporal variations and characteristics on both grid and event bases. We find that persistent, extensive and intense heatwave has become more frequent over the last four decades due to the increase of temperature. Apart from global warming, the association between heatwave and its key dynamic driver on synoptic scale (i.e., blocking) is also analyzed. Two prevailing blocking indices are applied to the association analysis to ensure the robustness of analysis results, examining 500hpa geopotential height (TM index) and vertically averaged potential vorticity anomaly (PV index), respectively. Although certain discrepancy between blocking climatology of TM index and PV index is exhibited, they both agreeably suggest that blocking favors the occurrence of heatwave, especially in the high-latitude region where blocking often occurs. Moreover, the extended temporal association with time lags between heatwave and blocking reveals that heatwave mostly starts no earlier and ends no earlier than blocking. It indicates that blocking is more of a favorable environmental condition to trigger heatwave than maintain it. Lastly, the impact of blocking on the characteristics of heatwave events is explored on an event basis using a 3D spatiotemporal object model. We find that blocking related heatwave events are more likely to be more persistent, extensive and intense than unrelated events. Flash drought that featured by abrupt onset and rapid intensification is also suggested to increase under global warming in the past four decades. Moreover, the temporal evolving and spatial varying contribution of individual meteorological forcing to the development of flash drought is unraveled by a deep learning method—long short-term memory (LSTM) networks with an interpretation method — SHAP approach. We find that generally precipitation deficit is the most important driver to flash drought intensification, which is closely followed by high temperature and low humidity. Given the spatial heterogeneity of feature importance, the whole study region over east China is further clustered into three sub-regions through K-means method. Interestingly, the spatial clustering result well reflects both topography and climatic background though no location information is directly given. Specifically, the three clusters center in Taihang Mountain region, North China Plain and Lower reaches of Yangtze River Basin (LRYRB), respectively. We find that precipitation contributions more to flash drought development over the humid sub-region, i.e., LRYRB than the other two semi-arid sub-regions, possibly due to the higher requirement of anomalous dryness to trigger flash drought in humid area. While high temperature and low humidity exhibit the opposite pattern. Moreover, global warming is found to not only magnify the flash drought occurrence but also escalate the contribution of high temperature to its intensification. Lastly, the enhanced climate variability might lead to more collision of drought/heat and pluvial amid a warming future. As the sudden swings between drought/heat and pluvial could cause adverse impacts far surpassing the sum of their individual effect, we propose a concept of intraseasonal “compound whiplash event” (CWE) to investigate sudden swings between wet and the compounding warm-dry events and their changes under climate change. We find that global warming will likely escalate the compound whiplash frequency to two to three and half times (two to three times) by the end of the 21^st century under the business-as-usual scenario (mitigated scenario). The growing threat of compound whiplash events not only stems from the increasing occurrence but also from its intensified severity and extended spatial coverage. Among all sub-regions over Asia, East Asian summer monsoon (EASM) region expects the largest intensification. The resulting population exposure would soar two-to-three-fold over Asia. Populous regions such as North India and EASM region might face a much worse situation than the western China where population is sparse and projected to decline. Moreover, the seasonality of swings with opposite directions will further split as a response to the skewed Asian monsoon annual cycle, leading to more frequent heat-drought to pluvial swings in spring, and more opposite- direction swings in autumn, disrupting cultivation and water management convention. To summary, based on the comprehensive investigation on the changing risks of heat-related hazards over Asia under global warming, we provide an integrated database and useful information directly relevant to climate adaptation and infrastructure/water resource planning. We also strongly call for effective mitigations in place to reduce carbon emissions to diminish the risk of aforementioned hazards that potentially damaging.

Date of Award	2022
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Mengqian LU (Supervisor)