This study reveals that 18 of Earth’s largest river deltas, including the Nile and Amazon, are sinking at rates significantly faster than global sea levels are rising, driven primarily by localized human activities like groundwater extraction and sediment starvation from dams. This research answers the pivotal question of whether global climate change or local intervention is the dominant threat to these regions; it concludes that for many deltas, human-induced land subsidence is the primary driver of "relative sea-level rise." This finding is critical because it suggests that while global warming remains a threat, the immediate survival of these deltas and their 236 million inhabitants depends on local management of water and sediment resources.