中国气候变化的科学新认知

了解和认识百年来中国气候发生的变化、引起其变化的驱动因素以及未来的可能变化,可以更好地适应和减缓气候变化。本文综合评估了观测到的中国气候变化事实、中国气候变化的驱动力、中国未来气候变化预估三大方面,分析了气候变暖的趋势、水循环以及降水和冰川变化、极端天气气候事件变化、生物化学循环、海洋和土地覆盖变化及其气候效应以及未来气候变化的特点和趋势等最新科学进展。在中国百年温度趋势、气候系统多气候指标变化特征、极端天气气候事件中的人类活动作用以及气候系统模拟能力等方面的研究有了新的进展。可以看到中国气候变暖趋势持续、大气二氧化碳等长寿命温室气体浓度继续增长、人为强迫影响了多种气候要素在强度和频率的变化,中国陆地生态系统的固碳量增加。本文最后提出未来中国气候变化研究需要进一步加强的问题,包括:中国气候变化中的城市化效应、气候系统内部变率在年代际变化中的作用、气溶胶-云-降雨相互作用的机理、大范围土地利用变化(如大规模生态恢复工程)的气候效应,以及云辐射反馈、海洋环流对气候变化的响应与反馈、气候-碳循环反馈等过程对气候模拟不确定的影响等。

近54 a蒙古高原降水变化趋势及区域分异特征

近半个世纪,有关全球气候的话题一直是科学界争论的焦点,拥有世界最大温带草原的蒙古高原降水变化是属于全球变化问题,又是其脆弱环境变化的最主要驱动因子之一。通过利用蒙古高原1961-2014年136个气象站点的月降水量数据,采用Sen’s斜率法、Mann-Kendall趋势检验法和空间地统计方法,研究了该地区近54 a降水要素基本气候特征及其时空变化规律。结果表明:(1)近54 a蒙古高原年降水量呈减少趋势,趋势为-2.30 mm·(10 a)^-1(P>0.05),整体上年降水量东南及西北显著减少,东北及中南明显增加(2)夏季和秋季降水量呈减少趋势,趋势分别为-5.75 mm·(10 a)^-1和-0.42 mm·(10 a)^-1(P>0.05);春季和冬季降水量呈显著增加趋势,趋势分别为1.95 mm·(10 a)^-1和0.50 mm·(10 a)^-1(P<0.05);季节降水量出现正负距平的年份和周期有所不同。(3)春季和冬季降水量呈增加趋势的站点居多,占全部站点的89.0%和84.6%,主要分布于高原东北部和中南部地区;夏季和秋季降水量呈减少趋势的站点居多,占全部站点的80.1%和57.4%,主要分布于高原东南部和西北部地区。为准确评估蒙古高原气候变化以及合理提出生态环境决策提供科学参考。

亚洲热带森林减少的增温效应及其影响机制

由于森林砍伐和火灾,近年来热带森林面积呈减少趋势,且森林减少的速率不断上升。热带森林覆盖的减少通过改变地表的热量、水分和物质循环,影响着热带气候。为了解热带森林减少对热带气候的影响,本研究采用窗口搜索的方法,利用森林变化遥感产品识别亚洲森林覆盖损失,计算同个窗口内相邻森林减少区与未改变森林覆盖区的地表温度(LST),地表净短波辐射(SW)和潜热通量(LE)的差异,从而评估2000-2017年亚洲热带雨林和季雨林两种森林类型的减少对地表温度的影响及其影响机制。结果表明:(1)亚洲热带雨林累计减少(2000-2017)使日均地表温度上升了1.7±0.7℃,地表净短波辐射下降7.2±0.9W×m^(-2),潜热通量增加8.9±4.4W×m^(-2)。(2)热带季雨林减少使日均地表温度上升2.1±0.9℃,地表净短波辐射下降7.1±1.0W×m^(-2),潜热通量与热带雨林表现出相反的变化趋势,减少了3.9±9.2W×m^(-2)。(3)热带雨林的减少对地表温度、地表净短波辐射和潜热通量变化影响的季节差异很小;热带季雨林的减少使地表温度、地表净短波辐射和潜热通量变化则存在明显的季节差异,这主要是由于季雨林区域降水对地表水分供应的季节性差异导致。(4)地表净短波辐射和潜热通量的净热量变化(△SW-△LE)一定程度上能够决定热带季雨林区地表温度的变化,但不能解释热带雨林区地表温度的变化。因此,在气候变化背景下,热带地区应采取措施阻止森林减少,缓解土地覆盖变化带来的变暖压力。

