| Global warming has become a true fact.The global scale of climate change is becoming more and more serious and will have a profound impact on human production and life.The surface albedo is the ratio of reflected to the incident solar radiation upon the Earth's surface.It defines the radiation transmission of the lower boundary of the atmosphere and can regulate the balance of surface energy.Also,albedo is being able to sensitively record the global climate change.Therefore,it is very important to study the temporal and spatial changes of surface albedo,which is of great significance for learning the changes of the global land surface process.The trend of global surface albedo,vegetation cover and snow cover extent are estimating using the Moderate Resolution Imaging Spectroradiometer(MODIS)short-wave albedo,Normalized Difference Vegetation Index(NDVI)and snow cover extent long-term sequence data from 2002 to2016.And the changes of surface albedo,vegetation cover and snow cover extent are discussed from three aspects,which are the global spatial distribution,the de-seasoned anomalies in High-latitudes and Mid-latitudes of Northern Hemisphere and the distribution of latitudinal variation.Then,the correlation coefficients and partial correlation coefficients of surface albedo and vegetation cover,surface albedo and snow cover extent are analyzed to explore how the variation of vegetation and snow cover can explain the trend of albedo.The dominant factor for albedo are obtained from the comparison of the absolute values of the partial correlation coefficients.Finally,the area where the surface albedo changed significantly(albedo increase significantly:Kazakhstan,the central part of the United States and the southeast of China and the Sichuan Basin;albedo decrease significantly:Central Europe and Southeast of Australia)are discussed with the help of Google Earth Engine(GEE).Through the above analysis,the paper draws the following conclusions.Surface albedo and snow cover both declined from 2002 to 2016,with an average decrease of0.0004 and 0.97%/pixel globally.The Seasonally Integrated Normalized Difference Vegetation Index(SINDVI)showed an upward trend,with an average increase of 0.175on a global scale.From the perspective of the global proportion of different trends,the surface albedo and SINDVI show a growth trend in more than half of the regions,and the regions with increasing trends account for 51.7%and 69.3%of the global area,respectively.As for the snow cover extent,about 54.1%of the areas showed a decreasing trend.The surface albedo showed a significant growth trend in Central Asia,northeastern China and the central part of the United States;however,in Siberia,Northern islands of Canada,southeastern Australia and Europe have seen significant reductions in albedo.Vegetation has improved significantly over the world,especially in southeastern China,the Indian Peninsula,Western Europe and southeastern Australia.However,in eastern Brazil,northwestern Canada,east coast of Africa and Central Asia,vegetation has been significantly degraded.The snow cover extent showed a decreasing trend in all latitudes,but the change was most obvious in the mid-latitudes of the northern hemisphere.The snow cover extent decreased by-1.064%/pixel in 2016 when compared with snow cover extent in 2002.In the southeastern part of Australia,the Mediterranean coast,southern Africa,the Gulf Coast,etc.,the surface albedo has a significant negative correlation with SINDVI.However,in the central United States,central Europe,Mongolia and the Qinghai-Tibet Plateau,surface albedo and snow cover extents show a significant positive correlation.By calculating the partial correlation coefficient,the influence of another factor on its correlation is eliminated.The study found that the surface albedo from 2002 to 2016 was mainly affected by fluctuations in snow cover extent in the continental latitudes of the northern hemisphere,for example,Turkey,the Qinghai-Tibet Plateau,northeastern China,Mongolia and the central United States.In these areas,the snow cover extent changes drastically,resulting in significant impact on the surface albedo.Areas where surface albedo is more strongly affected by changes in vegetation cover are located in Alaska,Canada,Siberia and Kazakhstan.Kazakhstan is a region with a significant increase in surface albedo.Through higher-resolution remote sensing data and land-use data,the upward trend in surface albedo(0.048)and snow cover extent(1.99%/pixel)in 2002-2016 are obtained respectively.The land use data showed that although the forest land in this area increased by 4.41×10~3 km~2,the reduction of cultivated land and grassland was more obvious,and the total reduction was 21.67×10~3 km~2.The summer drought in Central Asia and the extreme cold weather in winter is the main reason that SINDVI reduced and the area covered by snow increased in Kazakhstan.So the combined effect of SINDVI and snow cover extent together increased the surface albedo in Kazakhstan.The increase in surface albedo in the central United States is due to extreme winter temperatures in the year of 2009/2010,2010/2011and 2013/2014.The decrease in temperature leads to an increase in the area covered by snow,which in turn increases the surface albedo in this area.The surface albedo and SINDVI in the southeastern part of China and the Sichuan Basin showed an upward trend,and the snow cover extent showed a downward trend.The correlation between the surface albedo and the two factors were different from the previous knowledge.Due to the increasing air pollution in this region,the albedo product may not be perfectly convincing.The increasing in aerosol optical depth in the atmosphere has led to insufficient clear-sky observations for satellite products,thus reducing the accuracy of the data.Therefore,accurate retrieval of remote sensing data is critical to the study of changes in surface albedo in the region.The surface albedo in central Europe declined by around 0.0171 from 2002 to 2016,and the change in surface albedo was mainly affected by the increase in vegetation in the area.A large number of studies have analyzed the increase of vegetation in continental Europe.The land use data shows that the area of cultivated land decreased by 12.35×10~3km~2 from 2002 to 2016,while the forest land and grassland increased by 10.69×10~3 km~2and 0.22×10~3 km~2 respectively.For the reduction of surface albedo in southeast Australia,through the analysis of time series data,it was found that the surface albedo decreased sharply in 2009-2011,which was caused by the strong rainfall resulted from the strong El Ni?o phenomenon in 2010.Flooding weather has led to a significant increase in vegetation during this period.Thus,the increase in precipitation and vegetation is the main reason for the reduction in surface albedo in this region. |
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