Global distribution of coastal cliffs and retreat rates

Previous publications estimated coastal cliffs comprise about 80% of the world's coast, however this estimate has not been validated on a global scale. This study uses two approaches to capture information on the worldwide existence and erosion of coastal cliffs: a detailed literature survey and imagery search, and a GIS-based global mapping analysis. The literature and imagery review showed that coastal cliffs exist in 93% of the combined recognized independent coastal states and non-independent coastal regions worldwide. Additionally, cliff retreat rates have been quantified in at least one location within 33% of independent coastal states. The GIS-based mapping used a near-global digital elevation model to obtain backshore coastal elevations at 1 km alongshore intervals comprising about 1,350,000 locations worldwide. Backshore coastal elevations were compared with existing maps of coastal landforms to produce a model training set, and this relationship was extended globally to map the likelihood of coastal cliff locations. The results were combined with estimates of cliff percentages for Greenland and Antarctica from the literature, extending the global coverage to estimate the likelihood of coastal cliffs across 89% of the world's total shoreline. The results suggest that about half of the world's coasts are likely composed of coastal cliffs.