Spatiotemporal variability of precipitation during 1961-2014 across the Mongolian Plateau

Precipitation is the dominant factor that controls vegetation growth and land-use practices in the arid and semiarid Mongolian Plateau(MP), so the spatiotemporal heterogeneity of precipitation change has been an important scientific question in the region. This study investigated the spatiotemporal characteristics of annual and seasonal precipitation across the entire MP based on monthly precipitation data from 136 meteorological stations during 1961–2014 by using a modified Mann–Kendall test, Sen's slope, Morlet Wavelet Transform, and geostatistical methods. Results show the following: 1) Annual precipitation decreased slightly from 1961 to 2014.Stations with positive and negative trends were 41.9%and 58.1%, respectively. Significant positive trends were mainly in the southwestern and northeastern regions of the plateau, whereas significant negative trends were in the northern and southeastern regions.2) Precipitation decreased at rates of-5.65 and-0.41 mm/decade in summer and autumn, respectively, but increased at 1.91 and 0.51 mm/decade in spring and winter. The contribution of spring and winter precipitation to the annual amount increased significantly, but that of summer precipitation decreased significantly. 3) A large majority of stations(80.2%) showed decreasing trends in summer,whereas 89.7% and 83.1% of stations showed increasing trends in spring and winter. The spatial distribution of trend magnitude in seasonal precipitation amount was strongly heterogeneous. 4)By climatic zones, precipitation increased in humid and arid zones, but decreased in a semiarid zone. On the whole, the MP experienced a drying trend, with significant regional differentiation and seasonal variations.

Evaluation of AMSR-E——Derived Soil Moisture over Northern China

In this study,the authors evaluated two re-motely sensed surface soil moisture datasets derived from the Advanced Microwave Scanning Radiometer of the Earth Observing System (AMSR-E) over northern China.The soil moisture datasets were derived from algorithms developed by the National Snow and Ice Data Center (NSIDC) and jointly developed by the Vrije Universiteit Amsterdam and NASA Goddard Space Flight Center (VUA-NASA).The NSIDC and VUA-NASA products were compared to in situ soil moisture data from nine enhanced coordinated observation stations.The VUA-NASA dataset presented a strong correlation with top layer in situ soil moisture observations,and the correla-tion coefficients ranged from 0.34 to 0.73 (p<0.01).The correlation coefficients decreased as the observed soil layer depth increased.The correlation coefficients be-tween the NSIDC retrievals and the top layer in situ ob-servations were between 0.10 and 0.62 (p<0.01).Fur-thermore,VUA-NASA soil moisture variations agreed well with in situ soil moisture dynamics and responded sensitively to precipitation events.In contrast,the NSIDC dataset failed to capture signals of soil moisture dynamics.The analyses demonstrated that the VUA-NASA product was capable of representing soil moisture conditions over northern China.

Spatial and Quantitative Comparison of Satellite-Derived Land Cover Products over China

