More extreme-heat occurrences related to humidity in China

Wenyue He ,Huopo Chen

a Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

b Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters/Key Laboratory of Meteorological Disaster, Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China

c University of Chinese Academy of Sciences, Beijing, China

Keywords: Compound extremes Humidity Exposure Heat stress

ABSTRACT The co-occurrence of day and night compound heat extremes has attracted much attention because of the amplified socioeconomic and human health impacts.Based on ERA5 hourly reanalysis data,this study characterized and compared extreme day–night compound humid-heat/high-temperature events (CHHEs/CHTEs) in China as well as the associated impacts.Results indicated that the spatial patterns of summer mean extreme CHHEs are consistent with those of extreme CHTEs,except in northwestern China.A greater magnitude of these two types of events dominates over southern China,but the high-frequency centers are mainly observed over northern China.Significant increasing trends in frequency are captured nationwide,but with much stronger trends detected in northern and western China.Further analysis shows that the anomalies of humidity play a more important role than those of temperature in the occurrence of extreme CHHEs in most parts of China,but particularly in eastern regions.Since 1961,the human population and land areas of China have experienced strongly increasing compound heat extremes,with a faster rate of exposure to extreme CHHEs than to extreme CHTEs.This study highlights the importance of understanding regional changes in humidity when considering heat stress in the future.

1.Introduction

In the context of global warming,extreme heat events are occurring around the world,which have important impacts on socioeconomic development,human health,population migration,food security,and terrestrial and marine ecosystems (e.g.,Shaposhnikov et al.,2014;Xu et al.,2016;Lu and Chen,2016;Huang et al.,2020),thus arousing the attention of all sectors of society.Most current studies on extreme heat events are based on dry-bulb temperature measurements (e.g.,Perkins et al.,2012;Chen and Zhai,2017;Hao et al.,2018;He et al.,2021);however,the effect of humidity can exacerbate the effects of some extreme heat,especially on humans.Most of the heat dissipation required by the body for thermoregulation takes place through evaporative cooling by sweating.High temperatures lead to more heat input into the body,and high levels of humidity can limit the body’s ability to cool through evaporation,making extreme humid heat more physiologically stressful than extreme heat alone (Basu and Samet,2002;Sherwood and Huber,2010;Willett and Sherwood,2012).Therefore,understanding the current status of hygrothermal stress,including dry-bulb temperature and humidity,is important for understanding human and ecosystem adaptation to future climate change.

Combining temperature and humidity,the wet-bulb temperature(Twb)is a popular index of hygrothermal stress(Raymond et al.,2017;Li,2020;Chen et al.,2022).Extreme high Twb has a significant impact on aspects of human society,such as labor capacity(Kang and Eltahir,2018;Liu,2020;Parsons et al.,2022) and human health (Wehner et al.,2016;Mora et al.,2017).In the relevant literature to date,some studies have investigated the spatiotemporal patterns and causes of change in the historical extreme Twb around the world(e.g.,Raymond et al.,2017;Monteiro and Caballero,2019;Mishra et al.,2020).Unsurprisingly,China has experienced salient increases in summertime humid-heat extremes since 1961(Freychet et al.,2020;Li et al.,2020;Wang and Sun,2022;He et al.,2023),and these are expected to continue to increase in the future(Li et al.,2017,2020;Wang et al.,2021b;Chen et al.,2022).For example,Chen etal.(2022)indicatedthatextremehumid-heat eventswith maximum Twb above the 35?C physiological survival limit are expected to intensify throughout the 21st century.It is also suggested that almost every summer in China will be at least consistent with historically recorded humid-heat levels by the 2040s,and by the 2060s the summer mean wet bulb globe temperature will be generally (on average every other year) 3?C above historically recorded humid-heat levels under the RCP8.5 emission scenario(Li et al.,2020).However,previous studies associated with extreme humid-heat events over China mostly used daily-scale data and ignored the daytime and nighttime details (e.g.,Wang et al.,2019;Liu,2020;Ning et al.,2022),meaning we currently have little knowledge of day–night compound humid-heat extremes in China.Day–night compound extremes combine and amplify the adverse impacts of daytime and nighttime extremes,which is deserving of more attention.Thus,at least two issues arise here,including (a) the characteristics and changes of summer extreme compound humid-heat events(CHHEs)in China,and(b)the implications of theseextremeCHHEsfor humansandsocietyin China.By understanding the spatial and temporal patterns and variability of extreme CHHEs relative to extremecompoundhigh-temperatureevents(CHTEs),the areas atriskof hygrothermal threats can be revealed,which cannot be captured by considering temperature alone.

