低排放情景下全球极端气候事件变化在温升过冲前后达到1.5℃的差异

基于耦合模式比较计划第六阶段(CMIP6)全球气候模式在SSP1-1.9温室气体排放情景下的模拟结果,预估了全球平均气温相对于工业化前达到1.5℃温升(P1阶段)后继续增暖然后再次返回(过冲)到1.5℃温升时的(P2阶段)全球气温、降水及极端气候指数的可能变化,并且分析其预估不确定性。结果表明:P1和P2两阶段间气温、降水及极端气候的多模式一致的差异在全球各分区广泛出现,且区域性和局地性特征明显。各指标表征的多模式一致的极端温度变化普遍接近或者超过全球陆地面积的15%。极端低温的P1和P2两阶段差异的空间分布与冬季平均气温差异的空间分布有一定相似度,极端高温变化的分布则更凸显局地性。全球范围内低温和高温多模式一致增加的面积都高于其减少的面积,预计欧亚大陆中高纬的西部、北美洲、中国东北等区域的低温风险以及青藏高原、中国东部、南亚、东非、北美洲、南极洲等区域的高温风险会升高。各指标表征的多模式一致的极端降水变化普遍超过全球陆地面积的20%,其中增加与减少的面积接近。强降水差异的分布特征与年降水的有部分相似,一致增多主要分布在中国南方、南亚、东南亚、南美洲东部和西南端、北美、澳洲和中东欧的部分地区等,一致减少主要分布在中国华北到东北、青藏高原南麓、非洲南部、南美洲北部和澳洲北部等。连续干旱日数在多数区域表现为增多,集中且连续地分布在中亚、南亚、青藏高原、俄罗斯中北部、非洲撒哈拉以北和中部、澳洲中部、南极洲部分地区等。这些都表明即使全球在2030年前达到碳排放峰值,并立即开始减少碳排放,但由于各个区域气候对全球变暖的响应有明显差异,温升过冲后部分区域的极端事件发生频率并未及时回退到过冲前的水平,需警惕区域和局地尺度气象灾害及其影响的增加。

Changes in temperature extremes over China under 1.5 ℃ and 2 ℃ global warming targets

The long-term goal of the 2015 Paris Agreement is to limit global warming to well below 2 ℃above pre-industrial levels and to pursue efforts to limit it to 1.5 ℃. However, for climate mitigation and adaption efforts, further studies are still needed to understand the regional consequences between the two global warming limits. Here we provide an assessment of changes in temperature extremes over China （relative to 1986-2005） at 1.5 ℃ and 2 ℃ warming levels （relative to 1861-1900） by using the 5th phase of the Coupled Model Intercomparison Project （CMIP5） models under three RCP scenarios （RCP2.6, RCP4.5, RCP8.5）. Results show that the increases in mean temperature and temperature extremes over China are greater than that in global mean temperature. With respect to 1986-2005, the temperature of hottest day （TXx） and coldest night （TNn） are projected to increase about 1/1.6 ℃ and 1.1/1.8 ℃, whereas warm days （TX90p） and warm spell duration （WSDI） will increase about 7.5/13.8% and 15/30 d for the 1.5/2 ℃ global warming target, respectively. Under an additional 0.5 ℃ global warming, the projected increases of temperature in warmest day/night and coldest day/night are both more than 0.5 ℃ across almost the whole China. In Northwest China, Northeast China and the Tibetan Plateau, the projected changes are particularly sensitive to the additional 0.5 ℃ global warming, for example, multi-model mean increase in coldest day （TXn） and coldest night （TNn） will be about 2 times higher than a change of 0.5 ℃ global warming. Although the area-averaged changes in temperature extremes are very similar for different scenarios, spatial hotspot still exists, such as in Northwest China and North China, the increases in temperatures are apparently larger in RCP8.5 than that in RCP4.5.