Because land cover plays an important role in global climate change studies, assessing the agreemen among different land cover products is critical. Significan discrepancies have been reported among satellite-derived land cover products, especially at the regional scale. Different classification schemes are a key obstacle to the comparison of products and are considered the main factor behind the disagreement among the different products Using a feature-based overlap metric, we investigated the degree of spatial agreement and quantified the overall and class-specific agreement among the Moderate Resolution Imaging Spectoradiometer (MODIS), Global Land Cover 2000 (GLC2000), and the National Land Cover/Use Data-sets (NLCD) products, and the author assessed the products by ground reference data at the regional scale over China. The areas with a low degree of agreement mostly occurred in heterogeneous terrain and transition zones while the areas with a high degree of agreement occurred in major plains and areas with homogeneous vegetation The overall agreement of the MODIS and GLC2000 products was 50.8% and 52.9%, and the overall accuracy was 50.3% and 41.9%, respectively. Class-specific agreement or accuracy varied significantly. The high-agreemen classes are water, grassland, cropland, snow and ice, and bare areas, whereas classes with low agreement are shru-bland and wetland in both MODIS and GLC2000. These characteristics of spatial patterns and quantitative agreement could be partly explained by the complex landscapes mixed vegetation, low separability of spectro-temporal-texture signals, and coarse pixels. The differences of class definition among different the classification schemes also affects the agreement. Each product had its advantages and limitations, but neither the overall accuracy nor the class-specific accuracy could meet the requirements of climate modeling.

Precipitation as a control of vegetation phenology for temperate steppes in China

已有的物候研究主要针对温度的影响,而降水量的物候效应研究相对较少。文中用NDVI比率法分析内蒙古草原两个同温样带的年降水量与草原物候之间的关系。结果表明,在中国温带草原区,年降雨量增加能显著促进春季植物的萌芽和夏季最高生物量日期的到来,而年平均温度起抑制的作用。在低纬度的同温样带(内蒙古中部草原),年降水量增加能延迟生长期结束日期,而在高纬度地区(呼伦贝尔草原)年降水量增加会轻微提前结束生长期。因此在内蒙古中部草原年降水量增加能明显延长草原植被的生长期。

A Dynamic Method for Quantifying Natural Warming in Urban Areas

In the study of global warming, one of the main issues is the quantification of the urbanization effect in climate records. Previous studies have contributed much to removing the impact of urbanization from surface air temperature by carefully selecting reference stations. However, due to the insufficient number of stations free from the influence of urbanization and the different criteria used to select reference stations, there are still significant controversies about the intensity of the impact of urbanization on temperature records. This study proposes a dynamic method for quantifying natural warming using information on urbanization from every station acquired from remote sensing (RS) data instead of selecting reference stations. Two different spatial scales were applied to examine the impact of urbanization, but little difference was found, indicating the stability of this method. The results showed a significant difference in original temperature data and the homogenized data-urban warming accounted for approximately 64% in the original temperature warming but only approximately 20% in the homogenized temperature records.

Modeling Gross Primary Production by Integrating Satellite Data and Coordinated Flux Measurements in Arid and Semi-Arid China

Assessing large-scale patterns of gross primary production (GPP) in arid and semi-arid (ASA) areas is important for both scientific and practical purposes.Remote sensing-based models,which integrate satellite data with input from ground-based meteorological measurements and vegetation characteristics,improve spatially extended estimates of vegetation productivity with high accuracy.In this study,the authors simulated GPP in ASA areas by integrating moderate resolution imaging spectral radiometer (MODIS) data with eddy covariance and meteorological measurements at the flux tower sites using the Vegetation Photosynthesis Model (VPM),which is a remote sensing-based model for analyzing the spatial pattern of GPP in different land cover types.The field data were collected by coordinating observations at nine stations in 2008.The results indicate that in the region during the growing season GPP was highest in cropland sites,second highest in woodland sites,and lowest in grassland sites.VPM captured the temporal and spatial characteristics of GPP for different land covers in ASA areas.Further,Enhanced Vegetation Index (EVI) had a strong liner relationship with GPP in densely vegetated areas,while the Normalized Difference Vegetation Index (NDVI) had a strong liner relationship with GPP over less dense vegetation.This study demonstrates the potential of satellite-driven models for scaling-up GPP,which is a key component for studying the carbon cycle at regional and global scales.