The outline of this paper is as follows.Section 2 introduces the main data sources and dynamic methods used in this study.The main results are provided in section 3,including the characteristics of two types of compound-heat extremes and their impacts.Conclusions are presented in section 4.

2.Data and methods

2.1.Data

The hourly reanalysis datasets used in this study are derived from the fifth major global reanalysis produced by ECMWF (ERA5;Hersbach et al.,2020;https://www.ecmwf.int/en/forecasts/datasets/reanalysi s-datasets/era5),including 2-m temperature,2-m dewpoint temperature,and surface pressure,at a horizontal resolution of 0.25?× 0.25?.The gridded population data are from NASA’s Socioeconomic Data and Applications Center(https://doi.org/10.7927/H45Q4T5F)for the years 2000,2005,2010,2015,and 2020,and have the same spatial grid resolution as the ERA5 data.Land cover data were obtained from the Land Processes Distributed Active Archive Center (https://doi.org/10.5067/MODIS/MCD12Q1.006) for 2001–2020,the resolution of which was upscaled to match that of the ERA5 data.

2.2.Methods

The hourly relative humidity and Twb were calculated according to the method of Davies-Jones (2008),i.e.,by using the 2-m dewpoint temperature,2-m air temperature(T),and surface pressure.The hourly specific humidity(q)was computed with the relative humidity,2-m air temperature,and surface pressure in conjunction with the Matlab code package developed by Dr.Robert Kopp (available at http://www.bobkopp.net/software/code/WetBulb.m).

Extreme compound events were defined as an extreme day(0800–2000 BT (Beijing Time)) with a sequential extreme night(2000–0800 BT).The extreme heat threshold was regarded as the local 95th percentile from 1 June to 31 August for 1981–2010.This definition identified two compound-heat types: (a) CHHEs,selected using the maximum Twb during daytime and nighttime,respectively;and (b)CHTEs,selected using the maximumTduring daytime and nighttime,respectively.We defined extreme CHHEs/CHTEs as the daytime maximum Twb/Texceeding its daytime threshold and the nighttime maximum Twb/Texceeding its nighttime threshold.Similar to the definition of extreme compound heat events,extreme compound wet events(CWEs)indicate the maximumqof the target daytime and nighttime exceeds the corresponding threshold(the 95th percentile),respectively.We calculated the linear trends over 1961–2020 and estimated the significance using the Mann–Kendall test (Fatichi,2020).The frequency of compound heat extremes refers to the number of extreme CHHEs/CHTEs during a certain period.The magnitude of compound heat extremes was defined as the mean dailyT/Twb of all extreme CHHEs/CHTEs.A regional extreme CHHE was calculated as the regional mean maximum Twb exceeding the local 95th percentile over the 1981–2010 baseline during daytime and nighttime,respectively.

The impact of compound heat extremes on the human population and land areas (including urban and crop areas) can be measured by heat exposure.Rogers et al.(2021)suggested that the exposure to compound heat extremes can be measured in two ways: (a) the average number of compound heat extreme days per person or unit of land area;and(b)the total number of people or land area exposed to compound heat extremes simultaneously per day.Based on this method,further studies were conducted in China.When calculating the population exposure,the missing population data are replaced by the population data for the year greater than and most recent to the fixed year.For example,the population data for 2001–2004 were replaced by the population data for 2005.

3.Results

3.1.Changes in compound heat extremes

Firstly,the temporal characteristics of extreme CHHEs/CHTEs are investigated.In most parts of China,extreme CHHEs occur in the peak of summer (late July/early August).Similarly,extreme CHTEs are also common in the peak of summer,except in the southwest and Qinghai–Tibet regions (early summer) (Fig.S1).The magnitude of extreme CHHEs and CHTEs has a similar spatial pattern,characterized by southhigh and north-low features except in northwestern China(Fig.1(a,b)).The warm and humid air due to the summer monsoon causes higher Twb andTin southeastern China,ultimately leading to more severe CHHEs and CHTEs.The regions of northwestern China are characterized by the high temperature but with insufficient water vapor,resulting in lower Twb as well as a relatively smaller magnitude of CHHEs(Fig.1(a)).This difference shows the importance of humidity in exacerbating the thermal conditions of high temperatures that lead to thermal stress.The frequency of extreme CHHEs shows north-high and south-low patterns(Fig.1(c,d)).Specifically,extreme CHHEs occur frequently over Qinghai,Inner Mongolia,and Northeast China,while the lowest frequency of extreme CHHEs appears in Southwest China and the Tibetan region.In contrast,the frequency of extreme high-temperature events exhibits a more consistent pattern in China,and its high-value areas are more dispersed,such as in northwestern China and the Yangtze River basin.

