Department Seminar: Leonard Ohenhen

Title: Subsidence Across Scales: Quantifying the Contribution of Land Motion to Coastal Change

Abstract: As sea levels accelerate and ocean storms intensify, coastal communities require detailed assessments of compound coastal hazards to inform adaptation and mitigation measures. To comprehensively assess coastal risk, we must account for the combined influence of all processes that determine relative sea-level (RSL) change – the change in sea level relative to the local land surface. Yet most current global and regional assessments overlook land subsidence – the downward vertical change in local land elevation. This omission persists largely due to a fundamental data gap: the global lack of dense, long-term instrumentation capable of measuring vertical land motion (VLM) with sufficient spatial and temporal resolution. As a result, many of the world’s most vulnerable coastlines remain poorly characterized, despite the increasing availability of ocean and climate data. In this talk, I will show how advances in satellite geodetic observations are transforming our ability to monitor and understand land motion across scales, filling this longstanding void in coastal hazard assessment. First, I'll present a global analysis of 40 river deltas hosting more than half a billion people worldwide. Using radar interferometry and machine learning, we reveal the prevalence, magnitude, and dominant anthropogenic drivers of subsidence, showing that widespread, high-magnitude subsidence rates (95th percentile) exceed projected sea level rise by factors of 2-7 across most deltas. Secondly, at the regional scale, I will focus on Java Island, Indonesia, where we developed a high-resolution framework for constructing virtual tide gauges by combining radar satellites with satellite altimetry and probabilistic sea-level projections. This approach enables spatially dense monitoring of RSL rise, revealing that subsidence contributes 27-85% of current and projected RSL change along more than three-quarters of Java's northern coast. Together, these assessments demonstrate that subsidence is a first-order control on coastal hazard evolution that, if unaccounted for, can misrepresent exposure, understate risk, and undermine adaptation strategies.