Human influence on the duration of extreme temperature events in Asia's hotspot regions

Observations and models indicate that human activities exert a considerable impact on the frequency and intensity of extreme temperature events,which are associated with global warming.However,changes in the duration of extreme temperature events and their association with human influence have not been considered in most studies.Thus,the possible relationship between the observed changes in the warm and cold spell duration(WSDI and CSDI)in hotspot regions during 1960-2014 and human influence was investigated based on the NCEP/NCAR reanalysis version 1 and Coupled Model Inter-comparison Project Phase 6(CMIP6)data.Constraint projection based on these attribution results was also performed.The optimal fingerprinting technique was used to compare observed changes in WSDI and CSDI to simulated changes averaged across eight CMIP6 models.Results show that anthropogenic(ANT)forcing contributed to the observed increase in WSDI in the three hotspot regions(West Asia,South Asia and Southeast Asia),with the majority of the changes being attributed to greenhouse gas forcing.However,a generally weak ANT signal can be observed in the decreasing trend of CSDI and can be detected in South and Southeast Asia.The influence of aerosol forcing remains undetected in either WSDI or CSDI,which differs from the results for frequency and intensity of extreme temperatures.The attribution results revealed that the constrained projection of WSDI is lower than the raw projection for 2015-2100 in West Asia and Southeast Asia.However,no differences in future CSDI changes are found in Southeast Asia between the constrained and raw projections.

Impact of cropland degradation in the rural-urban fringe on urban heat island and heat stress during summer heat waves in the Yangtze River Delta

China has experienced rapid urbanization since the 1980s,which has significantly reduced croplands around cities.Apart from transition to urban land,croplands have also experienced degradation into cropland/natural vegetation mosaic in the rural-urban fringe.However,scant attention has been paid to the climatic effect of such land cover change so far.This study thus investigated the potential impacts of the conversion from irrigated cropland into the mosaic on regional hot extremes and heat stress by taking the Yangtze River Delta(YRD)as an example.Four heatwave events in the summer of 2013 and 2017 were simulated using the Weather Research and Forecasting Model(WRF).Three group experiments with various representative land cover scenarios,i.e.,a default setting experiment with outdated land cover(Exp USGS),a control experiment with updated land cover(CTL),and a sensitivity experiment with modified mosaic land cover(Exp MOS),were performed for comparative analyses.Results show that CTL using the updated land cover data from China(ChinaLC)reproduced the spatial variation of observed 2-m air temperature and relative humidity better than Exp USGS.After the irrigated cropland was converted into the mosaic,the 2-m temperatures in most areas of the YRD became significantly higher than those in Exp MOS due to the lower albedo and leaf area index(LAI)in the newly emerged mosaic areas.The land cover change produced a negative contribution to the urban heat island(UHI)intensity,while it had a positive effect on extreme high temperature under heat wave conditions.This suggests a more cautious usage of the traditional definition of the UHI index is required when investigating the heat island effect,because the rural warming around an urban area may expand the heat island and enhance its heat effect.During the heatwave periods,the high temperature areas in CTL were larger than those in MOS,and the hot day areas increased by an average of 25.9%.Increase in air temperature further enhanced regional heat stress.Those results imply that the effects of land cover change in the rural-urban fringe may increase the risks of extreme hot events and heat stress for urban residents under global warming.