When knowledge of a system is imperfect, thresholds are extremely useful tools for decision-makers. Sea level rise (SLR) threatens intertidal coastal wetlands because these aquatic plants will drown as water depths exceed their physiological tolerance limits. For mangroves—trees and shrubs that grow in tidal waters of the tropics or subtropics—the threshold for SLR has been elusive. On page 1118 of this issue, Saintilan et al. (1) deduce that the threshold SLR for mangrove ecosystems is 6 to 7 mm/year. This discovery can help inform decisions on how to sustain mangroves, which provide 200 million coastal people with essential ecosystem services. These include protection from intense storms and waves, reduction in the impact of coastal flooding, sequestering of carbon, improvement in water quality, and preservation of biodiversity and fisheries (2).