Fig.1.Mean (a,b) magnitude (units:?C) and (c,d) frequency (units: d) of extreme (a,c) CHHEs and (b,d) CHTEs over China during 1961–2020.

Fig.2 shows the long-term trends of compound heat extremes in summer during 1961– 2020.As shown in Fig.2(a,b),the frequency of summer extreme CHHEs has experienced a significant increasing trend in most of China during the past few decades.Compared with extreme CHTEs,the trend of extreme CHHE frequency shows a similar pattern but with a smaller increasing rate.Specifically,the frequency of compound heat extremes has increased by 0.12 d/10 yr for extreme CHHEs and 0.25 d/10 yr for extreme CHTEs(Fig.2(c)).The greatest increasing trend in the two types of heat extremes mainly occurs in western and northeastern China,which is related to its stronger warming signal.Meanwhile,the southeastern regions of China experience insignificant or even weak decreasing trends,though the magnitude of extreme events is at a high level.Note that the trends in CHHE and CHTE frequency are consistent in sign in most parts of China (Fig.2(d)).The regions in eastern China with opposite trends show an increasing trend in CHTEs but a decreasing trend in CHHEs,which may be due to a decrease in precipitation and humidity (Hartmann et al.,2013).The above results imply that the humidity may be responsible for the diverging climatological characteristics between extreme CHHEs and CHTEs.

Fig.2.Linear trends in summer (a) extreme CHHEs (units: d/10 yr) and (b) extreme CHTEs (units: d/10 yr) during 1961–2020.(c) Time series of extreme CHHEs(solid red) and CHTEs (solid blue) heat days over China during 1961–2020.Dashed lines show the corresponding linear trends.(d) Level of consistency between trends in extreme CHHEs and CHTEs(1:same sign,both significant;2:same sign,one significant;3:both not significant;4:different signs,one significant;5:different signs,both significant).Trend significance in (a),(b),and (d) is determined using the Mann–Kendall test at the 95% confidence level.

3.2.The role of humidity

Extreme CHHEs using the Twb metric are regulated by a combination of temperature and humidity.To quantify the effects of temperature and humidity on extreme CHHEs,the co-occurrence of extreme CHHEs with extreme CHTEs and extreme CWEs is calculated,respectively.As shown in Fig.3(a),the percentage overlap between extreme CHHEs and extreme CHTEs is less than 20% in most parts of China.In contrast,extreme CHHEs and CWEs are often observed simultaneously,especially in Northeast China and North China (Fig.3(b)).

Fig.3.The percentage overlap (%) between extreme CHHEs and (a) extreme CHTEs and (b) extreme CWEs in each grid cell over China during 1961–2020.(c–f)Scatterplots of the relationship between the local daily mean T (units: ?C) and q (units: g kg-1) in (c) northwestern China (NWC;38?–49?N,80?–98?E),(d)northeastern China(NEC;40?–54?N,119?–135?E),(e)Huang-Huai(HH;34?–40?N,105?–122?E),and(f)southern China(SC;20?–34?N,105?–122?E),where the red(blue) dots represent the regional extreme CHHEs (non-extreme CHHEs).The large black dots within the scatter are the means of each set and are connected by a solid yellow line.The more vertical the vector from the blue square to the red square,the more q-dominated a region’s extreme CHHEs.

To further identify the roles ofTandqin extreme CHHEs in different regions over China,the daily meanTandqassociated with the regional extreme CHHEs are further calculated.As shown in Fig.3(c),extreme CHHEs in northwestern China have a wider range ofqandT.In contrast,there is a stronger linear correlation betweenTandqover eastern parts of China (Fig.3(d–f)).The increasing magnitude of regional extreme CHHEs from north to south is mainly associated with the increases ofTandqfrom north to south in the eastern regions.The high sensitivity of Twb to humidity in eastern China is consistent with the higher cooccurrences of extremeTandqin these regions (Fig.3(b)).Also,the sensitivity is ranked as Huang-Huai>Northeast China>South China.Thus,it is important to understand the humidity changes in a warming world.

