Holocene Hydroclimatic Variations in the Asian Drylands:Current Understanding and Future Perspectives

0 INTRODUCTION.The intensification of global warming is generating increasing risks to human society and sustainable development,which are of growing concern to the international scientific community and policymakers(Sun,2023).The ecosystems of the Asian drylands are fragile and highly sensitive to global changes;moreover,while these drylands are the most important areas of oasis agriculture on Earth,their water resources are being adversely impacted by ongoing climate change(Zhang et al.,2023).Hence,an improved understanding of the patterns of climate change in this region is important and attracting increasing research attention.

陆地地表水卫星遥感监测进展与展望

陆地地表水(简称地表水)是地球系统的重要组成部分,也是人类赖以生存的重要水资源储备地.然而,地表水的分布范围广,空间异质性强,受到自然和人类诸多因素的综合影响,正经历着剧烈变化,对生态系统和社会的可持续发展产生着深远影响.在这样的背景下,遥感技术因其广泛覆盖、长时序观测的特性而日益成为研究地表水时空变化不可或缺的关键工具.本文从地表水遥感监测的数据源、方法、主要内容和发展趋势等方面对现有研究进行了综述.研究发现,近十年来,地表水遥感监测正处于黄金发展期.光学遥感数据源,如Landsat、Sentinel-2等,在地表水环境监测中发挥着主导作用.全球地表水数据产品,如Global Surface Water(GSW),为大区域地表水研究提供了有力支持.然而,新兴的大数据方法,例如深度学习,在地表水研究中仍显薄弱.受到光学遥感监测方法的局限,对于水库、湿地等特殊类型地表水的研究滞后于对大型湖泊等水体的研究.同时,中国、美国、欧洲和加拿大等地成为地表水研究的热点区域,而在经济欠发达、气候较干旱的地区,对地表水的研究相对较少.未来的研究需要更多关注极端气候变化下的地表水动态,提高对野外调查困难地表水(如热融湖和冰川湖)的识别能力.特别需要加强深度学习等大数据新方法在地表水研究中的应用,以推动对地表水更全面的认识,为地球系统研究和可持续发展提供关键支持.

荒漠化防治措施如何影响我国干旱半干旱地区植被恢复和农牧民收入?

我国潜在荒漠化地区以流动、半流动、半固定和固定沙地为主体,主要位于东北地区西部、华北地区北部和西北地区西部年降水量<400 mm的干旱、半干旱地区,总面积约120万平方公里.该区域气候干旱、植被稀疏、风沙活动强烈、沙尘暴频发,生存环境严酷,经济相对落后,是受人类活动和气候变化双重影响的生态脆弱区.同时,该地区的人类活动历史悠久,农业开发和弃耕还草/还牧在此处多次交替出现.目前,这一区域超过60%的土地沿用传统畜牧和农业耕作方式进行生产[1],承载着约4800万人口的生产和生活.自2000年初期实行“退耕还林/还草”和围封禁牧的荒漠化防治政策和措施以来,各级政府投资超过7800亿元,在抑制沙尘暴、减少水土流失、促进生态恢复等方面取得了显著成效[2~4].然而,潜在荒漠化地区近20年来植被显著恢复的贡献主要来自于自然环境变化还是人类干预,相关的荒漠化防治措施对区域经济和可持续发展又带来了怎样的影响等问题,尚缺少系统科学的评估.

Discussion of the“warming and wetting”trend and its future variation in the drylands of Northwest China under global warming