Changes in surface air temperature over China under the 1.5 and 2.0 ℃ global warming targets

This study investigated the projected changes in the annual mean surface air temperature （SAT） over China under the 1.5 and 2.0 ℃ targets, by analyzing the outputs from 22 models of the Coupled Model Intercomparison Project Phase 5. Under the 1.5 ℃ target, the scope of changes in the average SAT over China is quite narrow and has the largest probability to increase by 1.7-2.0 ℃ under the various RCP pathways, although the time of occurrence of the 1.5 ℃ target has a large spread of 40-60 years. Similarly, the models consistently show that the average SAT over China would most likely increase by 2.4-2.7 ℃ under the 2.0 ℃ target. Furthermore, the warming shows a clear spatial distinction over China： being stronger in the northwest part and weaker in the southeast part. Under all RCP pathways, the SAT over the northwest part would increase by 1.9-2.1 ℃ for the 1.5℃ target, which is much stronger than the SAT increase over the southeast part （1.3-1.5 ℃）. A similar spatial pattern appears for the 2.0 ℃ target.

Changes of heating and cooling degree days over China in response toglobal warming of 1.5℃, 2℃, 3℃ and 4℃

Future changes of heating degree days （HDD） and cooling degree days （CDD） in the 21st century with and without considering populationfactor are investigated based on four sets of climate change simulations over East Asia using the regional climate model version 4.4 （RegCM4.4）driven by the global models of CSIRO-Mk3-6-0, EC-EARTH, HadGEM2-ES, and MPI-ESM-MR. Under global warming of 1.5℃, 2℃, 3℃,and 4℃, significant decrease of HDD can be found over China without considering population factor, with greater decrease over high elevationand high latitude regions, including the Tibetan Plateau, the northern part of Northeast China, and Northwest China; while population-weightedHDD increased in areas where population will increase in the future, such as Beijing, Tianjin, parts of southern Hebei, northern Shandong andHenan provinces. Similarly, the CDD projections with and without considering population factor are largely different. Specifically, withoutconsidering population, increase of CDD were observed over most parts of China except the Tibetan Plateau where the CDD remained zerobecause of the cold climate even under global warming; while considering population factor, the future CDD decreases in South China andincreases in North China, the Sichuan Basin, and the southeastern coastal areas, which is directly related to the population changes. The differentfuture changes of HDD and CDD when considering and disregarding the effects of population show that population distribution plays animportant role in energy consumption, which should be considered in future research.

Future Changes in Extreme High Temperature over China at 1.5℃-5℃ Global Warming Based on CMIP6 Simulations

Extreme high temperature(EHT)events are among the most impact-related consequences related to climate change,especially for China,a nation with a large population that is vulnerable to the climate warming.Based on the latest Coupled Model Intercomparison Project Phase 6(CMIP6),this study assesses future EHT changes across China at five specific global warming thresholds(1.5℃-5℃).The results indicate that global mean temperature will increase by 1.5℃/2℃ before 2030/2050 relative to pre-industrial levels(1861-1900)under three future scenarios(SSP1-2.6,SSP2-4.5,and SSP5-8.5),and warming will occur faster under SSP5-8.5 compared to SSP1-2.6 and SSP2-4.5.Under SSP5-8.5,global warming will eventually exceed 5℃ by 2100,while under SSP1-2.6,it will stabilize around 2℃ after 2050.In China,most of the areas where warming exceeds global average levels will be located in Tibet and northern China(Northwest China,North China and Northeast China),covering 50%-70%of the country.Furthermore,about 0.19-0.44 billion people(accounting for 16%-41%of the national population)will experience warming above the global average.Compared to present-day(1995-2014),the warmest day(TXx)will increase most notably in northern China,while the number of warm days(TX90p)and warm spell duration indicator(WSDI)will increase most profoundly in southern China.For example,relative to the present-day,TXx will increase by 1℃-5℃ in northern China,and TX90p(WSDI)will increase by 25-150(10-80)days in southern China at 1.5℃-5℃ global warming.Compared to 2℃-5℃,limiting global warming to 1.5℃ will help avoid about 36%-87%of the EHT increases in China.

