Detection and Attribution of Changes in Thermal Discomfort over China during 1961-2014 and Future Projections

In this study, variation in the frequency of thermal discomfort days over China during the period of 1961-2014,including heat discomfort days(HDDs) and cold discomfort days(CDDs), and the influence of external forcings on it are discussed. HDDs are the conditions of overheating and overhumidity(represented by humidity index), and CDDs reflect the dangers from cold temperatures and winds(represented by wind chill index). Observations show significant increases(decreases) in the frequency of HDDs(CDDs) over China from 1961 to 2014, with clear regional distinctions. The historical ALL and greenhouse gas(GHG) simulations can sufficiently reproduce the spatial patterns of the observational trend in the frequency of both HDDs and CDDs over China. Further, the impacts of GHG and anthropogenic forcings on the HDDs(CDDs) are detectable over China, except for central and eastern China, based on the optimal fingerprinting method. GHG forcing is identified as a dominant factor for the observational changes in the frequency of HDDs over southern China;GHG and anthropogenic forcings have dominant effects on the variation in the frequency of CDDs over southwestern China. Although trends in the frequency of HDDs and CDDs in historical aerosol forcing simulations seems to be opposite to observations, an aerosol signal fails to be detected. Natural forcing contributes to the observational variation in the frequency of HDDs over northwestern China. In addition, the future projections of thermal discomfort days indicate that Chinese residents will face more threats of heat discomfort and fewer threats of cold discomfort in the future under global warming.

Contribution of Global Warming and Atmospheric Circulation to the Hottest Spring in Eastern China in 2018

The spring of 2018 was the hottest on record since 1951 over eastern China based on station observations,being 2.5°C higher than the 1961−90 mean and with more than 900 stations reaching the record spring mean temperature.This event exerted serious impacts in the region on agriculture,plant phenology,electricity transmission systems,and human health.In this paper,the contributions of human-induced climate change and anomalous anticyclonic circulation to this event are investigated using the newly homogenized observations and updated Met Office Hadley Centre system for attribution of extreme events,as well as CanESM2(Second Generation Canadian Earth System Model)simulations.Results indicate that both anthropogenic influences and anomalous anticyclonic circulation played significant roles in increasing the probability of the 2018 hottest spring.Quantitative estimates of the probability ratio show that anthropogenic forcing may have increased the chance of this event by ten-fold,while the anomalous circulation increased it by approximately two-fold.The persistent anomalous anticyclonic circulation located on the north side of China blocked the air with lower temperature from high latitudes into eastern China.Without anthropogenic forcing or without the anomalous circulation in northern China,the occurrence probability of the extreme warm spring is significantly reduced.

Future changes in daily snowfall events over China based on CMIP6 models

China has been frequently affected by severe snowstorms in recent years that have particularly large economic and human impacts.It is thus of great importance to increase our understanding of the underlying mechanisms of and future changes in snowfall occurrences over China.In this study,the effects of anthropogenic influences on snowfall and the associated future changes are explored using new simulations from CMIP6(phase 6 of the Coupled Model Intercomparison Project)models.Observational evidence reveals a decrease in the annual total snowfall days and an increase in intense snowfall days over the snowfall-dominated regions in China during recent decades.Fingerprints of anthropogenic influences on these changes are detectable,especially the impacts of increased greenhouse gas emissions.During the winter seasons,low temperatures still cover the regions of northern China,and the associated precipitation days show an increase due to anthropogenic warming,which substantially benefits the occurrence of snowfall over these regions,particularly for intense snowfall events.This is also true in the future,despite rapid warming being projected.By the end of this century,approximately 23%of grids centered over northern China are projected to still experience an increase in daily intense snowfall events in winters.Additionally,the length of the snowfall season is projected to narrow by nearly 41 days compared to the current climate.Thus,in the future,regions of China,especially northern China,are likely to experience more intense snowfall days over a more concentrated period of time during the winter seasons.