Assessing MODIS Land Cover Products over China with Probability of Interannual Change

Accurate and up-to-date land cover data are important for climate-change modeling. Quality assessment is becoming critical, as many satellite-based land cover products of differing scales have been released to meet the needs of scientific studies. In this study, the authors assessed the Moderate Resolution Imaging Spectroradiometer(MODIS) land cover products by analyzing the probability of interannual change from 2001 to 2012. The authors found that, cumulatively, 43.0% of MODIS land cover had changed over China from 2001 to 2012 at least once. Of this percentage, 12.1% was considered unreasonable change, 6.1% was considered reasonable change, and areas of confusion accounted for about 24.8%, giving rise to great uncertainty in the products. MODIS Collection 51 products clearly have less uncertainty than the Collection 5 products. Areas of reasonable change occurred in transition zones of ecological, biophysical, and climate gradients, while areas of unreasonable change appeared in heterogeneous landscapes. The misclassifications at three spatial scales of horizontal grids used in regional climate models occurred largely in the heterogeneous landscapes, and the areal percentage of misclassification decreased with larger horizontal grid spacing. In addition, the misclassifications in MODIS products often occurred among specific classes, which are geographically, ecologically, and spectrally similar, with low discriminative spectral-temporal signals. The effect of classification uncertainty should be made known, and further improvements are still needed for application in regional climate models. The authors' findings have important implications for better understanding the uncertainties of MODIS land cover products, and for improving the land surface parameterization for regional climate models.

Validation of Land Surface Temperature Derived from 37-GHz AMSR-E over Northern China

A validation study of land surface temperature (LST) obtained from the Ka band (37 GHz) vertically polarized brightness temperature over northern China is presented.The remotely sensed LST derived jointly by the Vrije Universiteit Amsterdam and the NASA Goddard Space Flight Center (VUA-NASA) from the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) were compared to the daily in-situ top soil temperature/infrared surface temperature observations from eleven/three Enhanced Coordinated Observation stations in arid and semi-arid regions of northern China.The VUA-NASA LST from the descending path exhibited a stronger correspondence to the in-situ infrared surface temperature than soil temperature observations,whereas correlations (R 2) of the latter ranged from 0.41 to 0.86.Meanwhile,the ascending overpass LST was generally warmer than the in-situ soil temperature observations at all stations,and the correlation (R 2) was between 0.07 and 0.72.Furthermore,the correlation of the descending path was generally greater than that of the ascending path at the same station.The descending path VUA-NASA LST was sensitive to precipitation and presented good agreement with ground temperature dynamics.The analyses demonstrated that the descending overpass LST was reliable to reflect reasonable patterns of temperature dynamics for land surface temperature in the region.

Increased Browning of Woody Vegetation due to Continuous Seasonal Droughts in Yunnan Province, China

In this paper, based on the analysis of satellite measurements, the authors conclude that the continuous seasonal droughts intensify the browning of woody vegetation and that evergreen needleleaf forest(ENF) shows a larger browning percentage than other woody vegetation types over Yunnan Province. Based on the Tropical Rainfall Measuring Mission(TRMM) precipitation standardized anomaly, in the dry season, which is from October to March, the 2010 drought affected an area of Yunnan Province 1.77 times larger than the 2012 drought, but in the post-drought months(April to June), the browning area of all woody vegetation in 2012 was 1.11 times larger than that in 2010 on the basis of the enhanced vegetation index(EVI) standardized anomaly. The reduction of vegetation greenness over large areas of Yunnan Province represents a photosynthetic capacity loss which will have an impact on carbon fluxes to the atmosphere.

基于TRMM卫星数据的蒙古高原降水精度评估与季节分布特征

采用蒙古高原134个气象站点的降水量数据作为地面验证数据,对新一代TRMM卫星降水产品（3B43V7）进行了精度评估,并分析了最近19 a蒙古高原降水的空间和季节分布特征。结果表明：月尺度TRMM 3B43数据与站点数据之间呈显著正相关,拟合优度R2达到0.88（P〈0.01）,具有较好的一致性;近19 a,基于TRMM的年均降水量总体上呈显著上升趋势,上升速率为32.1 mm·（10a）-1（P〈0.05）,但存在一定的空间差异,其中,高原东北部、中北部和西南地区降水量呈增加趋势,高原西部、东北部大兴安岭以西和东南部降水量呈减少趋势;季节分布上,除了冬季呈略微减少趋势外,其他季节都呈增加趋势,且四季降水量的年际变化也具有明显的空间分异。