Intensified East Asian summer monsoon and associated precipitation mode shift under the 1.5 ℃ global warming target

In this study, the East Asian summer climate changes under the 1.5 ℃ global warming （1.5 GW） target in 30 simulations derived from 15 coupled models within the Coupled Model Intercomparison Program phase 5 （CMIP5） are examined. Compared with the current summer climate （1975-2005）, both surface air temperature and precipitation increase significantly over the East Asian continent during the 1.5 GW period （average period 2021-2051）. In northeastern China this is particularly pronounced with regional averaged precipitation increases of more than 7.2%, which is greater than that for the whole East Asian continent （approximately 4.2%）. Due to stronger enhancement of precipitation north of 40°N, the leading empirical orthogonal function （EOF） mode of summer precipitation over the East Asian continent changes from tripolar-like mode to dipole mode. As there is stronger surface warming over the East Asian continent than that over surrounding ocean, the land-sea thermal contrast is enhanced during the 1.5 GW period. As a result, the monsoon circulation in the lower troposphere is significantly strengthened, which causes the increased summer precipitation over the East Asian continent. In addition, larger interannual variabilities of East Asian summer monsoon circulation and associated precipitation are also suggested for the 1.5 GW period.

Responses and changes in the permafrost and snow water equivalent in the Northern Hemisphere under a scenario of 1.5℃ warming

In this study, the period that corresponds to the threshold of a 1.5℃ rise (relative to 1861e1880) in surface temperature is validated using a multi-model ensemble mean from 17 global climate models in the Coupled Model Intercomparison Project Phase 5 (CMIP5). On this basis, the changes in permafrost and snow cover in the Northern Hemisphere are investigated under a scenario in which the global surface temperature has risen by 1.5℃, and the uncertainties of the results are further discussed. The results show that the threshold of 1.5℃ warming will be reached in 2027, 2026, and 2023 under RCP2.6, RCP4.5, RCP8.5, respectively. When the global average surface temperature rises by 1.5℃, the southern boundary of the permafrost will move 1e3.5
northward (relative to 1986e2005), particularly in the southern Central Siberian Plateau. The permafrost area will be reduced by 3.43x106 km2 (21.12%), 3.91x106 km2 (24.1%) and 4.15x106 km2 (25.55%) relative to 1986e2005 in RCP2.6, RCP4.5 and RCP8.5, respectively. The snow water equivalent will decrease in over half of the regions in the Northern Hemisphere but increase only slightly in the Central Siberian Plateau. The snow water equivalent will decrease significantly (more than 40% relative to 1986e2005) in central North America, western Europe, and northwestern Russia. The permafrost area in the QinghaieTibet Plateau will decrease by 0.15x106 km2 (7.28%), 0.18x 106 km2 (8.74%), and 0.17x106 km2 (8.25%), respectively, in RCP2.6, RCP4.5, RCP8.5. The snow water equivalent in winter (DJF) and spring (MAM) over the QinghaieTibet Plateau will decrease by 14.9% and 13.8%, respectively.

When and how will the Millennium Silk Road witness 1.5 °C and 2 °C warmer worlds?

Western China and central Asia are positioned centrally along the Millennium Silk Road,which is regarded as a core region bridging the East and the West.Understanding the potential changes in climate over this core region is important to the successful implementation of the so-called＇Belt and Road Initiative＇（a $1 trillion regional investment in infrastructure）.In this study,both mean and extreme climate changes are projected using the ensemble mean of CMIP5 models.The results show a warming of ～1.5,2.9,3.6,and 6.0 ℃ under RCP2.6,4.5,6.0,and 8.5,respectively,by the end of the twenty-first century,with respect to the 1986-2005 baseline period.Meanwhile,the annual mean precipitation amount increases consistently across all RCPs,with an increase by ～14% with respect to 1986-2005 under RCP8.5.The warming over the Millennium Silk Road region reaches 1.5 ℃ before 2020 under all the emission scenarios.The 2020s （2030s） see a 2 ℃ warming under the RCP8.5 （RCP4.5） scenario.Global warming that is 0.5 ℃ lower （i.e.a warming of 1.5 ℃） could result in the avoidance of otherwise significant impacts in the Silk Road core region-specifically,a further warming of 0.73 ℃ （with an interquartile range of 0.49%-0.94 ℃） and an increase in the number of extreme heat days by 4.2,at a cost of a reduced increase of 2.72% （0.47%-3.82%） in annual precipitation.The change in consecutive dry days is region-dependent

