Coastal mangrove forests stretch through the intertidal zone between land and ocean, where they provide a buffer to storms and floods and bind vast amounts of so called ‘blue carbon’) with their often conspicuous roots. These aboveground roots are also the ‘lungs’ of the mangroves, breathing air for the part of the day that they are not submerged (on land, the soil contains air that the roots benefit from, but mangrove soil is wet, muddy and poor in oxygen).
However, not even mangroves can coop with too much water without drowning. When the sea level rises, as it currently does (by ~3.4 mm per year) the mangroves compensate through vertical growth, but if the rate of sea level rise continues to accelerate, the mangroves will not be able to keep up. The new study was able to predict just how much sea level rise we can expect mangroves to be able to coop with, about 6mm per year, and when this threshold will be reached, given different emission scenarios.
Models suggest that if we manage to reduce carbon emissions (low-emissions scenario), the rate of sea level rise will increase to ~5 mm per year by 2100, which would mean mangroves can adjust by vertical growth. But if we continue to let carbon emissions accelerate (high-emissions, or “business-as-usual” scenarios), sea level rise could be increasing by as much as ~10 mm per year by 2100, threatening the existence of many mangrove forests, as well as the human societies that depend on them.
Estimating what rate of sea level rise that mangroves can tolerate is not a simple task; the researchers used ancient coastal sediment samples to look back 10.000 years in time to when the sea level varied in response to (non-anthropogenic) climate change at the end of the last glaciation. Periods of mangrove growth and dieback could be traced in their carbon remains preserved in the sediment.
Link to original article: Thresholds of mangrove survival under rapid sea level rise
Perspectives article: Blue carbon from the past forecasts the future