Monitoring Global Coastline Changes Over 1987-2016 Using Landsat Data

Coastline is the boundary between the land and ocean,and shows strong dynamic characteristics owing to the sea level rise,tide,wave and storm surge.Coastline change is an important indicator for monitoring changes in the ecological environment under the influence of various factors in coastal zones.Over the past decades,sea level rise and its acceleration from regional to global scale have been widely observed.Many local studies have observed the coastline retreat to the land.Rising sea levels may also increase the vulnerability of coastal zones.Global coastline change monitoring can help to reveal the potential driving forces of coastline changes.In addition,these data can help managers and policy makers to make more scientific and reasonable policies for coastal zone management and sustainable development.However,existing studies have mainly focused on coastline change monitoring,assessment and factor analysis at a regional scale.However,studies at the regional scale cannot answer the question of coastline change at a global scale.Therefore,this study collected 30 m resolution Landsat TM/ETM+imagery,used remotely sensed imagery time series composition method to conduct global coastline mapping and change monitoring based on Google Earth Engine cloud computing platform.We concluded as follows:(1)We used the spectral difference based land-water boundary as the proxy of coastline,and proposed a method of coastline detection and change monitoring using Landsat TM/ETM+time series based on water index,thresholding segmentation and edge detection.We found that our method could effectively reduce the influence of sea level fluctuation on the estimation of coastline position.(2)Based on Google Earth Engine cloud computing platform we collected all Landsat 5 TM and Landsat 7 ETM+data in the coastal zones around the world from 1987 to 2016,and used the proposed method to perform global coastline mapping and change detection between 1987 and 2016 with a 5-year interval.20 1°×1°coastal zones were selected to obtain the manually interpreted results for validations on coastline change over 30 years,and these exhibited a good performance.(3)Based on the produced dataset,we demonstrated and analyzed coastline changes at the global scale,continental scale and national scale.We found approximately 65%of the coasts gained land around the world.The total net land gain was approximately 12577±2061km2.All continents exhibited a positive coastline change.Asia exhibited the greatest coastline change of 7933±1470km2 and Oceania exhibited the lowest coastline change of 510±94km2.Among 147 maritime countries(or regions),121(82.3%)experienced a coastline expansion.The net land gain over 1987-2016 in China was 5210±1615km2,accounting for approximately 41%of the change in the world's coastal areas under 65° latitudes.Our results presented the coastline change at a global scale for the first time,and this can help the study of vulnerability of global coastal zones under the influence of climate change and human activities.