Intraseasonal oscillation intensity over the western North Pacific:Projected changes under global warming

The 30-60-day intraseasonal oscillation(ISO) and 10-20-day ISO are two dominant oscillation modes over the western North Pacific during boreal summer.With daily data derived from eight CMIP5 models,changes of the ISO intensities are projected under the 1.5 and 2.0℃ global warming levels under the Representative Concentration Pathway(RCP) 4.5 and RCP8.5 scenarios.Most of the models agree that the ISO intensities increase along a belt region from the south Indochina Peninsula(ICP) to the east to the Philippines.The variation pattern shows little difference between different warming levels or scenarios.Results indicate that the spatial distribution of ISO anomalies is related with the variation of background fields.Enriched lower-level humidity and moist static energy favor the intensity increases of ISOs,which are projected to be larger over the whole western North Pacific,with the most conspicuous changes located over the east to the Philippines for humidity but over the south of the ICP for moist static energy.In contrast,the ISOs over the west to Indonesia and northeast to the Philippines decrease,which is consistent with the local descending motions.

Future sea level rise along the coast of China and adjacent region under 1.5℃ and 2.0℃ global warming

Although future sea level rise along the China coast has been projected by various studies for different representative concentration pathways(RCPs),the projections for different warming thresholds,e.g.1.5℃ and 2.0℃,have not been done specifically for this region,to the best of our knowledge.We provide such a projection here based on the climate projections of Coupled Model Intercomparison Project Phase 5(CMIP5).The projections are given for 20 tide-gauge stations along the coast of China,Korea,Japan,and Vietnam.Vertical land motion(VLM)is also estimated for stations that have tide gauge records and satellite altimetry both covering the period of 1993-2018.Local land motion(LLM)is then estimated by subtracting the land motion due to glacial isostatic adjustment(GIA)from VLM.Without considering LLM,sea level rise by 2100 at median probability is projected to be 38-49 cm relative to the average sea level over 1986-2005 under warming of 1.5℃,and increase to 46-57 cm when the warming threshold is increased to 2.0℃.The steric component is the main contributor to this increase in sea level.Inclusion of LLM will not affect the sea level increase between the two warming thresholds,but it will make the local sea level rise by 2100 at certain locations substantially higher(up to 36 cm)or lower(up to 13 cm).

Risks of temperature extremes over China under 1.5℃ and 2℃ global warming

The Paris Agreement aims to keep global warming to well below 2℃ above pre-industrial levels and to pursue efforts to limit it to 1.5℃,recognizing this will reduce the risks of natural disasters significantly.As changes in the risks of temperature extremes are often associated with changes in the temperature probability distribution,further analysis is still needed to improve understanding of the warm extremes over China.In this study,changes in the occurrence probability of temperature extremes and statistic characteristics of the temperature distribution are investigated using the fifth phase of the Coupled Model Intercomparison Project(CMIP5)multimodel simulations from 1861 to 2100.The risks of the once-in-100-year TXx and TNx events are projected to increase by 14.4 and 31.4 times at 1.5℃ warming.Even,the corresponding risks under 2℃ global warming are 23.3 and 50.6,implying that the once-in-100-year TXx and TNx events are expected to occur about every 5 and 2 years over China,respectively.The Tibetan Plateau,Northwest China and south of the Yangtze River are in greater risks suffering hot extremes(both day and night extremes).Changes in the occurrence probability of warm extremes are generally well explained by the combination of the shifts in location and scale parameters in areas with grown variability,i.e.,the Tibetan Plateau for TXx,south of the Yangtze River for both TXx and TNx.The location(scale)parameter leading the risks of once-in-20-year TXx to increase by more than 5(0.25)and 3(0.75)times under 2℃ warming in the Tibetan Plateau and south of the Yangtze River,respectively.The location parameter is more important for regions with decreased variability e.g.,the Tibetan Plateau for TNx,Northwest China for both TXx and TNx,with risks increase by more than 3,6 and 4 times due to changes in location.