Since Shi et al.proposed that the climate in the drylands of Northwest China experienced a significant transition from a“warming and drying”trend to a“warming and wetting”trend in the 1980s,researchers have conducted numerous studies on the variations in precipitation and humidity in the region and even in arid Central Asia.In particular,the process of the“warming and wetting”trend by using obtained measurement data received much attention.However,there remain uncertainties about whether the“warming and wetting”trend has paused and what its future variations may be.In this study,we examined the spatiotemporal variations in temperature,precipitation,the aridity index(AI),vegetation,and runoff during 1950-2019.The results showed that the climate in the drylands of Northwest China and the northern Tibetan Plateau is persistently warming and wetting since the 1980s,with an acceleration since the 1990s.The precipitation/humidity variations in North China,which are mainly influenced by summer monsoon,are generally opposite to those in the drylands of Northwest China.This reverse change is mainly controlled by an anomalous anticyclone over Mongolia,which leads to an anomalous easterly wind,reduced water vapor output,and increased precipitation in the drylands of Northwest China.While it also causes an anomalous descending motion,increased water vapor divergence,and decreased precipitation in North China.Precipitation is the primary controlling factor of humidity,which ultimately forms the spatiotemporal pattern of the“westerlies-dominated climatic regime”of antiphase precipitation/humidity variations between the drylands of Northwest China and monsoonal region of North China.The primary reasons behind the debate of the“warming and wetting”trend in Northwest China were due to the use of different time series lengths,regional ranges,and humidity indices in previous analyses.Since the EC-Earth3 has a good performance for simulating precipitation and humidity in Northwest and North China.By using its simulated results,we found a wetting trend in the drylands of Northwest China under low emission scenarios,but the climate will gradually transition to a“warming and drying”trend as emissions increase.This study suggests that moderate warming can be beneficial for improving the ecological environment in the drylands of Northwest China,while precipitation and humidity in monsoon-dominated North China will persistently increase under scenarios of increased emissions.

中国北方全新世沙尘暴演化:从自然驱动转向人为驱动

亚洲强沙尘暴事件能显著地降低空气质量,改变到达地面的太阳辐射,对人体健康产生严重的潜在危害,受到国内外的广泛关注.然而,目前对于全新世不同时间尺度亚洲沙尘暴的驱动机制以及是否存在由自然驱动向人为驱动的转变仍不清楚.黄土高原是世界上最大的粉尘堆积区,该区域未受人类活动直接影响的湖泊沉积是研究沙尘暴变化的理想载体.基于黄土高原上的高山湖泊岩芯,我们发现全新世沙尘暴频率总体呈增加趋势,并在过去2000年达到最高值且伴随着剧烈波动.其中,11~2 cal ka BP沙尘暴频率与东亚夏季风降水、植被覆盖、东亚冬季风强度均呈负相关,揭示出不同于冬季风控制沙尘暴变化的观点,认为东亚夏季风是控制千年尺度沙尘暴变化的自然驱动因子.从~2 cal ka BP开始,沙尘暴频率与粉尘源区人口数量、季风降雨量同步变化,人类活动的增强使得在强夏季风阶段却出现强沙尘暴现象,揭示出年代际尺度人类活动对地表系统的影响已超过夏季风的影响.沙尘暴由自然驱动向人为驱动的转变,支持中国北方"早期人为假说",可能至少开始于~2 cal ka BP.

Differences in the evolutionary pattern of dust storms over the past 2000 years between eastern and western China and the driving mechanisms

The arid and semi-arid region of East Asia,principally including the deserts in eastern and western China,is the main dust source of the Northern Hemisphere.Dust storm activity in the region has attracted much research attention because of its impacts on air quality.

Dust storms in northern China and their significance for the concept of the Anthropocene