3.3.The impact of compound heat extremes

Changes in the frequency of compound heat extremes have resulted in increased population and land surface exposure,which may lead to devastating impacts (Kornhuber et al.,2019;Wang et al.,2021a).To explore this issue,the exposure of population-weighted and region-weighted compound heat extremes in China is calculated.It is clear that the regions of the eastern part of China are most severely affected by compound heat extremes,which is partly attributable to the dense population and rapid urbanization(Fig.4(a–c)).Specifically,the population exposed to extreme CHHEs is mainly concentrated in East China,especially in the regions around Shandong and Jiangsu provinces.Meanwhile,the population affected by extreme CHTEs is relatively dispersed,with the high values mainly located in the Huang-Huai,South China,and Southwest China regions.The urban area affected by extreme CHHEs and CHTEs is mainly found in East China,especially in Shandong Province,while the crop area is concentrated in East and Northeast China.Interestingly,some regions,such as Shandong and Jiangsu provinces,have significantly higher population and land exposure to extreme CHHEs than to extreme CHTEs,which illustrates the importance of considering humidity in assessing the impact of compound heat extremes against the background of climate change.Furthermore,the average number of days of extreme CHHE exposure increased by about 1.4 days per person per decade,and the average number of days of extreme CHHE exposure to urban areas increased by 1.7 days per unit land area per decade,while for the crop area it was 1.5 days.Similar to the increasing trend in exposure to extreme CHHEs,the exposure of the human population and land areas to extreme CHTEs has increased but with a slower rate for population(0.7 days per person per decade),urban area (0.8 days per unit land area per decade),and crop area(0.7 days per unit land area per decade),respectively.Note that the urban area exposure to compound heat extremes increased faster than that of the crop area,indicating that urbanization increases the risk of compound heat extremes in cities.

Fig.4.The (a,d) population,(b,e) urban land surface (units: km2) and (c,f) crop land surface (units: km2) exposure to (a–c) extreme CHHEs and (d–f) extreme CHTEs during summer 2020.(g–i) The number of extreme heat days (g) per person,(h) per unit urban land area (km2),and (i) per unit crop land area (km2) for extreme CHHEs (solid red) and extreme CHTEs (solid blue).Dashed lines in (g–i) show significant linear trends.

Therefore,extreme CHHEs and CHTEs have become more frequent and cause more exposure to the human population and land areas.Given the higher population and land exposure to extreme CHHEs and its faster increasing trend(Fig.4 and Fig.S2),particularly in vulnerable areas,our findings emphasize the need to better understand the challenges of heat stress for human health and society.

4.Conclusions

In this study,the changing characteristics of summer compound heat extremes (including extreme CHHEs and CHTEs) over China during 1960-2020 and their impacts were examined.In most parts of China,extreme CHHEs mainly occur in the peak of summer (late July/early August).The timing of extreme CHTEs follows a similar pattern but is focused in early summer in Southwest China and the Tibetan Plateau region.Generally,compound heat extremes show higher magnitude in southern China but higher occurrence in northern China.Aligning with the present climate,extreme CHHEs have experienced a significant increasing trend over most parts of China,but at a lower rate than extreme CHTEs.

The spatial and temporal characteristics of extreme CHHEs are highly similar to those of extreme CHTEs,but they have a low percentage overlap;whereas,extreme CHHEs and extreme CWEs are always simultaneously observed in much of eastern China,especially in Northeast China and North China.The range of variation in temperature and humidity in northwestern China is larger than that in the east,which is consistent with previous findings (Wang et al.,2019).The degree of sensitivity of humidity variation to extreme CHHEs varies in different regions of China.Except in the Qinghai–Tibet regions,extreme CHHEs are more sensitive to humidity changes in Huang-Huai.On the contrary,changes in humidity have less impact on extreme CHHEs in South China and Northwest China.

Further analyses indicated that the population and land exposure to compound heat extremes has increased disproportionately in China.The population and land in some vulnerable areas,such as Shandong Province and Jiangsu Province,have experienced more severe humid-heat challenges than high temperatures.A greater increase was identified in the exposure of the population,cities,and crop land to CHHEs.Our study identifies and highlights the importance of humidity in compound heat extremes.Therefore,the understanding and limitation of humidity may be an important step in dealing with the regional changes of heat stress in the future.

Funding

This study was jointly supported by the National Key Research and Development Program of China [grant number 2022YFF0801303] and the National Natural Science Foundation of China [grant numbers 41991284 and 42075021].

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.aosl.2023.100391.