Regional changes in extreme heat events in China under stabilized 1.5℃ and 2.0℃ global warming

Extreme heat events(EHEs)have a significant impact on the social economy and human health.China is a country with a large population and diverse terrain,and it is necessary to project future extreme heat changes in the sub-regions.This study used a specially designed dataset,the Community Earth System Model(CESM)simulations,namely CESM low-warming,to investigate the EHEs in China under 1.5℃ and 2.0℃ global warming.The results indicate that the regional mean warming over China will exceed the global average,about 1.63℃ and 2.24℃ in 1.5℃ and 2.0℃ warmer futures.Compared to the present-day(1976–2005),the frequency and duration of the EHEs in South China are projected to increase the most among the sub-regions.For example,the frequency of EHEs in South China at 1.5℃ and 2.0℃ warming will exceed 3 and 3.5 times the present-day level.However,when global warming rises from 1.5℃ to 2.0℃,the increased impacts relative to the 1.5℃ warming level will be the lowest in South China(less than 40%),and the highest increased impacts are projected to appear in Northeast China(53%-84%)and Northwest China(53%–107%).The main reason for this situation is that compared with the 1.5℃ scenario,the upper zonal westerly in northern China weakens and the continental high pressure enhances under the 2.0℃ scenario.Therefore,limiting global warming at 1.5℃ instead of 2.0℃ is beneficial for eliminating extreme heat events,especially for Northeast China and Northwest China.

Increased Tibetan Plateau vortex activities under 2℃warming compared to 1.5℃warming:NCAR CESM low-warming experiments

The Tibetan Plateau vortices(TPVs)are the major rain-producing systems over the Tibetan Plateau(TP).The activities of TPVs are closely related to TP's water source,which supplies fresh water to millions of people in Asia.Projection of the TPVs can increase understanding about the future of water supply in Asia under global warming.In this study,the possible activities of TPVs under 1.5℃and 2℃warming scenarios above the pre-industrial level are evaluated through the NCAR CESM(Community Earth System Model)Low-warming(CESM-LW)Experiments.The results show that the CESM-LW well reproduces the spatio-temporal characteristics of TPVs in the historical run from 1985 to 2000.The CESM-LW suggests TPVs in warm season(May-September)increase by 15%due to the additional 0.5℃warming by the end of this century(2071—2100).It implies the greater importance of TPVs to the precipitation over the TP in the future.The changes of TPVs are closely related to the large-scale circulations adjustments.The additional 0.5℃warming strengthens the temperature difference between the TP and its surrounding areas,which results in an enhanced convergence near the TP's surface and divergence in the upper troposphere by about-0.1×10^(-6)and 0.22×10^(6)s^(-1),respectively.The assessment of future TPVs provides a synoptic dynamic perspective on the climate change of precipitation and water resources.

Multi-Model Ensemble Projection of Precipitation Changes over China under Global Warming of 1.5 and 2℃ with Consideration of Model Performance and Independence

A weighting scheme jointly considering model performance and independence(PI-based weighting scheme) is employed to deal with multi-model ensemble prediction of precipitation over China from 17 global climate models. Four precipitation metrics on mean and extremes are used to evaluate the model performance and independence. The PIbased scheme is also compared with a rank-based weighting scheme and the simple arithmetic mean(AM) scheme. It is shown that the PI-based scheme achieves notable improvements in western China, with biases decreasing for all parameters. However, improvements are small and almost insignificant in eastern China. After calibration and validation, the scheme is used for future precipitation projection under the 1.5 and 2℃ global warming targets(above preindustrial level). There is a general tendency to wetness for most regions in China, especially in terms of extreme precipitation. The PI scheme shows larger inhomogeneity in spatial distribution. For the total precipitation PRCPTOT(95 th percentile extreme precipitation R95 P), the land fraction for a change larger than 10%(20%) is 22.8%(53.4%)in PI, while 13.3%(36.8%) in AM, under 2℃ global warming. Most noticeable increase exists in central and east parts of western China.