A key scientific issue in the study of the Anthropocene is the determination of the corresponding stratigraphic marker in geological archives.The arid and semi-arid regions of Asia are the second largest dust source on Earth,and their release,transport and deposition of dust affect global climate change,as well as marine and terrestrial biogeochemical cycles.Over the past~2000 years,human activity has outpaced natural climatic variability as the dominant control of dust storms in northern China.Thus,exploring the potential of anthropogenic Asian dust as a marker of the Anthropocene and its impacts on lake ecosystems may contribute to an improved definition of the characteristics and timing of the Anthropocene.In this context,we measured spectrally-inferred chlorophyll a from the sediments of an undisturbed alpine lake in northern China,and compared the results with dust storm data from the same cores and with regional climatic records.Asian dust is a widely distributed,globally significant signal of human activity,and it is also well preserved in various geological archives;hence,we propose anthropogenic dust can be considered as a potential marker of the Anthropocene.Anthropogenic dust signals in stratigraphic records during the past~2000 years differ substantially from those during the early and middle Holocene,which demonstrates that,at least since~2000 years ago,human activity has exceeded the natural forcing of dust transport in northern China.We therefore propose that there are spatial and temporal differences in the onset of the Anthropocene,as defined by anthropogenic dust deposition,which is therefore time-transgressive.Our spectrally-inferred chlorophyll a record is consistent with dust storm activity over the past~2000 years(except since the 1950s),suggesting that anthropogenic dust storms were the dominant control on lake primary production.Prior to the 1950s the interactions of the East Asian summer monsoon(EASM),human activity,dust storms and lake ecosystems resulted in a shift from a pattern in which“human activity outpaced the EASM as the dominant control on the Earth surface system”to one in which,after the 1950s,“human activity became the dominant factor influencing the EASM and the Earth surface system”.In the future this pattern may trend towards one in which there is the“sustainable development of humans and the environment”.We suggest that,in order to better understand the interactions of human activity,climate and environment,future research on the Anthropocene should focus on its time-transgressive characteristics and regional differences,in addition to the“Great Acceleration”

“North-South”dipolar mode of precipitation changes in eastern China extends to the Last Deglaciation

Changes in the East Asian summer monsoon (EASM)are closely related to human wellbeing,and thus improving our knowledge of the evolution of the EASM is of major socioeconomic importance, as well as being of scientific interest.Previous studies have shown that during the Holocene (the present interglacial)a "North-South" dipolar mode of precipitation change occurred over eastern China (i.e.the so-called "northern drought-southern flood"scenario, or vice versa)on decadal [1,2],centennial [3]and millennial timescales ([4]and references therein).A recent high-resolution speleothem trace element-based precipitation record from Haozhu Cave in the Yangtze River Valley (YRV)[5],together with a pollenbased quantitative precipitation reconstruction from Lake Gonghai in North China [6],further suggests that this dipolar mode extended to the Last Deglaciation [5].This inference is based on the recorded anti-phased variation of precipitation between North China and the YRV during the Younger Dryas (YD),Bolling-Allerod (B-A),and part of Heinrich Event 1(H1)(Fig.1a,b).

Increased water vapor supply in winter and spring leading to the arid Central Asian wetting in last 6000 years

Paleoclimate reconstructions show that the arid Central Asia(ACA)is characterized by a wetting trend from the midHolocene(MH)to the Preindustrial period(PI),which has been acknowledged to be a result of increased mean precipitation.However,a systemic understanding of its governing dynamics remains elusive.Based on model outputs from 13 climate models from the Paleoclimate Model Intercomparison Project phase 4(PMIP4)and proxy records from ACA,here we show that increase in mean precipitation in ACA can be attributed to changes in water vapor source and its transport intensity in winter(December,January,and February)and spring(March,April,and May).In particular,the increase in water vapor supply in winter is associated with the southerly wind anomaly over the northwestern Indian Ocean and Central Asia,caused by an overall weakening of the Asian winter monsoon.This is conducive to water vapor transport from the upwind regions(e.g.,Mediterranean)to ACA.Meanwhile,water vapor supply from the eastern Iceland is also enhanced due to a negative North Atlantic Oscillation-like(NAO-like)atmospheric circulation pattern caused by sea ice expansion in the North Atlantic.In spring,evaporation over land and inland lakes is enhanced by increased insolation in the Northern Hemisphere,which increases atmospheric humidity that fuels midlatitude westerlies to enhance ACA precipitation.In addition,weakened atmospheric subsidence over ACA in winter and spring also contributes to the increased precipitation.Overall,our results indicate that paleoclimate modeling is of great importance for disentangling governing dynamics accounting for reconstructed climate phenomena that might be a synergic consequence of several processes operating in different seasons.