The growth and decline of mangrove forests during the final stages of Holocene deglaciation offers a glimpse into how the ecosystems will respond to the rapidly rising seas projected for the future, according to a new study. Using ancient mangrove sediments, the authors reveal key rates of sea-level rise - a maximum of 7 millimeters (mm) per year - a limit beyond which mangrove ecosystems cease to function. Under current climate scenarios, this threshold is expected to be surpassed within the next 30 years, the authors say. Mangrove forests, which grow in warm shallow, tidal waters, provide essential ecosystem services to humans, plants and animals around the world, including protection from intense ocean storms and waves. They also support some of the highest rates of carbon sequestration of all ecosystems. They are also threatened by rapid sea-level rise (RSLR). However, their response to rising waters remains elusive due to a lack of long-term observational data. To better gauge it, Neil Saintilan and colleagues investigated mangrove accretion at the end of the last glaciation, 10,000-7,000 years before present, when rates of sea-level rise were even higher than they are today. Because sea-level rise during this period was so dynamic, the approach allowed the authors to identify critical RSLR thresholds of mangrove survival. Saintillan et al. used sediment cores from 78 tropical and subtropical sites around the world to model mangrove growth and decline relative to the sea-level rise. Their analysis revealed an upper threshold of 7 mm per year as the maximum rate of sea-level rise associated with mangrove forest survival. Although in some shallow reef settings, this limit was as low as 5mm per year. What's more, the sediment core data allowed the authors to estimate the amount of carbon sequestered as mangrove forests expanded during the early Holocene, suggesting a conservative 85 gigatons, which corresponded with a five parts-per-million reduction in atmospheric carbon dioxide. In a Related Perspective, Catherine Lovelock discusses the study's findings in detail.
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Journal
Science