Mid-summer surface air temperature and its internal variability over China at 1.5℃ and 2℃ global warming

Recently,extremely hot summers occurred frequently across China,and the mean mid-summer surface air temperature(SAT)continuously broke the records of the past decades,causing huge social and economic losses.As global warming accelerates,these extremely hot summers will undoubtedly occur more frequently.However,the issue of what will happen to the mid-summer SAT over China in the near future remains unclear.Therefore,we investigate the changes of mid-summer SAT and related internal variabilities over China at 1.5℃ and 2℃ global warming above preindustrial level by using the MPI-ESM Grand Ensemble simulations.The results indicate that compared to the present-day(1986–2005),national averaged mid-summer SAT will increase by 1.1℃ and 2.0℃,in 1.5℃ and 2℃ warming scenarios respectively.This means that the mid-summer SAT is projected to increase by 0.9℃ due to an additional 0.5℃ global warming,which is higher than the annual value(0.8℃)and almost two times the global warming rate.Regionally,in the two warming targets,the increase in mid-summer SAT will be more enhanced over the northwestern part of China.In addition,the extremely high monthly SAT would increase nationwide due to an additional 0.5℃ in global warming.Among all areas,the Qinghai and Xinjiang provinces would experience the strongest increase in extremely high monthly SAT.It is important to find that,from 1.5℃ to 2℃ global warming,changes of the internal variability of the mid-summer SAT differs across China.It would decrease over some parts of western Northwest China,North China,Northeast China and the Tibetan Plateau.However,it would significantly increase over Qinghai,Sichuan,and northern parts of Inner Mongolia.As a result,at 2℃ global warming,the increase of extreme SAT in Qinghai is caused by the synergistic effect of stronger warming rate and larger internal variability.Differently,the increase in Xinjiang province is mainly caused by the stronger local warming.Further analysis suggests that we can effectively reduce the intensity of extremely hot months over most regions of Northwest China by limiting global warming to 1.5℃,rather than to 2℃.

Spatiotemporal variations of aridity index over the Belt and Road region under the 1.5℃ and 2.0℃ warming scenarios

Aridity index reflects the exchanges of energy and water between the land surface and the atmosphere,and its variation can be used to forecast drought and flood patterns,which makes it of great significance for agricultural production.The ratio of potential evapotranspiration and precipitation is applied to analyse the spatial and temporal distributions of the aridity index in the Belt and Road region under the 1.5℃and 2.0℃global warming scenarios on the basis of outputs from four downscaled global climate models.The results show that:(1)Under the 1.5℃warming scenario,the area-averaged aridity index will be similar to that in 1986-2005(around 1.58),but the changes vary spatially.The aridity index will increase by more than 5%in Central-Eastern Europe,north of West Asia,the monsoon region of East Asia and northwest of Southeast Asia,while it is projected to decrease obviously in the southeast of West Asia.Regarding the seasonal scale,spring and winter will be more arid in South Asia,and the monsoon region of East Asia will be slightly drier in summer compared with the reference period.While,West Asia will be wetter in all seasons,except winter.(2)Relative to 1986-2005,both areal averaged annual potential evapotranspiration and precipitation are projected to increase,and the spatial variation of aridity index will become more obvious as well at the 2.0℃warming level.Although the aridity index over the entire region will be maintained at approximately 1.57 as that in 1.5℃,the index in Central-Eastern Europe,north of West Asia and Central Asia will grow rapidly at a rate of more than 20%,while that in West Siberia,northwest of China,the southern part of South Asia and West Asia will show a declining trend.At the seasonal scale,the increase of the aridity index in Central-Eastern Europe,Central Asia,West Asia,South Asia and the northern part of Siberia in winter will be obvious,and the monsoon region in East Asia will be drier in both summer and autumn.(3)Under the scenario of an additional 0.5℃increase in global temperature from 1.5℃to 2.0℃,the aridity index will increase significantly in Central Asia and north of West Asia but decrease in Southeast Asia and Central Siberia.Seasonally,the aridity index in the Belt and Road region will slightly increase in all other seasons except spring.Central Asia will become drier annually at a rate of more than 20%.The aridity index in South Asia will increase in spring and winter,and that in East Asia will increase in autumn and winter.(4)To changes of the aridity index,the attribution of precipitation and potential evapotranspiration will vary regionally.Precipitation will be the major influencing factor over southern West Asia,southern South Asia,Central-Eastern Siberia,the non-monsoon region of East Asia and the border between West Asia and Central Asia,while potential evapotranspiration will exert greater effects over Central-Eastern Europe,West Siberia,Central Asia and the monsoon region of East Asia.