Effects of anthropogenic forcing and atmospheric circulation on the record-breaking welt bulb heat event over southern China in September 2021

In September 2021,southern China witnessed an extreme high-temperature and high-humidity event.The average regional wet bulb globe temperature(WBGT)anomaly(relative to 1961-1990 mean)in 110.0°-120.0°E,27.5°-32.5°N region was the highest on record at 3.28°C and exceeded three times the observed standard deviation.To investigate the underlying causes,we examine the effects of anthropogenic forcings and anomalous circulation patterns on this event using the multi-model ensembles from the Coupled Model Intercomparison Project Phase 6.Results indicate that 2021-like events would happen extremely rarely without anthropogenic warming(would not occur in counterfactual world simulations)and have become a 1-in-16-year event in the factual world.For the threshold of the second most extreme year,the occurrence probability of extreme WBGT events increases approximately 50 times due to the impact of anthropogenic forcings.The effect of anthropogenic warming under similar atmosphere circulation increases the probability of extreme WBGT events by 13-60 times,and that of corresponding circulation patterns under the same anthropogenic warming increases the probability by 1.3-1.8 times.

Understanding Differences in Event Attribution Results Arising from Modeling Strategy

While there is high confidence that human activities have increased the likelihood and severity of hot extreme events over many parts of the world,there is notable spread in quantitative estimates of anthropogenic influence even on a single event.To better understand the uncertainty of attribution results,here we compare different event attribution methods using the 2015 July-August record-breaking heat event in northwestern China as a case study.To address the anthropogenic influence on the likelihood of the extreme event,we employ attribution runs with two modeling strategies—atmosphere-only and coupled simulations—with different conditioning.In atmosphere-only attribution runs,given the observed sea surface boundary conditions and external forcings in 2015,it is estimated that anthropogenic forcing has increased the likelihood of hot extremes such as that observed in 2015 in the target region,by approximately 27 and 12 times in MIROC5 and HadGEM3-A-N216,respectively.In Coupled Model Intercomparison Project phase 5(CMIP5)fully coupled attribution runs,given the external forcing at the 1961-2015 level and regardless of sea surface boundary conditions,there is a 21-fold increase in the likelihood of similar heat events due to anthropogenic forcing.The differences in quantitative attribution results can arise from modeling strategies,which are tightly linked to different conditioning in attribution.Specifically,different ocean boundary conditions,external forcings,and air-sea coupling processes contribute to different attribution results between the two modeling strategies.Within each modeling strategy,model uncertainty affects quantitative attribution conclusions.The comparison of different attribution methods provides a better understanding of the uncertainty of attribution results,which is useful in synthesizing and interpreting attribution results.

Simulations of Water Resource Changes in Eastern and Central China in the Past 130 Years by a Regional Climate Model

Anthropogenic influences on regional climate and water resources over East Asia are simulated by using a regional model nested to a global model. The changes of land use/land cover （LULC） and CO2 concentration are considered. The results show that variations of LULC and CO2 concentration during the past 130 years caused a warming trend in many regions of East Asia. The most remarkable temperature increase occurred in Inner Mongolia, Northeast and North China, whereas temperature decreased in Gansu Province and north of Sichuan Province. LULC and CO2 changes over the past 130 years resulted in a decreasing trend of precipitation in the Huaihe River valley, Shandong Byland, and Yunnan-Guizhou Plateau, but precipitation increased along the middle reaches of the Yangtze River, the middle reaches of the Yellow River, and parts of South China. This pattern of precipitation change with changes in surface evapotranspiration may have caused a more severe drought in the lower reaches of the Yellow River and the Huaihe River valley. The drought trend, however, weakened in the mid and upper reaches of the Yellow River valley, and the Yangtze River valley floods were increasing. In addition, changes in LULC and CO2 concentration during the past 130 years led to adjustments in the East Asian monsoon circulation, which further affected water vapor transport and budget, making North China warm and dry, the Sichuan basin cold and wet, and East China warm and wet.