Response Of Coral Reefs And Islands In The South China Sea To Climate Warming In The Past 40 Years Recorded By Remote Sensing Images

Coral reefs are the important supports for China's reef engineering in the South China Sea(SCS).However,due to the global warming,sea levels rising and severe coral reef degradation,the reefs and lime-snad islands in SCS are at risk of being submerged.This study choose the representative coral reefs and lime-sand islands in SCS as the research objects,including Huangyan Island of Zhongsha islands,lime-sand islands of Xisha Islands(Ganquan Island,Jinqing Island,Jinyin Island,Shanhu Island,Xishazhou,Bei Island,Zhong Island and Nan Island),and lime-sand islands of Nansha Islands(Beizi Island,Mahuan Island,Xiyue Island).Based on the satellite remote sensing images,the dynamic change history of coral reefs and lime-sand islands in SCS over the past 40 years was reconstructed to analyze their response to climate changes.Research methods include collecting Landsat and Sentinel-2 satellite remote sensing images in recent 40 years,using gradient vetor flow based active contour model(GVF-Snake)and manual vectorizing to extract boundary information,using Digital Shoreline Analysis System(DSAS)to quantitatively analyze the changes of geomorphic boundaries,and using the linear fitting method to explore the change trends of the area of reefs and lime-sand islands.And draw the following conclusions:(1)The geomorphic zone boundaries of Huangyan Island atoll have changed.The boundary between the outer sea and the fore-reef slope,the boundary between the fore-reef slope and the reef flat,and the boundary between the lagoon slope and the lagoon bottom,were seen to move outward with the rates of 0.36 m/a,0.06 m/a and 0.23 m/a,respectively,while the boundary between the reef flat and the lagoon slope had a trend of inward movement with a rate of 0.16 m/a.(2)The area of geomorphic zones of Huangyan Island atoll have also changed.The area of the lagoon bottom and the total area of the atoll(the part of the water depth within 17 m)were increased at the rates of 12676 m~2/a and 39078m~2/a,the area of the lagoon slope was decreased at a rate of 57362 m~2/a,and the area of the reef flat was increased at a rate of 114523 m~2/a during 1977 to 1997,but decreased at a rate of 11289 m~2/a during 1997 to 2019.(3)The area of lime-sand islands in SCS have shown a decreasing trend.The area change speeds were ranged from-2568.40 m~2/a to 42.41 m~2/a.Moreover,the area growth rates of lime-sand islands were ranged from-0.60%/a to-0.04%/a.(4)The vegetation area of lime-sand islands in SCS have shown an increasing trend.The area change speeds of vegetation of lime-sand islands were positive,ranged from 307.37 m~2/a to 1487.63 m~2/a.Furthermore,the area growth rates ranged from 0.33%/a to 39.42%/a.(5)Coral reefs and islands in SCS will continue to be threatened by global warming in the future.Under the erosion of the waves,the reef flat and the fore-reef slope of Huangyan Island will gradually shrink,and the area of the lime-sand islands will gradually decrease too.As the calcification speed of coral reefs reduces,the lagoon slope area of Huangyan Island and the beach area of lime-sand islands will further decrease,while the area of lagoon bottom will increase correspondingly.At the same time,the vegetation area of lime-sand islands will expand due to the increase in precipitation,and will decrease as it approaches the edge of lime-sand islands.