Projection of weather potential for winter haze episodes in Beijing by 1.5℃ and 2.0℃ global warming

Haze episodes become very frequent in Beijing over the past decade,and such trend is related to favorable weather conditions.Here,we project the changes of weather conditions conducive to winter haze episodes in Beijing by 1.5℃ and 2.0℃ global warming using Haze Weather Index(HWI)and data of ensemble simulations from the Community Earth System Model(CESM)low-warming experiment.Compared to present day(2006–2015),the frequency in winter season is projected to increase by 14% for regular haze episodes(HWI>0)and 21% for severe haze episodes(HWI>1)at the 1.5℃ global warming.Projections shows larger increases of 27% for regular and 18%for severe haze events at the 2℃ global warming.The additional warming of 0.5℃ largely enhances the persistence of weather conditions conducive to haze episodes.The increased temperature contrast between near-surface and mid-troposphere in eastern Asia accounts for 57% and 81% of the change in HWI by 1.5℃ and 2℃ warming,respectively.Considering increased haze weather potential caused by climate warming,we suggest that additional efforts in emission reductions of carbon dioxide and air pollution are necessary to mitigate haze episodes in Beijing.

Changes in Extreme Maximum Temperature Events and Population Exposure in China under Global Warming Scenarios of 1.5 and 2.0°C: Analysis Using the Regional Climate Model COSMO-CLM

We used daily maximum temperature data（1986–2100） from the COSMO-CLM（COnsortium for Small-scale MOdeling in CLimate Mode） regional climate model and the population statistics for China in 2010 to determine the frequency, intensity, coverage, and population exposure of extreme maximum temperature events（EMTEs） with the intensity–area–duration method. Between 1986 and 2005（reference period）, the frequency, intensity, and coverage of EMTEs are 1330–1680 times yr^–1, 31.4–33.3℃, and 1.76–3.88 million km^2, respectively. The center of the most severe EMTEs is located in central China and 179.5–392.8 million people are exposed to EMTEs annually. Relative to 1986–2005, the frequency, intensity, and coverage of EMTEs increase by 1.13–6.84, 0.32–1.50, and15.98%–30.68%, respectively, under 1.5℃ warming; under 2.0℃ warming, the increases are 1.73–12.48, 0.64–2.76,and 31.96%–50.00%, respectively. It is possible that both the intensity and coverage of future EMTEs could exceed the most severe EMTEs currently observed. Two new centers of EMTEs are projected to develop under 1.5℃ warming, one in North China and the other in Southwest China. Under 2.0℃ warming, a fourth EMTE center is projected to develop in Northwest China. Under 1.5 and 2.0℃ warming, population exposure is projected to increase by 23.2%–39.2% and 26.6%–48%, respectively. From a regional perspective, population exposure is expected to increase most rapidly in Southwest China. A greater proportion of the population in North, Northeast, and Northwest China will be exposed to EMTEs under 2.0℃ warming. The results show that a warming world will lead to increases in the intensity, frequency, and coverage of EMTEs. Warming of 2.0℃ will lead to both more severe EMTEs and the exposure of more people to EMTEs. Given the probability of the increased occurrence of more severe EMTEs than in the past, it is vitally important to China that the global temperature increase is limited within 1.5℃.

