全球分光地表反照率的长期变化

Study on long-term change of global spectral surface albedo

为了精确地定量研究土地利用变化对地表反照率的影响,利用公元850–2100年期间土地利用数据集获取了一套长时期的全球分光(近红外、可见光、短波)地表反照率数据集,并进行了详细的分析和对比验证。验证结果表明,所得的分光地表反照率与MODIS数据对应结果的空间分布特征基本一致;在非冰雪覆盖区的可见光反照率差别在−0.0081∼0.0029之间,精度较高;差别最大的区域为南、北半球中高纬度的冰雪覆盖区。研究表明,自1860年以来,所有典型区域(包括中国东部、欧洲东南部、美国中东部和巴西南部)的城市建成区逐年增加,特别是巴西南部的自然植被逐年减少;在1860–1980年间,各区域主要表现为自然植被向城市建成区和耕地转化;在1980–2015年,巴西南部耕地类型持续增加,其余区域均表现出自然植被逐步恢复的变化趋势。此外,不同土地利用类型的分光反照率存在很大差别,例如,春季可见光反照率在冰雪表面为0.5069,混交林为0.0444,而城市建成区为0.0870;且受不同纬度和下垫面性质的影响,同一土地利用类型的分光地表反照率也存在明显的空间差异,例如,春季草原的可见光反照率最大为0.4915,最小为1.127×10^(−4)。这套长时期全球分光地表反照率数据集可以为土地利用变化驱动气候变化的相关定量研究奠定数据基础。

In order to study the impact of land use change on the surface albedo accurately and quantitatively,a long-term global spectral surface albedo dataset(near-infrared,visible,shortwave)is obtained by using land use types dataset from 850 A.D.to 2100 A.D.provided by land-use harmonization(LUH2)project,and then detailed analysis and comparative verification are carried out.The results show that,the spatial distribution characteristics of spectral surface albedo obtained in this work are consistent with those of MODIS.The visible albedo in non-snow and non-ice covered area varies from-0.0081 to 0.0029 with high precision,while the largest difference is found in snow and ice covered area in the middle and high latitudes of the southern and northern hemisphere.It is also shown that,since 1860,the urban built-up area has increased year by year in all typical regions(including Eastern China,Southeastern Europe,Mid-east Unite States and Southern Brazil),while the natural vegetations have decreased in Southern Brazil.From 1860 to 1980,natural vegetations were mainly transformed into urban and croplands in each region.In 1980~2015,croplands continued to increase in Southern Brazil,while the other regions showed a trend of gradual restoration of natural vegetations.In addition,the spectral albedo of different land cover types varies greatly.For example,the visible albedo in spring is 0.5069 on the surface of ice/snow,while it is 0.0444 in mixed forest and 0.0870 in urban.Under the influence of different latitudes and underlying surface properties,even the spectral albedo of the same land use type also has obvious spatial differences.For example,the maximum visible albedo of grassland in spring is 0.4915,and the minimum is 1.127×10^(−4).It is believed that the long-term global spectral albedo dataset can lay a data foundation for the quantitative research on the climate change driven by land use change.