Bias correction and projection of surface air temperature in LMDZ multiple simulation over central and eastern China

Based on LMDZ4 daily temperature dataset,equidistant cumulative distribution function matching method(EDCDFm)and cumulative distribution function-transform method(CDF-t)are used to evaluate the ability of models in simulating extreme temperature over central and eastern China.The future temperature change is then projected.The results show that the EDCDFm and CDF-t methods function effectively correct the spatial distribution of daily mean temperature and extreme temperature,significantly reduce the biases of the model simulation and effectively improve the capacity of models for spatial pattern of extreme temperature.However,the cold bias of the CDF-t method in winter is obviously higher than that of the EDCDFm method,and the temperature change curve of the EDCDFm method is closer to the observation than that of the CDF-t method.The projection based on the EDCDFm method shows that under the RCP4.5 emission scenario,the temperature in the study area shows a warming trend.Relative to 1986e2005,the mean temperature is projected to increase by 0.76,1.84,and 2.10℃during 2017e2036,2046e2065,and 2080e2099,respectively.The spatial change for the mean,maximum,and minimum temperature in the three future periods have good consistency;warming in northern China is higher than that in the south.Uncertainties in temperature projection are large in the Tibetan Plateau and Sichuan Basin.Frost days decrease significantly,especially in the Tibetan Plateau,and the frost days in the three periods decrease by more than 15,30,and 40 d,respectively.The variation of heat wave indice is the smallest;the increase of heat wave is mainly in eastern China,and the increase in South China is more than 2 d.Besides,under the global warming of 1.5℃and 2℃,the response characteristics of extreme temperature over central and eastern China are also analyzed.The results show that the mean temperature,maximum temperature and minimum temperature in the study area increase by more than 0.75℃under 1.5℃target and over 1.25℃under 2℃target,especially in the northwestern China and the Tibetan Plateau,relative to 1986e2005.Additionally,comparing two warming targets,the difference of three temperature indices in parts of northeastern China is over 1.5℃,while more than 3 d for heat wave.

Distinct response of Northern Hemisphere land monsoon precipitation to transient and stablized warming scenarios

To better understand the climate response under stabilized,overshoot,and transient global warming,four types of ensemble experiments on 1.5℃/2℃ global warming scenarios(i.e.,stabilized 1.5℃,1.5℃ overshoot,stabilized 2℃,and transient 2℃)are elaborately designed using the Nanjing University Information Science and Technology Earth System Model(NESM).Compared with the modern climate(1985–2014),the projected surface air temperature(SAT)change is characterized by a robust‘Northern Hemisphere(NH)-warmer than-Southern Hemisphere(SH)’and‘land-warmer than-ocean’patterns.The projected precipitation change exhibits‘NH-wetter than-SH’pattern in the tropics.Although the response of SAT and precipitation climatology show similar pattern between stabilized and overshoot scenarios,some significant differences are still found.The projected change in the Northern Hemisphere land monsoon precipitation(NHLMP)is 30% larger in the transient 2℃ experiment compared with that in the stabilized 2℃ experiment.The more vigorous NHLMP in the transient global warming scenario is mainly due to the enhanced land-sea thermal contrast and interhemispheric temperature difference.The enlarged land-sea thermal contrast increases the surface pressure gradient between the NH continents and its adjacent oceans,thus enhancing the NH monsoon circulation and moisture convergence.The enhanced interhemispheric temperature difference shifts the Hadley circulation and intertropical convergence zone northward,leading to the enhanced moisture convergence and the shifts of tropical rain band over the NH monsoon region.This result highlights that climate responses may depend on different warming trajectories and,which could facilitate the strategic planning of governments.